Study of the influence of water gaps between fuel assemblies on the activation of an aeroball measurement system (AMS)

Abstract The aeroball measuring system (AMS) at Siemens/KWU built pressurized water reactors is an important part of the in-core instrumentation. With the Monte Carlo program MCNP6 the effect of the water gap size between the fuel assemblies was determined with respect to the 51V(n,γ)52V reaction rate in the AMS. A 3 × 3 matrix of identical fuel assemblies was used. The AMS were simulated in the central fuel assembly. By shifting the outer assemblies, different water gaps were created and the effects on the reaction rate were calculated for different burn-up and boron contents in the cooling water. It was found that the change of reaction rates can reach up to 10% for a gap maximum of 1 cm. The changes are largest for burn-up at 30 and 45 GWd/t. The results are an important piece of information to assess the possibility of detecting non-nominal water gaps during reactor operation.