Successful Application and Development of a Burn-Up Credit Methodology For Use in a UK Transport Criticality Assessment

Rolls-Royce has obtained United Kingdom regulatory approval to transport fuel assemblies using a Burn-Up Credit (BUC) method. We believe this is the first time a transport criticality assessment using a BUC method has been obtained from a UK regulator.Extant transport criticality assessments for previous core designs have employed the Fresh Fuel Approximation (FFA), which involves taking credit for Start of Life (SoL) fissile loading and discounting the presence of burnable neutron absorbers. The FFA is straightforward, pessimistic and well known to the regulator. However, due to the relatively high SoL fuel loading of the latest assembly design the criticality criterion (keff < 0.95) could not be met using FFA. Therefore, a BUC method was developed to take credit for burn-up of fuel and neutron absorbers in order to use the extant transport flask (without the need for expensive re-design). The Regulator approved the transport licence application after the following steps were completed:a) Reviewed applications of BUC in the IAEA SSR-6 regulations and associated guidance.b) Developed a method to exploit the operating profiles of the reactors where assemblies were installed.c) Evaluated candidate codes that can perform both depletion and criticality calculations.d) Perform robust validation using:I. Reactivity measurements and criticality control rod positions taken from through-life measurements.II) In-Core Flux Measurements from the prototype core.III) Isotopic composition measurements from Post Irradiated Examination measurements from irradiated assemblies of similar design.e) Independent comparisons of bounding calculations were performed by the regulator using other well established criticality codes.This paper describes how the above steps were carried out to reach this success outcome.