Gamma radiation techniques for non-destructive post-irradiation examination of nuclear fuel : Predicting performance and optimizing instrument design

Sammanfattning: Collimated gamma-based techniques are widely used to study nuclear fuel, especially for non-destructive post-irradiation examination (PIE). Examples of such techniques include gamma emission tomography (GET) and gamma transmission densitometry (GTD).Further development in designing collimated GET and GTD setups can be foreseen, optimizing the spatial resolution from a millimetre scale to a hundred-micron scale. The enhanced performance would be an asset in PIE, providing a unique combination of high spatial resolution and low interrogation effort. The experimental data would provide a detailed insight into the fuel for modelling its performance at high burnup or in accident scenarios. However, optimizing new instrument designs is not trivial. Multiple performance metrics exist, and their interdependence with the setup configuration and sample characteristics requires evaluation. At the same time, trade-offs should be identified and considered according to the application requirements. Monte Carlo (MC) radiation transport tools are generally unsuitable for modelling such collimated setups due to the intrinsic transmission inefficiency of gammas through the collimator slit. Therefore, alternative methods are desired for evaluating the collimator response.  This study proposes a structured optimization methodology, where the trade-offs between the performance metrics are considered to suggest optimal collimator designs for GET applications. The method combines analytical methodologies for fast collimator response calculation and accurate MC simulations to evaluate sample self-attenuation and detector response. The results indicate that for sub-millimetric slits, a few hundred microns resolution can be achieved in suitable conditions (high burnup and short cooling time fuel samples) with reasonable investigation time and noise. Furthermore, the methodologies developed were used to evaluate the feasibility of radial gamma transmission micro-densitometry. Such a technique was also demonstrated through a first experimental campaign using calibration standards and an ADOPTTM irradiated nuclear fuel sample.