Publications Repository - Helmholtz-Zentrum Dresden-Rossendorf

The aeroball measuring system (AMS) at Siemens/KWU built pressurized water reactors (PWR) is an important part of the in-core instrumentation to determine in detail the local power distribution. Simulations were carried out with the help of the MCNP6 Monte Carlo program to determine the possible impact of an additional the water gap between the fuel assemblies with regard to the 51V(n,γ)52V reaction rate in the AMS. A simplified geometric model in a 3x3 matrix of identical fuel assemblies was used and four AMS lances were simulated in the central fuel assembly. By shifting the outer 8 fuel assemblies, different water gaps were created and the effects on the reaction rate of the AMS were calculated for different burn-up values and boron contents in the cooling water. It was found that the change of reaction rates can reach up to 10% for an assumed gap maximum of 1cm. The changes are largest for burn-up values at 30 and 45 GWd/t and slowly increase with decreasing boron concentration. The results are an important piece of information to assess the possibility of detecting non-nominal water gaps during reactor operation.