U-233 fission detectors for reactor instrumentation

Uranium-233 fission counters have been developed as an alternative to 235U counters for start up instrumentation for Fast Breeder Test Reactor (FBTR) and other reactors. To avoid alpha pile up problems, relatively low amounts of 233U have been used, resulting in neutron sensitivity ranging from 0.003 to 0.09 cps/nv. Tests have been carried out at Apsara and FBTR and the counters showed good compatibility with the instrumentation. With an electrode spacing of 3 mm and argon + 3% nitrogen gas filling, collection times of the order of 100 ns have been achieved. The low sensitivity counter has been tested in pulse, dc as well as Campbell mode. The signals showed good linearity with respect to neutron flux and reactor power. The voltage plateau showed a slope of 5% per 100 volts. The counters showed a 15% reduction in neutron sensitivity in a gamma field of 1000 r/h. Designs are also proposed for developing high sensitivity fission detectors for reactor applications using 233U as well as low grade 235U. (author). 9 refs., 9 figs