Validation

During 1999 the U.S. National Spent Nuclear Fuel Program (NSNFP) at the Idaho National Engineering and Environmental Laboratory (INEEL) sponsored the Institute of Physics and Power Engineering (IPPE), Obninsk, Russia to perform experiments designed to provide data applicable to the storage of spent nuclear fuel in dry underground stores. IPPE utilised their Big Physical Stand (BFS) facility to perform a number of critical experiments with either highly enriched uranium or plutonium metal contained in octoganol-section lattices.

The experiments consist of hexagonal-pitched lattices containing sets of tubes filled with pellets of stainless steel clad fuel, silicon dioxide, and polyethylene. Also, quartz (silicon dioxide) sand was used to fill tubes in the reflector at the core periphery. The main core of the experiment was contained within a cylindrical vessel, with on one side a solid graphite thermal column and on the opposite side a bay for a metal column, which was an extension of the reflector, composed of tubes containing depleted UO2.

Thin polyethylene dowels were inserted into the space between the tubes in some cases, to provide well-defined moderation similar to water. The resulting range of variation in neutron spectrum can be quantified by comparing the average energy of neutrons causing fission events, which varies from 5.6keV to 1.3eV.

Having been defined in the ICSBEP Benchmark Handbook, these unique experiments are currently being studied as part of the on-going MONK Validation Programme. The MONK models make extensive use of the new MONK8B features of HOLE naming, alphanumeric material names, parametric input, file inclusion, and the GHOLE and GH options.

Dry Fuel Store Design

As part of the design safety case for a proposed dry store for UK AGR fuel, MONK was used to investigate the criticality safety of the proposed design under both normal and potential accident scenarios during the envisaged lifetime of the store. The investigations covered all stages of the proposed operations, including flask arrival, unloading, fuel transfer and medium term storage.

Fuel Pond Storage

A series of MONK calculations were performed to determine possible criticality implications of the degredation of Boraflex neutron absorber panels used in a BWR spent fuel storage pond.

Reactor Analysis

A series of MONK calculations were performed to investigate fuel handling safety issues in an experimental reactor facility. The powerful visualisation tools available to support MONK enabled the range of scenarios covered to be readily described and documented.

Reprocessing

The design of fuel dissolvers requires that the fuel remains safely subcritical under all conditions. MONK and WIMS have been used to demonstrate the criticality safety of dissolvers. Calculations identify the most reactive configuration and determine the poison required to maintain safety under this configuration and hence ensure substantial safety margins for typical dissolver conditions.

Burnup Credit

Criticality safety assessments for spent fuel operations have traditionally been based on the 'fresh fuel assumption' which ignores reactivity losses associated with fuel burnup. The CERES International Collaboration supported by the UK, France and USA was set up to provide experimental validation of codes used to calculate the reactivity of spent fuel. The methods may then be used as part of criticality assessments which take account of fissile depletion and the build up of neutrons poisons during burnup (Burnup Credit).

The programme has covered all main commercial fuel types, with reactivity and PIE measurements on CAGR, PWR and BWR fuel samples. The most recent phase of the programme has included measurements on samples of Mixed Oxide fuel (MOX). The use of Burnup Credit in criticality assessment can lead to considerable savings in transport and storage costs by removing unnecessary pessimisims in the derivation of loading limits.