Detector development for PFBR

Development of high-temperature fission detectors and boron-counters is being carried out for neutron flux measurement at PFBR for reactor control instrumentation and delayed neutron monitoring. Thermal neutron flux levels (235U equivalent) from 0.1 to 1010nv are to be monitored from start up to full power. Fission detectors are designed with integral triaxial mineral insulated (MI) cable assembly to withstand operating temperatures up to 650 degC and gamma background up to 1 MR/h. The location and dimensional specifications require the use of enriched uranium for neutron sensing. The 0.8mm electrode spacing in the counters will be filled with argon+3% nitrogen at 8 bars for fast collection time and adequate pulse-height. Inconel 600 is used as construction material to minimise nitrogen absorption. The mechanical design provides for thermal expansion of the internals of the detectors. Hot Isostatic Pressed (HIP) high-purity alumina insulators are to be used to ensure minimum of 106U between the electrodes at the highest temperature. To carry out preliminary tests, one counter (50mm dia x 220mm) has been developed with natural uranium coating on the electrodes. Tests at Apsara Thermal column in 105nv flux showed neutron sensitivity of 6x10-4 cps/nv..Tests in gamma fields in ambient gamma background of 2MR/h did not show any deterioration in the neutron sensitivity. The counter has charge collection time of 50ns and requires 400V DC polarising voltage. MI cables and ceramic components for the detectors have been tested at temperatures up to 650 degC. Insulation resistance reduced from 1012U to 109U and no breakdown noise pulses were observed at 400V. (author)