Numerical prediction of the fission product plate-out for a VHTR application

In order to identify and quantify the key parameters affecting the predictions of fission product transport and plate-out behavior in the coolant circuits of a very high temperature reactor (VHTR) system, systematic and intensive analyses were performed based on numerical predictions as well as the existing experimental data. For the purpose, the computational module for modeling fission product transport phenomena was developed and incorporated into the system analysis code, GAMMA+ for an integrated analysis. This integration can provide more realistic boundary conditions such as velocity, temperature, etc., during off-normal conditions as well as normal operations in a given VHTR system. The developed module has been applied to predict the fission product transport and plate-out in the experimental loops of VAMPYR-I and OGL-1. A good agreement is found in general between the predictions by the present module and the measured values. The results of the sensitivity analysis have shown that the surface conditions of the coolant circuit can be major source of the uncertainty in the prediction of the plate-out activity of I-131. In the high temperature regions, in particular, it is found that the predicted plate-out activities of I-131 on the oxidized surface result in about 100 times smaller values than those on the unoxidized surface. On the other hand, Cs-137 is found to be less affected by the surface oxidation. It is also confirmed that the diffusion coefficient significantly affects the fission product distribution in the coolant and plate-out activities over the large surface area like heat exchangers.