Download
1 / 14
E N D
Severe Accident Severe Accident Definition: An event largely exceeding design basis event (*) assumed in safety evaluation, a condition where appropriate core cooling or reactivity control is impossible with a means assumed in evaluation of safety design, and an event where, consequently, the core is severely damaged. (*) An event as what should be taken into consideration during safety design of reactor facility and its evaluation, which is extracted out of the events that may cause abnormal condition to the reactor facility. TMI-2炉心最終状態
Severe Accident Containment Load at Severe Accident Containment load - DCH (making molten debris smaller and direct heating of gas in containment due to rupture of pressure vessel under high pressure conditions) - Hydrogen burning and detonation - FCI (conversion to high energy due to contact between hot molten materials and water: outside and inside of reactor) - Recriticality (outside and inside reactor) - MCCI (core-concrete reaction) - Humidification and pressurization - Shell attack Relevant items Bypass leakage IVR
Severe Accident Direct Containment Heating (DCH) An event where molten core that has become minute particles is dispersed into space of the containment and the containment is rapidly pressurized as the containment atmosphere is heated, when reactor vessel is damaged under high pressure condition of reactor coolant.
Severe Accident Hydrogen Burning and Detonation Hydrogen is generated due to metal-water reaction, core and concrete interaction and water radiolysis. Generated hydrogen pressurizes and heats the containment by releasing thermal energy due to burning by reaction with oxygen in containment atmosphere. It is known that there is a possibility of occurrence of transition to deflagration and detonation (DDT), as flame is accelerated in narrow compartment, in case hydrogen concentration is above 13%. When detonation occurs, there is a possibility of occurrence of shock wave and missiles due to shock wave, and dynamic load is applied to the containment.
Severe Accident In-Vessel Steam Explosion (FCI) An event of damage of containment by missiles generated by rupture of reactor vessel lid due to occurrence of hydrogen explosion by contact between molted core and coolant (water) in reactor vessel.
Severe Accident Ex-Vessel Steam Explosion (FCI) When the molten core drops onto containment due to damage of reactor vessel, steam explosion occurrence is possible if water exists on floor and this water contacts molten core. An event where, due to this, the primary coolant pipes are largely displaced by thrusting-up of reactor vessel or loss of supporting function, and the confinement function of containment is lost by damage of CV penetration of main steam pipes, etc.
Severe Accident Recriticality An event where pressure and temperature in reactor pressure vessel and containment rise due to increase of fuel heating value by mixing of molten core with low concentration boric acid water in reactor vessel or on containment floor of lower part of reactor vessel.
Severe Accident Molten Core Concrete Interaction (MCCI) An event where molten core drops from reactor vessel onto concrete of floor surface of containment, and concrete is thermally discomposed by high temperature molten core and is molten to be eroded. Also, non-condensable gases are generated such as hydrogen, carbon monoxide, etc. by reaction between the gases (steam and carbon dioxide) generated by thermal decomposition of concrete and the molten core.
Severe Accident Pressurization and Heating of Containment An event of pressurization and humidification due to heat generation by release of primary coolant, steam generation by decay heat, hydrogen burning, and chemical reactions such as metal-water reaction, etc. Pressurization is made also by generation of non-condensable gases due to molten core-concrete interaction (MCCI).
Severe Accident Containment Shell Attack An event where molten core dispersed during rupture of reactor vessel sticks to main body of containment and confinement function is lost as liner is molten and eroded.
Severe Accident Containment Bypass SGTR and interface An event where leakage of radioactive materials occurs due to LOCA, even if main body of containment keeps integrity.
Severe Accident ＩＶＲ （In-Vessel Retention） Overview of RV Outside Cooling
Example of Plant damage state : BWR • *1：炉心損傷以前に格納容器破損が発生する場合と，炉心損傷時には格納容器は健全である場合とでは，その後の挙動が異なるため，格納容器破損のタイミングによる分岐を先に設けている。 • *2：---------は，格納容器イベントツリー内で，デブリの冷却手段，格納容器除熱手段の可能性を評価することを示す。
