CHAPTER 2: SEVERE ACCIDENT PHENOMENA AND MITIGATION STRATEGIES

Core cooling, UHS and RPV melt-through

Challenges

The lack of adequate core cooling will result, as has been discussed before, in melting of the fuel cladding, melting of the control rods, collapse of the fuel stack, melting of the debris and relocation of the molten core debris to the RPV lower plenum.

While relocating to the lower plenum, the debris also attacks the lower grid plate, which may fail and also melt into the pool of core debris.

The molten material in the lower plenum may be stratified, in that the heavier oxides (UO2) fill the lower part and the lighter molten metals form a layer on top of the oxides. Due to natural circulation phenomena in the oxide molten pool and the high thermal conductivity of the upper metallic layer, the heat flux to the vessel wall from this metal layer can be larger than the thermal load from the oxides on the bottom and the lower side of the RPV ('focusing effect'). The focusing effect may potentially lead to the vessel failure at the location of the metal layer. Note that, depending on the core materials more complex density driven layering of materials in the lower head can occur. See OECD MASCA Project.

The corium pool may also attack the vessel bottom penetrations and initiate a vessel failure at those locations. Alternatively, the vessel wall may fail at the side. Experiments have shown that often a fish-mouth opening may be expected on the vessel lower head. More information is povided in the Chapter 2.4. Lower Head Failure Read more →.

For BWRs with recirculation piping, part of the corium debris may flow to the recirculation lines, which have little resistance against thermal failures.

A prime strategy is, hence, to prevent or delay vessel failure. It should be noted that any means of restoring in whole or in part core debris cooling will automatically include the need to find an ultimate heat sink, where the heat can be rejected.

Strategies

In order to prevent or delay vessel failure, a major SAMG strategy is to restore core cooling. An important support to this strategy is to depressurise the RCS, so as to enable low-pressure sources to inject into the RCS. A major objective is also to avoid high pressure melt ejection (see preceding strategy). It also helps to prevent steam generator tube creep rupture.

It should be noted that injection of water on a highly overheated core may generate a large mass of hydrogen, and the generated heat can even promote melting of the core (as was found in Fukushima-Daiichi, Unit 2). The injection may also generate an RCS pressure spike, which may cause steam generator tubes to creep rupture or may interrupt low pressure water injection and cause the injection to be intermittent. Such injection could for example be generated by an RCP restart, as it will pump remaining loop seal water through the core. Without such risk, RCP restart may delay vessel failure for considerable time.

Injection into the RCS may not be available or not be (totally) successful. Therefore, another important strategy has been developed to flood the vessel lower head from outside. This strategy has been demonstrated to be effective for about 600 MWe PWR plants, and has been analysed to be largely effective up to 1000 MWe. At higher power levels, special provisions are studied such as guided convection and use of nanofluids or RPV coatings. The most recent status is available in Proceedings of the International Seminar “In-vessel Retention: Outcomes of the EU IVMR project”, Juan-les-Pins, France, January 2020. Learn more → (IVMR Project)

Cooling debris either inside or outside the reactor vessel, or both, requires the availability and identification of an ultimate heat sink. This could be the atmosphere (e.g. by steam generator Auxiliary Feed Water cooling, AFW or the containment atmosphere (e.g. via PORVs). In the latter case, repeated venting may be needed to provide the ultimate heat sink. Should a once-though cooling mode be selected, then the run-off water should be kept and stored.

This strategy is to be pursued totally or largely independently of other measures to mitigate the core damage accident. If restoration of core cooling competes with other measures to mitigate the severe accident, a careful balance should be sought between restoring core cooling and these other measures. For example, power / water may also be needed to feed the steam generators or spray the containment. In various SAMG, therefore, the efforts to cool the core go uninterruptedly together with other efforts to mitigate the accident.