Criticality safety analysis of the design of spent fuel cask, its manipulation and placement in a long-term storage

Abstract

Spent nuclear fuel storage is gradually becoming a nightmare for nuclear reactors which were commissioned in the 1980s. This leaves the nuclear facility management with the dilemma of having to choose between pursuing the cask storage option to relieve the demand pressure on the spent fuel pool, or to opt for the more radical but unpopular option of shutting down the reactor compromising the energy supply, and South Africa is no exception. In a bid to minimise the risk of reactor shut down, the Nuclear Analysis Section (NAS) of Eskom launched the present study of investigating the design requirements of spent fuel casks suitable for the storage and transportation of spent fuel assemblies that have an initial enrichment of up to 5 wt% and much higher burnup of between 50 and 60 GWD/MTU. The aim of the present study is to investigate the suitability of the existing casks for use in 5 wt% enriched fuel, given that they are licensed for a maximum enrichment of 3.5 wt%. As a result of the huge number of casks required, there is potentially a risk of shortage of cask storage space and, therefore, it was prudent that the study also investigates the most optimum storage array that will maximise the storage space, while keeping the effective neutron multiplication factor (keff) below the internationally recommended value of 0.95 [IAEA, 2014]. As such, it is also necessary to identify parameters which have the greatest effect on the neutron multiplication factor. These include determining the effect of changes in moderator and fuel temperature on the neutron multiplication factor and also what the effect of an increase in the concentration in 10B of the boral plate will have on the neutron multiplication factor.