Comparison of prediction models for Cherenkov light emissions from nuclear fuel assemblies

Abstract

The Digital Cherenkov Viewing Device (DCVD) [5] is a tool used by nuclear safeguards inspectors to verify irradiated nuclear fuel assemblies in wet storage based on the Cherenkov light produced by the assembly. Verifying that no rods have been substituted in the fuel, so-called partialdefect verification, is done by comparing the intensity measured with a DCVD with a predicted intensity, based on operator fuel declaration. The prediction model currently used by inspectors is based on simulations of Cherenkov light production in a BWR 8x8 geometry. This work investigates prediction models based on simulated Cherenkov light production in a BWR 8x8 and a PWR 17x17 assembly, as well as a simplified model based on a single rod in water. Cherenkov light caused by both fission product gamma and beta decays was considered. The simulations reveal that there are systematic differences between the model used by safeguards inspectors and the models described in this publication, most noticeably with respect to the fuel assembly cooling time. Consequently, if the intensity predictions are based on another fuel type than the fuel type being measured, a systematic bias in intensity with respect to burnup and cooling time is introduced. While a simplified model may be accurate enough for a set of fuel assemblies with nearly identical cooling times, the prediction models may differ systematically by up to 18 % for fuels with more varied cooling times. Accordingly, these investigations indicate that the currently used model may need to be exchanged with a set of more detailed, fuel-type specific models, in order minimize the model dependent systematic deviations.

On the inclusion of light transport in prediction tools for Cherenkov light intensity assessment of irradiated nuclear fuel assemblies

“…In this work, ORIGEN [8] has been used for this pur-pose, and the irradiation history simulated has been similar to the "standard" histories used in the prediction model currently in use by the IAEA [4]. This work has neglected the direct Cherenkov light contribution due to beta-decays, though it can in principle be handled in the same way as gamma-emissions in the methodology [3].…”

Experimental evaluation of models for predicting Cherenkov light intensities from short-cooled nuclear fuel assemblies

“…As a consequence of this calibration, the predictions do not correspond to the absolute measured intensity, but to a relative intensity measure for assemblies of similar design. Additional details regarding the DCVD and the verification procedure can be found in [4].…”

“…However, it also involves relatively large simplifications, which may limit its precision, in particular for assemblies with short CT and assemblies with irregular irradiation history. The two-parameter model also assumes that all assemblies behave like a BWR 8x8 fuel assembly, neglecting the effects that different assembly geometries have on Cherenkov light production [4].…”

“…These simulations are time-consuming, however they only have to be done once per assembly design and they are done in advance so that tabulated values can be used to make rapid computations of predicted intensities. In this prediction model, simulations are performed for different geometrical assembly configurations in order to take into account differences in Cherenkov light production caused by the physical design of the assembly [4]. However, in order to finish these simulations in reasonable time, one fundamental simplification is made: that the detected Cherenkov light intensity is proportional to the produced intensity, i.e.…”

“…The first prediction model brought into use was a two-parameter model, basing the predictions on declared integral burnup (BU) 1 and cooling time (CT) 2 [3]. Recently, a new gamma-spectrum based model has been introduced for improved predictions of the Cherenkov light intensity [4]. This recent model takes more operator-declared data into account compared to the previously used two-parameter model, such as assembly geometry, Initial Enrichment (IE) and irradiation history (i.e.…”