Nuclear Energy and Safety Research Division

The Paul Scherrer Institute has a long tradition in energy research. With respect to nuclear energy, PSI has a unique position in Switzerland. This is due to its heavy infrastructure, namely the Hot Laboratory with so-called hot cells, well equipped and shielded zones for work and research on radioactive material. In addition, the nuclear energy division takes advantage of PSI's large facilities like the Swiss Light Source (SLS) and the Swiss Spallation Neutron Source (SINQ).

Based on this infrastructure and the know-how of its collaborators the Division is involved in three main topics of research: Safety of currently operating light-water reactors, safety characteristics of future reactor concepts and related fuel cycles, and long-term safety of deep geological repositories for nuclear wastes of all kind.

The work is being done on behalf of the Federal Government and in close cooperation with the Swiss nuclear utilities, the national waste management organization, Nagra, and the national regulatory authority, ENSI. It also includes scientific services for the nuclear power plants. Most of the research is connected with international projects on a multi- or bi-national cooperation basis.

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Recent News from NES

27 September 2018

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Consistent criticality and radiation studies of Swiss spent nuclear fuel: The CS2M approach

Nuclear Power Plant Safety Matter and Material Particle Physics

Spent fuel management is becoming one of the major concerns in many countries with a nuclear program. The radiation aspect as well as the safe and economical part of the long-term storage of the spent nuclear fuel has to be evaluated with a high degree of confidence. To assist such project from the neutronic simulation side, a new method is proposed to systematically calculate at the same time canister loading curves and radiation sources, based on the inventory information from an in-core fuel management system. The CS2M approach consists in coupling the CMSYS core simulation platform, developed over the year within the LRT/STARS to provide reference validated core simulation methodologies based on CASMO and SIMULATE, with the downstream state-of-the-art codes SNF and MCNP for decay- and criticality analyses respectively. In the present study, the considered cases cover enrichments from 1.9 to 5.0% for the UO2 assemblies and 4.8% for the MOX, with assembly burnup values from 7 to 74 MWd/kgU. Because it is based on the individual fuel assembly history, it opens the possibility to optimize canister loadings from the point-of-view of criticality, decay heat and emission sources. It also allows to perform a full uncertainty propagation at once: from the first irradiation to the entry in the storage facility.

24 July 2018

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Material from PSI helps to check inconsistencies in the Big Bang theory

Media Releases Research Using Neutrons Matter and Material Particle Physics

Shortly after the Big Bang, radioactive Beryllium-7 atoms were formed, which today, throughout the universe, they have long since decayed. A sample of beryllium-7 artificially produced at PSI has now helped researchers to better understand the first minutes of the universe.

20 July 2018

Collective magnetism in an artificial 2D XY spin system

Two-dimensional magnetic systems with continuous spin degrees of freedom exhibit a rich spectrum of thermal behaviour due to the strong competition between fluctuations and correlations. When such systems incorporate coupling via the anisotropic dipolar interaction, a discrete symmetry emerges, which can be spontaneously broken leading to a low-temperature ordered phase. However, the experimental realisation of such two-dimensional spin systems in crystalline materials is difficult since the dipolar coupling is usually much weaker than the exchange interaction. Here we realise two-dimensional magnetostatically coupled XY spin systems with nanoscale thermally active magnetic discs placed on square lattices. Using low-energy muon-spin relaxation and soft X-ray scattering, we observe correlated dynamics at the critical temperature and the emergence of static long-range order at low temperatures, which is compatible with theoretical predictions for dipolar-coupled XY spin systems. Furthermore, by modifying the sample design, we demonstrate the possibility to tune the collective magnetic behaviour in thermally active artificial spin systems with continuous degrees of freedom.
Facility: SμS, SLS

Reference: N. Leo et al, Nature Communications 9, 2850 (2018)

Read full article: here

4 June 2018

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Nuclear data for nuclear installations: Radiochemistry improves the precision of the cross-section data of long-lived radionuclides

Matter and Material

Knowledge about the cross sections data of the target materials used for spallation neutron facilities (SNF) and accelerator driven systems (ADS) is essential for the licensing, safe operation and decommissioning of these facilities. In addition, these data are important to evaluate and improve the existing computer simulation codes. Especially the α-emitter 148Gd has a large contribution to radio-toxicity of spallation target facilities with its 74.6 years of half-life. As the laboratory of radiochemistry, we used radiochemical methods to improve the precision of the production cross section data of long-lived radionuclides from proton irradiated lead, tantalum and tungsten targets. These results are long awaited in the nuclear data community and are of paramount importance for the evaluation of the theoretical codes. They will have a high impact on the design of high-power spallation neutron facilities, in particular the ADS prototype MYRRHA and the European Spallation Source, which is going to be the world`s most powerful neutron source. Our work has recently been published in the internationally high ranking journal Analytical Chemistry.

24 May 2018

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ETH Medal for outstanding MSc thesis: Beam Characterization of Low Energy Electrons from a Laser Wakefield Accelerator by N. Sauerwein

Large Research Facilities

The characteristics of low energy electrons accelerated by a laser wakefield (Laser Wakefield Acceleration LWFA) has been studied. The work included understanding the acceleration process, setting up the experiment and measuring properties like charge, divergence and energy of the accelerated electrons. The experiment included diagnostics for the laser and the electrons. In order to make high-resolution energy distribution measurements with relative errors ∆E/E of below 10%, a tunable electron spectrometer has been designed, built and characterized. A tunable permanent magnet quadrupole triplet has been designed for stigmatic focusing in a range of 5 keV to 5 MeV. The thesis can be found here: MSc Thesis N. Sauerwein

16 May 2018

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Global Sensitivity Analysis and Registration Strategy for Temperature Profiles of Reflood Experiment Simulations

Nuclear Power Plant Safety

Global sensitivity analysis (GSA) is routinely applied in engineering to determine the sensitivity of a simulation output to the input parameters. Typically, GSA methods require the code output to be a scalar. In the context of thermal-hydraulic system code, however, simulation outputs are often not scalar but time-dependent (e.g. temperature profile). How to perform GSA on these outputs? A common solution is to represent the profile with a few selected scalar values and perform GSA on these scalar values. This is illustrated in the figure where stochastic samples of a temperature profile resulting from a quenching transient are obtained with the TRACE code in grey and where the maximum temperature and quenching time are chosen as scalars of interest. Their probability distributions, respectively shown in red and blue, can be used to perform GSA. However, these scalars might not be representative of the mixed variations in time and amplitude observed in the output of a typical transient. In this work we address the issue by studying two registration procedures – Landmark and Square Root Slope Function (SRSF) – which separate the amplitude and phase/time variations of the temperature profiles. We then perform dimension reduction by principal component analysis (PCA). PCA allows us to represent the variability of the phase-aligned time series in a few fixed eigenvectors representative of the transient physics and in their associated scalar eigenvalues, which are suitable for GSA. We compare the two registration procedures and the classical “scalar-of-interest” approaches using the Sobol’ indices sensitivity measure. This work received the best paper award at the Best Estimate Plus Uncertainty (BEPU) conference held in May 2018 in Lucca, Italy.

13 March 2018

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BSAF-2: Analysis of Unit 3 accident and fission product transport

Nuclear Power Plant Safety

The Accident at the Fukushima Daiichi Nuclear Power Station, which occurred in March 2011, had a very strong impact on the nuclear community. Three reactors suffered core damage and fission products were released to the environment. Paul Scherrer Institute (PSI) has participated in an Organisation for Economic Cooperation and Development (OECD) project, Benchmark Study of the Accident at the Fukushima (BSAF). The project aimed to evaluate and analyse the accident progression, likely end-state of the reactor core after the accidents, and the release of radioactivity to the environment. PSI has concentrated on the analysis of unit 3 using MELCOR 2.1. Hundreds of calculations have been performed and a plausible scenario which predicted remarkably well the main signatures has been selected.

24 January 2018

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Neutron radiography of detrimental hydrogen in nuclear fuel claddings

Materials Research Energy and Environment Nuclear Power Plant Safety

Hydrogen is at the source of degradation mechanisms affecting mechanical properties of many structural metal materials. In nuclear power plants, zirconium alloy fuel cladding tubes take up a part of the hydrogen from coolant water due to oxidation. Because of the high mobility of hydrogen interstitial atoms down temperature and concentration gradients and up stress gradients, hydrogen distribution in fuel claddings can often be non-uniform, arising the risk for the integrity of spent fuel rods under mechanical load. At the Laboratory of Nuclear Materials (LNM) in collaboration with the Laboratory of Neutron Scattering and Imaging (LNS), hydrogen redistribution in zirconium alloys was quantified by neutron radiography using the state-of-the-art detector of PSI Neutron Microscope, and the concentration was computed based on thermodynamics, to predict hydrogen diffusion and precipitation for used nuclear fuel.

5 January 2018

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UO2 fuel behavior at very high burnup

The investigation of the nuclear fuel at very high burnup is critical for evaluating the safety margin for the evaluated fuel in normal as well as in accidental conditions. PSI is one of the very few hot laboratories which possess access to irradiated UO2 fuel with very high burnup from commercial reactors. The application of relevant tools for the investigation, handling and analysis of those highly irradiated materials emphasize the necessary expertise.

5 January 2018

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New Focused Ion Beam (FIB) in the Hot Laboratory

The implementation of Focused Ion Beam instruments in material research laboratories during the last decade has not only strongly improved the preparation of very thin specimens for the Transmission Electron Microscope (TEM), in particular at interfaces, but also led to the development of new analysis methods inside the instrument itself. It became a powerful instrument for the analyses of highly radioactive materials, because it allows for the production and analysis of very small specimens that can be then analyzed with very sensitive detectors without strong interference from the radiation field of the specimen itself.
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