Safety Design Requirements and design concepts for SFR

Transcription

1 Safety Design Requirements and design concepts for SFR Reflection of lessons learned from the Fukushima Dai-ichi accident Advanced Nuclear System Research & Development Directorate Japan Atomic Energy Agency (JAEA) Shigenobu KUBO

2 Contents 1. Introduction 2. Lessons learned from Fukushima Dai-ichi NPP s accident 3. Safety design requirements and design concepts Reactor Shutdown System Decay Heat Removal System Containment System Seismic countermeasures Tsunami and Flood Countermeasures 4. Concluding Remarks 2

3 Introduction (1/2) To achieve Generation IV reactor safety goals, followings are required. Simple and robust structural design, Ensuring inspection, maintenance, and repair performance, Deviations from normal operation not escalating to accidents, Protection of core damage in accidents, Protection of significant radioactive materials release under design extension conditions (DECs) Taking lessons learned from the accident at Fukushima Daiichi Nuclear Power Plants (NPPs) into account, countermeasures under DECs including those of external events such as Earthquakes and Tsunami should be taken. These countermeasures should be taken in order to practicably eliminate any off-site emergency responses. 3

4 Introduction (2/2) Measures for core damage prevention and measures for mitigation of core damage consequences under DECs shall be taken. Passive system should be adopted for these measures. Active systems should be diverse in order to prevent common cause failures. They shall be effectively operated under loss of electric power situations. Taking the experience of JSFR conceptual design into account, reflecting lessons learned from the Fukushima Daiichi NPPs accident, Safety design requirements and design concepts to achieve Generation IV reactor safety goals are addressed. 4

5 Lessons learned from Fukushima Dai-ichi NPP s accident (1/2) The design basis level big earthquake hit broad coastal area of northeast Japan. Stem from unanticipated Tsunami accompanied, long term total black out occur in the multiple units. Generation IV reactor shall be designed to avoid significant radioactive materials release to the environment even under such severe condition. In particular, diverse decay heat removal facilities are called for to ensure cooling of the core under situation such as long term loss of electric power supply and failures in auxiliary facilities such as sea water cooling systems. 5

6 Lessons learned from Fukushima Dai-ichi NPP s accident (2/2) Even though the Fukushima Dai-ichi NPPs accident escalated to a severe accident accompanying core melt, external water injection as an accident management measure is leading the reactors to cold state. Differ from LWR, external water injection directly to the core is prohibited in SFR. Taking this into consideration, heat removal measures under core damage situations shall be considered in design. Countermeasures against natural disasters beyond design basis shall be considered in design. 6

7 Safety design requirements and design concepts Reactor Shutdown System Decay Heat Removal System Containment System Seismic countermeasures Tsunami and Flood Countermeasures 7

8 Reactor Shutdown System (1/2) Diverse active dual system shall be installed. Mechanical rod jamming shall be prevented to ensure its insertion considering earthquakes and core deformations. Adoption of passive reactor shutdown features. Detection Signals A B C D Detection Signals a b c d Logic control Logic control Actuator Actuator Control Rods Control Rods Passive Mechanism Main reactor shutdown system Backup reactor shutdown system 8

9 Reactor Shutdown System (2/2) Requirements for passive reactor shutdown features Effective against all ATWSs; loss of flow, loss of heat sink, over power Provide sufficient irreversible negative reactivity No interruption on normal operation Easy to Reset after actuation Provide testability Sensitive physical quantity Ambient temperature Ambient liquid pressure or peressure difference Activation mechanism Loss of electro-magnetic force aroud Cuie point Difference of thermal expansion Thermal expansion Melting Loss of pressure or suspension force Types Installed above the core Installed inside the core Circuit breaker switch Mechanical release Mechanical release and pushing down Mechanical elongation Thermal expansion of absorbing liquid Solid absorber injection Liquid absorber injection Gas Expansion Liquid suspension Neutron Absorber to be inserted Control rods Liquid lithium Boron balls Liquid lithium Inert gas (enhancement of neutron leakage) Control rods Boron balls 9

10 Decay Heat Removal System (1/2) Utilization of inherent natural circulation characteristics Providing redundancy and diversity Alternative measures to prevent common cause failures shall be provided. Heat sink diversity; air cooler + water cooler Coolant diversity; Na + NaK System diversity; Cooling circuit + component outer surface cooling 10

11 Decay Heat Removal System (2/2) 11

12 Containment System (1/5) Challenging factors for containment feature are mechanical loads from energetics due to prompt criticality and thermal loads from molten fuel. For mechanical loads, avoiding energetics and retaining damaged core in the reactor vessel are should be persued. For thermal load, retention in any of reactor vessel or guard vessel or containment vessel should be considered. Measures against each initiating events; Anticipated Transient Without Scram (ATWS), Loss Of Heat Sink (LOHS), Loss Of Reactor Level (LORL) are as follows. 12

13 Containment System (2/5); ATWS In fast reactors, early failure of containment function by energetics due to severe prompt criticality shall be prevented. Reactor vessel breach should be prevented since the damage could lead to severe sodium combustion causing damage on containment. 13

14 Containment System (3/5); ATWS Energy release reduction measures (void reactivity limit, molten fuel release features) and fuel retention features (invessel core catcher) are needed. CDA Mitigation CDA phases Approach for Mitigation Key points (1) Initiating Phase Prevent prompt criticality caused by coolant boiling Core & Fuel characteristics (2) Early-Discharge Phase Prevent severe recriticality caused by large molten-fuel compaction Mechanism for early molten fuel discharge (3) Material Relocation & Decay Heat Removal Phases Ensure stable long-term cooling of debris, by natural circulation Molten-fuel relocation/quench Core debris retention and heat removal 14

15 Containment System (4/5); LOHS Possible measures (under investigation) Cooling measures from outer surface of guard vessel in order to terminate events in guard vessel Installing core catcher in the containment. This case calls for heat resistance of concrete structure and prevention of sodium and fuel debris contacts with concrete. Cooling measure from outer surface of GV Core Catcher in CV 15

16 Containment System (5/5); LORL Necessity for postulation of double vessel failures depends on their reliability. If needed, sodium shall be retained in the reactor vessel cavity to keep sodium level necessary for core cooling. This case calls for heat resistance of concrete structure and prevention of sodium contact with concrete. Sodium retention in RV cavity 16

17 Seismic countermeasures Seismic isolation technology should be adopted to ensure integrity of the thin-walled structures of SFR under severe earthquake condition. It should be adopted not only for reactor vessel and primary system but also for secondary system since it contains considerable amount of sodium. Also, External Vessel fuel Storage Tank (EVST) calls for it. Thicker laminated rugger bearings + oil damper 17

18 Tsunami and Flood Countermeasures Basic countermeasures are to have sufficient site elevation and to provide dike in order to prevent submersion of the reactor building. Countermeasures against DECs shall be taken. Waterproof measures shall be applied for rooms for safety grade electricity facilities and for sodium contained components such as dump tanks, which are on lower level of the building. Natural circulation heat removal by air cooling is effective under loss of electric power and water splash situations. 18

19 Concluding Remarks To achieve safety goals for Generation IV reactor, design measures should be taken under DECs including those for external events considering the lessons learned from the Fukushima Dai-ichi NPPs accident. In particular, core cooling under long term loss of electric power or failure of auxiliary systems such as sea water cooling systems shall be ensured by utilizing diverse decay heat removal facilities. Alternative heat removal measures under core damage situations are needed to be considered. Also countermeasures against natural disasters beyond design basis shall be considered in design. Concepts of design measure to meet these requirements were presented. International consensus for these kind of requirements and design concepts is necessary to ensure the global safety of the nuclear power. 19