Investigation of a fast partial defect detection method for safeguarding PWR spent fuel assemblies

Highlights

• A novel detector was designed for fast spent fuel verification examining partial defects.

• Detection criterion was setup based on uncertainty analysis.

• The partial defect verification capability of the developed detector was analyzed using test case assemblies.

• Cost – effectiveness of the developed detector in comparison to conventional detectors was analyzed.

Abstract

As the number of countries with their spent fuel inventory in storage increases, spent fuel verification for nuclear safeguards becomes important. In particular, detection of partial defect, which is the result of local diversion of fuel rods, deserves special attention. This is because accumulation of small scale fuel rod diversions could lead to undesirable consequences. While the necessary detection technologies are available, the cost and detection time associated make it difficult for the partial defect detection technologies to be applied to all spent fuel assemblies. This research investigated the feasibility of developing an efficient and cost-effective method for detecting partial defects of spent fuel assemblies. The approach is based on using the gamma radiation emitted from spent fuel and converting its energy to electric energy. Such approach was incorporated into a new detector concept called, “scintillator based partial defect detector (SPDD)”. SPDD detects the intensity of passive gamma by converting gamma radiation into photons using a CdWO₄ scintillator and then into electric current by using amorphous silicon photodiode. Along with the use of detector measurements, a parallel approach of estimating generated electric current by using computation models using the declared spent fuel information is implemented. Detection of partial defects is based on comparing the differences in the electric current between measurements and the expected results. The proposed method was tested using the scenario of detecting 1 SQ (significant quantity) of Pu missing among spent fuel assemblies loaded for a shipment in a typical transportation cask. Results from selected test cases indicated the feasibility of as well as limitations in detecting partial defects by using the proposed method for screening purposes. The irradiation damage to the CdWO₄ scintillator of SPDD was also examined for the periods of in-reactor application. In addition, the cost associated with SPDD was estimated in comparison to that of existing partial defect technologies.

Purpose of the research

To design a cost-effective partial defect detector with fast screening capability. To demonstrate the feasibility of applying the detector in high radiation environment. To setup partial defect detection criterion. To examine the feasibility of applying the developed detector to partial defect detection.

Approaches

Design a scintillator – photodiode based gamma detector. Develop a method for detecting partial defects. Setup detection criterion for partial defect detection. Analyze radiation damage issue and cost – effectiveness of the proposed detection method. Demonstrate the feasibility of partial defect detection base on examining test case assemblies.

Introduction

Nuclear safeguards continues to face challenges as the number of nuclear facilities, the amount of nuclear material, and types of facilities to be safeguarded increase. Similarly, the diversion pathways of nuclear material may become complex as the experiences in nuclear reactor operation accumulate and the capabilities of plant operating personnel increase. This may translate into increase in inspection time for nuclear material verification. Such considerations are reflected in the creation of research projects, such as the U.S. Department of Energy Next Generation Safeguards Initiative Spent Fuel (NGSI-SF), conducted specifically for establishing the baseline of future nuclear safeguards and non-destructive assay (NDA) detectors.

The objective of nuclear safeguards is “timely detection of diversion of significant quantities of nuclear material and deterrence of such diversion by the risk of early detection”, according to the IAEA (IAEA, 1972). To meet the requirements, spent fuel assemblies have to be verified before they are emplaced in a geological repository or an encapsulation cask for storage (Tarvainen et al., 1997). Verification of a spent fuel assembly is accomplished by comparing the amount of the declared and existing nuclear material in an assembly (International Atomic Energy Agency, 2011). But challenges in nuclear safeguards of spent fuel remain. Such challenges include improving the capabilities of NDA instruments for partial defect detection, autonomous spent fuel information verification, and plutonium inventory measurement (Hu et al., 2014). This study takes note of the partial defect issue as the focus of study. Partial defect is defined as “an item or a batch that has been falsified to such an extent that some fraction of the declared amount of material is actually present” (International Atomic Energy Agency, 2011). Conventional instruments for partial defect detection measure the localized intensity of passive gamma or passive neutrons. Instruments for spent fuel information verification count the passive gamma, passive neutron, or active neutron to measure initial enrichment, burnup, and cooling time. Instruments for plutonium inventory verification count active neutrons since ²⁴⁴Cm dominates passive neutrons from spent fuel. Since there is no quantified or declared objective for detection limits in the literature, partial defect detection remains particularly challenging in NDA instrument development.

There are a number of NDA instruments developed to detect partial defects. These instruments include the digital Cerenkov viewing device (DCVD), fork detector (FDET), partial defect detector (PDET), passive gamma emission tomography (PGET), or spent fuel MOX python (SMOPY) (Tarvainen et al., 1997, International Atomic Energy Agency, 2011, Siskind, 2013). These instruments distinguish defective spent fuel assemblies from normal spent fuel assemblies by using a combination of neutron and gamma detection, multiple gamma detectors, or by visualization of photon intensity. These methods are summarized in Table 1.

While these methods are useful and have been practically implemented, limitations still remain. These methods often come with poor spatial resolution for detecting partial defects or long detection time. Or the cost of equipment or maintenance requirements are high. In the case of using passive gamma measurements, the NDA instruments used require calibrating count rate. The calibration process delays inspection and requires the presence of an inspector or operator. This prevents the application of the technique in an autonomous manner for spent fuel verification. In the case of DCVD, the detector is not applicable to spent fuels under the dry storage conditions. Key characteristics and limitations of existing partial defect detection methods are summarized in Table 1.

The objective of this research is to investigate the development of a cost-effective NDA measurement technique for detecting spent fuel partial defects without the need for the calibration process. The proposed technique is based on direct conversion of the local gamma intensity of a spent fuel assembly into electric current, using a scintillator and photodiode (Lee and Yim, 2017). The technique is termed “scintillator based partial defect detector (SPDD)”.