Development of wide-range in-core fission counter-chamber for HTGR

A wide-range and high-temperature in-core fission counter-chamber (HTIC) has been developed for in-core flux monitoring in high-temperature gas-cooled reactors (HTGRs). Good results were obtained in the development of the HTIC using a new type of gas composed of Xe and N₂. Characteristics of the Xe-N₂ mixture were investigated and compared to those of a mixed gas of Ar and N₂, which is widely used for conventional fission counter-chambers. The characteristics of these gases were examined by measuring a current-pulse output and a charge-pulse output of a counter-chamber filled with the gas of 2 bars (abs) with a polarizing voltage of 100 V for 1-mm electrode spacing. When Xe-N₂ filling gas is used, the average current-pulse output and charge-pulse output were 1.6×10^-6 A and 0.8×10^-13 C, respectively. These outputs were about two times larger than those of a counter-chamber filled with the Ar-N₂ mixture gas of the same condition. Following the experiment, an HTIC with an outer diameter of 10 mm was designed and fabricated using the Xe-N₂ filling gas. The performance of the HTIC was tested up to 800°C in a high-temperature irradiation rig installed in the research reactor JRR-4 of JAERI (Japan Atomic Energy Research Institute). Test results showed that the HTIC has a good linearity over a measuring range of eight decades in a high-temperature environment of up to 800°C