The containment as we have seen is constructed to certain design bases, one of which is, that compliance with siting radiation limits must be shown even at raised design pressure and temperature conditions within the containment. These conditions are more severe than those following the worst accident to which the plant could ever be subjected. Thus, in order to calculate the containment safety design margin, it is necessary to follow the analysis of a containment evaluation or design basis accident from initiation to its possible final consequences.

5.4.1 Design Basis Accident Initiators

To date, design basis or evaluation accidents for containments have been selected hypothetical occurrences. A result was postulated either in terms of gross core damage or in terms of an ultimate energy release, and in many cases an initiator was not specifically isolated as the cause of the incident.

Table 5.8 shows that EBR-II and the Enrico Fermi Reactor, both built under the shadow of the EBR-I melt-down, were evaluated on the basis of a postulated worst compaction of the core. In both cases the top of the core fell onto an already molten and slumped core, the top being presumed to have hung in position for some time before dropping.

However Table 5.8 also shows that in the case of SEFOR the first glim­merings of light were entering into containment evaluation analysis; in this case a sequential slumping of annular sections of the core was assumed.

The fact is, that although smaller experimental earlier cores could be postulated to slump in a most unrealistic manner and still give rise to only moderate energy releases, the large commercial power plants cannot assume such all-enveloping pessimism without paying a severe economic penalty in the provision of hypothetical safety features and safeguards.

Thus it is important to start with the initiator and inject some plausibility into the accident chronology and at the same time obtain a better idea of where and how safety features may be provided to prevent the occurrence of such an incident.

Therefore the present trend in containment evaluation analysis is away from the hypothetical and toward the sequential description of the accident behavior. The analyst attempts to follow the course of the core disruptive accident from initiation through to the final energy release and the distribu­tion of that energy. In this way too he is better able to specify the required research and development to confirm his evaluation than he could have done with an upper limit analysis.

The following occurrences are incidents which might be considered to have consequences severe enough to be considered as candidates for a core disruptive accident (CDA):

(a) A total assembly blockage.

(b) A refueling accident in which an assembly is dropped into a near­critical core or in which a control rod is withdrawn or ejected from a near­critical core.

(c) A rod ejection from a critical core.

(d) Large bubbles passing through the core.

(e) A local subassembly blockage arising from a structural failure or a defective fuel pin.

(f) A pipe rupture.

(g) Scram failure in conjunction with some more probable accident such as a flow failure due to loss of power to the pumps.