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To identify functions and facilities that are critical for ensuring safety in advanced reactors, for which only a limited usage history is available, it is necessary to consider the characteristics of each reactor type by utilizing more risk-related information. In this study, we compare the response capability during an accident for light water reactors and fast reactors to quantitatively evaluate the impact of the reactor type on resilience. A resilience evaluation procedure is developed for assessing the response margin and response reliability of a nuclear power plant during an accident; this procedure was then applied to a simple pressurized-water reactor and a sodium-cooled fast reactor plant model using an accident scenario involving an earthquake that generated a leakage of coolant from the primary heat-transport system. Our research indicated a relatively large difference between the response reliability with respect to time for the two reactor types.

The authors are developing the design concept of the pool-type sodium-cooled fast reactor (SFR) that addresses Japan’s specific siting conditions such as earthquakes and meets safety design criteria (SDC) and safety design guidelines (SDG) for Generation IV SFRs. The development of this concept will broaden not only options for reactor types in Japan but also the range and depth of international cooperation. A design concept of 1,500 MWt (650 MWe) class pool-type SFR was thought up by applying design technology obtained from the design of advanced loop-type SFR, named JSFR, equipped with safety measures that reflect results from the feasibility study on commercialized fast reactor cycle systems and fast reactor cycle technology development, improved maintainability and repairability, and lessons learned from the Fukushima Daiichi Nuclear Power Plants accident. The design concepts of a reactor vessel (RV) and its internal structures have been investigated whether they could withstand severe seismic conditions in Japan and thermal loads. The design adopted enhanced RV support structure, enhanced conical-shaped core support structure, a thickened knuckle part of the RV, and a flat plenum separator with ribs. A three-dimensional steady-state thermal hydraulic analysis of the RV revealed that the temperature difference of the upper and lower surfaces of the flat plenum separator could be effectively reduced by installing layers of thermal insulation plates. The authors have also conducted a transient analysis of loss of flow type anticipated transients without scram events to evaluate the feasibility of a self-actuated shutdown system. Moreover, the configuration of the decay heat removal system has been investigated considering sufficient utilization of natural circulation capability of sodium coolant, heat removal capacity of each cooling system, conformance with design requirements, and recommendations of SDC and SDG such as diversity and redundancy of components.

Development of an inspection technique in opaque liquid-metal coolant is one of the important issues to ensure the safety of the liquid-metal fast breeder reactor (LMFBR). Performance tests of an under sodium viewer (USV), which was developed to detect an obstacle in the reactor vessel (RV) of the LMFBR Monju, have been carried out. Ultrasonic sensors and reflectors are located across the core inside Monju's RV. The USV can detect an obstacle existing between the core top and the upper core structure bottom by identifying differences of echo signals. This paper describes the USV performance tests. In the tests, the reference echo signals under various conditions were accumulated, and the signal-to-noise ratio successfully exceeded the target value. Measured signals clearly differed with and without an obstacle. These experimental results show the performance of the USV for detecting an obstacle in a specified place. [ABSTRACT FROM AUTHOR]

The Japan Sodium-cooled Fast Reactor (JSFR) adopts a double boundary for all sodium components. In this paper, design measures are investigated against a secondary sodium fire inside the reactor building, which might be assumed as design extension conditions. Candidate sodium fire measures for the secondary sodium systems compared in terms of safety are the sodium drain, nitrogen injection, pressure release valve, catch pan, and drain system for leaked sodium. Various sodium fires in the steam generator room have been analyzed by the SPHINCS code to evaluate the performance of the candidate sodium fire measures. [ABSTRACT FROM AUTHOR]

Severe external hazards on the Japan Sodium-cooled Fast Reactor (JSFR) have been analyzed and evaluated. For seismic design, safety components are confirmed to maintain their functions even against recent strong earthquakes. Integrity of the major components has been confirmed covering recent earthquake conditions. In the case of a tsunami, the seawater pumps for the component cooling water system (CCWS) could be damaged by the tsunami, since they are located at sea level. In the JSFR design with full natural convection decay heat removal systems (DHRSs) and an air-cooling emergency gas turbine, safety-grade components are independent of CCWS, and loss of CCWS does not affect reactor cooling. As a consei-vative case, hypothetical station blackout (SBO) has been evaluated. In the case ofSBO, decay heat is removed by natural convection DHRS, but control of the air cooler (AC) damper is lost after the battery power is out. The analysis has revealed that freezing at one of three ACs could happen due to loss of automatic control of AC dampers. However, the time margin to protected loss of heat sink is evaluated to be >10 days. Manual control of the AC damper is also investigated. Transient analyses show that the AC dampers can be controlled manually adopting a simple operation procedure with sufficient operation time. Decay heat cooling in the case of collapse in all air stacks of AC has been evaluated. The result shows that decay heat could be removed maintaining air paths in two of three ACs by accident management. In conclusion, JSFR in the 2010 design version has enough external hazard toughness mainly thanks to passive safety features and a seismic isolation system. [ABSTRACT FROM AUTHOR]

Failed fuel detection for the Japan Sodium-cooled Fast Reactor (JSFR) has been studied. The present JSFR design adopts a selector-valve (SV) failed fuel detection and location (FFDL) system. In this study, a tagging-gas (Tag) FFDL (Tag-FFDL) system has been investigated as an alternative. Although the identification performance of the Tag-FFDL system has been demonstrated in small and medium-sized reactors, the Tag-FFDL system has not been demonstrated yet in a large reactor like JSFR, which has 1500-MW(electric) power and 562 core fuel sub-assemblies. Major issues of the JSFR Tag-FFDL system are affected by high-burnup fuel and large cover gas volume. High-burnup fuel leads to a large change of the isotope ratio, which is important for the detection performance of the Tag-FFDL system. Since the cover gas volume in JSFR is larger than that in previous reactors, the tagging-gas concentration in the cover gas is lower than that in previous reactors. Thus, a requirement of the background value is more strict in JSFR. This study investigates whether two issues of the Tag-FFDL system for JSFR would be solvable. Tag gas isotope change in a high-burnup condition has been evaluated regarding transmutation and fission gas release. Taking into account tag gas isotope change due to the high-burnup conditions and large cover gas space, JSFR tagging gas has been designed. The investigation results showed that the JSFR FFDL system can provide an identification capability for 672 subassem-blies, which is larger than the number of subassemblies in JSFR combining tagging-gas and burnup estimation. In addition, an allowable background concentration of natural Kr and Xe in the cover gas has been evaluated. [ABSTRACT FROM AUTHOR]

In response to the accident at the Fukushima Daiichi (IF) nuclear power plant, designers of the 2010 version of the Japan sodium-cooled fast reactor (JSFR) have been studying the robustness of JSFR's fuel handling system (FHS) against an earthquake and a tsunami. In the earthquake evaluation, the margin of seismic resistance and sloshing in the ex-vessel fuel storage tank (EVST) against an earthquake of the IF-envelop condition were estimated. In the tsunami evaluation, for the case of loss of emergency power supply, a scenario is studied where fuel subassemblies are led to a stable cooling state, and potentialities for the cooling system are examined. As a result, it is shown that the original design of the JSFR FHS already had the potential to prevent the release of radioactive material. Additionally, some design improvements of the EVST cooling system are introduced. [ABSTRACT FROM AUTHOR]