In-air and in-water performance comparison of Passive Gamma Emission Tomography with activated Co-60 rods

Abstract A first-of-a-kind geological repository for spent nuclear fuel is being built in Finland and will soon start operations. To make sure all nuclear material stays in peaceful use, the fuel is measured with two complementary non-destructive methods to verify the integrity and the fissile content of the fuel prior to disposal. For pin-wise identification of active fuel material, a Passive Gamma Emission Tomography (PGET) device is used. Gamma radiation emitted by the fuel is assayed from 360 angles around the assembly with highly collimated CdZnTe detectors, and a 2D cross-sectional image is reconstructed from the data. At the encapsulation plant in Finland, there will be the possibility to measure in air. Since the performance of the method has only been studied in water, measurements with mock-up fuel were conducted at the Atominstitut in Vienna, Austria. Four different arrangements of activated Co-60 rods, steel rods and empty positions were investigated both in air and in water to confirm the functionality of the method. The measurement medium was not observed to affect the ability of the method to distinguish modified rod positions from filled rod positions. More extended conclusions about the method performance with real spent nuclear fuel cannot be drawn from the mock-up studies, since the gamma energies, activities, material attenuations and assembly dimensions are different, but full-scale measurements with spent nuclear fuel are planned for 2023

Improved Passive Gamma Emission Tomography image quality in the central region of spent nuclear fuel

Reliable non-destructive methods for verifying spent nuclear fuel are essential to draw credible nuclear safeguards conclusions from spent fuel. In Finland, spent fuel items are verified prior to the soon starting disposal in a geological repository with Passive Gamma Emission Tomography (PGET), a uniquely accurate method capable of rod-level detection of missing active material. The PGET device consists of two highly collimated detector banks, collecting gamma emission data from a 360 degrees rotation around a fuel assembly. 2D cross-sectional activity and attenuation images are simultaneously computed. We present methods for improving reconstructed image quality in the central parts of the fuel. The results are based on data collected from 2017 to 2021 at the Finnish nuclear power plants with 10 fuel assembly types of varying characteristics, for example burnups from 5.7 to 55 GWd/tU and cooling times from 1.9 to 37 years. Data is acquired in different gamma energy windows, capturing the peaks of Cs-137 (at 662 keV) and Eu-154 (at 1274 keV), abundant isotopes in long-cooled spent nuclear fuel. Data from these gamma energy windows at well-chosen angles are used for higher-quality images, resulting in more accurate detection of empty rod positions. The method is shown to detect partial diversion of nuclear material also in the axial direction, demonstrated with a novel measurement series scanning over the edge of partial-length rods.Peer reviewe

Measuring spent fuel assembly multiplication in borated water with a passive neutron albedo reactivity instrument

Abstract The performance of a passive neutron albedo reactivity (PNAR) instrument to measure neutron multiplication of spent nuclear fuel in borated water is investigated as part of an integrated non-destructive assay safeguards system. To measure the PNAR Ratio, which is proportional to the neutron multiplication, the total neutron count rate is measured in high- and low-multiplying environments by the PNAR instrument. The integrated system also contains a load cell and a passive gamma emission tomograph, and as such meets all the recommendations of the IAEA’s recent ASTOR Experts Group report. A virtual spent fuel library for VVER-440 fuel was used in conjunction with MCNP simulations of the PNAR instrument to estimate the measurement uncertainties from (1) variation in the water boron content, (2) assembly positioning in the detector and (3) counting statistics. The estimated aggregate measurement uncertainty on the PNAR Ratio measurement is 0.008, to put this uncertainty in context, the difference in the PNAR Ratio between a fully irradiated assembly and this same assembly when fissile isotopes only absorb neutrons, but do not emit neutrons, is 0.106, a 13-sigma effect. The 1-sigma variation of 0.008 in the PNAR Ratio is estimated to correspond to a 3.2 GWd/tU change in assembly burnup.Peer reviewe

Passive neutron albedo reactivity measurements of spent nuclear fuel

The upcoming disposal of spent nuclear fuel in Finland creates new challenges for nuclear safeguards. Part of the national safeguards concept for geological repositories, developed by STUK — Radiation and Nuclear Safety Authority, is non-destructive assay (NDA) verification of all fuel items before disposal. The proposed verification system is a combination of PGET (Passive Gamma Emission Tomography), PNAR (Passive Neutron Albedo Reactivity) and weight measuring NDA-instruments. PGET takes a pin-level image of the fission products inside of a fuel assembly and PNAR verifies the multiplication of the assembly, a quantity that correlates with the fissile content. PGET is approved by IAEA (International Atomic Energy Agency) for safeguards measurements, but the feasibility of PNAR has not yet been established. A first of its kind PNAR prototype instrument was built in a collaboration coordinated by STUK. This paper concludes the results of the first measurements of spent BWR (Boiling Water Reactor) nuclear fuel with the prototype in July 2019. Based on the measurements, the ability of the PNAR instrument to detect the presence of fissile material in a repeatable manner in a reasonable amount of time was demonstrated. Furthermore, the instrument was able to detect differences in multiplication between partially and fully spent fuel assemblies, and axial differences in multiplication within a single assembly.Peer reviewe

The World’s First Spent Fuel Repository : How to tackle safety, security and safeguards needs?

How to dispose of spent nuclear fuel safely and permanently? This is one of the fundamental questions related to the use of nuclear energy, that has been waiting for an answer since criticality of the first commercial reactors some sixty years ago. Also, in Finland, discussion on the question of nuclear waste was on the public agenda already when the first reactor was commissioned in the late 1970s and nuclear waste management policy and strategy were actively developed on the national level. In 1978, the Finnish Government decided that each producer of nuclear waste is responsible for the management of spent nuclear fuel. This decision was the beginning of a long process, the result of which is the world’s first spent spent nuclear fuel repository Onkalo, where the final disposal of spent nuclear fuel inside the Finnish bedrock is expected to start in 2025. This paper describes from the regulatory perspective how Finland changed the game and how Finland is developing a safe1 and sustainable solution for disposal of spent nuclear fuel. It will explain how this became politically acceptable, how the long-term safety of the solution is being demonstrated and how regulatory challenges related to safety, security and safeguards are being resolved in this first-of-its-kind facility. In broad terms, it will illustrate how the progress in geological disposal has been made possible in Finland and further highlight topical issues that are of interest to professionals and policymakers. The first chapter is focused on public acceptance and development of nuclear waste management policy and strategy in Finland. The second chapter explains how the long-term safety of the final repository has been handled and what the supporting technical solutions are. In the third and final chapter, an overview of safeguards of the disposal process is provided. Safeguards, a prerequisite for peaceful use of nuclear energy, is a topic of utmost importance also in the last leg of the nuclear fuel cycle. The scope of this short paper is rather limited and far from complete, but hopefully it manages to pass on certain lessons: responsible decision-making and a long-term political commitment to the chosen method, together with the research and development of the technical solution and enabling regulatory framework, are the keys for accomplishing the difficult task of disposing of spent nuclear fuel safely and permanently

Passive Neutron Albedo Reactivity in the Finnish encapsulation context

1 Introduction The purpose of this report is to document the Monte Carlo simulation and analytic results for the implementation of the Passive Neutron Albedo Reactivity (PNAR) nondestructive assay (NDA) technique in the context of Finnish spent fuel encapsulation needs. This document is offered as partial fulfillment of the contract between Encapsulation NDA Services and Radiation and Nuclear Safety Authority (STUK) of Finland. The end goal of this research effort is two conceptual designs of the PNAR technique for both BWR and PWR fuel in Finland

Passive Neutron Albedo Reactivity (PNAR) Prototype for Spent Nuclear Fuel Verification

The Finnish safeguards concept for encapsulation plant and geological repository includes verification of all fuel items before encapsulation and disposal. The verification process is carried out according to IAEA Group of Experts ASTOR (Application of Safeguards TO Repositories) recommendations. One of the non-destructive assay (NDA) instruments featured in the concept is Passive Neutron Albedo Reactivity (PNAR). The PNAR device will be used to verify the presence of fissile material in spent nuclear fuel assemblies. The PNAR instrument makes two relative neutron flux measurements. One in a neutron albedo maximizing an one in a neutron albedo minimizing configuration. From the ratio of the two measurements, a measure of the presence of fissile material can be calculated. The albedo minimizing configuration is achieved by surrounding the fuel assembly with cadmium. A prototype of the PNAR device is being built by STUK. The prototype is designed for underwater measurements of BWR fuel used in Olkiluoto nuclear power plants. This article presents the final design of the prototype device and presents the upcoming measurement plans with the detector. A mock-up PNAR detector pod was assembled and tested under laboratory conditions to quantify the detector signals and find potential sources of errors in the design