A monitor solution to characterize a fissile material in a glove box
Résumé
Handling fissionable material requires special know-how in managing irradiation and contamination risks while controlling any potential chain reaction, i.e. maintaining sub-criticality. If the risks associated with cleanup and dismantling operations had to be ranked in order of seriousness, keeping control of sub-criticality would probably be number one. Unlike contamination or irradiation, management of sub-criticality during the D&D phase is mainly ensured by control of the fissile material mass present; because management approaches by geometry or by concentration and moderation no longer apply. The control modes for management are therefore reduced to the question of the fissile mass. This important subject represents an opportunity for the development of fissile matter mass control, or in other words, the development of innovative radiological characterization systems. More precisely, D&D operations require the design and deployment of innovative ways to ensure accurate measurement of the mass of matter in a hostile environment. More specifically, this means radiological characterization techniques are needed to evaluate decontamination factors or material holdup in equipment undergoing cleanup, based on an initial radiological inventory. The technical proposal described in this document is based on gamma spectrometry applied to the characterization of the factors of decontamination or of fissile material accumulation in a glove box. The system's originality lies in the development of a compact detector setup functioning at ambient temperature, and requiring no modification of the glove box. Evaluation of the source term contained in the glove box results from gamma spectrum processing in order to obtain the expression of a decontamination factor. An original method was also developed to achieve this, enabling the decontamination factor to be realistically assessed. Trials carried out in an industrial location have demonstrated the validity of the approaches implemented in the system setup and in processing the gamma spectra. Gamma spectrometry is adapted to monitoring the evolution of the main radionuclides, $^{241}$Am and $^{241}$Pu. The quality of the spectra (resolution) and the sensitivity of the technique in the configurations retained for this study confirm the suitability of this in situ analysis technique. The work has confirmed that detectors equipped with CdZnTe crystals are devices which are well-adapted to on-line monitoring of glove box cleanup.
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