Many sensitivity calculations are necessary to evaluate the effect of the uncertainties of Core Disruptive Accident (CDA) scenarios on debris bed coolability for Sodium-cooled Fast Reactors (SFR). This paper describes a calculation model and a technique for high-speed calculations of debris bed coolability. Firstly, the detailed coolability calculation model taking into account phase change and vapor-liquid advection for a boiling zone in the debris bed is derived as a single equation. The vapor-liquid advection is based on the momentum equation including inertial, viscous, capillary and gravitational effects. In addition, the streamlined calculation which ignores inertial and gravitational effects for the boiling zone is proposed. Secondly, both the detailed and streamlined calculation models are validated through the analysis of the Sandia ACRR-D10 in-pile experiments. Then, the effects of the ignoring on debris bed coolability are evaluated through sensitivity calculations. Thirdly, the speed-up technique for the streamlined calculation model is proposed which solves the boiling zone by the combination of transient and steady state calculations and allows using coarse mesh size and time step. Owing to the technique, the calculation time becomes about 1/50. The application of the speed-up technique to the streamlined calculation model is confirmed under typical SFR conditions. Finally, it is demonstrated that the streamlined calculation model and the speed-up technique enable us to perform many sensitivity calculations in a realistic time. The model and technique developed in this study is practical to evaluate the effect of the uncertainties of CDA scenarios on debris bed coolability for SFR.

No abstract

Understanding the effect of uncertainties of Core Disruptive Accident (CDA) scenarios on debris bed coolability on a core catcher is required for decision making on design options to mitigate a CDA consequence. For the understanding, a huge number of calculations are required but are extremely difficult to perform because a huge number of calculations require much calculation time to solve non-steady equations in the coolability calculation model. Thus, we applied Artificial Neural Network (ANN), which is one of models for machine learning, to debris bed coolability calculations. The application of ANN is expected to exponentially improve the calculation speed of debris bed coolability because ANN provides results from experimental rules learned through training without solving non-steady equations. The application is in three steps. Firstly, we created many data for training ANN and validating the trained ANN through coolability calculations parameterizing main dominant inputs (particle diameter of debris bed, porosity of debris bed, etc.) by using Latin hypercube sampling. Secondly, ANN was trained and validated with the created data. The accuracy rate of the results by the ANN to the validation data exceeded 99%. In addition, the calculation time using ANN was micro seconds order. Finally, through demonstration calculations, it was confirmed that we can easily understand the effect of uncertainties of CDA scenarios on debris bed coolability owing to results visualization based on a huge number of parametric calculations using ANN. Thus, the application of ANN to debris bed coolability calculations should contribute to the decision making on design options to mitigate a CDA consequence.

This paper describes coolability evaluations of a debris bed with a variety of decay heat removal system (DHRS) operating conditions with a whole vessel model assuming fuel accumulation on the core catcher in a short term. The DHRS consists of two circuits of direct reactor auxiliary cooling system and one circuit of reactor vessel auxiliary cooling system. The evaluation tool is a one-dimensional plant dynamics code, Super-COPD, with a debris bed module. The coolability evaluations have indicated that the current core catcher design secures sufficient natural circulation flows around the core catcher to ensure the debris bed cooling when at least one circuit of DHRS was activated. Sensitivity analyses under a pessimistic condition have shown that the debris bed is coolable with at least one circuit of improved DHRS even if most fuel accumulates on the core catcher in a short term.

No abstract