Fission product behavior in high-temperature reactors under normal operating conditions and in accident situations

Automatic translation: The development of high-temperature reactors (HTRs) has been intensively pursued for many years, particularly in the USA and the Federal Republic of Germany (FRG). In both countries, smaller experimental HTR reactors (Peach Bottom in the USA; AVR Jülich in Germany) have been successfully tested for years, and prototype HTR plants with an output of around 300 MWe are now either operational (Fort St. Vrain, USA) or in advanced stages of construction (THTR-300, FRG). Both countries have also developed ^on-paper commercial HTR nuclear power plants (HTR-1160) for electricity generation, designed as two-circuit systems with a thermal output of 3000 MW. These plants have the necessary safety reports required to initiate licensing procedures. These reports describe the plant and its behavior under normal operating conditions, operational disturbances, and accident scenarios up to the ^maximum credible accident (MCA). In the licensing process, the plant's controllability—primarily compliance with legally defined maximum permissible activity release limits—must be demonstrated for all these scenarios, potentially utilizing appropriate and sufficiently reliable safety systems. Moreover, a probabilistic analysis of a broader spectrum of accident scenarios has been presented in the USA for the HTGR-1160 plant through the AIPA study /1/. This study aims, among other things, to determine risk values for some accident sequences considered dominant. The risk values are derived—similar to the well-known risk study for light water reactors /2/ (WASH-1400)—as the product of the probability of occurrence and the consequences of the respective accident sequence. The results indicate that analyzing relatively few dominant accident sequences suffices to capture the main risk contributors. Similarly, in Germany, a probabilistic ^HTR Safety Study /3/ has been initiated, commissioned by the Federal Ministry of the Interior (BMI) and carried out by the Institute for Nuclear Safety Research (ISF) at KFA Jülich and the Gesellschaft für Reaktorsicherheit (GRS). HTGR Accident Initiation and Progression Analysis: Another method for the safety assessment of nuclear power plants is the so-called damage scope analysis. This method primarily examines very hypothetical accident scenarios, which contribute little to risk in probabilistic analyses due to their low likelihood and are not relevant to the licensing process. The goals of this analysis are to identify the maximum potential consequences of the most severe accidents and to determine the timeline of radiological impacts. The first goal aims to establish the upper limit of possible impacts, while the second provides time intervals available for interventions in the accident sequence or measures to protect the public. In Germany, damage scope analyses have so far been cooperatively conducted by Hochtemperaturreaktor Bau GmbH (HRB), Mannheim, and ISF-KFA Jülich for two highly hypothetical accident scenarios in the German HTR-1160 plant /4/, /5/. Further scenarios with potentially greater impacts are yet to be analyzed. Finally, it should be noted that the three mentioned methods for nuclear power plant safety evaluation—licensing procedures, probabilistic risk analysis, and damage scope analysis—should not be regarded as alternatives but rather as complementary approaches. Each has specific objectives and focuses on different accident scenarios. A common requirement of all three methods is the need to analyze the impacts, such as expected environmental contamination due to activity releases, according to the respective accident sequence. This, in particular, necessitates knowledge of the behavior of fission products in HTRs under both normal and accident conditions. The following will provide a detailed discussion of this topic, drawing on results obtained so far.