Validation of an argon fire suppression system
Issue
A current safety case exists for the storage of Magnox Fuel Element Debris in the Waste Vaults of Berkeley Power Station. However the retrieval of the waste from the vaults presents new challenges. Of particular importance is the fire suppression system, as much of the waste consists of a mixture of graphite and magnesium, both of which are flammable.
The current fire suppression system works by injecting argon into the vault containing a fire, thereby starving it of oxygen. Under storage conditions the vault covers are closed and the argon can be used to displace the air in the vault quite efficiently due to its greater density. Under retrieval conditions however, the vault covers could be open and the concern was that the thermal plume from any fire could lift argon out of the vault into the retrieval tunnel and additional air could enter to feed the fire.
Solution
In order to support the fire suppression system design and safety case, Frazer-Nash used Computational Fluid Dynamics (CFD) to analyse the dispersion of argon during a fire scenario. The work was particularly challenging due to the need to model the fire accurately. In order to accurately capture the convection it was important to get the correct heat transfer from the fire to the air. We developed a unique simulation protocol to incorporate Magnox best practice for the prediction of fire strength and propagation into CFD modelling software. We also developed active fire suppression in the model by linking the fire parameters to the concentration of argon in the vicinity of the fire.
Benefits
Our work enabled Magnox to confirm that modifications to the current fire suppression system will be necessary for the retrieval process. In addition we were able to define which parts of the system design and procedure were most important to successfully extinguish the fire. A number of hypothetical fire system designs were analysed using the CFD modelling tool and we were able to recommend the best way forward for future design work.