Masutake Sotsu

ATWS Frequency Evaluation of FBR Monju

MONJU is a sodium-cooled, loop-type prototype fast breeder reactor which can supply 280MW of electricity. The frequency of anticipated transient without scram (ATWS) of MONJU that used to be based on conservative assumption was re-evaluated by appropriately providing the event sequence from the result of plant response analysis. As a result, detailed evaluation of the ATWS frequency according to the result of the plant response analysis and reviewing event sequence scenarios have enabled realistic ATWS frequency estimation.Copyright

Assessment of FBR MONJU Accident Management Reliability in Causing Reactor Trips

This paper describes a method and application of quantitatively evaluating Accident Management (AM) reliability upon a reactor trip failure for the MONJU fast breeder reactor using a PSA technique. The present method comprises an allowable time estimation that is based on plant transient response analysis using the Super-COPD code that was developed for use in best estimates of the plant dynamics of MONJU and in estimating failure probability of operators actions in AMs within the allowable time based on time records obtained from simulator training. Application of this method to MONJU resulted in the estimation that the allowable time for an unprotected loss-of-heat sink event would be more than the longest observed time of 326 s. The corresponding operation failure probability would be less than 0.1 even after taking the uncertainty into consideration. Combining this with a level 1 PSA revealed that the total frequency of core damage accompanying a reactor trip failure at MONJU could be decreased by at least 50 percent due to the reactor trip AM.

Multidimensional Thermal-Hydraulic Analysis on Natural Circulation Behavior in Ex-Vessel Fuel Storage Tank of MONJU

Since the accident of the Fukushima Dai-ichi Nuclear Power Station, analysis evaluations for stress tests have been conducted on the prototype fast breed reactor MONJU. In these evaluations, it has been necessary to evaluate the plant characteristics including the Ex-Vessel fuel Storage Tank (EVST) under the severe accident conditions such as station blackout. The EVST is a coaxial cylindrical tank to store spent fuels (SFs) stored in fuel transfer pots (FTPs) until the radioactivity enough decays. It is able to store 252 fuel transfer pots in the rotating rack and cool them by internal natural circulation of sodium coolant under both the severe accident and normal operation. The heat produced by SFs removed by the helical cooling coils installed in the annular space.Evaluations of natural circulation in the EVST have been performed with a one-dimensional flow-network code. However, it would be difficult to predict its behavior exactly, because it would include multidimensional flow such as local natural convection. Then, in order to clarify the natural circulation behavior and multidimensional effects and evaluate appropriateness of this flow network model, we have performed a thermal-hydraulic analysis using a three-dimensional model which has high resolution meshes and the almost same geometry as the actual equipment. This model makes it possible to take into account the following multidimensional phenomena, the heat distribution of the FTPs, the mixing in plenums, the bypass flow through the flow holes and the other geometry effects.In this study, we have used a commercial computational thermal-hydraulics code, “FrontFlow/red”. As a result of steady analyses, we have confirmed the following: The coolant temperature in the plenums is almost uniform and its difference is in a few degrees. The influence of the flow holes is also limited because its flow rate is relatively low to main flow rate. On the other hand, pressure loss at supporting plates of the rotating rack, which are main causes of flow resistance in the EVST, is larger than the case without multidimensional effects because of the natural convection concentrated in the high temperature region near heated FTPs. The result leads to our presumption that the flow network model of the EVST is almost appropriate. It should be noted that flow resistance coefficient of the supporting plates or the heat transfer center of the cooling coils should be set to conservative for the safety analysis on the EVST.© 2013 ASME

Plant Dynamics Evaluation of a Monju Ex-Vessel Fuel Storage System During a Station Blackout

The prototype fast breeder reactor “Monju” has an ex-vessel fuel storage system (EVSS) which consists mainly of an ex-vessel fuel storage tank (EVST) and an EVST sodium cooling system. EVST uses natural circulation of sodium for decay heat removal. Natural circulation in the EVST is generated by the decay heat from the spent fuel assemblies and the cooling of the cooling coils installed in the EVST. The EVST sodium cooling system consists of three independent loops. In each loop, sodium is circulated by electromagnetic pumps and the heat is removed by an air cooler with blowers. This system has the ability to remove the maximum decay heat using two loops, and thus, it uses two of the three loops for normal operation. During a station blackout (SBO), the pumps and blowers are stopped. However, the three air coolers are installed about 13.5 m higher than the cooling coils, and therefore, the EVST sodium cooling system potentially retains some cooling ability because of natural circulation. In this study, an analysis and evaluation of the plant dynamics for the spent fuel and the EVSS structural integrity during an SBO were performed.The ultimate heat sink for the EVST sodium cooling system is the atmosphere, and the air coolers have an exhaust stack for efficient natural circulation caused by the chimney effect. However, the EVST sodium cooling system loses pressure and the heat transfer characteristics change if the flow rate is low. It was, therefore, necessary to confirm the temperature and flow rate behavior of EVSS in this analysis.In the present calculations, the plant dynamics analysis program “Super-COPD” was used. The factors affecting the cooling ability were investigated and analytical cases were determined. In one case, the two operated loops were switched to natural circulation after an SBO. The number of cooling loops was then changed from two to three by having an operator open the vane and dampers of the standby loop. In this case, sodium temperature in the EVST increased to approximately 320°C. When the number of cooling loops was not changed and natural circulation occurred in only two loops, the sodium temperature in the EVST increased to approximately 450°C. In both cases, however, the structural integrity of the EVSS was maintained. These analytical results, therefore, help clarify the number of necessary cooling loops for efficient decay heat removal and sodium temperature behavior in an SBO.Copyright

Evaluation of Monju Core Damage Risk With Change of AOT Using Probabilistic Method

MONJU is a sodium-cooled, loop-type prototype fast breeder reactor with three primary cooling loops which can supply 280 MW of electricity. Limiting conditions of operation (LCO) defined in the safety regulations in MONJU given the allowed outage time (AOT) are evaluated using a PSA technique. The result indicates the possibility of limit extension and some prospects that we should examine.Copyright