Odds Are On The Monte Carlo System - Dounreay

28th January 2021

Dounreay features in a new leaflet from the Nuclear Decommissioning Agency showing recent research, development and innovation. See the leaflet link at the bottom of this page to read about some of the other investments by the NDA.

Before the experimental Dounreay Fast Reactor (DFR) can be dismantled, a clear understanding is required of radiation levels, and their distribution, inside the shielded vault that houses the

radioactive core.

DFR was the world's first fast breeder reactor to supply electricity to a national grid, playing an important part in research for fuel and coolant technology.

Decommissioning the DFR is one of the UK's most significant nuclear challenges. As workforce access to DFR's interior is

impossible, measurements were taken using a variety of tools deployed through the dozens of openings. Among other

items, the vault contains graphite, tanks, vessels, heat exchangers and ring main contaminated with residual sodium potassium liquid metal coolant, NaK.

Almost 70 tonnes of highly radioactive and chemically reactive NaK were destroyed over 10 years in a purpose-built plant, and

dealing with remaining contamination is a key challenge.

Four different systems were used to collect data, including the compact N-visage gamma imaging scanner, which was developed by Cumbria-based Createc with support from the NDA's research budget and Innovate UK.

Senior Characterisation Specialist Vittoria Baldioli said: "Work to build a comprehensive picture of the vault's inventory, including radioactive hotspots, has been under way for a number of years.

"It's also important to predict how conditions will change once decommissioning starts and items are retrieved. This will enable us

to assess the possibility of future workforce access."

Data from all techniques provided large quantities of information but did not represent the entire vault. The team opted to use the Monte Carlo N-particle (MCNP) code, a powerful and highly specialised

simulation software developed by the Los Alamos national lab, used globally for safety, radiation and shielding problems.

A 3D MCNP model of DFR's vault was constructed using all available information, and gradually adjusted to achieve a match

between measured and theoretical dose rate, indicating accurate modelling of activity distribution.

The model will support decommissioning, modified when necessary, and used to determine dose rate changes outside the vault, through the effects of shielding, and inside as components are removed.

It will be of particular value for addressing contamination levels, and can also be applied at other decommissioning sites in the NDA group.

See the full leaflet HERE