The Effect of Temperature on the Neutron Multiplication Factor for Pressurised Water Reactor Fuel Assemblies

         There is significant international interest in the effects of temperature on criticality safety evaluations. Improved access to nuclear data, notably thermal scattering data S(α,β) for hydrogen in ice, has made it possible to assess and deepen the understanding of the variation of criticality calculations associated with temperature.         Under the auspices of the Nuclear Energy Agency (NEA) Nuclear Science Committee (NSC) and of the Working Party on Nuclear Criticality Safety (WPNCS), Subgroup 3 was launched to study the impact of temperature through a wide range of codes, data libraries and methods. The group studied a pressurised water reactor (PWR) assembly in a thick water reflector at five different temperatures, ranging from ice to room temperature and up to reactor operation temperature. Ten organisations in eight countries participated, providing 34 sets of results.         With a view to launching the activity and obtaining results in a timely fashion, geometrical and material data from a previous WPNCS benchmark model were used in a new study to help examine the effect of temperature on criticality safety calculations.         The exercise was successful in bringing together a large number of institutions and employing a wide range of calculation codes and nuclear data libraries, thereby providing solid evidence for the conclusions drawn. The participants’ dedication made it possible to complete the exercise in two years, providing timely support to end users.         This report contains the benchmark study and results, and was endorsed at the WPNCS Subgroup 3 meeting in July 2020.      