Fission Gas Release Determination Using an Anti-Compton Shield Detector

Poolside measurements of fission gas release (FGR) in fuel pins have been made using gamma-ray spectroscopy with a Ge detector, measuring ⁸⁵Kr activity in the fuel rod plenum. The gamma-ray energy spectra from irradiated nuclear fuel are characterized by prominent Compton distributions that can obscure the weak 514-keV ⁸⁵Kr peak. To improve the sensitivity, the detector has been provided with an anti-Compton shield of six Bi₃Ge₄O₁₂ detectors. Laboratory tests of the detector system showed that the maximum peak-to-Compton (p/c) ratio was improved by a factor of ~6. The results of the poolside measurement p/c ratio showed a somewhat smaller improvement (a factor of ~4) because of scattered gamma radiation from the surrounding material. However, the precision in the poolside FGR measurements was improved substantially utilizing the Compton shield.