Active reduction of the end effect by local installation of neutron absorbers

In the burnup credit analyses of interim or long-term spent fuel (SF) storage facilities and transport casks, when the average burnup value is greater than approximately 30 GWd/t, the neutron multiplication factor becomes greater if we consider the axial burnup distribution of the spent fuel assembly rather than assuming an average burnup. This phenomenon is called the “end effect” and it is one of the main technical issues in burnup credit research. The end effect is characterized by an increase of the neutron flux around the end regions of the spent fuel assemblies in the criticality calculation. However, such increase of the neutron flux has not been observed in experiments using actual spent fuel assemblies. In this study, the reason why the end effect occurs in the criticality calculation of spent fuel assemblies is discussed based on analyses of the neutron distribution measurement using both fixed source and eigenvalue calculations. It is shown that actual neutron flux increase occurs only when the neutron multiplication factor is close to unity. However it is always found in the eigenvalue calculation. The eigenvalue calculation evaluates the fundamental mode flux and the maximum eigenvalue which is assumed to be the neutron multiplication factor, and therefore an increased neutron flux around the end regions is obtained because the end regions have more importance, i.e., generate more neutrons than the central regions of higher burnup. Based on this consideration, it is expected that we would not have an increase of the neutron flux around the end regions when obtaining maximum eigenvalue, if we are able to reduce the importance of the end regions. Finally, we propose reducing the end effect actively by local installation of neutron absorbers (LINA) around the end regions of the fuel assemblies; their effectiveness was confirmed by several criticality calculations.