Benchtop Zone Refinement of Simulated Future Spent Nuclear Fuel Pyroprocessing Waste.

The UK's adoption of pyroprocessing of spent nuclear fuel as an alternative to the current aqueous processing routes requires a robust scientific underpinning of all relevant processes. One key process is the clean-up of the contaminated salt from the electroreducing and electrorefining processes. A proposed method for this clean-up is zone refining, whereby the tendency of the contaminants to remain in the liquid phase during melting and freezing is exploited to 'sweep' the contaminants to one end of the sample. Experiments were performed, utilising off-the-shelf laboratory equipment, to demonstrate the feasibility of zone refining for clean-up of electroreducing and electrorefining wastes. This was successful for the electrorefining simulant samples, with effective segregation coefficient, keff, values, which provide a measure of the degree of separation in the sample, between 0 and 1. Lower values indicate greater separation, with values of as low as 0.542 achieved here, corresponding to a reduction in RECl3 content from 10.0 wt.% to 8.4 wt.% (for 80% salt reuse). Due to difficulties in obtaining a fully homogeneous electroreducing simulant waste, it was not possible to demonstrate the feasibility of zone refining using the current experimental setup. Further research is required to elucidate the correct preparation conditions for production of homogeneous electroreducing waste simulants.

Pyroprocessing and the optimum mix ratio of rice husks, broken bricks and spent bleaching earth to make pozzolanic cement.

This paper presents the findings of an experimental investigation on optimizing pozzolanic activity of a blend of Rice Husks (RH), Spent Bleaching Earth (SBE) and Broken Bricks (BB) to form pozzolana that would have pozzolanic activity comparable to natural pozzolanas. Four ratios of RH, BB, and SBE were burnt in the Fixed Bed Kiln (FBK). The starting ratio had 20 kg of RH, 0 kg of BB and 4 kg of SBE. The amount of BB was increased by 2 kg each to a maximum of 6 kg as the mass of SBE was kept constant. The resultant ashes were subjected to various pozzolanic tests. This included; saturated lime test and compressive strength analysis. It was observed that the calcined blend with 10: 1: 2 mix of RH: BB: SBE exhibited the highest pozzolanic activity. This sample was mixed with acetylene lime sludge (ALS) in the ratio of 2:1 pozzolana: ALS. The compressive strengths for these cements were tested at 2 and 28 days of curing. The compressive strengths of this cement met the required EN standards for Portland pozzolana cement.

Laser-Induced Breakdown Spectroscopy (LIBS) Measurement of Uranium in Molten Salt.

In this current study, the molten salt aerosol-laser-induced breakdown spectroscopy (LIBS) system was used to measure the uranium (U) content in a ternary UCl3-LiCl-KCl salt to investigate and assess a near real-time analytical approach for material safeguards and accountability. Experiments were conducted using five different U concentrations to determine the analytical figures of merit for the system with respect to U. In the analysis, three U lines were used to develop univariate calibration curves at the 367.01 nm, 385.96 nm, and 387.10 nm lines. The 367.01 nm line had the lowest limit of detection (LOD) of 0.065 wt% U. The 385.96 nm line had the best root mean square error of cross-validation (RMSECV) of 0.20 wt% U. In addition to the univariate calibration approach, a multivariate partial least squares (PLS) model was developed to further analyze the data. Using partial least squares (PLS) modeling, an RMSECV of 0.085 wt% U was determined. The RMSECV from the multivariate approach was significantly better than the univariate case and the PLS model is recommended for future LIBS analysis. Overall, the aerosol-LIBS system performed well in monitoring the U concentration and it is expected that the system could be used to quantitatively determine the U compositions within the normal operational concentrations of U in pyroprocessing molten salts.

Measurement of Cerium and Gadolinium in Solid Lithium Chloride-Potassium Chloride Salt Using Laser-Induced Breakdown Spectroscopy (LIBS).

Pyroprocessing of used nuclear fuel (UNF) has many advantages-including that it is proliferation resistant. However, as part of the process, special nuclear materials accumulate in the electrolyte salt and present material accountability and safeguards concerns. The main motivation of this work was to explore a laser-induced breakdown spectroscopy (LIBS) approach as an online monitoring technique to enhance the material accountability of special nuclear materials in pyroprocessing. In this work, a vacuum extraction method was used to draw the molten salt (CeCl3-GdCl3-LiCl-KCl) up into 4 mm diameter Pyrex tubes where it froze. The salt was then removed and the solid salt was measured using LIBS and inductively coupled plasma mass spectroscopy (ICP-MS). A total of 36 samples were made that varied the CeCl3 and GdCl3 (surrogates for uranium and plutonium, respectively) concentrations from 0.5 wt% to 5 wt%. From these samples, univariate calibration curves for Ce and Gd were generated using peak area and peak intensity methods. For Ce, the Ce 551.1 nm line using the peak area provided the best calibration curve with a limit of detection (LOD) of 0.099 wt% and a root mean squared error of cross-validation (RMSECV) of 0.197 wt%. For Gd, the best curve was generated using the peak intensities of the Gd 564.2 nm line resulting in a LOD of 0.027 wt% and a RMSECV of 0.295 wt%. The RMSECV for the univariate cases were determined using leave-one-out cross-validation. In addition to the univariate calibration curves, partial least squares (PLS) regression was done to develop a calibration model. The PLS models yielded similar results with RMSECV (determined using Venetian blind cross-validation with 17% left out per split) values of 0.30 wt% and 0.29 wt% for Ce and Gd, respectively. This work has shown that solid pyroprocessing salt can be qualitatively and quantitatively monitored using LIBS. This work has the potential of significantly enhancing the material monitoring and safeguards of special nuclear materials in pyroprocessing.

Performance evaluation of PRIDE UNDA system with pyroprocessing feed material.

The PRIDE (PyRoprocessing Integrated inactive DEmonstration) is an engineering-scale pyroprocessing test-bed facility that utilizes depleted uranium (DU) instead of spent fuel as a process material. As part of the ongoing effort to enhance pyroprocessing safeguardability, UNDA (Unified Non-Destructive Assay), a system integrating three different non-destructive assay techniques, namely, neutron, gamma-ray, and mass measurement, for nuclear material accountancy (NMA) was developed. In the present study, UNDA's NMA capability was evaluated by measurement of the weight, 238U mass, and U enrichment of oxide-reduction-process feed material (i.e., porous pellets). In the 238U mass determination, the total neutron counts for porous pellets of six different weights were measured. The U enrichment of the porous pellets, meanwhile, was determined according to the gamma spectrums acquired using UNDA's NaI-based enrichment measurement system. The results demonstrated that the UNDA system, after appropriate corrections, could be used in PRIDE NMA applications with reasonable uncertainty. It is expected that in the near future, the UNDA system will be tested with next-step materials such as the products of the oxide-reduction and electro-refining processes.

Laser-Induced Breakdown Spectroscopy (LIBS) in a Novel Molten Salt Aerosol System.

In the pyrochemical separation of used nuclear fuel (UNF), fission product, rare earth, and actinide chlorides accumulate in the molten salt electrolyte over time. Measuring this salt composition in near real-time is advantageous for operational efficiency, material accountability, and nuclear safeguards. Laser-induced breakdown spectroscopy (LIBS) has been proposed and demonstrated as a potential analytical approach for molten LiCl-KCl salts. However, all the studies conducted to date have used a static surface approach which can lead to issues with splashing, low repeatability, and poor sample homogeneity. In this initial study, a novel molten salt aerosol approach has been developed and explored to measure the composition of the salt via LIBS. The functionality of the system has been demonstrated as well as a basic optimization of the laser energy and nebulizer gas pressure used. Initial results have shown that this molten salt aerosol-LIBS system has a great potential as an analytical technique for measuring the molten salt electrolyte used in this UNF reprocessing technology.

Optimization of hybrid-type instrumentation for Pu accountancy of U/TRU ingot in pyroprocessing.

One of the final products of pyroprocessing for spent nuclear fuel recycling is a U/TRU ingot consisting of rare earth (RE), uranium (U), and transuranic (TRU) elements. The amounts of nuclear materials in a U/TRU ingot must be measured as precisely as possible in order to secure the safeguardability of a pyroprocessing facility, as it contains the most amount of Pu among spent nuclear fuels. In this paper, we propose a new nuclear material accountancy method for measurement of Pu mass in a U/TRU ingot. This is a hybrid system combining two techniques, based on measurement of neutrons from both (1) fast- and (2) thermal-neutron-induced fission events. In technique #1, the change in the average neutron energy is a signature that is determined using the so-called ring ratio method, according to which two detector rings are positioned close to and far from the sample, respectively, to measure the increase of the average neutron energy due to the increased number of fast-neutron-induced fission events and, in turn, the Pu mass in the ingot. We call this technique, fast-neutron energy multiplication (FNEM). In technique #2, which is well known as Passive Neutron Albedo Reactivity (PNAR), a neutron population's changes resulting from thermal-neutron-induced fission events due to the presence or absence of a cadmium (Cd) liner in the sample's cavity wall, and reflected in the Cd ratio, is the signature that is measured. In the present study, it was considered that the use of a hybrid, FNEM×PNAR technique would significantly enhance the signature of a Pu mass. Therefore, the performance of such a system was investigated for different detector parameters in order to determine the optimal geometry. The performance was additionally evaluated by MCNP6 Monte Carlo simulations for different U/TRU compositions reflecting different burnups (BU), initial enrichments (IE), and cooling times (CT) to estimate its performance in real situations.

A study on artificial rare earth (RE2O3) based neutron absorber.

A new concept of a neutron absorption material (i.e., an artificial rare earth compound) was introduced for criticality control in a spent fuel storage system. In particular, spent nuclear fuels were considered as a potential source of rare earth elements because the nuclear fission of uranium produces a full range of nuclides. It was also found that an artificial rare earth compound (RE2O3) as a High-Level Waste (HLW) was naturally extracted from pyroprocessing technology developed for recovering uranium and transuranic elements (TRU) from spent fuels. In this study, various characteristics (e.g., activity, neutron absorption cross-section) were analyzed for validating the application possibility of this waste compound as a neutron absorption material. As a result, the artificial rare earth compound had a higher neutron absorption probability in the entire energy range, and it can be used for maintaining sub-criticality for more than 40 years on the basis of the neutron absorption capability of Boral™. Therefore, this approach is expected to vastly improve the efficiency of radioactive waste management by simultaneously keeping HLW and spent nuclear fuel in a restricted space.