[en] Highlights: • Phenomenology focusing on BWR fuel degradation is discussed, including several concerns arisen from the FDNPS accident.

[en] This report fulfills the milestone (M4SF-17PN010501041 Report on Radiolysis Modeling for the Defense Repository) to discuss continued integration of the PNNL Radiolysis Model and the ANL Mixed Potential Model. This work concerns the development of an integrated Radiolysis Model (RM) for evaluating defense waste materials (oxide and metal) degradation and radionuclide mobilization. Within an anoxic repository environment, primary oxidizing species (e.g., hydrogen peroxide (H₂O₂), OH• radicals, as well as chlorate and other oxidizing species depending on the disposal environment) will be generated at the surface of the nuclear waste forms as a function of their specific activity. RM development has included expansion of chemical environments considered to encompass species for various disposal environments. PNNL has been coordinating this effort with ANL on the integration of the radiolysis work with the fuel degradation model. In this study, we demonstrate and approximate possible effects of iodide on H₂O₂ generation. As has been shown these are conditions for which H₂O₂ generation is reduced. We find that the presence of the iodide ion reduces the steady-state H₂O₂ concentration, but not to the same degree as bromide at micro-molar concentrations.

[en] Highlights: • Development by thermomechanical treatment of new generation of RAFM steels for fusion applications. • Effects of tempering on secondary precipitation (M₂₃C₆, M₂X, and MX) and martensite transformation. • Influence of alloy chemistry refinement on amount, distribution, and size of precipitates.

[en] The objective of this Safety Guide is to provide recommendations for the design of new nuclear power plants related to defence in depth and the practical elimination of plant event sequences that could lead to an early radioactive release or a large radioactive release. It also provides recommendations in relation to design aspects of defence in depth, in particular on those aspects associated with design extension conditions. Since the introduction of the term ''practical elimination'' in the early 90's and the recognition that accident conditions could include design extension conditions without significant fuel degradation and design extension conditions with core melting, those topics have been the subject of extensive discussions and several publications. The purpose of this publication is to facilitate international consensus on the understanding of those topics among regulators and designers and to provide recommendations for their consistent implementation in relevant nuclear power plant designs. In particular, this Specific Safety Guide gives recommendations related to the demonstration of the implementation of the practical elimination concept for those plant event sequences that could lead to an early radioactive release or a large radioactive release, which relies on the physical impossibility or on the high-level confidence that they are extremely unlikely to arise.

[en] The objective of this Safety Guide is to provide recommendations for the design of new nuclear power plants related to defence in depth and the practical elimination of plant event sequences that could lead to an early radioactive release or a large radioactive release. It also provides recommendations in relation to design aspects of defence in depth, in particular on those aspects associated with design extension conditions. Since the introduction of the term ''practical elimination'' in the early 90's and the recognition that accident conditions could include design extension conditions without significant fuel degradation and design extension conditions with core melting, those topics have been the subject of extensive discussions and several publications. The purpose of this publication is to facilitate international consensus on the understanding of those topics among regulators and designers and to provide recommendations for their consistent implementation in relevant nuclear power plant designs. In particular, this Specific Safety Guide gives recommendations related to the demonstration of the implementation of the practical elimination concept for those plant event sequences that could lead to an early radioactive release or a large radioactive release, which relies on the physical impossibility or on the high-level confidence that they are extremely unlikely to arise.

[en] In this work, we quantify the impact of the randomness of the traversed medium for a simple benchmark problem of neutron transport in a stochastic mixture composed of low-enrichment (3.7%) UO₂ fuel fragments dispersed in water. A tool that allows sampling a large set of stochastic material configurations by Monte Carlo methods has been recently developed. By using this tool, we assess the effects of the stochastic geometries as opposed to lattice dispersions of fuel in a moderator material. The findings of our preliminary investigation show that lattice models are not always conservative with respect to the reactivity. (authors)

[en] Anticipated operational occurrence (AOO) events have “the potential to challenge the safety of the reactor and might be reasonably expected to happen during the lifetime of a plant, with frequencies of occurrence equal to or greater than 10^-2 per reactor year” [9]. It needs to be ensured that the fuel remains fit for service when the reactor comes back to normal operation following an AOO. For this purpose, in Canadian nuclear industry, so-called ‘fitness-for-service criteria’ are used. The concept of fitness-for-service criteria for fuel is an integral approach to ensure that the fuel and fuel channel maintain their structural integrity under the environment that several fuel degradation mechanisms are stimulated by an AOO. Therefore, the fitness-for-service criteria are derived on the basis of assessments of associated fuel degradation mechanisms using a stylized conservative analysis approach. In the stylized analysis, the fuel thermomechanical behaviour is simulated taking into account coolant pressures, bundle powers and power ramps typical of AOOs, while keeping the fuel sheath temperature constant throughout the 60 seconds simulation time of the event. The contents of this Appendix are primarily taken from COG-12-2049. Fitness-for-service criteria are usually used by reactor operators as a simple and reliable measure to provide assurance that the performance of the fuel returned to normal operation following an AOO still remains within its acceptable range. Therefore, fitness-for-service criteria are specific to fuel design and also to transients considered.

[en] A generic severe accident model of a PWR is developed to support analyses of cyber-fault scenarios that may lead to extensive core damage. For this purpose, a MELCOR 2.1 model of a generic PWR is created that is capable of simulating primary and secondary thermal-hydraulics, core heatup and fuel degradation, RPV failure, containment response, and radionuclide release and transport to the environment. The size and detail of the severe accident model are intended to facilitate scoping analyses of various cyber-based scenarios in an automated, dynamic framework. A dynamic job scheduler such as ADAPT will be used to evaluate many accident scenarios in an automated fashion utilizing parallel computing. Individual test calculations are first conducted of a basic unmitigated scenario (i.e. no significant operator intervention) with a cyber-based accident initiator. This preliminary assessment is used to gauge modelling capabilities for further, expanded cyber accident scenarios in conjunction with ADAPT. (authors)

[en] In the decommissioning of Fukushima Daiichi Nuclear Power Station (NPS), the removal of fuel debris cannot be avoided. The fuel debris is currently considered to be sub-critical. Volume ratio of debris/water has strong sensitivity to the criticality. Thus, the criticality estimation is required in the debris removal process. While, the best estimation of the criticality is difficult due to the unclear properties and/or geometries of debris, therefore, random sampling with Monte Carlo calculation was used in current evaluation. However, the Monte Carlo calculation spend three days to one week for the evaluation due to its high calculation cost. Therefore, it is difficult that Monte Carlo apply to on-demand statistical criticality evaluation. For these background, multidimensional interpolation was applied to the statistical criticality evaluation. In this study, fundamental validation was examined by comparing with Monte Carlo results. The proposed method was 614,400 times faster than Monte Carlo calculation, additionally, difference of mean value was 0.9 % δk. As a result, proposed multidimensional interpolation showed good agreement with direct Monte Carlo calculation. In the future work, simplification of the model, evaluation of applicable limit and operation procedure will be examined. (authors)

[en] Vapour risks are best assessed utilising soil vapour concentrations, as this approach removes the uncertainty associated with estimating partitioning between soil and soil vapour. However, on redevelopment sites with shallow impacts (e.g. <1.5 m below ground level), it may not be possible to collect soil vapour samples which are considered robustly representative of soil vapour concentrations which would underlie a future building (given the absence of the building and the potential for atmospheric effects). The aim of this presentation is to outline possible approaches for assessing potential vapour risks from shallow impacts, with particular emphasis on coal tar sources and heavily degraded hydrocarbon fuels (for which the assessment options based on currently available screening levels may be limited). The presentation provides general results regarding the likely levels of vapour risk associated with these sources, and suggested approaches for undertaking site-specific assessments. Appropriate assessment of potential vapour risks from these sources is critical for reducing unnecessary remediation and facilitating site closures. (author)