Enhancing the performance of the Digital Cherenkov Viewing Device : Detecting partial defects in irradiated nuclear fuel assemblies using Cherenkov light

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ACTA UNIVERSITATIS

UPSALIENSIS UPPSALA

Digital Comprehensive Summaries of Uppsala Dissertations

from the Faculty of Science and Technology

1708

Enhancing the performance of the

Digital Cherenkov Viewing Device

Detecting partial defects in irradiated nuclear fuel

assemblies using Cherenkov light

ERIK BRANGER

ISSN 1651-6214 ISBN 978-91-513-0415-1

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Dissertation presented at Uppsala University to be publicly examined in Room 2005, Ångströmlaboratoriet, Lägerhyddsvägen 1, Uppsala, Friday, 12 October 2018 at 13:00 for the degree of Doctor of Philosophy. The examination will be conducted in English. Faculty examiner: Dr. Christopher Orton (Pacific Northwest National Laboratory, PNNL).

Abstract

Branger, E. 2018. Enhancing the performance of the Digital Cherenkov Viewing Device. Detecting partial defects in irradiated nuclear fuel assemblies using Cherenkov light. Digital

Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology 1708. 97 pp. Uppsala: Acta Universitatis Upsaliensis. ISBN 978-91-513-0415-1.

The Digital Cherenkov Viewing Device (DCVD) is an instrument used by authority safeguards inspectors to verify irradiated nuclear fuel assemblies in wet storage based on Cherenkov light emission. It is frequently used to verify that parts of an assembly have not been diverted, which is done by comparing the measured Cherenkov light intensity to a predicted one.

This thesis presents work done to further enhance the verification capability of the DCVD, and has focused on developing a second-generation prediction model (2GM), used to predict the Cherenkov light intensity of an assembly. The 2GM was developed to take into account the irradiation history, assembly type and beta decays, while still being usable to an inspector in-field. The 2GM also introduces a method to correct for the Cherenkov light intensity emanating from neighbouring assemblies. Additionally, a method to simulate DCVD images has been seamlessly incorporated into the 2GM.

The capabilities of the 2GM has been demonstrated on experimental data. In one verification campaign on fuel assemblies with short cooling time, the first-generation model showed a Root Mean Square error of 15.2% when comparing predictions and measurements. This was reduced by the 2GM to 7.8% and 8.1%, for predictions with and without near-neighbour corrections. A simplified version of the 2GM for single assemblies will be included in the next version of the official DCVD software, which will be available to inspectors shortly. The inclusion of the 2GM allows the DCVD to be used to verify short-cooled assemblies and assemblies with unusual irradiation history, with increased accuracy.

Experimental measurements show that there are situations when the intensity contribution due to neighbours is significant, and should be included in the intensity predictions. The image simulation method has been demonstrated to also allow the effect of structural differences in the assemblies to be considered in the predictions, allowing assemblies of different designs to be compared with enhanced accuracy.

Keywords: DCVD, Nuclear safeguards, Cherenkov light, Nuclear fuel assembly, Partial defect

verification

Erik Branger, Department of Physics and Astronomy, Applied Nuclear Physics, Box 516, Uppsala University, SE-751 20 Uppsala, Sweden.

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There can be no doubt that the usefulness of this radiation [Cherenkov light] will in the future be rapidly extended. -Pavel ˘Cerenkov [1]

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List of papers

This thesis is based on the following papers, which are referred to in the text by their Roman numerals.

I E. Branger, S. Grape, S. Jacobsson Svärd, P. Jansson, E. Andersson

Sundén, On Cherenkov light production by irradiated nuclear fuel rods. Journal of Instrumentation, June 2017. DOI:

10.1088/1748-0221/12/06/T06001

My contribution: I made the simulations and analysed the results. I am the main author of the paper.

II E. Branger, S. Grape, S. Jacobsson Svärd, P. Jansson, E. Andersson

Sundén, Comparison of prediction models for Cherenkov light emissions from nuclear fuel assemblies. Journal of Instrumentation, June 2017. DOI: 10.1088/1748-0221/12/06/P06007

My contribution: I made the simulations and analysed the results. I am the main author of the paper.

III E. Branger, S. Grape, P. Jansson, S. Jacobsson Svärd, Improving the

prediction model for Cherenkov light generation by irradiated nuclear fuel assemblies in wet storage for enhanced partial-defect veriﬁcation capability. The ESARDA Bulletin issue no. 53, June 2016.

My contribution: I made the simulations and proposed the prediction method. I am the main author of the paper.

IV E. Branger, S. Grape, P. Jansson, E. Andersson Sundén, S. Jacobsson

Svärd, Investigating the Cherenkov light production due to cross-talk in closely stored nuclear fuel assemblies in wet storage. Paper presented

at the 39thESARDA Annual Meeting, 16-18 May 2017, Düsseldorf,

Germany. Accepted for publication in the ESARDA Bulletin.

My contribution: I made the simulations and proposed the prediction method. I am the main author of the paper.

V E. Branger, S. Grape, P. Jansson, S. Jacobsson Svärd, Experimental

evaluation of models for predicting Cherenkov light intensities from short-cooled nuclear fuel assemblies. Journal of Instrumentation, February 2018. DOI: 10.1088/1748-0221/13/02/P02022

My contribution: I made the analyses of the results and the simulations. I am the main author of the paper.

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VI E. Branger, S. Grape, P. Jansson, S. Jacobsson Svärd, On the inclusion of light transport in prediction tools for Cherenkov light intensity assessment of irradiated nuclear fuel assemblies. Manuscript. My contribution: I made the simulations and the analyses. I am the main author of the paper.

VII E. Branger, S. Grape, P. Jansson, S. Jacobsson Svärd, Experimental

study of background subtraction in Digital Cherenkov Viewing Device measurements. Journal of Instrumentation, August 2018. DOI: 10.1088/1748-0221/13/08/T08008

My contribution: I did the measurements and the analyses. I am the main author of the paper.

Reprints were made with permission from the publishers.

Additional papers part of this work, but not included in the thesis:

i E. Branger, E. L. G. Wernersson, S. Grape, S. Jacobsson Svärd, Image analysis as a tool for improved use of the Digital Cherenkov Viewing Device for inspection of irradiated PWR fuel assemblies. Report, June 2014. Available in DiVA: diva2:3A766776

My contribution: I assisted in writing the report.

ii E. Branger, S. Grape, S. Jacobsson Svärd, E. L. G. Wernersson, Im-proved DCVD assessments of irradiated nuclear fuel using image anal-ysis techniques. Paper presented at the 55th INMM Annual Meeting, Atlanta, USA, 2014.

My contribution: I wrote and presented the paper. I am the main author of the paper.

iii E. Branger, S. Grape, S. Jacobsson Svärd, E. L. G. Wernersson, To-wards unattended partial-defect veriﬁcation of irradiated nuclear fuel assemblies using the DCVD. Paper presented at the IAEA Symposium on International Safeguards, Vienna, Austria, 2014.

My contribution: I wrote and presented the paper. I am the main author of the paper.

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List of abbreviations

1GM First-generation prediction model

2GM Second-generation prediction model

AP Additional Protocol

BU Burnup

BWR Boiling Water Reactor

Clab Swedish Central Interim Storage Facility for Spent Nuclear Fuel

CT Cooling time

DA Destructive Assay

DCVD Digital Cherenkov Viewing Device

IAEA International Atomic Energy Agency

ICVD Improved Cherenkov Viewing Device

IE Initial enrichment

NDA Non-Destructive Assay

NN Near-Neighbour

NNWS Non Nuclear Weapons State

NPT Non-Proliferation Treaty

NWS Nuclear Weapons State

PWR Pressurized Water Reactor

ROI Region Of Interest

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1. Introduction

1.1 The need for nuclear safeguards

Shortly after the discovery of nuclear fission [2], i.e. the splitting of atomic nuclei, researchers realized that vast amounts of energy could be released by creating a fission chain reaction. This energy has found peaceful applications in terms of electricity production in nuclear power plants, but also destructive use in terms of nuclear weapons. Whether used peacefully or for military purposes, nuclear energy requires nuclear material, i.e. material that is fissile or can be converted to fissile material through nuclear reactions. A fissile material contains nuclei that can fission following the absorption of a neutron, while emitting one or several neutrons following the fission, thus allowing a chain reaction to take place.

To promote peaceful use of nuclear energy and nuclear technology, and to inhibit its use for nuclear weapons or other military purposes, the Inter-national Atomic Energy Agency (IAEA) was founded in 1957 [3]. About a decade later, the work of the IAEA was significantly expanded when the Treaty on Non-Proliferation of Nuclear Weapons (NPT) opened up for sig-natures in 1968. To ensure that nuclear materials and technologies are used peacefully, the NPT signatory states are required to sign a nuclear safeguards agreement with the IAEA. The safeguards agreement give the IAEA the right to inspect and verify a state’s nuclear facilities, and to verify a state’s posses-sion of nuclear material. The IAEA also verifies that no undeclared nuclear activities take place within the state. Through these inspections the IAEA can provide credible, independent confirmation that states are using their nuclear technologies and materials only for peaceful purposes, and that no nuclear ma-terial is diverted to any clandestine nuclear weapons program, or for any other non-peaceful purposes.

The civilian nuclear fuel cycle contains vast amounts of nuclear material that is placed under safeguards. As of 2018, there are about 450 commercial electricity-producing nuclear reactors in operation worldwide, with another 60 under construction [4]. The nuclear material under safeguards is moni-tored and verified throughout the entire nuclear fuel cycle, i.e. from when it is mined, through its conversion to and usage as nuclear fuel, as well as though any reprocessing to make new fuel from used fuel, and until its disposal in a final repository. Verifying that all nuclear material is accounted for and only used peacefully in each step of the nuclear fuel cycle is a massive undertaking, and consequently the inspections executed by the IAEA need to be efficient, accurate and comprehensive.

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To aid the authority inspectors in verifying nuclear material in its various forms, a multitude of instruments have been developed to independently ver-ify, describe, quantify or characterize nuclear material [5]. This thesis cov-ers developments of analysis tools related to one such instrument, the Digi-tal Cherenkov Viewing Device (DCVD). The DCVD is used to measure the Cherenkov light produced by spent nuclear fuel assemblies in wet storage, i.e. stored in water pools. Based on the presence, characteristics and intensity of the Cherenkov light, the properties of the nuclear fuel assembly can be veri-fied. Considering the vast amounts of nuclear material existing in the form of spent nuclear fuel assemblies in this type of storages worldwide, it is important that the verification tools are both accurate and time-efficient. Accordingly, at-tention is paid to both the practical aspects of the developed tools and to the accuracy and precision in comparison to the currently used tools.

1.2 Outline of the thesis

This thesis is based on seven scientific papers, which can be found at the end of this thesis. The key findings of all the papers are presented in the compre-hensive summary, of which you are now reading the first chapter.

Chapter 2 presents the fundamentals of nuclear safeguards, in terms of his-tory, aims, and techniques and methods used. It also presents which nuclear materials are under safeguards, how the materials are veriﬁed, and provides the context in which the work summarized in this thesis should be seen.

Chapter 3 introduces nuclear fuel assemblies, their design, and safeguards aspects that should be considered when verifying spent nuclear fuel assem-blies. Important parameters describing the assemblies are presented, and dif-ferences in physical design for assemblies of different reactor types are dis-cussed.

Chapter 4 presents Cherenkov light and the physics behind its occurrence, and discusses how the Cherenkov light can be used to verify nuclear fuel as-semblies.

Chapter 5 introduces the DCVD and the measurement methodology used, and details how the DCVD data is used to verify spent nuclear fuel assem-blies in wet storage. The chapter also presents some earlier research that has been done on safeguards veriﬁcation with the DCVD prior to this thesis, and presents how this work allows for the capabilities of the instrument and asso-ciated analyses to be extended.

Chapter 6 summarizes the Monte-Carlo simulations that have been per-formed as part of this work. The code used for the simulations is presented, and its capabilities are shown. Simulation results are presented for nuclear fuel rods, assemblies, and assemblies stored close to other neighbouring as-semblies in wet storage. These results are primarily based on papers I and II, but also includes papers IV and VI.

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Chapter 7 presents the Cherenkov light prediction tools that have been de-veloped in this work, and how these tools extend the capabilities of the DCVD veriﬁcation methodology compared to the previously used one. The tools de-veloped can be used to predict the Cherenkov light production in isolated as-semblies, to predict the light contribution due to nearby neighbouring assem-blies, and to predict the effect on the measured light intensity due to various structural components of the assemblies. The performance of the prediction tools have been evaluated based on experimental data. The results presented in this chapter are based on papers III, IV, V and VI.

Chapter 8 presents work done on improving the background-subtraction method used in DCVD measurements. An improved method is proposed and evaluated using experimental data. This chapter is based on paper VII.

Finally, chapter 9 provides some concluding remarks on what have been considered the most important outcomes of this work, and chapter 10 discusses possible future work that can be done to further improve the performance of the DCVD and its associated analysis, based on the key findings of this thesis. For formal reasons, it should be noted that parts of chapter 2 and 4 are based on the author’s licentiate thesis [6]. The material has been adapted to better fit into this work, but some portions of the text and some figures may have remained identical to [6].

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2. Nuclear safeguards

2.1 The legal framework for nuclear safeguards

The Nuclear Non-Proliferation Treaty (NPT) opened for signatures in 1968 and entered into force in 1970. To date, the treaty has 191 signatory states [7], making it one of the most adhered to arms limitation and disarmament treaties in history. Currently, only India, Israel, Pakistan and South Sudan have not signed the treaty, and North Korea signed the treaty in 1985 but withdrew in 2003. The NPT serves three main purposes:

• To prevent the proliferation of nuclear weapons.

The NPT prohibits the ﬁve nuclear weapon states (NWS) recognized by the treaty (China, France, Russia, the United Kingdom and the United States) from transferring nuclear weapons to other states. The treaty also prohibits the NWS from transferring equipment that can be used to produce nuclear weapons, or to encourage other states to obtain nu-clear weapons. The signatory non-nunu-clear weapons states (NNWS) are obliged to refrain from receiving assistance in or trying to develop nu-clear weapons.

• To promote nuclear disarmament.

The NPT states that the NWS shall pursue negotiations in good faith for nuclear disarmament, though the NPT does not impose any nuclear disarmament agreements itself, and it does not set any time limit on when the disarmament should be completed.

• To promote peaceful use of nuclear technology.

The NPT acknowledges the right of all parties to the treaty to develop nuclear technology for peaceful purposes. The treaty also encourages in-ternational cooperation on nuclear development, provided that the states can demonstrate that their nuclear programs are not being used for the development or production of nuclear weapons.

As part of the NPT, the signatory NNWS are required to sign a safeguards agreement with the International Atomic Energy Agency. Under this agree-ment, all nuclear material and nuclear activities shall have safeguards applied to them, to verify that no nuclear material is diverted for production of nuclear weapons, and that the nuclear facilities are used only for peaceful purposes. The objectives of nuclear safeguards is the timely detection of diversion of nu-clear material for the manufacture of nunu-clear weapons, or for other unknown purposes, and the deterrence of diversion by the risk of early detection.

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The safeguards agreement signed by the NNWS is called the Comprehen-sive Safeguards Agreement [8], and under this agreement the IAEA has the right and obligation to ensure that safeguards measures are applied to all nu-clear material in the state, and to verify that no material diversion takes place. Consequently, the IAEA can and shall provide credible and independent assur-ances that states are honouring their obligations and do not pursue obtaining nuclear weapons.

In addition to the comprehensive safeguards agreement, a total of 146 states have also signed the Additional Protocol (AP), further extending the rights of the IAEA to inspect facilities suspected of being used for nuclear activities. More information on the IAEA safeguards legal framework can be found in [9].

In addition to the IAEA, other organizations are also part of the international safeguards work. For example, Euratom is a European organization founded under the Euratom treaty, with the purpose of creating a specialist market for nuclear power in Europe, and developing nuclear energy in Europe. As part of their work, Euratom perform inspections at nuclear facilities, and the goals of the inspections includes verifying that no nuclear material has been diverted, and that no nuclear facility is used for other purposes than intended. Euratom frequently performs safeguards inspections together with the IAEA in the Eu-ropean States.

There are also national organizations working with domestic safeguards; one example is the Swedish Radiation Safety Authority, or Strålsäkerhetsmyn-digheten (SSM). SSM has a mandate to work proactively and preventively with nuclear safety, radiation protection and nuclear non-proliferation in Swe-den [10]. Within nuclear non-proliferation the authority works with export control, safeguards as well as illicit trafficking of nuclear material. SSM has also been appointed the task by the Swedish parliament of providing the IAEA with a support program concerning safeguards, where research and training, specifically for spent nuclear fuel verification, plays a major role. Further-more, SSM supports scientific research of value for the work of the authority, which provides a scientific foundation to its recommendations and regulations.

2.2 Material and facilities under safeguards

The foundation for verifying that no diversion of nuclear material has occurred lies in material accountancy. Under a safeguards agreement, a State must es-tablish a bookkeeping system containing all nuclear material present in the State, and any material entering or exiting the state. The IAEA performs in-spections to verify that the bookkeeping is correct and complete, and that all material is accounted for, thus verifying that no material has been diverted. During 2016, the IAEA collected and evaluated over 1 million nuclear mate-rials reports [11].

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A central concept in IAEA safeguards is the "Signiﬁcant Quantity" (SQ), which is "the approximate amount of nuclear material for which the possibility of manufacturing a nuclear explosive device cannot be excluded" [12]. The concept of a SQ takes into account unavoidable losses in the conversion and manufacturing processes required to produce a nuclear weapon. A SQ should not be confused with the critical mass of a nuclear material, which is lower.

Two elements of particular importance for nuclear safeguards are uranium and plutonium. For uranium, there is one naturally occurring ﬁssile isotope,

235_{U. This isotope can be used in nuclear weapons, but it must ﬁrst be}

sepa-rated from the much more abundant isotope238U, since natural uranium

con-tains about 99.3%238U and only 0.7%235U. The process of removing238U to

increase the fraction of 235U is called enrichment, and for a nuclear weapon

the uranium is typically enriched to more than about 90% 235U. Plutonium,

on the other hand, does not occur in nature, but can be produced in a nuclear

reactor. If238U absorbs a neutron, it will turn into239U, which will beta decay

twice, at a half-life of in the order of a few days, turning it into 239Pu, i.e.

uranium is transmuted to plutonium. Once plutonium has been produced in a reactor, it can be chemically separated from the fuel, which is less complicated than uranium enrichment.

The mass of one SQ depends on the type of material, and the SQ values for

235_{U and plutonium are given in table 2.1. Note that for plutonium, certain}

iso-topes are undesirable in a nuclear weapon for practical reasons, but within the safeguards framework all Pu isotopes are conservatively considered equally useful for weapons manufacturing [13]. By the end of 2016, there were a total of 204 000 SQs of nuclear material under IAEA safeguards [11].

Table 2.1. Signiﬁcant quantities for ﬁssile materials of relevance in the context of nuclear fuel [12].

Material Signiﬁcant Quantity

Pu 8 kg

High-enriched U (235U≥ 20%) 25 kg235U

Low-enriched U (235U< 20%) 75 kg235U

The IAEA has also set up detection timeliness goals, to ensure that the di-version of any nuclear material can be detected before the diverter has had enough time to convert it to a nuclear weapon. Different materials have dif-ferent timeliness goals, reﬂecting the estimated time required to convert the material to a weapons-useable form, and these timeliness goals determine the frequency of inspections for these materials. Nuclear materials that can be used in the manufacturing of nuclear weapons without further enrichment or transmutation are referred to as direct-use materials, and are associated with short timeliness goals and frequent inspections. Materials falling in this cat-egory are e.g. Pu in fresh nuclear fuel assemblies of mixed-oxide type (see

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addi-tional enrichment or transmutation before being usable in a nuclear weapon,

such as low-enriched U (235U< 20%), have longer timeliness goals. Irradiated

nuclear material such as spent nuclear fuel is also associated with longer time-liness goals, due to the difficulty of handling the strongly radioactive fission products present, and the difficulty of separating the nuclear material from all other fission products. The timeliness goals specified by the IAEA are pre-sented in table 2.2.

Table 2.2. Timeliness goals deﬁned by the IAEA for detecting diversion of one or more SQ of nuclear material [12]. Direct use material refers to material that can be used to manufacture a nuclear weapon without transmutation or further enrichment.

Material Example of material Timeliness goal

Unirradiated direct- MOX fuel 1 month

use material High-enriched U (235U≥ 20%)

Irradiated direct- Spent fuel 3 months

use material

Indirect- Low-enriched U (235U< 20%) 12 months

use material

In addition to material accountancy, design schematics of all nuclear facil-ities must be provided to the IAEA by the signatory States, and the physical design of the facilities is veriﬁed through inspections. This allows the IAEA to verify that the facilities are being used as declared. The inspections are also aimed at verifying the absence of undeclared activities, and with the introduc-tion of the AP, the IAEA has gained extended rights to inspect also undeclared facilities in a State, further strengthening this capacity.

2.3 Safeguards veriﬁcation of nuclear material

Several instruments have been developed to assess nuclear material, in order to allow inspectors to independently characterize and verify it. A comprehensive survey of the safeguards techniques and equipment used by the IAEA can be found in [5].

Two general types of methods are commonly used when verifying nuclear material, Destructive Assay (DA) and Non-Destructive Assay (NDA). DA measurements are typically performed on samples of nuclear materials, which are sent to laboratories for analysis. Parts of, or the full sample, is consumed in the analysis, since DA methods requires that the samples are altered chem-ically or physchem-ically. DA usually offers superior precision compared to NDA, being able to identify a material and its isotopic composition with a high level of detail. Some downsides are that DA can only be used when it is possible to extract samples for analysis, such as when the nuclear material is handled

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in bulk, and that sending a sample to a laboratory and analysing it is time-consuming and sometimes difﬁcult.

NDA measurements typically make use of the gamma or neutron radiation emissions from a nuclear material. This radiation carries information about the material, and since it can penetrate relatively thick layers of materials, such as some storage containers, the radiation can be detected from the outside of the container. Thus, NDA measurements can verify and characterize nuclear material without altering the material itself, and there is often no need to open up a container to verify its contents. NDA measurements are typically quick, and may give immediate information about the measurement results, but they are rarely as precise as DA measurements.

Veriﬁcation of nuclear material and its accountancy can be done at different levels of precision, depending on the inspection goals, the material properties and on which instruments are available to assess them. The three main levels of veriﬁcation used by the IAEA are [12]:

• Gross defect verification, where the inspector verifies the presence or absence of nuclear material in an inspected item. This level of verifi-cation is often performed using NDA techniques, since low precision is required and fast measurements are preferred.

• Partial defect verification, where the inspector verifies that a fraction of the nuclear material has not been diverted from an inspected item. The current IAEA requirement on instruments used to perform this level of verification is that they must be able to reliably detect a 50% removal or substitution of nuclear material from the item.

• Bias defect veriﬁcation, where the inspector veriﬁes that small portions of the nuclear material has not been diverted from an inspected item. This requires instruments and measurements with a high degree of pre-cision.

Once the nuclear material has been successfully verified, the IAEA deploys containment and surveillance (C/S) techniques to verify that no material is di-verted at a later stage. Commonly used C/S techniques involve seals used to verify that containers remain sealed, or video surveillance to monitor that no diversion activities occurs at a site. The seals and surveillance ensure that the authorities maintain Continuity of Knowledge (CoK), by knowing that the ma-terial has not been diverted or altered since the last inspection. By maintaining CoK, the absence of diversion can be verified by inspecting the applied C/S devices, without having to re-verify the nuclear material. However, should the CoK be lost, all material affected will need to be re-verified, to ensure that no material has been diverted during the time that the CoK was lost.

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3. Nuclear fuel assemblies

The nuclear material used in a civilian power-producing reactor is in the form of Nuclear Fuel Assemblies. The design of these assemblies ensures that a controlled fission chain reaction can take place, and that the heat produced in the fuel material can be transported away, to be used to produce electricity. The design also ensures that radioactive fission products produced by fission events stay inside the assembly, and that no radioactive isotopes leak out into the reactor. A nuclear fuel assembly is normally the smallest unit of nuclear material handled at a reactor site, and is often referred to as an item in a safe-guards inspection (see section 2.3).

The two most common types of commercial reactors in the world are Boil-ing Water Reactor (BWR) and Pressurized Water Reactor (PWR). In a BWR, the energy released by the nuclear chain reaction produces steam directly in-side the reactor vessel, which is led to a turbine where the steam is used to produce electricity. In a PWR, the high pressure in the reactor vessel keeps the water from boiling. Instead, the hot water is led to a steam generator, where it produces steam in a secondary loop. Due to the different modes of operation of these facilities, BWR and PWR nuclear fuel assemblies have noticeably different designs. Still, many of their properties are similar, and safeguards veriﬁcation procedures do not differ signiﬁcantly between fuel as-sembly types.

3.1 Physical design of nuclear fuel assemblies

Most of the commercial nuclear reactors in the world use uranium dioxide

(UO2) as fuel, enriched to about 4-5% of235U, in order to increase the ﬁssile

content. A few reactors use a mixture of UO2and plutonium dioxide (PuO2),

which is referred to as Mixed-Oxide Fuel (MOX). In MOX fuel,239Pu is

gen-erally the dominant ﬁssile isotope.

To manufacture a nuclear fuel assembly, the fuel material is ﬁrst turned into

cylindrical pellets, with a height and diameter of typically about 1 cm. UO2

and MOX are ceramic materials, which can withstand high temperatures with-out melting, and which can trap ﬁssion products inside the fuel material to ensure that they do not leak out. Next, several hundred pellets are stacked inside a metal tube, forming a fuel rod or pin, which is typically around 4 m in length. The tube, or cladding, is generally made of zircaloy, a metal al-loy consisting primarily of zirconium, with a thickness of about 1 mm. Since

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zirconium has a small neutron capture cross-section, few neutrons are lost by being absorbed by the zirconium, which is beneﬁcial for sustaining the ﬁssion chain reaction. The fuel rods are assembled into a fuel assembly, with a BWR assembly typically containing 60-100 rods and a PWR assembly typically con-taining 200-300 rods. The fuel rods are held in position by spacers, which are manufactured from zircaloy or stainless steel (typically Inconel steel), and by steel top and bottom plates. At the top plate there is also a lifting handle, used when moving the assembly. In total, the length of an assembly is on the order of 4 m, and the width is around 10-25 cm.

As a gross means to control the neutron flux in the reactor, a BWR uses control rods (sometimes called control blades for a BWR), containing a neu-tron absorbing material, which can be inserted or removed from the reactor core during operation. The main use of the control rods are for stopping and starting the reactor. The control rods are inserted in between the assemblies, consequently the outer dimensions of all BWR assemblies in one reactor core must be similar to provide space for the control rods. However, the configura-tion of fuel rods inside the assembly can be chosen more freely. Thus, BWR assemblies of noticeably different designs can be present in the same reactor, and the number of fuel rods and their dimension may vary from assembly to assembly. In addition to the fuel rods, which are typically arranged in a square lattice in the assembly, there may also be water channels present to provide a higher flow of water in desired parts of the assembly. Some rod positions may contain part-length rods, to ensure that there is more space between the fuel rods at the top of the assembly as water turns into steam, which requires more space. BWR assemblies also feature a fuel channel, i.e. the entire assembly is placed inside a zircaloy containment box, to ensure that water does not escape the assembly when boiling occurs. In total, a fresh BWR assembly typically

contains 200-300 kg of UO2or MOX.

A PWR also uses control rods, but these are inserted into the assemblies rather than in between them. As for the BWR, their main use is for starting and stopping the reactor. Since the control rods are inserted into the assemblies, the PWR assemblies feature guide tubes into which the control rods are inserted. For all assemblies accepting the same control rod type, the guide tubes must be placed in the same position, and because of optimizations, the fuel rod placement and dimensions will vary little for these assemblies. However, there are still some variations. One notable difference in the context of this work is that the top plate designs may vary noticeably, which affect the transport of Cherenkov light (see section 7.2.3). PWR assemblies also tend to be bigger than BWR ones, as illustrated in ﬁgure 3.1, and they typically contain around

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Figure 3.1. Left: Example of the fuel rod placement in a PWR 17x17 assembly Right: Example of the fuel rod placement in a BWR 8x8 assembly. The Cherenkov light production in these two assembly types have been studied in chapter 6. The BWR assembly has one rod position functioning as a water channel, and is enclosed by a fuel channel, illustrated schematically in the ﬁgure. The PWR assembly features a central instrumentation tube, and 24 control rod guide tubes. The BWR assembly is 13.6 cm wide, and the PWR assembly is 21.4 cm wide. The assembly height is around 4 m.

3.2 Fuel usage in a reactor

Nuclear power plants typically operate in cycles, with a long period of running the reactor, and a short period of downtime for replacing spent fuel assemblies and performing maintenance work. For Swedish reactors, the running time is typically about 11 months, with one month of downtime. The downtime is generally scheduled in the summer when the Swedish electricity consumption is lower. For countries with less pronounced seasonal electricity usage differ-ences, longer cycles are often used, and 18 or even 24 months are common. Signiﬁcantly longer running times are however not possible in most commer-cial reactors, since the reactor core cannot be loaded with too much ﬁssile material, and the fuel must be replaced once it is used up.

As the fuel material undergoes fission in the reactor, fission products are created, which build up as the fuel is irradiated. Many of the fission prod-ucts are radioactive, and will decay until they have turned into stable isotopes. Several fission products are relatively long-lived, and consequently the fuel as-semblies will emit radiation also after being discharged from the reactor. The activity is high enough that a noticeable amount of decay heat is produced. To shield the environment from the radiation, and to cool the residual heat, the assemblies are often stored in water. For this reason, reactors generally have a storage pool next to the reactor to store recently discharged assemblies. A civilian nuclear reactor, such as the ones in Sweden, will typically replace 20-25% of its fuel assembly inventory each year, corresponding to 40-140 as-semblies, or 20-40 significant quantities of Pu. These assemblies are stored at the reactor for one to two years, after which their radioactivity has decayed

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to a level low enough that they can be moved to the central Swedish interim storage for spent nuclear fuel, Clab. Worldwide, dry storages are also com-mon, where the spent fuel assemblies are put in massive canisters to shield the surrounding environment from the radiation.

Other than undergoing nuclear ﬁssion, the elements in the nuclear fuel may instead absorb neutrons, and turn into heavier isotopes. Thus, plutonium is produced, as described in section 2.2, as well as even heavier isotopes. These heavy metals are radioactive, and some have very long half-lives resulting in a

need for long-term storage of the used fuel material. When a UO2fuel

assem-bly is discharged, it contains about 1% 235U, 1% Pu, 3-4% ﬁssion products,

and the rest is238U, with some trace amounts of elements heavier than

pluto-nium present.

Two parameters used to describe a spent nuclear fuel assembly are its bur-nup (BU) and cooling time (CT). The BU is a measure of the amount of energy that has been released from the fuel material through ﬁssion, and it is given in units of MWd/kgU (Mega-Watt days per kilogram of uranium) or GWd/tU (Giga-Watt days per ton of uranium). Typical BU values of discharged com-mercial nuclear fuel assemblies are in the order of 40-50 MWd/kgU, or equiv-alently around 1 million kWh per kg uranium. CT is the time since the assem-bly was discharged from the reactor. High BU results in a large production of radioactive isotopes in the fuel, while long CT implies that a relatively larger fraction of the activity has decayed away.

Once a fuel assembly has been discharged from a reactor following its ﬁnal cycle, it is often referred to as either a spent or a used fuel assembly. In this work, assemblies are generally referred to as irradiated, since assemblies that will be further irradiated in the reactor are also of interest.

Since the nuclear power plants optimize the use of the nuclear fuel, most assemblies are used in a similar way. Depending on the reactor type, the fuel assemblies are used for 3-6 years, until they have reached their design BU, after which they are discharged. Occasionally, assemblies will be used in a different way from this standard usage. As an example, a fuel assembly may require reparation after suffering damage, which will result in it spending one or a few cycles outside the reactor, before being used again. Furthermore, some assemblies from the ﬁrst core loading of a reactor may need to be re-placed more quickly, resulting in a low burnup at discharge.

3.3 Safeguards veriﬁcation of irradiated nuclear fuel

assemblies

Depending on the design of a nuclear fuel assembly, one or a few irradiated as-semblies will contain one SQ of nuclear material. Accordingly, an important task carried out by safeguards inspectors is verifying that no nuclear mate-rial is diverted from the irradiated fuel assemblies. Of particular importance

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is plutonium, since it is abundant in spent fuel, and can relatively easily be chemically separated from the other elements in the fuel assembly.

Verifying the nuclear material in an irradiated fuel assembly is a challeng-ing task, since the material is not accessible, preventchalleng-ing DA techniques from being used, and since the intense gamma and neutron radiation emitted by the fission products and minor actinides present interferes with direct NDA mea-surements of the fissile material. For this reason, safeguards verification of irradiated nuclear fuel assemblies often aim at verifying the BU and CT of the assemblies. This is often done by measuring the radiation emitted by the fission products, to verify that the abundance of fission products is consistent with irradiated nuclear fuel. While such indirect measurements do not assess the quantity of nuclear material, which is what is of interest to safeguards, the measurements can indicate that the nuclear fuel assembly has been used as declared, and that it has not been tampered with. The measurements can thus give an indication that no diversion has taken place, even if the nuclear material is not measured. An additional complication is that the intense radi-ation emitted by the assembly necessitates that it is stored in strong radiradi-ation shielding, which may make it difficult to place a detector close to the assem-bly. In the case of assemblies in wet storage, the measuring equipment may have to be submerged in the water to get close to the assembly, which presents additional technical challenges.

As mentioned in section 2.3, different diversion scenarios are considered, requiring different instruments and measurement methodologies to detect di-version in irradiated nuclear fuel assemblies:

• For gross defects, diverting one or a few irradiated assemblies is suffi-cient to divert one SQ of Pu. To detect this type of diversion, the entire inventory needs to be verified to find if any assemblies are missing or replaced with non-radioactive substitutes. Consequently, the measure-ments must be fast to be able to cover a large assembly inventory, but they do not need to be very precise, since they only have to determine if an item under study is radioactive or not. Out of the safeguards in-struments used by the IAEA to verify irradiated fuel assemblies [5], a majority are used for gross defect verification.

• For partial defects on the 50% level, diverting fuel rods from about 4-10 irradiated assemblies is sufficient to divert one SQ of Pu. This is still rel-atively few assemblies, and to ensure that these assemblies are covered in a verification campaign, a large part of the fuel assembly inventory needs to be measured. Thus, detecting this type of diversion calls for a fast measurement technique, which must also be sensitive enough to detect if 50% or more of the fuel rods have been removed or replaced with non-radioactive ones. The DCVD is suited for this scenario, since measurements are fast and since it can be used for partial defect verifi-cation [14], as further detailed in chapter 5. Other than the DCVD, the

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FORK detector is occasionally used for partial defect veriﬁcation at this level [15].

• For bias defects, diverting single rods from 200-300 irradiated assem-blies is required to divert one SQ of Pu. This is a substantial number of assemblies, and thus a sampling of a relatively small fraction of the assembly inventory is sufﬁcient to detect with high probability if this type of diversion has occurred. Consequently, measurement techniques for detecting this type of diversions can be more time-consuming, but they have to be highly precise to detect a single removed or substituted rod in an assembly. At present, gamma tomography is the only method approved by the IAEA to perform this type of veriﬁcation [16].

One may also identify diversion scenarios with defect levels in between the three presented levels. This opens up for additional considerations with respect to time consumption and precision of the techniques used. In this context, one may note that enhancing the precision of the DCVD assessments would open up for additional segments of the defect levels to be covered, which is one motivation for the work of this thesis.

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4. Cherenkov light

4.1 The physics of Cherenkov light

In 1934 the Soviet scientist Pavel ˘Cerenkov observed that when water was

subjected to ionizing radiation, it emitted blue light. While initially thought to be caused by ﬂuorescence, through careful observation he concluded that this light was produced by other means, and between 1934 and 1937 he published his investigations. In 1937 Ilya Frank and Igor Tamm provided a theoretical explanation of this light [17], explaining the mechanism behind its production and characterizing its properties. For this discovery and explanation, the three were awarded the Nobel Prize in physics in 1958 [18].

Cherenkov light is produced when a charged particle moves faster than the speed of light in a medium. While nothing can move faster than the speed of light in vacuum (c), the effective speed of light in a medium is lower. For

a medium with refractive index n it is vl = c/n, and for example in water

vl≈ 0.75c for visible light. Thus, any charged particle with velocity vpin the

range 0.75c < vp < c will radiate visible Cherenkov light in water. For an

electron, this corresponds to a threshold kinetic energy of about 250 keV. When a charged particle propagates in a dielectric medium, it will disrupt the local electromagnetic ﬁeld, polarizing the medium. If the particle moves slowly, this disruption will elastically relax back to an equilibrium state, and no photons are radiated. However, if the particle is moving faster than the speed of light in the medium, a disturbance is left in the wake of the particle, which will emit its energy in the form of a coherent shockwave, i.e. Cherenkov light is emitted. A common analogy to this phenomenon is the sonic boom of a supersonic aircraft.

When a charged particle emits Cherenkov light, the radiated photons form

an angle θ to the particle propagation direction. For a particle with velocity

β = vp/c travelling in a medium with refractive index n, this angle will follow the relation given by equation 4.1, as illustrated in ﬁgure 4.1.

cosθ = 1

βn (4.1)

The spectral characteristic of the Cherenkov light is given by the Frank-Tamm formula [17], presented in equation 4.2 [19]. This equation gives the

number of Cherenkov photons N emitted in a wavelength range dλ, for a

charged particle with electric charge z traversing a distance dx in the medium.

In equation 4.2, α is the so-called ﬁne-structure constant (which is

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θ vl·t =nc·t

vp·t = β · c ·t

Figure 4.1. Cherenkov light is produced at an angleθ to the charged particle propa-gation direction, determined by equation 4.1. Consequently, the produced Cherenkov light forms a cone. The length of the sides of the triangles are marked in the ﬁg-ure as a function of the particle and light velocity. Note that the refractive index n of a medium typically depends on photon wavelength, and as a result the angleθ is wavelength-speciﬁc.

larger than zero, which corresponds to particles moving fast enough to radiate

Cherenkov light, i.e. vp≥ vl.

d2N dxdλ = 2παz2 λ2 1− 1 β2_n2(λ) (4.2)

Equation 4.2 also show that the spectral intensity is proportional to 1/λ2.

As a result, the number of radiated photons of a short wavelength (such as blue) is higher than that of longer wavelengths (such as red). Consequently, Cherenkov light appears blue to the naked eye, although the Cherenkov light intensity can be even higher at shorter wavelengths, such as ultraviolet (UV). For Cherenkov light in water, one should note that at wavelengths shorter than UV, water is no longer transparent, and radiated Cherenkov photons of such wavelengths are immediately absorbed. No matter the medium, Cherenkov light cannot be produced with energy above roughly that of x-rays.

4.2 Cherenkov light from irradiated nuclear fuel

assemblies

As mentioned in chapter 3, irradiated nuclear fuel assemblies are often stored in water, both for radiation protection and for decay heat removal. The in-tense radiation emitted by these assemblies cause high-energy electrons to be

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200 250 300 350 400 450 500 550 600 650 0 0.2 0.4 0.6 0.8 1 Wavelength [nm] Relati v e Cherenk o v intensity [arb . unit] 10−4 10−3 10−2 10−1 100 101 Attenuation coef ﬁcient [cm − 1] Cherenkov spectrum (linear scale)

Attenuation (log scale)

Figure 4.2. The green line show the attenuation coefﬁcient of water for soft-UV and visible light (on the log scale on the right axis) based on data from [20] and [21]. The blue line show the Cherenkov light spectrum measurable by the safeguards instrument considered in this work (on the left axis). The spectrum was calculated using equation 4.2 including the refractive index of water, and assuming a 480 keV electron, corre-sponding to the maximum energy of an electron after Compton-scattering of a 662 keV

137_{Cs gamma. It was also assumed that the light must traverse 10 m of water, which}

attenuates the light, corresponding to the depth at which irradiated fuel assemblies are typically stored.

released in the water surrounding the fuel assembly. These electrons will in turn radiate Cherenkov light as they propagate through the water. Thus, the presence of Cherenkov light surrounding an item indicates that the item is ra-dioactive, and the quantity of the Cherenkov light depend on the quantity and energy of the radiation emitted by the item. Several safeguards instruments have been developed to measure the Cherenkov light emitted by a nuclear fuel assembly assembly, to verify that it is a strongly radioactive item and to quan-tify the Cherenkov light emitted. One such instrument, the DCVD, which is the subject of this thesis, is further described in chapter 5.

As shown in ﬁgure 4.2, the Cherenkov light intensity peaks in the soft-UV range in water. For this reason, the safeguards instrument considered in this work was designed to be sensitive to the UV-light component of the Cherenkov light, where the intensity is the highest. By using a UV ﬁlter, visible light com-ponents are also excluded from the measurements, making them less sensitive to background light such as facility lighting.

In irradiated nuclear fuel, several sources of ionizing radiation are present that cause Cherenkov light to be produced. One signiﬁcant source of radiation is electrons produced in beta decays. These electrons frequently have more than 250 keV of kinetic energy [22], allowing them to produce Cherenkov light in water. However, electrons are effectively stopped in materials as they

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continuously lose energy when interacting with electrons in the material. Elec-tron ranges tend to be on the order of 1 mm per MeV in dense material, and on the order of 2 mm per MeV in low-density materials [23]. Since the cladding thickness is on the order of 1 mm and typically beta energies are lower than 1 MeV, electrons produced in the fuel material through beta decays may be expected to contribute negligibly to the Cherenkov light intensity in the water surrounding the fuel assembly. However, such contributions to the Cherenkov light production may still require attention.

Another major source of ionizing radiation is gamma rays, which are fre-quently emitted in radioactive decays. Since gamma rays are high-energy photons, they carry no charge and do not directly emit Cherenkov light, but they can interact with matter, causing secondary high-energy electrons to be released. The gamma rays can also penetrate the fuel material and cladding relatively easily, hence they can interact in the water. The interactions of rele-vance to this work, i.e. those that can produce electrons with a kinetic energy above the 250 keV threshold for Cherenkov light production in water, are [23]: • Photoelectric absorption. An atom absorbs the gamma-ray photon, and as a result it ejects one of its electrons. Any energy carried by the initial photon above the binding energy of the ejected electron becomes kinetic energy. Since electrons are relatively loosely bound in light materials such as oxygen and hydrogen, a gamma ray with only slightly more energy than the threshold energy can cause production of Cherenkov light.

• Compton Scattering. The gamma ray scatters on an electron, transfer-ring parts of its energy to the electron. A gamma ray must have above roughly 420 keV of kinetic energy for a Compton-scattered electron to be able to recieve kinetic energy above the threshold for Cherenkov light production. Note that the cross section for scattering (as given by the Klein-Nishina formula [19]) gives at hand that the most likely scattering modes change the gamma-ray direction little, and consequently transfer very little energy to the electron. Instead the lower-probability, high-angle scattering is what can produce electrons of sufﬁcient energy to radiate Cherenkov light.

• Pair production. If a gamma ray has energy above twice the rest mass of an electron (i.e. above 1.02 MeV) it can be converted into an electron-positron pair, and any excess energy is converted into kinetic energy of the newly produced particles. A gamma ray with energy above roughly 1.5 MeV can then create an electron-positron pair having sufﬁcient ki-netic energy that they can radiate Cherenkov light in water. The positron will also annihilate on an electron in the material after it has lost most of its kinetic energy, which produces two 511 keV annihilation photons that may in turn result in the production of additional Cherenkov light.

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Figure 4.3. A schematic of the dominant path of Cherenkov light production caused by gamma decays in nuclear fuel assembly. (1) A gamma ray is emitted following a radioactive decay. (2) The gamma ray Compton-scatters on an electron in the water, transferring its energy to the electron. (3) The electron radiates Cherenkov light.

It has been shown in paper I that Compton scattering is the dominant type of interaction in water, when considering the radiation emitted by the fuel material with sufficient energy to produce Cherenkov light in water. This pro-cedure is illustrated in figure 4.3. In the fuel material, photoelectric absorption will dominate up to about 800 keV, which is low enough that the photoelec-tron is not expected to escape the fuel rod with sufficient energy to produce Cherenkov light in the water. At energies above 800 keV, Compton scattering becomes dominant, and such high-energy electrons could potentially escape the fuel with more than 250 keV of kinetic energy. Pair production is not expected to contribute significantly to the interactions for the gamma-ray en-ergies encountered in decays of fission product isotopes.

Heavier isotopes present in the fuel material may spontaneously fission, and will emit neutrons while doing so. These neutrons cannot directly pro-duce Cherenkov light due to their lack of charge, but they can cause other nuclear reactions to occur. The most likely neutron interaction of relevance to Cherenkov light production is absorption in a nuclei in the fuel material, which may produce new nuclei that decay with beta and/or gamma emissions. How-ever, the intensity of neutron emission is expected to be much lower compared to beta and gamma emissions, and consequently the neutrons are not expected to contribute significantly to the Cherenkov light production. Heavier isotopes can also alpha decay, but due to the low intensity and since the alpha par-ticles are easily stopped, they are not expected to contribute significantly to Cherenkov light production.

For the produced Cherenkov light to be detected, it must escape the fuel assembly and reach the detector. However, due to oxidation in the harsh re-actor environment, and due to material depositions (CRUD), the surfaces of any fuel rods or other structural components are typically dark, and thus the surfaces absorb any incoming visible or UV light to a large extent. Since Cherenkov light measurements are done from above the assembly, only the

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vertical Cherenkov light can escape the assembly and be detected, as more horizontally directed light is likely to encounter a fuel assembly surface and be absorbed. As an additional consequence, the Cherenkov light that can be detected will be highly collimated along the direction of the fuel rods, and the intensity that can be measured depends strongly upon how the instrument is aligned above the assembly. Thus, when investigating the Cherenkov light produced in an assembly and its propagation to a detector, the vertical light component is of primary interest.

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5. The Digital Cherenkov Viewing Device,

DCVD

5.1 History

Several instruments have been developed to assess irradiated nuclear fuel in wet storage based on the Cherenkov light produced, and the two instruments currently in use by the IAEA are the Improved Cherenkov Viewing Device (ICVD) and the Digital Cherenkov Viewing Device (DCVD). The ICVD is an analogue instrument, which converts UV light to visible light, allowing an inspector to visually inspect the Cherenkov light emitted by an assembly. The ICVD does not allow for storage of images for further processing or documen-tation. Furthermore, due to the modest efﬁciency of the light conversion, the ICVD is not sensitive enough to allow an inspector to verify assemblies with weak Cherenkov light emission, such as assemblies with long CT and low BU. The ICVD currently used (as of 2018) by the inspectors is the Mark IV CVD, which has been in use for close to thirty years [24].

The DCVD was originally developed to enable verification also of low-intensity assemblies using Cherenkov light. The initial target was the ability to reliably verify assemblies with a cooling time of 40 years and a burnup of 10 MWd/kgU, corresponding to low-BU assemblies from the first core loading of a reactor. In field tests, the initial DCVD prototype could not only measure the Cherenkov light from such assemblies, but it was also able to measure the light from assemblies with cooling times of 30 years and a burnup of only 1.1 MWd/kgU [25].

It was later realized that the DCVD could not only be used to qualitatively provide an image of the Cherenkov light emitted by an assembly, but it could also be used for quantitative measurements of the emitted Cherenkov light in-tensity. The recorded intensity is used to detect partial defects in an assembly, based on the change in Cherenkov light intensity caused by removing radioac-tive rods from an assembly. The DCVD is most frequently used for partial defect veriﬁcation, as the ICVD is easier to use for gross defect veriﬁcation.

5.2 Measuring fuel assemblies with a DCVD

During a measurement with the DCVD, the instrument is normally mounted onto the railing of a fuel handling machine, as shown in ﬁgure 5.1. The DCVD is looking down into the fuel assembly storage pool, where the assemblies are

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typically covered by around 10 meters of water, as schematically shown to the left in ﬁgure 5.2. The result of a measurement is an image of the Cherenkov light emitted by an assembly, an example of which is shown to the right in ﬁgure 5.2. The digital image obtained from the measurement can then be analysed, as further detailed in section 5.3.

When qualitatively measuring the Cherenkov light emission from an as-sembly, using either an ICVD or a DCVD, the instrument is positioned above and then moved across an assembly, to conﬁrm the presence of Cherenkov light, and to verify that the light is collimated, as discussed in section 4.2. The presence and characteristics of the Cherenkov light then conﬁrms whether or not the item under study is an irradiated nuclear fuel assembly, as opposed to a non-radioactive item.

When quantitatively measuring an assembly with the DCVD, the inspector will first align the DCVD above the centre of the assembly, along the direction of the fuel rods (which is close the vertical direction, but the assemblies may be slightly tilted). Next, the inspector manually selects a Region Of Interest (ROI) in the image, containing the fuel assembly and excluding its surround-ings. After that, the measurements are performed, including typically three to five measurements per assembly. For each measurement, a background-subtraction is performed, aimed at removing an electronics-induced offset in the pixel values (further described in section 8). The pixel values inside the ROI are then summed to provide a total emitted intensity value of the assem-bly. The reported assembly intensity value is the average of the three to five measurements. The reason for performing multiple measurements of each as-sembly is to reduce the effect of noise and changing conditions over time, such as the effect of ripples on the water surface that can slightly distort an image. Note that the pixel values are expected to be proportional to the measured light intensity (further discussed in section 8), but no calibration is done to convert the pixel values to a photon flux.

The DCVD detector consists of a Charged-Coupled Device (CCD) chip sensitive to UV-light. The DCVD optics consists primarily of a motorized zoom lens, which allows different zoom levels to be set, and for the focus to be adjusted to ensure that the top of the fuel assembly is in focus. The optics also contains a UV-ﬁlter for selecting relevant wavelengths, ensuring that visible light will not be detected. The detector setup can be extended or tilted, to allow the detector to be positioned vertically above a fuel assembly. The DCVD can be powered either by two hot-swappable batteries, or by a power cable if a wall-socket is available.

The DCVD contains an on-board computer, which handles the data output from the detector. The user interacts with the DCVD software through a touch-sensitive LCD screen. Customised software performs image processing and calculates the total Cherenkov light intensity based on the recorded images provided by the detector. The software also keeps track of which measurement

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Figure 5.1. Top: A DCVD instrument. To the right is the CCD detector and optics, mounted on a yoke that can be extended and tilted. The computer and electronics are housed inside the casing, and the batteries are seen to the left. On top is an LCD screen providing the user interface. Bottom: A DCVD in use. Images courtesy of Dennis Parcey and Channel Systems Inc.

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Figure 5.2. Left: A schematic illustration of the measurement situation when using the DCVD. Note that the assemblies are typically around 4 m in length, and covered by about 10 m of water. Right: An example of an image obtained from a measurement of a PWR fuel assembly with the DCVD. The bright, circular regions are the guide tubes of the PWR assembly, the smaller bright spots are the regions in between fuel rods.

corresponds to which assembly, if the user provides information about the assemblies veriﬁed in a measurement campaign.

The DCVD has several advantages and disadvantages compared to other safeguards instruments, which are considered when selecting an instrument for a veriﬁcation campaign. Advantages include:

• Non-intrusiveness:

The fuel assemblies are measured where they are stored, and there is no need to move the assemblies to a dedicated measurement station. There is neither any need to insert the device or any associated equipment into the water to get close to an assembly, which reduces the risk of contam-ination.

• Speed:

Since there is no need for moving any assemblies, measurements are quick, typically requiring approximately 10-30 seconds for one assem-bly, depending on the assembly intensity and storage conditions. This speed makes it feasible to verify a large assembly inventory using the DCVD.

The DCVD also has some limitations, which must be considered when ver-ifying a fuel assembly inventory:

• Limited information:

The currently used methodology analyses the total Cherenkov light in-tensity, which depends on the intensity of the ionizing radiation emitted by the assembly. It does not provide any information about the source of the radiation, such as the presence or abundance of ﬁssion product isotopes, nor of the ﬁssile contents of the assembly.

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• Limited sensitivity:

In order to detect a partial defect, sufﬁciently many rods must have been diverted to affect the total Cherenkov light intensity of an assembly no-ticeably. Furthermore, some rods may be hidden under the top plate or lifting handle and contribute little to the measured Cherenkov light intensity, making the diversion of such rods difﬁcult to detect using Cherenkov light.

As a consequence of these characteristics, the DCVD is well suited for detecting partial defects (as described in section 3.3) in scenarios where a large fraction of the fuel rods in an assembly have been removed or replaced with non-radioactive ones. Previous work has shown that the DCVD is sensitive enough to detect diversions on the order of 50-100% removed or substituted rods [14]. Furthermore, the measurements are fast enough that the DCVD can be used to verify a large assembly inventory, to ﬁnd the relatively few assemblies where such diversion may have taken place.

5.3 Detecting partial defects using a DCVD

There are two methods in use to detect partial defects with the DCVD. The first method uses image analysis to identify removed rods, which is based on the identification of bright regions in the images which should be dark due to the expected presence of a fuel rod in that position. This method can thus detect removed rods in visible positions, and it is frequently used for BWR fuel assemblies since individual rods are often visible in BWR assembly designs. An example of an intact BWR assembly and one with two removed rods is shown in figure 5.3.

Figure 5.3. Left: A DCVD measurement of a complete BWR assembly. Right: A BWR assembly with two removed rods. Images courtesy of Dennis Parcey and Channel Systems Inc.

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The second method, which is the focus of this thesis, is based on quanti-tatively measuring the total Cherenkov light intensity from an assembly, and comparing it to a predicted intensity, which is calculated based on the declared assembly information [26]. This method is capable of detecting diversion sce-narios where fuel rods have been substituted with non-radioactive replace-ments. Previously performed simulations [14] have shown that a 50% substi-tution of irradiated fuel rods with non-radioactive steel rods will decrease the Cherenkov light intensity by at least 30%. Thus, if a measured intensity is 30% or more below the predicted intensity, the fuel is flagged by the DCVD software as an outlier requiring additional investigation, as illustrated in fig-ure 5.4. Key to this procedfig-ure is a prediction method with high accuracy, and improving the prediction model is a major part of this work. Furthermore, the predictions must be quickly executed on modest hardware, to allow for in-field use by an inspector. 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 0 0.2 0.4 0.6 0.8 1

Predicted intenisty [arb. unit]

Measured

intenisty

[arb

.

unit]

Figure 5.4. Example of the analysis performed as part of quantitative Cherenkov light verification. Once the measurements and predictions are available, a least-square fit is performed to find the multiplier that relates the two. The solid line indicates the expected agreement between predictions and measurement, after adjustment with the multiplier. The dashed lines indicate a±30% deviation. The data point marked with a red circle has a measured intensity more than 30% below expected, and is flagged as an outlier requiring further investigation.

An inspector in the ﬁeld performs quantitative intensity veriﬁcation accord-ing to the followaccord-ing steps:

1. The inspector obtains information about the fuel assemblies present at the facility, including parameters such as assembly type, BU, and CT. These parameters are used to predict the Cherenkov light intensity of the assembly, as further detailed in section 5.4. Based on the

Enhancing the performance of the Digital Cherenkov Viewing Device : Detecting partial defects in irradiated nuclear fuel assemblies using Cherenkov light

Digital Comprehensive Summaries of Uppsala Dissertations

from the Faculty of Science and Technology

1708

Enhancing the performance of the

Digital Cherenkov Viewing Device

Detecting partial defects in irradiated nuclear fuel

assemblies using Cherenkov light

ERIK BRANGER

List of papers

Contents

List of abbreviations

1. Introduction

1.1 The need for nuclear safeguards

1.2 Outline of the thesis

2. Nuclear safeguards

2.1 The legal framework for nuclear safeguards

2.2 Material and facilities under safeguards

2.3 Safeguards veriﬁcation of nuclear material

3. Nuclear fuel assemblies

3.1 Physical design of nuclear fuel assemblies

3.2 Fuel usage in a reactor

3.3 Safeguards veriﬁcation of irradiated nuclear fuel

assemblies

4. Cherenkov light

4.1 The physics of Cherenkov light

4.2 Cherenkov light from irradiated nuclear fuel

assemblies

5. The Digital Cherenkov Viewing Device,

DCVD

5.1 History

5.2 Measuring fuel assemblies with a DCVD

5.3 Detecting partial defects using a DCVD