Description of experiments on low enriched uranium oxide powder at low moderation

The need to assign realistic criticality margins for low-enriched low moderation materials used in the BNFL uranium oxide fuel plants production and storage areas led to a series of experiments. These experiments were conducted at BNFL Springfields by a joint BNFL/AEE Winfrith team using equipment designed and supplied by Winfrith and with fuel supplied by BNFL. Low enriched UO2 powder in the form of spray-dried granules was used with four values of hydrogen/uranium ratio in the range 0.1 to 0.4% by weight. The amount of hydrogen present was determined by the quantities of organic binder used during the spray drying process. The investigation was based on a subcritical mass of 25 tonnes of fuel arranged in the form of an exponential experiments used to derive the material bucklings for the fuel. Additionally fission chamber and detector foil reaction rate measurements were completed in each experiment in order to assist in the theoretical interpretation of the results. An analysis of the results has been completed using the latest version of the WIMS code and data library with good agreement achieved over the range of experiments. Values are presented for low enriched systems up to 5% and H/U up to 0.4%, calculated using the WIMS code and data. It is shown that ksub(INF) varie almost linearly with H/U ratio for values above 0.2% with a slope which is essentially independent of enrichment