SIMULATE5-K (S5K) is the newest state-of-the-art, best-estimate, transient reactor analysis software developed by Studsvik Scandpower, Inc. (SSP). When used in conjunction with CASMO5 to generate multi-group neutronic data for steady-state and transient calculations, S5K can be used to accurately model reactor transient conditions in Pressurized Water Reactors (PWRs) and Boiling Water Reactors (BWRs). Recently, the methods in S5K and CASMO5 were validated by simulating reactor transients from the SPERT-III E-core experiments. These experiments were performed on a small Light Water Reactor (LWR) core and were designed to resemble Reactivity Insertion Accidents (RIAs) in PWRs. Overall, the results calculated with S5K agree well with the experimentally measured data and any differences are much smaller than the reported uncertainties in the measurements, especially the uncertainty in the initial condition. Additionally, S5K supports general multi-group time-dependent neutron diffusion calculations and the SPERT-III E-core experiments were used to show that there were no obvious trends or discrepancies when the two-, four-, and eight-group calculations were compared.

Geopolymers, aluminosilicate materials formed by alkali activation, have drawn interest because of their unique mechanical, chemical, and thermal characteristics. They are interesting for adsorption applications due to their similar chemical structure to zeolite. This study investigates the synthesis and characterization of hybrid geopolymer/zeolite composites to remove lead ions (Pb(II)) and brilliant green (BG) dye from aqueous solutions. Sodium hydroxide and sodium silicate were used to activate fly ash and blast furnace slag blends. This was followed by hydrothermal treatment to encourage the conversion of amorphous geopolymeric gel to crystalline zeolites. Several variables were systematically changed, such as foaming agents, alkali molarity, and bead size to compare adsorption performance. The formation of zeolite phases was confirmed by structural and morphological investigations, such as XRD, FT-IR, SEM, and BET, which also shed light on the porous character of the composite. The geopolymer/zeolite composites demonstrated notable removal efficiency for Pb(II) (up to 123 mg/g) and BG dye (up to 115 mg/g) in adsorption studies. Importantly, this work reveals that average pore diameter plays a more critical role than surface area in determining adsorption capacity of bulk-type adsorbents, contrasting conventional assumptions in the field. The work provides possibilities for creating long-lasting, efficient adsorbents for the treatment of water by highlighting the roles that pore size and surface area play in the adsorption mechanism. Given the structural similarity between heavy metals and certain radionuclides, these findings have broader implications for developing geopolymer-based materials for radioactive waste treatment applications.

The release of radionuclides initially encapsulated in a slowly degrading solid waste form and contained in an eventually corroding canister defines the source term for numerical simulations for the assessment of a geologic repository for high-level radioactive waste. While the details of waste degradation, canister corrosion, and dissolution and mobilization of the radionuclides in pore water include complex chemical reaction and transport processes that are coupled to the thermal, hydrological, microbiological, and mechanical conditions in the repository, the source-term model suitable for use in a numerical performance assessment model should be a defensible abstraction of these mechanisms. We developed a radiological source-term model and implemented it into a non-isothermal flow and transport simulator. While the proposed source-term model is applicable to various waste forms, canister systems, and disposal concepts, we specifically considered radionuclide releases from vitrified high-level waste placed in a cylindrical canister disposed in a deep vertical borehole repository. In this model, waste degradation is a function of temperature, and it can be adjusted to evaluate the influence of and propagate uncertainties in pH, passivation reactions, and chemical conditions as well as geometrical factors. The time-dependent, congruent release of safety-relevant radionuclides present in the decaying inventory is then calculated. Finally, the radionuclides are mobilized by diffusive and advective transport according to the thermo-hydraulic conditions prevailing in the near field of the repository, from where they migrate through the geosphere to the accessible environment. We examine the influence of the source-term model’s parameters on performance assessment calculations through sensitivity and uncertainty propagation analyses, identifying influential factors and confirming the upper bound of their impact. These considerations align with the overarching goal of repository design, which is to demonstrate that engineered and natural barriers can collectively delay radionuclide migration for timescales far exceeding human planning, thereby providing multiple, redundant barriers against environmental contamination.

The molten salt experiment (MSE) is a 30-day 1 MW (thermal) molten salt criticality experiment designed by Copenhagen Atomics that will be conducted at the Paul Scherrer Institute (PSI) Swiss research center. A detailed analysis of the radioactive inventory available during the operation and after shutdown has been carried out for future estimates of potential source term release in case of undesired accidents using the Monte Carlo transport and depletion code OpenMC. To benchmark the radioactive inventory obtained through the OpenMC burn-up calculation (version 0.15.2), code-to-code comparison with the Serpent Monte Carlo and depletion code (v2.2.1), as well as with Serpent coupled to the EQL0D depletion module developed at PSI, were performed. All of the utilized codes support neutron transport on CAD-based geometries and allow continuous material reprocessing during depletion. This feature is essential for modeling the MSE concept, where gaseous fission products are continuously extracted from the circulating fuel salt. For the present verification study, three models of increasing geometric and physical detail were developed: (1) a nested hollow sphere model in constructive solid geometry (CSG), with the scope of aligning basic settings beyond geometry; (2) a geometrically simplified CAD model of the Copenhagen Atomics Onion Core, mainly utilized for testing the integration of CAD models within the above depletion codes; (3) the same simplified CAD model but with the implementation of reactor fluid flows from the inner active core to the outer inactive region, mainly to account for the effect of out-of-core decay. The results were compared to benchmark the original OpenMC model. Particular attention was given to the end-of-cycle (EOC) isotopic inventory, with detailed comparisons for key radionuclides and total radiotoxicity.

Understanding how environmental variables influence the dissolution rate of nuclear waste materials in aqueous systems is crucial for developing durable nuclear waste forms. In experiments to estimate dissolution rates, the amount of aqueous solution reacting with the material surface is often used as a convenient variable to control the solution saturation state, which then controls the dissolution rate. An exponential function between the dissolution rate and the solution volume-to-surface area ratio was derived, based on an empirical relation of a power function between the Gibbs free energy of dissolution and the volume-to-surface ratio. The relationship was employed to model the dissolution rates of several oxide minerals. The results suggest that the relationship is robust in numerically describing the dissolution rates as a function of the volume-to-surface ratio. Applying the relationship to the dissolution datasets of a nuclear glass and a ceramic nuclear waste form demonstrates its applicability to nuclear materials, providing important insights into the saturation state of the experimental conditions and the chemical durability of these materials. The proposed empirical relationship provides a convenient tool to help design dissolution experiments and offers important insights into the dissolution behavior of materials.

Site selection procedures for deep geological repositories are driven by the rejection of candidate sites whose degree of long-term protection is insufficient or less sufficient. If long-term protection is defined in relation to future exposures, it has to be operationalized, that is, translated into measurable indicators, such as dose or degree of containment , which, again, have to be evaluated by safety assessments. Site selection procedures, therefore, depend on the quality with which long-term protection is operationalized and assessed. Although it is widely acknowledged that operationalizations and assessments of long-term protection are inherently inaccurate, little attention has been paid to the question whether these inaccuracies prevent site selection procedures from improving long-term protection. Still, there is no theory of site selection that could specify the conditions under which site selection procedures are rational with regard to the target of long-term protection. To contribute to such a theory, a conceptual model is presented that explores how site rejection decisions can be justified by inaccurate operationalizations and safety assessments. The model rests on the assumption that site rejections are justified by logical arguments. By explicating what is needed to support the arguments, the model displays the complex structure of the justification, which, amongst others, rests on the quality of operationalization, safety assessment and system understanding. The presented argument-based approach is novel in the context of site selection. However, it is not meant as an alternative to multi-criteria decision-making, but as a necessary complement to understand the potential and limitations of safety-related decision criteria. The presented model identifies which types of errors are tolerable in the context of site selection and it explains why error tolerance is lowest for safety comparisons. The model points out that the frequently used assessment strategy of conservatism is not suitable for rejecting sites for reasons of insufficient or lower safety. It also shows that consensual requirements for the conditions under which long-term protection is achieved may be powerful tools for site selection.

Nuclear forensics relies on the integration of complementary signatures to constrain the origins and history of materials. Outcomes benefit from the timeliness and precision of the disparate methods that form typical analysis chains. Sample forms are often either minute in quantity or contain signatures like morphology or composition heterogeneity encoded on a microscale, so many analysis techniques focus on resolving signatures on ever-smaller length scales. The new hyperspectral x-ray imaging (HXI) instrument developed at Los Alamos National Laboratory seeks to improve the information available from scanning electron microscopy (SEM) x-ray spectrum analysis through superior spectral energy resolution vs. typical energy dispersive spectroscopy (EDS) systems in common use in nuclear forensics and other microanalysis fields. Based on arrays of transition-edge sensor (TES) microcalorimeter detectors, this instrument achieves a typical energy resolution of 7 eV full-width at half-maximum (FWHM) at 2 keV, opening new possibilities in trace element detection/analysis and chemical state determination through spectral shape shifts. We present here some of the first applications of the HXI instrument to actinide samples and discuss potential maturation of this nascent technology for future analysis pipelines.

Most safety cases for radioactive waste disposal consider a temperature limit of 100°C for the clay buffers. Given that being able to tolerate higher temperatures would have significant advantages, the work package HITEC of the EURAD project aimed at determining the influence of temperature above 100°C on buffer properties, trying to establish if the safety functions are unacceptably impaired. A synthesis of the state of knowledge on the thermo-hydro-mechanical and chemical behaviours of different buffer materials at different temperatures is presented, along with the progress made in this area during HITEC. The changes in the properties of the preheated material and the hydromechanical properties of bentonite at high temperatures were assessed. To cover the first instance, bentonite was heated at 150°C in dry and wet conditions for different periods of time up to 2 years. The clay mineralogy was significantly preserved. The slight changes observed in the other properties were opposite depending on the heating conditions: in the case of evaporation, the cation exchange capacity, specific surface area, sorption coefficients, and sometimes swelling pressure decreased. These changes likely resulted from the strong drying induced by the elevated temperatures. Bentonite was also subjected to hydration under a thermal gradient in field and laboratory tests. No post-mortem structural modifications of the smectite were observed; however, dissolution and precipitation of species occurred, conditioned by the type of bentonite and hydration water. These processes were accompanied by the modification of the exchangeable cation complex. Determination of the hydromechanical properties of expansive clay at elevated temperatures is challenging owing to experimental and interpretation issues. In most cases, a reduced swelling pressure was obtained when the temperature increased, particularly at higher dry densities. These results may have been affected by the experimental protocols, use of bentonite or purified smectite, and exchangeable cations. Even at the highest temperatures, bentonite can fill voids and develop large swelling pressures at high densities. Thermo-hydro-mechanical models were developed or upgraded during the project to include thermal phenomena and dependencies and were applied to the simulation of new laboratory thermo-hydraulic tests in cells.

“We have opinions, we make observations, we change our opinions.” Stephen Senn, Dicing with Death, 2003. The purpose of this review paper is to inform debate and provide snapshot opinions about challenges and solutions for the development of nuclear fission technology. We review the socio-political and techno-economic status of nuclear power technology, which, as almost always during its history, is on an edge from which it may jump to success or fall into the abyss of oblivion. Recent nuclear projects and claims are reviewed in light of lessons learned from the international aircraft and minerals processing industries on manufacturing, construction, and technology maturity. For new nuclear reactor projects and designs, these experiences assist in systematically and objectively quantifying investor risk. We strongly recommend the deployment of nuclear fission for energy production everywhere in the world, depending on the scale and dominant economic factors.