Maximum credible accident

You can quickly identify these plant sections by reviewing process flow diagrams and valving arrangements. Isolation points are defined by control valves or powered block valves that can be remotely activated. Process hazard analysis techniques help you identify the maximum credible accident scenarios. (Note that manual valves should not be considered reliable isolation points unless they are located to be accessible following a major accident. However, remotely-activated valves can only be considered reliable isolation points if there are adequate reliability engineering and maintenance programs in place.)... 

A. The regulation of the agent quantity in CAMDS OPERATIONS assures that any released agent is maintained on Tooele Army Depot property during a maximum credible accident. 

This is a design-based accident that could lead to the largest off-site release of activity and was originally referred to as the maximum credible accident. 

Consequently, the reactor must be cooled even after shutdown and even in case of an accident. Therefore several independent cooling systems are incorporated. The worst accident that is considered (maximum credible accident MCA ) is the complete shearing off of the primary circuit line. If this should occur, then a mixture of steam and air would be blown into the containment for a time span of 12 s (blowdown phase). When the pressure falls below 26 bar, water tanks maintained at this pressure automatically would empty into the reactor vessel from both sides (high and low temperature) of the circuit. If this water should evaporate or leak out again, at a pressure of 10 bar, a pump would provide water from other reservoirs. Water leaking out from the primary circuit will have to accumulate on the bottom of the... 

Calculations have been made of the areas and distances Involved on the release of fission products resulting from the maximum credible accidents using the assuiqptlons alrei tabulated. 

On the question of accidents that might breach the containment, the staff agreed with the Safeguards Committee that more theoretical and experimental research was needed to evaluate all possible contingencies. The staff argued, however, that such studies were unlikely to "remove all possibilities of meltdown," and it doubted that design features "can remove all probability of a secondary critical accumulation if meltdown does occur." Yet on the basis of studies completed, the staff concluded that the containment could hold the "maximum credible accident in this reactor. 

The draft criteria, presented to the environmental subcommittee in late August 1960 and to the full Safeguards Committee a month later, established three distances for a reactor of any given power level. Beck labeled them "benchmarks"—spedfically, an exclusion distance, an evacuation distance, and a population-center distance. The exclusion area, under the complete control of the reactor owner, had an outer-limit distance at which, following the onset of a maximum credible accident, the total radiation dose received by an individual in two hours... 

Since a small theoretical possibility existed that a more serious accident than the maximum credible accident might occur. Beck s group thought it prudent to provide the third benchmark in the criteria. The population-center distance, the length from tiie reactor to the nearest boundary of a population area containing more than twenty-five thousand residents, was defined either in terms of the projected effects in the event of a contained maximum credible accident or in terms of the distance that would prevent any lethal exposure in the event of a conceivable accident in which the containment was breached. In the most serious accident of the latter kind, the scenario assumed a complete puff release from the containment of 100 percent of the noble gases, 25 percent of the halogens, and 1 percent of the solids. Beck s staff fixed the population center distance at 133% percent of the evacuation distance. ... 

The maximum credible accident was a concept introduced in the draft to strike a balance between two extremes. If the worst conceivable accident was postulated (e.g., an uncontained meltdown as in WASH 740), only sites isolated from populated areas by hundreds of miles would offer sufficient protection. As noted earlier, this would have effectively precluded the commercialization of nuclear power. On the other hand, if engineered safety features (ESFs) to protect against all possible accidents were included in the facility design, then it could be argued that every site would be satisfactory. Of course, in the latter case no potentially serious accidents could be overlooked and the ESFs would have to be fail proof. Such omnipotence was not defensible. This led to the idea of designing for what was subjectively assessed to be the maximum credible accident. 

When 10 CFR 100 was issued (April 1962), the term maximum credible accident was dropped, but the notion was retained in 100.11 (a) and an associated footnote ... 

This maximum credible accident has, at various times, also been referred to as the design basis accident (DBA), the design-basis loss of coolant accident (LOCA), and the siting-basis LOCA. Given the rather prescriptive assumptions that evolved for demonstrating compliance with 10 CFR 100, the term design-basis LOCA is adopted here. This hypothetical accident is invariably initiated by the reactor-coolant system pipe break that would yield the highest containment pressure. 

The annual radiation exposure of nuclear workers is limited to 5rem, and sites for nuclear operations are selected so that in the event of the maximum credible accident the exposure of the public will be less than 25 rem. The levels of radiation exposure were first established on the basis of historical data, and since then they have been periodically evaluated by the International Commission on the basis of continuing experience. 

It has to protect the control room in the case of a maximum credible accident with fission products being released into the containment vessel so that limited operations should be possible in the control room during the first seven days after the accident. 

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