Maximum credible event

This same model is used to calculate the maximum credible event (MCE) which is defined as that unintended, unplanned, or accidental adverse occurrence which causes release of agent from an ammunition item, bulk container, or process. It must be realistic with a reasonable probability of occurrence. It is necessary to hypothesize an MCE to enable calculation of the magnitude of a worst case hazard. The hypothetical MCE for any given situation will be based upon the nature and characteristics of the agent involved, ammunition, container, configuration, and... 

The DOD Ammunition and Explosives Safety Standards DOD 5154.4S describes standards applying to combined toxic chemical and explosive hazards to a maximum credible event and its effect on hazard zone distance calculations. Also within the standards are found safety criteria relating to selection of the type of operational containment required, total versus vapor containment. 

None of the munitions at Pueblo contains M28 propellant therefore, this incident has no direct bearing on planned disposal activities at Pueblo. However, the incident does show that even though the maximum credible event may not result from every process upset, sound engineering practices must always be used. This will reduce the likelihood of an accident and mitigate the consequences of accidents that do occur. The incident also highlights the need for training personnel involved in such operations to become aware of all possible hazards. 

A consequence-based approach that takes into consideration the impact of explosions, fires, and toxic releases based on maximum credible events for each budding and type of hazard considered. 

Can equipment be designed sufficiently strong to totally contain the maximum pressure generated, even if the worst credible event occurs ... 

Normal loss prevention practices are to design protection measures for the worst case fire event that can occur at the facility. To interpret this literally would mean that an oil or gas facility is completely on fire or totally destroyed by an explosion. Practical, economical and historical review considerations indicate this rational should be redefined to the Worst Case Credible Event (WCCE) or the as referenced in the insurance industry, the Probable Maximum Loss (PML), that could occur at the facility. 

In an effort to increase the reaction rate of the energetic materials in the hydrolysis reactor, strong caustic solutions (pH greater than 12), elevated temperatures (60°C to 155°C [MOT to 311°F]), and elevated pressures (up to 14 atm) have been used. The hydrolysis reaction is exothermic, so process controls are necessary to maintain the reactor temperature and respond to thermal excursions in order to prevent a runaway reaction. Because upsets are always possible, the reactors and containment buildings must be designed to contain the maximum credible explosive event. ... 

An accident scenario is one of many specific situations that might evolve at the end of an accident sequence. Here, we have considered eight accident scenarios of Vapour Cloud Explosion (VCE), Boiling Liquid Expanding Vapour Explosion (BLEVE) and toxic release events originating from different hazardous industries located in the study area (Fig. 2). The as reference scenarios were selected with an inherent assumption of having the potential of causing Maximum Credible Loss (MCL) (Khan and Abbasi, 2002) as identified on the basis of facility level risk assessment (Table 1). 

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 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 is required that the structural integrity and the safe shutdown functions of the neutron control assemblies are maintained for the OBE and SSE events. At the OBE level, the neutron control assemblies must be able to perform their safety function. Additionally, the ability to perform their power generation function during and after the earthquake should also be oiaintained. Their operation must be unaffected by any credible misalignment of the core control channels due to core deflections as a result of the seismic disturbance. A maximum misalignment of 7.6 cm (3 in) is allowed. At the SSE level, the neutron control assemblies must retain their safety function during and after the earthquake. The failure modes which could cause the seismic requirements not to be satisfied are identified as follows ... 

Risk has different types, such as individual risk, societal risk, ecological risk, and economic risk. Risk can also have different meanings, such as the maximum thinkable (probable or credible) outcome of an undesirable event, worst-case outcome, and most... 

The emphasis in design has been to incorporate inherent and passive safety features to the maximum extent as a part of the defence in depth strategy. The main objective has been to establish a case for the elimination of evacuation planning following credible accident scenarios in the plant. Another major objective has been to provide a grace period for the absence of any operator or powered action in the event of a credible accident scenario. 

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