Criticality Safety Assessment for Reprocessed Uranium as a Waste
Objective:
RM Consultants Ltd (RMC’s) role was to provide a preliminary assessment to determine if criticality is a safety issue in drums of reprocessed uranium if classed as a waste.
Added value to client:
Computer models representing how fissile materials might be stored, handled or transported were developed and assessed using the MCNP5 code. Models were developed to represent the waste packages under normal conditions when no neutron moderator is contained inside the 200-litre drum. Credible accident scenarios involving mixing with water were also investigated.
MCNP 5 is a “computational tool” used world wide to identify the safety limits for a wide range of fissile material of almost any shape or form.
Activity:
Possible criticality safety issues were studied in the context of a 200-litre drum encapsulated in a standard 500-litre Nirex drum. RMC performed a series of criticality calculations representing the UO3 powder with an enrichment of 1.6%. The main activities involved for the criticality calculations were:
- Establishing detailed inventories of fissile material and other materials contained in a waste package.
- Obtaining accurate information on how the waste might be stored, in particular detailed geometric arrangements of waste packages to develop a physical model.
- Carrying out quality assurance checks on the physical model using the graphics software of MCNP.
- Converting the physical model into relevant input for mathematical modelling using MCNP 5 whilst applying pessimism in the data to allow for uncertainties where appropriate.
- Using MCNP 5 to calculate the values of the effective neutron multiplication factor (keff ) for the different scenarios including the “worst case scenario”.
- Examining results to highlight any issues arising due to use of the 500-litre Nirex drum as a waste package from the criticality safety point of view.
