A solution of CB6 benchmark on VVER-440 final disposal with Monte Carlo reactor physics code Serpent

Tuukka Lahtinen, Fortum Power and Heat Ltd, Finland

23rd Symposium of AER on VVER Reactor Physics and Reactor Safety (2013, Štrbské Pleso, Slovakia)
Spent fuel storage and disposal


The speci?cation of CB6 benchmark on VVER-440 ?nal disposal was published by OECD/NEA
Working Party on Nuclear Criticality Safety (WPNCS), Expert Group on Burn-up Credit Criticality Safety (EGBUC). The benchmark is part of the assignment of EGBUC in assessing the
ability of code systems to predict the reactivity of spent nuclear fuel systems.
The CB6 benchmark is divided in two parts. The ?rst part is aimed at investigating the evolution
of the isotopic composition of an irradiated VVER-440 fuel assembly over the time duration
relevant to long-term spent fuel disposal. The initial isotopic inventory for 4.25 % VVER-440
fuel at 50 MWd/kgU burn-up is given in the benchmark speci?cation.
The second part of the benchmark involves calculation of e?ective multiplication coe?cient
values for 3D cask geometry at various time points. The cask is loaded with 84 VVER-440 fuel
assemblies and ?ooded with pure water. Isotopic composition of the fuel, which is assumed to
be uniform, results from the decay calculation. The “actinides and ?ssion products” burn-up
credit approach is applied in the calculation.
In this study, the code Serpent is applied in solution of the benchmark. Serpent is a continuousenergy Monte Carlo reactor physics code developed at VTT Technical Research Center of Finland. In order to gain comparison data, the decay calculation part is also carried out with
point-depletion calculation code ORIGEN.
The paper demonstrates the decay and ke? calculation results as determined in the benchmark
speci?cation. The di?erences in results arising between the two calculation codes in decay
calculation, and between the ENDF/B-VII and JEFF-3.1.1 based cross section libraries in ke?
calculation, are depicted and discussed.

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