RWST Dose Analysis
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RWST Backleakage

Phenomenon and a Very Conservative Approach

RWST backleakage, including the above-water flowpath, contributes to the total radiological consequences of the DBA LOCA event. Once depleted following a LOCA, the RWST is isolated and the total sump fluid inventory is recirculated to provide cooling for the fuel. Backleakage past closed valves can allow contaminated sump water to enter the RWST. Absent detailed evaluation results for the physical conditions of the sump fluid entering the airspace, the backleakage must be conservatively considered to be consistent with the models for leakage into the auxiliary building which specify that 10% of the iodine is assumed to be instantly released to the air space (as elemental iodine gas) due to “flashing.” There are no considerations for travel time, decay, or mixing along the leakage path the RWST. This analysis approach leads to very low allowable leakage values and does not include physical processes that may reduce radiological consequences associated with the leakage.

Revised Analysis Approach

The Analysis Division can use GOTHIC™ to analyze the physical conditions of the sump fluid entering the airspace and thereby determine the leak rate that will preclude any amount of “flashing” at the above-water entrance to the RWST. The GOTHIC model will account for convective cooling of the pipes along the leak-path and the mixing effects of multiple leak-paths. This replaces the 10% instantaneous flashing with a much more realistic physics-based model which will allow for significantly higher leakage rates to the above-water RWST entrance.

When multiple leakage pathways are present the problem can be analyzed using a matrix of GOTHIC case runs that considers the many possible leakage configurations. From the result of these case runs (see graph ), a simplified model can be constructed that calculates the maximum allowable leakage at any given flow configuration. This method will provide the most margin for flow to the RWST through the above-water pathway.

These results are then fed into a RADTRAD-NAI™ model that uses the maximum backflow to transport Iodine to the RWST. RADTRAD-NAI then utilizes the compartment inventories accounting for decay and re-evolution to determine the dose impacts of RWST backleakage.

Analysis Support

For more information on our ability to evaluate RWST backleakage and how this might be used to analyze your plant, please contact Anita Gates or Steve Winter. The Analysis Division contact list is here, including phone numbers.