A Study on the Burnup Effect of Silver Self-Powered Neutron Detector

Abstract Self-Powered Neutron Detector (SPND) has been widely used in reactor core for in-core neutron flux monitoring. As a typical delayed type of SPND, the burnup behavior of Ag SPND is more complicated than Rh SPND and VSPND, because Ag is composed of two natural isotopes 107Ag (51.839%) and 109Ag (48.161%) with distinct neutron capture cross-sections. This will lead to change in neutron sensitivity of Ag SPND during operation, which in turn affects the monitoring of neutron flux. In this work, the burnup effect of Ag SPND utilized in CE-type assembly was studied by using SPNDSignal toolkit. By solving the burnup equations, we calculated the composition change of the emitter material over 10.5 operation years. Ag SPNDs with different burnup rates were modeled in SPNDSignal, and the components contributing to neutron sensitivity were calculated and analyzed. After 10.5 years of use, 7.84% of 107Ag and 21.08% of 109Ag are burned up, and correspondingly it leads to neutron sensitivity decrement by 16.68%; however, the sensitivity to external gamma is negatively small and remains almost unchanged during the whole operation period. On the other hand, signals’ timing analysis shows the rising time of the total signal current to a step neutron flux increases from 115 s to 125 s. All of these suggest that the neutron sensitivity and delay compensation coefficients need to be recalibrated during the use of Ag SPND.