Fuel with advanced burnable absorbers design for the IRIS reactor core: Combined Erbia and IFBA

IRIS is an advanced medium-size (1000 MW) PWR with integral primary system targeting deployment already around 2015-2017. Consistent with its aggressive development and deployment schedule, the “first IRIS” core design assumes current, licensed fuel technology, i.e., UO2 fuel with less than 5% 235U enrichment. The core consists of 89 fuel assemblies employing the 17×17 Westinghouse Robust Fuel Assembly (RFA) design and Standard Fuel dimensions. The adopted design enables to meet all the objectives of the first IRIS core, including over 3-year cycle length with low soluble boron concentration, within the envelope of licensed, readily available fuel technology. Alternative fuel designs are investigated for the subsequent waves of IRIS reactors in pursuit of further improving the fuel utilization and/or extending the cycle length. In particular, an increase in the lattice pitch from the current 0.496 in. for the Standard Fuel to 0.523 in. is among the objectives of this study. The larger fuel pitch and increased moderator-to-fuel volume ratio that it entails fosters better neutron thermalization in an altogether under-moderated lattice thereby offering the potential for considerable increase of fuel utilization and cycle length, up to 5% in the two-batch fuel management scheme considered for IRIS. However, the improved moderation also favors higher values of the Moderator Temperature Coefficient, MTC, which must be properly counteracted to avoid undesired repercussions on the plant safety parameters or controllability during transient operations. This paper investigates counterbalancing the increase in the MTC caused by the enhanced moderation lattice by adopting a suitable choice of fuel burnable absorber (BA). In particular, a fuel design combining erbia, which benefits MTC due to its resonant behavior but leads to residual reactivity penalty, and IFBA, which maximizes cycle length, is pursued. In the proposed approach, IFBA provides the bulk of the hold-down, with no penalty on cycle length, while the amount of erbia is adjusted to obtain the desired margin in the core peaking power and MTC. Preliminary economic analysis proves that within the IRIS design envelope, the combined BA fuel together with the enhanced moderation lattice offer the potential for considerable fuel cycle cost savings when compared to the current core design based on the Westinghouse Standard 17×17 lattice with IFBA. Therefore a combined BA fuel with the enhanced moderation lattice is a promising option to consider for future developments of the IRIS core.