Scientific Highlights

The PSI Laboratory for Reactor Physics and Thermal-Hydraulics (LRT) conducts computational and experimental research with focus on the safety of nuclear reactors and systems. In recent years, it established the EPSILON program to coordinate and consolidate its research activities on nuclear space applications. Among other things, developments were initiated towards an open-source European platform for high-fidelity simulations and experiments dedicated to space nuclear reactors. Referred to as the open^SPACE platform, its underlying concepts are a) to include not only solvers but also reference simulation models as well as experimental validation data; b) to make all of these available to the broader and combined nuclear- and space communities for usage and/or further developments. Through this, the goal is thus not only to facilitate collaborative research in this area but also to enable effective support to the European Space Agency for thorough design, safety and performance evaluations of nuclear reactor systems for in-space propulsion and/or surface power. A first development phase focused on nuclear electric propulsion was proposed and retained among the two projects selected in 2023 by the Swiss National Science Foundation (SNSF) for its MARVIS call (Multidisciplinary Advanced Research Ventures in Space) and funded by the Swiss Secretariat for Research and Innovation (SERI). This project, to be conducted via four inter-connected PhD theses, was launched in October 2024 and this marks thus a key milestone for the propulsion of PSI nuclear research towards space.

For water-cooled nuclear reactors, a loss of coolant accident constitutes one of the key scenarios to be evaluated for the design of the plant and associated safety systems. Even if these accidents are not expected to occur at all during reactor lifetime, their potential consequences include the heat up of the fuel in the reactor core. For the recovery of the plant to safe conditions, safety systems are in place to inject water in order to reflood the core and to quench the high temperature fuel. The two-phase flow behaviour during this reflooding phase is extremely complex. In particular, the prediction of the behaviour of small liquid droplets generated as the quench front propagates upwards has a significant effect on the fuel temperatures in the upper regions of the reactor core. In collaboration with the US Nuclear Regulatory Commission (NRC), we have been working to improve our modelling of the droplet behaviour and their impact on key safety parameters.

The European aviation sector stands at a pivotal juncture in its quest to achieve net-zero climate impacts. Focusing on flight CO₂ emissions overlooks up to 80% of the sector's climate repercussions.

Our research delves deep into the role of electricity-based synthetic jet fuels and direct air carbon capture and storage (DACCS) as potential game-changers. These solutions promise climate-neutral aviation, but there's a catch: the relentless rise in air traffic. Relying solely on renewables-derived synthetic fuels may strain both economic and natural resources. On the flip side, offsetting fossil jet fuel impacts via DACCS poses its own set of challenges. Our findings underscore one clear message: for a genuinely climate-neutral European aviation, we must reconsider the scale of air traffic.

The objective of this project was to determine the contribution from a variety of obstacles to moving dislocations to the nano-indentation stress necessary to initiate plastic flow. The obstacles are characterized by different length scales. Among these characteristic lengths, there are those associated with the material microstructure such as grain size, dislocations density, irradiation-induced defects, and those related to the size of the plastic zone beneath the indenter, or equivalently to the size of the indent. Thus, we can classify the size effects into two categories: structural size effect and indentation size effect (ISE). The underlying idea is to quantify and separate these two effects on the unirradiated material first to be able to properly isolate the contribution of the irradiation defect on the measured hardness from the tests on irradiated materials.

Chalk River Unidentified Deposits (CRUD) are dissolved and suspended solids, product of the corrosion of structural elements in water circuits of nuclear reactors.  

The chemical composition of CRUD is variable as it depends on the composition of the reactor’s structural material, as well as the types of refueling cycles.  Recent internal investigations have found unexpected but significant Si-amount in CRUD. The chemical composition of CRUD holds key information for an improved understanding of CRUD formation and possible impact in fuel reliability and contamination prevention.

The standard analytical methods available in the hot laboratory did not allow an easy quantitative determination of the Si-amount in CRUD. A new innovative procedure has been developed and tested with synthetic CRUD name Syntcrud.

The adapted flex-fusion digestion method presented here is able to provide reliable concentrations of several elements within CRUD, including Si, which was not possible in methods used previously for ICPMS measurement.

Mobility of water, sodium and gas molecules within a smectite nanopore

Various gases are produced by metal corrosion and organic material degradation in deep gelological repository for nuclear waste. To ensure repository safety, it's important to demonstrate that gases can be dissipated by diffusion in host rocks and prevent pressure buildup in repository near field. Smectite mineral particles form a pore network that is usually saturated with water, making gas diffusion the primary transport mechanism. Molecular simulations have shown that the diffusion of gases through the pore network depends on various factors, including pore size and temperature. For instance, smaller pores and lower temperatures tend to reduce gas diffusion. Interestingly, hydrogen and helium have been found to diffuse faster than argon, carbon dioxide, and methane, possibly due to interactions with the clay surface and water molecules. Ultimately, the diffusion coefficients for different gases and pore sizes can be predicted using an empirical relationship, which is useful for macroscopic simulations of gas transport.

During the last decades, computing power has been subject to tremendous progress due to the shrinking of transistor size as predicted by Moore’s law. However, as we approach the physical limits of this scaling, alternative techniques have to be deployed to increase computing performance. In this regard, the next big advance is envisioned to be the usage of approximate computing hardware based on field-programmable gate arrays and/or digital-analogue in-memory circuits. Such approximate computing can provide disproportional gain (x1000) in energy efficiency and/or execution time for acceptable loss of simulation accuracy. This could be highly beneficial in order to accelerate computational intensive simulations such as reactor core analyses with higher resolution multi-physics models. On the other hand, the execution of programming codes on low-precision hardware may result in inadequate outcomes due to quality degradation and/or algorithm divergence. To address these questions, studies on the stability and the performance of advanced reactor simulation algorithms as function of reduced floating-point arithmetic precision are being conducted at the laboratory for reactor physics and thermal-hydraulics. Results obtained so far indicate a large room for the acceleration of nuclear engineering applications using mixed-precision hardware. Therefore, research is now being enlarged towards assessing multiprecision computing methods for reactor core simulations with higher spatial resolution.

The increasing risk of extended electricity supply disruptions and severe electricity price fluctuations  strongly motivate an evaluation of electricity supply resilience. In this direction, this research proposes a multicriteria decision support framework to assess resilience at a country level, based on three major dimensions: Resist, Restabilize and Recover. In total, 35 European countries are ranked according to their performance on 17 indicators, through a synergy of MCDA methods, techniques and communication protocols. The assessment framework has been extended to incorporate the Choquet Integral method, in order to accommodate potentially interacting pairs of criteria and negate their arbitrary effects on the final evaluation results. The analysis incorporates country data from credible international databases, as well as the preference information of a European energy expert. The results are envisaged to support energy policymakers in Europe and provide guidelines and areas for improvement at a country level.

PSI has a unique (worldwide) environment for the investigation of highly radioactive / toxic materials:

> Materials (different fuel types, very high burn-up, different cladding materials, materials activated in SINQ).

> The hot lab with advanced tools for microsample analysis and preparation.

> The large-scale equipment for advanced material analysis.

This unique combination at PSI allows us to meet the needs of our industrial partners to improve plant safety / efficiency, up to fundamental research.

The quantitative distribution of fission products over the cross-section of a pellet with a shielded electron probe microanalyzer (EPMA) used for verification analysis of the material behavior to validate the model. In this context, Xe behavior during transients/failure (LOCA, RIA) is an important safety parameter that can’t be measured with the EPMA at the periphery. Microstructural EBSD investigations on a microsample extend the information horizon, which is deepened at the microXAS beamline by detailed X-ray analyses.