High Pressure Melt Ejection (HPME)
Challenges
As discussed earlier, without RPV coolant recovery, significant core damage can occur resulting in core debris relocation to the lower head of the RPV. On arrival to the lower vessel head, temporary debris quenching and energetic steam flashing, or explosion and pressurization may take place. After the remaining lower head water inventory has been boiled away, the debris can reheat, presenting a thermal threat to the vessel wall. In the event that the fuel debris in the lower head is molten at the time that the lower head is failed, either by creep rupture, penetration attack or simple wall melting, molten fuel materials can be released to the containment. If the RPV is at elevated pressure, this ejection under pressure of molten core materials can cause a rapid heat exchange to the containment atmosphere and a resultant large transient pressurization of the containment potentially beyond its failure pressure. The ejection of molten core materials through a failure point in the vessel is referred to as High Pressure Melt Ejection HPME and the resultant energetic pressurization of the containment atmosphere is called Direct Containment Heating DCH.
Should the accident have initiated from a large break LOCA, the pressure in the RCS will already be low, and the falling debris will just accumulate in the cavity. Such low pressure may also be the consequence of previous AM actions (e.g. RCS depressurisation).
Should the accident have initiated from a transient and the RCS still be at high pressure when the vessel melts through, the core debris will be expelled vigorously, and may even be spread over a large part of the containment volume. The stored heat of the debris will be transferred to the containment atmosphere almost instantaneously and cause a large pressure spike. This pressure spike is augmented by the exothermic metal/oxygen reaction (discussed later), which leads to a large mass of hydrogen and a possible hydrogen combustion.
For the issue of DCH, the most important factors associated with debris entrainment are the geometry of the reactor cavity, impingement of the debris on containment structures immediately downstream of the reactor cavity, re-entrainment of the debris and the dispersal of the debris to other containment compartments.
The High Pressure Melt Ejection phenomenon - beside short-term containment pressurization - may also have an impact on other aspects, such as the long term cooling capability of the core debris, or a partial or complete blockage of recirculation sump screens. A positive feature is the better possibility for long term cooling of such distributed debris.
A remote failure probability is the unzipping of the reactor vessel bottom weld, which may lead to the RPV being launched as a rocket and penetrate the containment dome. This scenario is considered to have a low probability and is not further discussed here (again under residual risk).
Strategies
The prime strategy against high pressure melt ejection is to depressurise the RCS. Various possibilities are available:
• Restore and establish cooling of the steam generators (e.g. via turbine driven AFW pump)
• Operate isolation condenser (IC) or reactor core isolation cooling system - BWR
• Depressurise the steam generator(s)
• Use (auxiliary) pressurizer sprays
• Open primary power operator relief valves (PORVs), force open primary safety valves; note: keep valves open, also at low RCS pressure
• Open letdown valves of the volume control system
• Open RPV head vent, vent BWR RPV (turbine condenser should be available)
• Provoke a creep failure of the hot leg or the steam generator tubes
The above strategies have also as an equally important goal to regain core cooling via intermediate pressure (accumulators) and low pressure injection (e.g. from the fire extinguishing system).
It should be noted that opening the RCS will inevitably lead to even more loss of cooling water from the RPV and may thereby decrease the time to core melt and vessel failure. Hence, depressurisation should be selected at a place that is compatible with other severe accident management measures. As an example, some NPPs open up only one PORV for RCS depressurisation, see ISAMM 2009, paper 8.4. Read more → If a slow depressurisation is selected, one can benefit from potential repairs, such as bringing a diesel back online before vessel failure is expected to occur.
Some PORVs close again at low pressure, and RCS re-pressurisation may occur. Measures should be taken to prevent this from happening.
It should be noted that provoking a creep failure of the hot leg or steam generator tubes includes the risk of high pressure melt ejection, if the predictive calculations are not accurate or even incorrect. In addition, steam generator tube creep failure may cause a large release early in the accident (see previous item).
See EPRI TBR, Vol. 2, Appendix N. Read more →
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