Scientific Highlights LRT

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.

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?

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.

Noise appears in many areas of science, and commonly has an unwanted and disturbing nature by deteriorating signals’ quality. Therefore, various techniques have been developed over the years for separating noise from pure signals. However, noise has a key role in signal analysis of nuclear reactors as its’ appropriate assessment can be used not only for exploring the normal and dynamic behaviour of nuclear cores, but also for identifying and detecting possible anomalies of reactor systems. State of the art methods have been recently implemented within the well-established signal analysis methodology of the STARS program, at the Laboratory for Reactor Physics and Thermal-Hydraulics (LRT), for investigating nuclear reactor noise and getting a better insight on analysing reactors’ operation.

Spent fuel characterization is necessary to improve nuclear fuel design, optimize core refueling patterns and manage the handling, transport and storage of spent fuel assemblies. The experimental characterization of spent fuels includes measuring their gamma and neutron emissions typically with high-purity Germanium and He-3 detectors. In the past few years, however, efforts to develop efficient and low-cost, fast and thermal, neutron detectors have guided the research to the development of new scintillation detectors. These scintillators offer good efficiency, fast-timing properties, and good pulse shape discrimination capabilities for dual gamma and neutron detection. Within the Laboratory for Reactor Physics and Thermal-Hydraulics (LRT), a preliminary analysis was performed through Monte Carlo simulations to design a measurement unit at the HOTLAB based on new scintillators for the detection of fast neutrons emitted by spent fuel. This semester work of Marianna Papadionysiou was presented at the ANS Student conference in April and received two awards for "Best Detection and Measurement" and "Best Overall Research".

The main threat to the reactor pressure vessel (RPV) operational safety is certainly the radiation damage that hardens and embrittles the steel it is made of. Four decades of research worldwide have allowed understanding and monitoring the phenomena that come into play. At the computational level, a simulation platform, PERFORM-60, has the ambitious aim of predicting the steel evolution for most RPV and operational conditions. It was initially elaborated in the frame of the EU project of the same acronym and is currently further developed to be the end-product of the on-going H2020 EU project SOTERIA. As a contribution of the Laboratory for Reactor Physics and Systems Behaviour (LRS) to SOTERIA, the platform has been rigorously assessed for the first time since its release on a real case study of a Swiss RPV. This critical assessment has been acknowledged by the community and serves as basis for improvements and future developments of the platform within SOTERIA.

Nuclear reactor dynamics deals with the transient behaviour of nuclear reactors which mostly refers to time changes of the imbalance between heat production and removal. Since the prediction of the dynamic behaviour is crucial for the safety of a reactor, computational models and methodologies have been developed in the framework of the STARS project, at the Laboratory for Reactor Physics and Thermal-Hydraulics (LRT), with the main goal to simulate the complex behaviours of reactors under various conditions with a high level of fidelity.