title: Optimising BWR spent fuel storage using the burnup credit method and exploring other solutions for the management of high-level waste
Creators: name: Detkina, Anna
Publisher: name: University of Liverpool Repository
Year: 2022

description: The volumes of accumulated spent fuel from commercial power plants have significantly increased over the last decades and are projected to grow further. With a small part of the spent fuel being reprocessed and with the lack of delivered solutions for the long-term spent fuel management, the demand for interim spent fuel storage facilities is expected to grow in the foreseeable future. Currently, LWRs – PWRs and BWRs, are considered as the major contributor to the spent fuel accumulation. The given research is focused on analysing BWR spent fuel properties since it is less researched than PWR due to the more complex fuel assembly design. The criticality safety analysis is a part of the safety assessment of the design of storage facilities. The criticality analysis can be performed by either the fresh fuel approach (unirradiated fuel) or by applying the burnup credit method (considers fuel burnup). The burnup credit method is widely applied to PWR storage systems and, in some countries, to BWR wet storage, but is still under development for BWR dry storage. The current study aims to develop a novel cutting-edge approach to the burnup credit analysis that can account for the realistic status of the BWR fuel assemblies through their lifetime in the reactor core. To achieve that, the approach incorporates the results of the reactor core nodal burnup simulations into the criticality safety assessment of the spent fuel storage cask. In addition, the study examines potential solutions for the long-term LWR spent fuel management. Prior to development of the burnup credit model, supporting investigations have been performed to determine the quality of the computational tools for the nodal reactor core analysis. The research included evaluation of potential lattice physics codes for BWR burnup calculations and comparison of nodal and high-fidelity methods for the full-core analysis with heterogenous BWR fuel assembly design. According to the results, the lattice physics modules TRITON and Polaris of SCALE-6.2 package showed to be robust for BWR analysis, while the UK code WIMS-10A produced the least satisfactory results among analysed codes. As expected, the outcome of the reactor core analysis by nodal and high-fidelity approaches for BWR core differed from each other. The leading reasons for that were found in the difference in the estimation of the axial power profiles for 3D fuel assembly models performed under both approaches. The main advantage of the nodal approach was the significantly lower computational demand which allowed more comprehensive reactor core analysis which was to date not possible in full-depth using the high-fidelity approach. The burnup credit study included the development of a 3D nodal fuel assembly model with coupled neutronics and thermal-hydraulics as well as steady-state reactor core analysis of four cycles of reactor operation with different initial parameters. The results of the reactor core analysis, such as fuel assembly’s discharge burnup or its approximated operational conditions (no actual operational data from a plant has been used), were used to define the composition of the spent fuel for further criticality analysis of the BWR storage cask. The variation of the initial parameters was conducted to estimate their effect on the criticality of the storage cask. The varied initial parameters included refuelling scenarios, fuel assembly types, and target burnups. The study demonstrated that the utilisation of the more detailed 3D nodal model of the BWR fuel assembly can improve the accuracy of the 2D lattice approach while analysing the fuel assembly status, and lead to a reduction of the conservatism of existing approaches of criticality analysis such as the fresh fuel approach. Although creating a detailed and realistic model of the fuel assembly for burnup credit analysis is more time-consuming and computationally expensive than just using a 2D lattice model or applying fresh fuel approach for the criticality analysis, it can potentially provide significant benefit in terms of reduction of the costs of spent fuel storage. The developed advanced modelling approach will mostly benefit the handling of complex and heterogeneous BWR fuel assemblies which are commonly used today in BWRs and are planned to be used in developing ABWRs. At the final part of the study, it was shown that the molten salt reactors working directly on a spent fuel can be a cost-effective, safe, and feasible alternative to the generally accepted strategies of the long-term spent fuel management such as completely closed nuclear fuel cycle and direct disposal of spent fuel.