In-vessel retention

Containment and Severe Accident -Double Containment -Cavity Flooding System(CFS) -Hydrogen Mitigation System - Single Containment In-Vessel Retention - Replacement of Fusible Plug with MOV (Motor Operated Valve) Passive Auto-catalytic Recombiner -i- Igniter Accident mitigation Measure such as IVR adopted... 

These designs use an in-vessel retention approach, that is, the reactor pressure vessel cavity is flooded and the decay heat is removed by boiling on the outer surface of fhe reactor pressure vessel. 

In-vessel retention of corium is considered a priority to limit severe accident consequences in the VBER-150, since they are to a large extent determined by the reactor vessel failure and the resulting initiation of additional loads on the containment under a release of corium. 

The VBER-150 design incorporates a special system of emergency vessel cooling to secure in-vessel retention of corium in severe accidents this system functions in a passive mode. 

The design calculations indicate that the problem of in-vessel retention of corium could be successfully solved for the VBER-150. 

A reactor caisson and a system of water supply to the reactor caisson supporting the reactor vessel integrity and core melt in-vessel retention in severe beyond design basis accidents and... 

There is in-vessel retention of core debris following core melt, which significantly reduces the uncertainty in the assessment of containment failure and radioactive release to the environment due to severe accident phenomena. 

The capability to flood the reactor cavity prevents the failure of the reactor vessel given a severe accident. The vessel and its insulation are designed so that the water in the cavity is able to cool the vessel and prevent it from failing that is, in-vessel retention (IVR). By maintaining die vessel integrity, the core debris in the vessel eliminates the potential of a large release due to ex-vessel phenomena and its potential to fail the containment. 

The insulation on the outside of the reactor vessel allows cooling water to penetrate through it to the outer surface of the vessel, thereby providing an in-vessel retention capability. The passive core cooling approach results in the introduction of large amounts of water into the lower portions of the contaimnent. The expected level of water in the containment after an accident is above the nozzles of the reactor vessel and, hence, above the top of the fuel. The water in the containment sump is able to flow into the reactor vessel insulation structure, and come into contact with the reactor vessel. It would then cool the reactor vessel by convection and evaporation. 

Some Beyond Design Basis fault sequences could result in a core melt in such circumstances reactor vessel integrity and therefore prevention of a large release of radioactivity is maintained by flooding the reactor cavity with water, thereby cooling the vessel by evaporation within its insulation. Changes have been made to the flow path between the outside of the reactor vessel and the reactor vessel insulation, and testing has confirmed the robustness of the heat transfer required for in-vessel retention (Section IB. 1.5 ofReference 6.1). 

The APIOOO has incorporated the capability of maintaining the containment integrity following a core melt PRA sequence. Testing and analysis has shown that the in-vessel retention capability of the AP1000 provides a robust means of keeping molten core debris in the reactor vessel (Section 19.34.2.1 of Reference 8.1). 

Post aceident in vessel retention would have been hard to demonstrate. 

Certain Beyond Design Basis accident sequences could lead to a core melt whilst this is extremely unlikely, it is not incredible. This possibility required some form of mitigation. The AP1 OOO s designers addressed this challenge by developing the capability for in-vessel retention. The alternative would have been to incorporate some form of core catcher outside the reactor vessel. A core catcher would have features that precluded re-criticality of the mixtures of core structural materials and building structures (known as corium), and cooled it to slow its reaction with materials around the reactor vessel. This could have been the design solution for APIOOO. 

The following advantages result from choosing the in-vessel retention option ... 

The disadvantage of choosing the in-vessel retention option is that there is a shght risk that the natural movement of the in-containment water over the reactor vessel in a real core melt accident sequence could be insufficient to prevent core melt-through. 

Implementing in-vessel retention in this way provides a safe, simple, natural cooling mechanism for the reactor vessel that maintains its integrity and obviates the need for an external core catcher. Avoiding the dispersion of radioactive material is the dominant benefit the ALARP option of adding a core catcher as well as in-vessel retention to the APlOOO s design would have no effect on this. The option to supplementing in-vessel retention with a core catcher has been considered, but found not to be ALARP. The in-vessel retention option alone is thus ALARP. 

The only disadvantage resulting from improving the design of the in-vessel retention is lhat it required additional structure and a shaped internal boundary for the reactor vessel insulation design, which incurred a development cost. 

The thermal hydraulic parameters associated with core melt demanded a change in the design of the APlOOO s In Vessel Retention features doing nothing was not an option. The chosen solution involves the minimum change from the previous design, and has resulted in improved safety performance. It is thus the ALARP option. 

As demonstrated in Seetion 8.4.3.8, the APIOOO design does not rely on contaimnent spray for post-aceident isotope eontrol. However, the US NRC requires that the APIOOO design include a manually initiated eontainment spray for certain Beyond Design Basis initiating events not only for isotope control but also as an alternate means for flooding the reactor vessel (in-vessel retention), for debris quenching should vessel failure occur and to control containment pressure should the passive contaimnent cooling system fail. 

Assuming appropriate timing, eontainment spray eould be used as an alternate means for flooding the reactor vessel (in-vessel retention) and for debris quenching should vessel feilure occur. 

JSCOBEL, J.E., THEOFANOUS, T.G., CONWAY, L.E., In-vessel retention of molten core debris in the Westinghouse APIOOO Advanced Passive PWR, Advanced Nuclear Power Plants (Proc. Int. Congress, Hollywood, FL, USA, 2002) ICAPP 02- ISBN 0-89448-663-2. 

Several designers of water cooled SMRs (e.g., MARS, SCOR, CCR) examine passive in-vessel retention of corium (Level 4 in Table 4). 

The majority of inputs for all SMRs correspond to the ongoing or planned R D for a variety of different passive systems. Specifically, the in-vessel retention of corium by passive means is indicated as a design objective for three water cooled SMRs. 

R D on in-vessel retention (FVR) by passive means AHWR (Annex XI)... 

In particular, thermal-mechanical analyses show that the reactor vessel of the MARS plant can guarantee the in-vessel retention and cooling of corium produced by the melting of 60% of core and vessel internals if the following conditions are met ... 

In-Vessel Retention (IVR) by passive means Evaluation in progress. 

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