Effect of low temperature on criticality calculation for the transport of fissile material

IAEA (International Atomic Energy Agency) is responsible for prescribing regulations for safety and protection of persons and the environment. Among those standards, the regulation n°SSR-6 stipulates that packages used to transport radioactive materials have to meet specific requirements.For FISSILE package, these requirements are to be ensured for an initial temperature of the package between -40 °C and +38 °C. But, the main criticality calculation tools used until now do not allow to perform easily and quickly calculation over the required range of temperature. Indeed, the nuclear data are primarily generated for positive temperature and validated mainly at room temperature (~20 °C) and sometimes for specific applications at higher temperature. Accordingly, criticality calculations are performed at +20 °C. Over the past, this hypothesis was considered valid since the Keff value decreases with temperature for temperature above +20 °C and because of the simplifications taken into account in the modeling. However, since neutron cross-section for low temperatures could now be generated, even if not perfect and not validated over benchmarks, and due to more and more accurate codes and models, it is necessary nowadays to verify if this statement is really valid.In order to do so, this study is related to the impact of low temperatures on the neutron effective multiplication factor (Keff) for simple configurations. This impact will take into consideration not only the effects of temperature on nuclear data (cross sections, Sαβ), but also the effects of material density variation of the materials with temperature.The aim of this paper is to present the impact on Keff of low temperatures (until -40°C and considering the formation of ice for temperature below 0°C) for different configurations:an infinite medium in order to understand effects for the fissile nuclides,a cylindrical containment vessel loaded with moderated uranium reflected by water in order to understand effects of reflector,an UO2 fuel assembly filled with water and surrounded by boronated resin in order to understand the effects of neutron poison material,a simplified packaging loaded with moderated UO2 fuel rods in order to understand the cumulative effects.