Yuanzineng kexue jishu (Aug 2023)
Impact Analysis of Vanadium SPND Prompt Gamma Current in PWR-core Measurement
Abstract
Vanadium self-powered neutron detector (SPND) is applied in Gen-Ⅲ commercial PWR AP1000 to simultaneously and continuously measure the in-core neutron flux density, and to control the safe and stable operation of nuclear reactors. However, not only neutron flux can interact with vanadium SPND, then generate the response current. But also, the gamma flux from fission, capture and decay reaction can generate the response current. And the prompt gamma current can impact the measured response current. The response current can reflect the operation condition based on an accurate SPND response current calculation. Thus, a SPND simulation analysis system was designed and developed based on the PWR-core analysis code system NECP-Bamboo code and Monte-Carlo code NECP-MCX. There are three principal steps in the simulation procedure of SPND response current, viz lattice simulation to obtain the nuclide composition and cross section of emitter and the few-group constants, SPND simulation to obtain electron escape probability and gamma sensitivity in different assemblies and burnup based on the conservation of electron number, core simulation to obtain the response current during core operation. The decay activity was employed to facilitate the accuracy based on micro-depletion. Based on this system, the response current and the gamma response current ratio of a vanadium SPND in the AP1000 core was simulated. The results were verified against measured values and perform well. The results show that the prompt gamma current ratio is different in different assemblies. Most of them can’t be ignored. It is necessary to consider the gamma response current in the response current simulation, since the absolute value of prompt gamma current ratio is above 6.3%. The impact of prompt gamma response current to some measurements in AP1000 core was analyzed. The error is large than 10% without gamma response current. Then, the impact of prompt gamma current on core axial flux deviation (AFD) measurement and online power reconstruction was analyzed. If the neutron current is only considered, the difference between the measured and calculated values of AFD is 1+α times (α is the ratio of prompt gamma response current to neutron response current), and the difference between the measured and calculated values of power reconstruction is 1/(1+α). The error of power reconstruction corrected by α performs well than the uncorrected one. Thus, it is necessary to use different α value in different assemblies to correct the response current.
