(A) study on optimal position for the secondary neutron source in PWRs

Abstract

During the subcritical operation of the PWRs, the potential reactivity increase is monitored by using the source-range excore detector signals. In the modern low-leakage core with many control rods inserted, the relationship between the detector count rate and the reactivity is usually quite non-linear and it is important to make sure the excore detector signal correctly indicates the core reactivity in a timely manner. Currently, the reactivity change during a subcritical startup operation of PWRs is determined by the so-called ICRR (In-verse Count Rate Ratio) curve. For the ICRR curve to be reliable, the excore detector count rate should be correctly related with the multiplication of the secondary neutron source in a highly subcritical core. This work is concerned with optimization of the position of the secondary neutron source in terms of the ICRR curve.The response of the ex-core detector greatly depends on the position of the secondary neutron source in two ways. First, the multiplication of the source is higher, the detector signal can be generally higher and the detector count rate may reflect the whole core characteristics. Secondly, a source located near excore detec-tor will lead to a high detector count rate. However, the detector count rate may just reflect the local charac-teristics of the core. In this sense, the optimal secondary source position is determined by taking into account the magnitude of the source multiplication in the core. As an efficient way to determine the source multiplica-tion, the homogeneous adjoint flux is chosen in this study, which can be obtained by solving the conventional homogeneous reactor adjoint equations. The KENO-VI code is used to solve the multi-group homogeneous adjoint equation. For a given secondary source position, the excore detector signal is also simulated by using the MAVRIC code. The new method is applied to two Korean OPR1000 cores using a Sb-Be secondary neu-tron source. For the adjoint flux analys...