Introduction
In the beginning of this chapter, the concept of Plant Damage Conditions or Descriptors (PDC) was used, to indicate the various stages along which a severe accident evolves and the associated threats to the fission product boundaries. In this section, a more detailed discussion is presented, to better place the various strategies just discussed.
As presented in EPRI TBR sec. 2.3 EPRI TBR, Ch. 3 Read more → we introduce the PDC as follows:
Plant damage condition descriptors are means of characterizing the progression of severe accidents and plant status with respect to the extent of fuel damage, fission product release from the fuel, and primary/secondary containment damage. In addition, a particular PDC specifies the availability of fission product barriers at which fuel melt progression could be arrested or consequences mitigated.
In general, the RCS, spent fuel, primary containment, and secondary containment barriers would be challenged separately; therefore, the descriptors characterize each barrier individually. Each fuel damage condition represents a substantial change in fuel damage severity and the potential for release of fission products to the environment. Each containment damage condition represents a major change in the capability of the containment function.
In the SAMG-D, we recognise the following PDCs for the RCS:
Fuel damage states
OX (means oxidised fuel): It represents degraded fuel conditions in which the fuel cladding has undergone oxidation but fuel degradation is not (yet) sufficient to lead to appreciable changes in the fuel geometry which, hence, is still coolable. Also the control material is still largely intact.
BD (means badly damaged fuel): It represents a degraded fuel condition in which significant fuel relocation starts to occur or has already occurred so that the coolability of the fuel geometry has degraded or largely disappeared. Fuel may also have relocated to the vessel lower head (or partly in BWR vessel recirculation lines, for BWRs that have such lines). The BD condition is a challenge for the RPV/RCS integrity.
EX: (means fuel ex-vessel or ex-RCS): It represents a degraded fuel condition in which a part or most of the core debris has relocated into the containment. This is the damage state in which direct attack of the concrete basemat of the containment can occur.
Containment damage states
CC (means containment closed and cooled): It represents the condition in which the containment is intact and no appreciable buildup of energy is occurring within the volume.
CH (means containment challenged): It represents a situation in which either appreciable quantities of energy have built up within the containment volume giving rise to a high containment pressure, or flammable gases are present in a mixture that could ignite given the presence of an ignition source. A late containment challenge may occur due to venting or leakage of the containment and subsequent condensation of the remaining steam, resulting in sub-atmospheric pressure.
I (means containment impaired): It represents an impaired containment state in which either the containment isolation is not complete or a breach of the containment has occurred by some other mechanism.
B: It represents conditions in which there is ab reach in the RCS that could bypass the containment boundary.
F: It represents a failure of the secondary containment structure in some manner, providing direct release path to the environment.
For the Spent Fuel Pool (SFP), separate damage descriptors have been developed, see EPRI TBR, sec. 2.3.2.3: Read more →
SFP-OX. Spent fuel is significantly oxidised but intact. The release of fission products from the fuel is limited to gap activity.
SFP-BD. Spent fuel has undergone significant oxidation and debris formation, and relocation has commenced. Alternatively, physical damage to a limited portion of the spent fuel racks could have occurred (for example, a crane failure event), dislocating fuel from spent fuel racks. For the secondary containment damage condition similar descriptors can be developed, EPRI TBR, sec. 2.3.4 and Table 2-0: Read more →
SC-CC: secondary containment is undamaged and is closed and cooled.
SC-CH: secondary containment is closed but challenged.
SC-F: Secondary containment has failed, with a large path existing to the environment.
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