Accidents In Fast Breeder Reactors Research Articles

Comprehensive and consistent interpretation of local fault experiments and application to hypothetical local overpower accident in Monju

Experimental studies on local fault (LF) accidents in fast breeder reactors have been performed in many countries because LFs have been historically considered as one of the possible causes of severe accidents. Comprehensive and consistent interpretations of in-pile and out-of-pile experiments related to LF were arrived at in this study based on state-of-the-art review and data analysis techniques. Safety margins for a hypothetical local overpower accident, which was evaluated as a LF accident in the licensing document of the construction permit for a prototype fast breeder reactor called Monju, were also studied. Based on comprehensive interpretations of the latest experimental database, including those performed after the permission of Monju construction, it was clarified that the evaluation of the hypothetical local overpower accident in the Monju licensing was sufficiently conservative. Furthermore, it incorporated adequate safety margins in terms of failure thresholds of the fuel pin, molten fuel ejection, fuel sweep-out behavior after molten fuel ejection, and pin-to-pin failure propagation. Moreover, these comprehensive interpretations are valid and applicable to the safety evaluation of LF accidents of other fast breeder reactors with various fuel and core designs.

The code RENONS for the prediction of nonvolatile species release from liquid surfaces into a gas

In the field of nuclear safety, the release of volatile and nonvolatile species from liquid surfaces into a gas atmosphere is important for aerosol source term considerations. In case of a hypothetical fast breeder reactor accident with tank failure, primary coolant sodium with suspended or solved fuel particles and fission products may be released into the containment. The code package REVOLS/RENONS calculates, based on theoretical mechanistic models, the release of these species from the pool surface into the inert gas atmosphere of the inner containment. Hereby, the module REVOLS calculates the RElease of VOlatile Species (Koch et al., 1991). Whereas the evaporative release of the volatile species is governed by diffusive and convective transport processes, the release of the nonvolatile may be governed by mechanic processes entraining droplets from the wavy surface under conditions of convection in the atmosphere. The modular structured code RENONS (RElease of NONvolatile Species) calculates the liquid entrainment rate of nonvolatile species from pool surfaces by a parallel gas flow. Hence, the influence of the system temperatures, the pool inventory and the geometry as well as the decisive influence of the thermal hydraulic conditions on the entrainment rate can be investigated.

Validation of Neutronic Codes for Distorted Core Configurations with the SNEAK-12 Critical Assemblies

The reactivity effect sof material rearrangements, simulating conditions in a postulated liquid-metal fast breeder reactor accident, were measured in three different critical assemblies: SNEAK-12A, a single-zone core, fueled with enriched uranium metal plates; SNEAK-12B, which had a central test zone fueled with PuO/sub 2/O/sub 2/ rod bundles surrounded by a buffer and a driver zone; and SNEAK-12C, which had nearly the same integral compositions as SNEAK-12B but was loaded totally with plates. The reactivity effects were calculated using current Kernforschungszentrum Karlsruhe methods and data and, in part, also using the corresponding modules of the SIMMER-II accident analysis system. Also, for some configurations, a comparison of measured and calculated fission rate distributions was performed. The evaluation yielded similar results for the three assemblies. For most cases investigated, satisfactory agreement between theory and experiment was reached when two-dimensional transport eigenvalue calculations or exact transport perturbation methods were used. As long as larger deviations occurred, transport results generally were on the conservative side. First-order transport perturbation theory only worked well in a limited number of cases. Diffusion calculations often led to large discrepancies, particularly when the experiments involved fuel dilution.

Investigation of material relocation phenomena during severe accidents in fast breeder reactors

The SIMBATH out-of-pile experiments simulate severe accidents in fast breeder reactors. In the tests the nuclear energy released is substituted by the exothermal energy of a thermite reaction. Single pin and small bundle experiments as well as freezing tests are performed. Material ejected from the fuel rod simulators in an early phase is finely dispersed. A portion penetrates the upper breeding zone without freezing. The bulk of molten material ejected afterwards leads to blockages in the colder zones of the bundle. Under these conditions bottled-up situations may occur in the SIMBATH experiments. Residual sodium may become entrapped. The current version of the computer code CALIPSO developed to interpret these experiments is verified by calculation of two single pin experiments. The computations show that the relocation mechanisms in the SIMBATH experiments are mainly controlled by expansion of noncondensible gases originally existing inside the pins. The contribution from fuel vapour pressure or from sodium evaporation due to fuel-coolant-interaction is of less importance during the first 100 ms after fuel pin failure.

Measurements and Computation of the Reactivity Effects of Accident-Caused Core Distortions in Liquid-Metal Fast Breeder Reactors

The reactivity effects of material rearrangements, simulating conditions in a postulated liquid-metal fast breeder reactor accident, were measured in SNEAK-12A, a single-zone uranium fueled critical assembly, and calculated using current Kernforschungszentrum Karlsruhe methods and data and, in part, also using the corresponding modules of the SIMMER-II accident analysis system. For all cases investigated, satisfactory agreement between theory and experiment was reached when two-dimensional transport eigenvalue calculations were used. The application of first-order perturbation theory or diffusion theory in a number of cases led to larger discrepancies, particularly when the experiments involved fuel compaction.

Analysis of Temperature Fluctuations in Liquid-Metal Systems

A phenomenological model is developed to analyze temperature fluctuations in experiments that simulate various liquid-metal fast breeder reactor accident scenarios. Simplified mass, momentum, and energy equations are used to provide a unified system for analysis. Predictions of temperature autocovariance are compared with those observed during a representative in-pile sodium loop experiment. 11 refs.