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 N/sub 2/. Characteristics of the Xe-N/sub 2/ mixture were investigated and compared to those of a mixed gas of Ar and N/sub 2/, 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-N/sub 2/ filling gas is used, the average current-pulse output and charge-pulse output were 1.6*10/sup -6/ A and 0.8*10/sup -13/ C, respectively. These outputs were about two times larger than those of a counter-chamber filled with the Ar-N/sub 2/ 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-N/sub 2/ filling gas. The performance of the HTIC was tested up to 800 degrees 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 degrees C.<
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