Molten Core Concrete Interaction (MCCI)
Challenges
Upon leaving the vessel, the fuel debris may either fall in a wet cavity /drywell or in a dry cavity /drywell, depending on the accident evolution and/or accident management measures (e.g., advance flooding of the cavity / drywell).
In a deep pool (several meters) the debris jet may break up and form a porous mass, which may be coolable by water. In a shallow pool or in the absence of water, the debris bed will not be coolable and attack the concrete.
Only if the debris is spread out widely to a relatively thin layer (~ 10 cm), it is coolable from the top only (part of the design basis of the core catcher of the EPR), but otherwise it will generally not be coolable, unless caught in a specifically designed core catcher (examples: AES-2006, GE Hitachi's BiMac).
The interaction of molten core material with the concrete (MCCI) causes concrete ablation, both vertically and horizontally. Progress is relatively slow, it may take several days before large parts of the cavity / drywell are ablated or even the basemat is penetrated.
MCCI can generate a large volume of CO2, plus CO, depending on the composition of concrete. Notably limestone type of concrete generates much CO2.
The consequence is a gradual pressurisation of the containment, which can reach and exceed the design pressure in several days (mostly - more quickly in small containments such as BWR Mark I or a PWR Ice Condenser plant).
In a suppression pool type reactor (BWR), the molten material may penetrate the drywell and cause a direct path from the drywell to the wetwell, i.e. a by-pass of the suppression pool. This eliminates the scrubbing function of the suppression pool, which otherwise would capture many fission products.
In a wet cavity, also steam is generated which contributes also to containment pressurization.
The expulsion of the debris from the vessel exposes still unoxidised Zr to steam / air, and will thereby generate additional hydrogen. Because of the significant rebar content of the basemat concrete, the hydrogen generated in the ex-vessel MCCI phase can even exceed the in-vessel hydrogen.
Further information is found in EPRI TBR, Vol. 2, section Q Read more → and the NEA/CSNI/R(2016)15 Read more →
Strategies
The primary strategy against MCCI is to flood the cavity / drywell, either before or after vessel meltthrough or both. If done before, this may also help to prevent or delay vessel failure, if the water level reaches and envelopes the RPV lower head, as has been discussed in the Section 2.4.
For reactors where the external vessel cooling may not be successful, core catchers have been developed and built, i.e. devices which are designed to cool and retain molten coium. Flooding can be accomplished by passive means, where the water is coming from adjacent volumes (e.g. a suppression pool), or by active means, for which then power and water must be available.
If the pool of water is deep enough, the jet of debris leaving the vessel may break up and result in a coolable geometry, as has been discussed previously. The mixture of debris with water may yield a sharp pressure peak in the cavity, so that it is necessary to analyse this pressure peak to determine whether the cavity can withstand the load. Subsequently, the pressure will load the containment. There is also a small risk that the cavity flooding will induce an ex-vessel steam explosion.
It may happen that the debris has already relocated to the cavity before water can be brought there. In this case, if water is subsequently added to the cavity, a sharp pressure rise may be expected in the containment. It is important to realize that if water is added to the cavity and the containment has already failed, the generation of steam at this time can increase the leakage of airborne radioactivity to the environment by advection of fission products with the escaping steam.
Where debris threatens the drywell integrity (BWRs) and may cause a bypass to the wetwell, the potential leakage area might be protected by flooding (presumably from outside).
The heat that is dissipated by the contact between water and debris should be carried away to an ultimate heat sink. If no such heat sink is available, this heat can alternatively be simply transferred to the environment by venting of the containment. Note that if this venting operation is not filtered or scrubbed by the suppression pool in the BWR, radiological releases to the environment can also occur. One must also consider the personnel dose that might occur in venting operations if the vent lines are in proximity to the control room or other manned areas of the plant. If a once-through cooling mode is selected (e.g. from a river or the sea), the run-off water should be captured and safely stored, as it will be highly radioactive.
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