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Effect of Gamma Radiation on LMFBR Subcriticality Measurements Using Noise Analysis

D. P. Roux, A. R. Buhl

Nuclear Technology

Volume 12 / Number 1 / September 1971 / Pages 137-140


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As a result of the effect of residual gamma radiation on neutron detectors in shutdown reactors, the precision of subcriticality measurements by noise analysis is degraded. An equation for this gamma degradation effect was developed for application to fast reactors. The gamma degradation factor D and the detection efficiency Wn were evaluated for 10B, 3He, and 235U detectors for application in the Fast Flux Test Facility (FFTF), and it was concluded that the 235U, the least sensitive of the three detectors, is the best choice. The 235U detector is 40 times less efficient than the 3He detector, but it should experience only a slight degradation in the FFTF. By contrast, degradation factors varying between 100 and a few thousand are anticipated for the 3He detector through the shutdotvn range of -1 to -10 dollars. Furthermore, D is unity up to 106 R/h and is independent of p for the fission chamber when used as a counter.

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