In this chapter the number of canisters that is required to deposit the spent fuel accounted for in Chapter 2 is assessed. The assessment is based on the requirements on the handling and selection of assemblies presented in Chapter 3 and Section 4.4.1 respectively. In addition, the encapsulation period is postulated.
5.1 Factors that will affect the number of canisters
The number of canisters to be deposited will depend on:
• the number of fuel assemblies in the canisters,
• the number of spent fuel assemblies to be encapsulated,
• the burnup of the assemblies,
• the allowed decay power in the canister,
• when the encapsulation is initiated and the encapsulation rate.
The maximum number of BWR and PWR assemblies in a canister is twelve and four, respectively.
The number and burnup of the assemblies to be deposited will depend on the thermal powers, number and type of assemblies in the reactor core and operation times of the nuclear power reactors (see Table 2-1), and on the past and planned average target burnups.
The allowed total decay power of the assemblies encapsulated in a canister will ultimately depend on the decay power that can be accepted for the temperature in the buffer not to exceed 100°C. The temperature in the buffer will depend on the thermal properties of the buffer and bedrock and on the distances between deposition holes; see Underground openings construction report, Section 4.2.1.
The current layout of the final repository is based on a maximum decay power in a canister of 1,700 W.
The decay power of the fuel assemblies will decrease with time; consequently, the age of the assemblies at encapsulation will affect the decay power. The age of the spent fuel assemblies at any time during the encapsulation period will depend on the operation times of the nuclear power plants and on when the encapsulation and deposition is initiated and completed.
The assemblies for encapsulation shall be selected with respect to their burnup and age so that they conform to the criterion for maximum allowed total decay power in a canister.
In Figure 5-1 the relation between burnup and age of the spent fuel assemblies to reach 1,700 W in a BWR and PWR canister, respectively, is shown, assuming that all positions in the canister are occupied by assemblies with the same decay power.
From the interpolation of data in Figure 5-1, the interim storage time required for the average burnup BWR and PWR assembly to reach 1,700 W in the canister can be derived. For the spent fuel from the reference scenario for the operation of the nuclear power plants presented in Section 2.1.1 this time is 38.7 years for the average burnup BWR assembly and 37.9 years for the average burnup PWR assembly. If all positions in all canisters are to be filled and their total decay powers not exceed 1,700 W, the average interim storage time must be at least 38.7 and 37.9 years for BWR and PWR canisters respectively. Shorter interim storage time will mean that all positions cannot be filled in all canisters and longer interim storage time will result in lower total decay power in some canisters.
5.2 Simulation of the encapsulation
The encapsulation of the spent nuclear fuel to be deposited has been simulated assuming that the encapsulation is initiated in 2023 and completed 25 years after the last nuclear power plant is taken out of operation (2070) / SKBdoc 1221567/. The assemblies were selected so that the conformity to the decay power criterion was ensured, see Section 4.4.1. Further, the simulation is based on the encapsulation rate given in Table 5-1.
The assemblies stored in Clab as of December 31th 2007 and the assemblies included in the SKB reference scenario for the future operation were included in the simulations. With exception of the fuel residues from Studsvik, which require all together seven canisters, the miscellaneous fuels pre-sented in Table 2-3 were also included in the simulation. The total numbers and kinds of assemblies included in the simulation are compiled in Table 5-2. The burnup and age of the BWR and PWR assemblies included in the simulation are illustrated in Figure 2-3 and Figure 2-4. In the simulations it is anticipated that no more than one MOX assembly will be encapsulated in any canister.
For each simulated year of encapsulation the inventory of assemblies in the Clink facility and their decay power was calculated. For assemblies in interim storage, documented data were used and for the remaining assemblies, the planned reactor operation information was used as input. Other data required for the calculations were based on typical BWR and PWR assemblies, i.e. Svea Optima 2 and Areva 17×17, respectively.
Figure 5‑1. Burnup and age of spent fuel assembly required to reach a decay power of 1,700 W in a BWR and PWR canister respectively assuming that all positions are occupied by assemblies with the same decay power.
Table 5-1. Encapsulation rate assumed in the simulation of canisters to be deposited in the KBS-3 repository.
Year Encapsulation rate (canisters/yr)
BWR PWR Total
2023 17 6 23
2024 58 22 80
2025–2027 87 33 120
2028–2054 109 41 150
2055–2069 73 27 100
2070 77 13 90
Age of spent fuel assembly
Burnup (MWd/kgU)
20 30 40 50 60
80
60
40
20
0
12 BWR fuel assemblies 4 PWR fuel assemblies
The assemblies were selected to give a combined decay power of 1,700 W at the time of disposal.
That is, assemblies with a combination of burnup/age lying above the curves in Figure 5-1 were combined with assemblies with a combination burnup/ age below the curves. Based on the burnup and age of the assemblies to be deposited and the assumed encapsulation period, it is not possible to fill all the assembly positions in all of the canisters if their summed up decay power shall not exceed 1,700 W. Consequently, there will be a number of canisters containing less than the maximum possible number of assemblies. The simulations were carried out to minimise the number of unfilled assembly positions in the canisters. In Figure 5-2 and Figure 5-3 the resulting number of BWR and PWR canisters to be deposited is illustrated. The canisters are divided into groups with respect to the average burnup of the encapsulated assemblies and the number of assemblies in the canisters.
A detailed account of the canisters to be deposited each year is given in Table C-3 and C-4 in Appendix C.
Table 5-2. Total numbers and kinds of spent fuel included in the simulation of the encapsulation.
Kind of spent fuel Number of
assemblies BWR UOX
From the reactors B1, B2, F1, F2, F3, O1, O2, O3 and R1 Including rod cassettes, i.e. dismounted fuel rods placed in fuel rod cassettes.
47,415
PWR UOX
From the reactors R2, R3 and R4 6,016
Ågesta (encapsulated in BWR canisters) 222 BWR MOX
From Oskarshamn (O1 and O3)
Swap from Germany 83
184 PWR MOX
Swap from Germany 33
Total for encapsulation in BWR canisters 47,904 Total for encapsulation in PWR canisters 6,049
Total for encapsulation 53,953
<38 MWd/kg HM 38–42 MWd/kg HM >42 MWd/kg HM
Number of BWR canisters
<10 10 11 12 Number of assemblies
Figure 5‑2. The BWR canisters to be deposited divided into groups with respect to the average burnup of the encapsulated assemblies. The number of assemblies in the canister is indicated by different colours.
Figure 5‑3. The PWR canisters to be deposited divided into groups with respect to the average burnup of the encapsulated assemblies. The number of assemblies in the canister is indicated by different colours.
The total number of PWR canisters is 1,652.
0 100 200 300 400 500 600 700 800 900 1000
<42 MWd/kg HM 42–47 MWd/kg HM >47 MWd/kg HM
Number of PWR canisters
<3 3 4 Number of assemblies