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[en] Highlights: • Review of the requirements and recommendations for BEPU methodology. • Summary of the advantages and limitations of the current deterministic bounding method for non-LOCA transient analysis. • Description of a pragmatic, graded approach for application of the BEPU methodology to non-LOCA transient analysis. • Proposal for a demonstration case. - Abstract: Since 1990’s, the use of best estimate plus uncertainty (BEPU) methodology is becoming a common practice for large-break Loss-Of-Coolant Accident (LOCA) analysis. However, the development and application of BEPU methodology requires a higher-level requirement on the verification and validation, and uncertainty quantification (VVUQ) of the used calculational method and computer codes. This may result in a high-cost for BEPU methodology development, and hence prevent the industry to take full benefit from the BEPU applications. This paper proposes a pragmatic, graded approach for application of the BEPU methodology to non-LOCA transient analyses.
[en] A series of examples is given illustrating the use of Bayes' Theorem to express out state of knowledge about the frequency of reactor transient events. The examples address the effect of the prior, the mesh spacing, and the interpretation of the data. (orig.)
[en] Highlights: • A coupled thermalhydraulic-neutronic flow stability criterion is established for SFRs. • Pressure drop and power evolutions are considered in the criterion. • A linear stability analysis of 2 equations expressing flow and power evolution is done. • Reactivity feedback of enthalpy change on SA power is modelled. • The capability of the criterion to predict ULOF flow behavior is shown. - Abstract: A coupled thermalhydraulic-neutronic extended criterion developed in order to assess the conditions of static instability of the sodium flow in a SFR core during the beginning of an unprotected loss of flow (ULOF) is presented in the first part of this paper. This extended criterion takes into account the evolution of the pressure drop in a sub-assembly (SA) as well as the evolution of its power when the reactor is affected by a flow rate (or power) perturbation by starting from a steady-state. The considered steady states are typical of quasi-steady states reached by a SFR during a ULOF. The temporal evolution of the flow and the temporal evolution of the power can be represented by a system of two differential equations whose linear stability analysis has enabled to define the aforementioned extended criterion. Then, in the second part of the paper, a verification of this extended criterion is performed by simulating the behavior of a GEN IV SFR of 1500 MWth. Four various steady states of the reactor have been investigated with a simulation tool devoted to unprotected loss of flow simulations (MACARENa). First simulations have enabled to calculate the various physical parameters needed to check the extended criterion. Perturbations have been applied to these four states. The extended criterion, proposed in this paper, has predicted flow stabilization for 3 states whereas it has predicted a flow excursion (sudden flow rate decrease leading to SA dry-out) for the last state. In this latter case, the classical Ledinegg criterion, which relies only on pressure drop evolution at constant power, has predicted a stable configuration. Then, the extended criterion predictions have been verified with MACARENa simulations, by slightly and temporary reducing the driving pressure head of the flow rate in the studied SA from each of the four initial states. For stable states, when setting again the pressure head of the flow to the value of the initial state, the initial flow rate is recovered. For the last initial state (predicted as unstable by the extended criterion), after a convergence period carried-out by imposing the SA steady flow rate, the solving of the transient momentum equation have shown that a flow excursion occurs in the MACARENa simulation as expected from the extended criterion. Consequently, the extended criterion proposed in this paper has been verified and could enable to define stable and unstable domain of couple flow rate/power in order to design future cores.
[en] A simplified estimation method for the first excursion probability of the secondary system with gap such as pipings, tanks and other mechanical equipment having mechanical supports and joints installed on the primary system such as buildings is proposed. First, an analytical method for the secondary system with gap is presented. This method is based on the equivalent linearization method. As input excitations, artificial time histories are used. Next, using the proposed method, some numerical examples are shown. It is found that when the tolerance level is normalized by the maximum standard deviation of response of the linear secondary system, the first excursion probability is independent of mass ratio of the secondary system to the primary system, the damping ratio and the natural period of the secondary system
[en] Within the COMAS (corium on material surfaces) project, representative experimental and analytical investigations on the ex-vessel spreading behaviour of prototypic corium were performed to provide a realistic technical basis for the development of core catchers for future nuclear reactor generations. In various large-scale experiments, melts in the Mg-range with real corium compositions and temperatures around 2000 deg. C were tested using different substratum materials (concrete, ceramics, cast iron). These tests were carried out at Siempelkamp's CARLA melting facility for radioactively contaminated metal scrap supported by laboratory experiments at Siemens, Erlangen. The COMAS results indicate that for a sufficiently high melt release rate a quick and homogeneous coverage of an ex-vessel spreading compartment as realized in the Franco-German EPR project can be expected. Regarding the analytical assessment, it can be concluded that some spreading codes have reached a sufficient level to provide guidelines for the evaluation of spreading concepts. Open questions are mainly related to segregation and immobilization of the melt
[en] In reactor protection systems based on minicomputers a central role is played by the diagnostic capability of selfchecking programs. It is thus of great importance to determine the efficiency that such programs must have with respect to fault detection in order to meet a certain reliability goal. Even though the content of this report is part of the safety study on a particular plant (Tapiro Research Reactor in service at C.S.N. Casaccia) it allows one to reach more general conclusions about the reliability of computerized protection systems. Another major aim of this paper is to point out the methodological difficulties met in the safety qualification of these systems. (orig.)
[en] The project ECOSTAR (acronym for ex-vessel core melt stabilization research) involves in total 17 organizations from five European countries. The objective of the project is to demonstrate the technical feasibility of core melt mitigation measures outside the reactor pressure vessel (RPV) as well as the validation of a selected set of codes in order to provide the necessary input for the definition of a convincing safety concept to control corium melts for both existing and future reactors. The ECOSTAR project is focused on three key phenomena for ex-vessel mitigation measures, namely melt release from RPV, ex-vessel corium transport and long-term corium stabilization. The extensive work program covers different scenarios and consists of a large number of experiments with both simulant and real materials, code development and validation as well as analytical activities. This paper presents the progress of work per end of the year 2001 and the results obtained since the launch of the project
[en] This paper describes two KROTOS tests in which ∼4 kg of corium melt was injected into a ∼1-m deep subcooled water pool. These tests were performed in a test vessel that allowed direct visual observations of melt injection and mixing conditions. Visual observations showed that the corium jet penetrated deep into the water while maintaining its shape. The tests did not produce spontaneous explosions. However, the second test with an external trigger produced an explosion with a relatively low efficiency of 0.15%. This is different from the behaviour of alumina melts, which exhibit one order of magnitude higher explosion efficiencies. This paper describes possible mechanisms that could have contributed to the reduced efficiency with corium melt
[en] A closed-form solution is presented for the probability of fast fracture of a pressure vessel in the presence of crack-like defects. The defect distributions are characterized by the total number of defects and the concentrations and lower threshold values of exponential size distributions. The effect of ultrasonic inspection on defect occurrence is taken into account. The structure of the failure probability is examined, and it is shown what types of defect distributions may be admitted in order to keep the likelihood of failure reasonably remote. Remarkably specific conclusions can be drawn given the limited data available. (orig.)