Credible event

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. 

Several additional factors are important when considering the worst case credible event. 

Before safety measures are applied to a facility, it is prudent to identify and evaluate the possible hazards that may evolve before spending considerable amounts on protection that may not be needed or overlooking requirements for protection measures that are needed. The first step in fire protection engineering should therefore be to always identify the major risks at a facility. When conducting these analyses it is prudent only to only consider credible events. Farfetched or outlandish event considerations (e.g., a meteor striking the facility) are not necessary or practical and lead to a less cost effective approach. 

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. 

The reader should note that a fault tree includes the following (1) works backward from an undesirable event or ultimate consequence to the possible causes and failures, (2) relates the occurrence of an undesired event to one or more preceding events, (3) chain links basic events to intermediate events that are in turn connected to the top event, (4) is used in the calculation of the probability of the top event, (5) is based on the most likely or credible events that lead to a particular failure or accident, and (6) analysis includes human error as well as equipment failure. 

As in the long film, the tone is far from realistic and in fact is quite free—it can be fantastic, or it can be very dark. In both cases, the tone has to make credible events and characters that do not exist in our everyday lives. The presentation of the material should sharpen the passion the author feels regarding the moral tale that underlies the narrative. 

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. 

The potential for a criticality accident in the HCF has been formally addressed in a criticality safety assessment (CSA) (Mitchell and Romero 1999). This assessment concluded, based on the quantities of fissile materials and the physical and administrative features implemented to handle fissile materials, that criticality is not a credible event in the HCF. 

Figure 14.2 shows the partially complete what-if analysis worksheet for the vapor degreaser facility. Note that many more what if questions can (and should be) asked to completely exhaust all possible events that could affect the safety of overall system operation. Quite typically, a properly completed what-if analysis will address many dozens of individual items and encompass numerous worksheet pages before all credible events have been evaluated. The intent of Figure 14.2 is to... 

The physical configuration of the reactor power module assures decay heat removal by passive means in the event that all normal heat sinks are lost, without the action of the operator or any active system, for all credible events. 

LEADIR-PS 200 does not pose a safety concern to the public for any credible event events that are a risk for other reactor types are non-concems. Coolant channel blockage for example, although very unlikely, does not have significant consequences. Even if all coolant chaimels in a fuel column were blocked peak fuel temperatures in the block (at full power) would only teach about 1000°C, well below the threshold for fuel failures. 

A directly-initiated hydrogen detonation in the APIOOO containment is not a credible event. 

Much discussion could be presented as the most credible worst case event at the facility. Obviously a multitude of unbelievable events can be postulated (industrial sabotage, insane employees, plane crash impacts, etc.). Only the most realistic and probable events should be considered. In most cases, historical evidence of similar facilities is used as a reference for the worst case events. Alternatively, the effect of the most probable high inventory hydrocarbon release could be postulated. The worst case event should be agreed upon with loss prevention, operational and senior executive management for the facility. The worst case credible event will normally define the highest hazard location(s) for the facility. From these hazards, suitable protection arrangements can be postulated to prevent or mitigate their effects. 

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