Releases and Dsperions: Mechanisms of Releases: The potential of many hazardous substances to cause harm can only be realised via a RELEASE of energy or of the substance to the surrounding environment. This can arise from failure of the containment system designed to hold the substance in a safe condition. Releases fall in a spectrum from deliberate and controlled discharges necessary for the operation of a process to inadvertent and uncontrolled escapes. The possible effects of all such releases should be considered, but it is particularly important to identify initiating events, which may cause inadvertent containment failure leading to an uncontrolled release. Initiating events fall into two general categories, those internal and those external to a system. Internal causes may be subdivided broadly as those arising from departures from design conditions during operation (e.g. overheating); failure of equipment operating within design conditions (e.g. due to defective or incorrect materials of construction) or from human error in operation. External causes can be similarly subdivided; examples being failure from mechanical damage, natural hazards, external corrosion, and domino effects (i.e. events arising at one plant affecting another). Releases from containment systems range from slow discharge through a small pinhole failure to rapid discharge resulting from a major break. Various mechanisms, such as fatigue, creep, or stress corrosion, may cause cracks or defects to grow, possibly leading to a through-wall failure and therefore a release. If the effect exceeds certain critical proportions, which depend on a number of factors, a propagating Fracture may rapidly result in a major failure. For example, perforation of an underground pipeline could result from a third party activity, such as digging with a pneumatic drill. Such Punctures may lead to a propagating fracture under certain conditions. The type of fracture may be described as brittle or ductile. A complete failure of pipework resulting in discharge from two open ends of pipe is often described as a Guillotine failure, a term which is usually used in the description of hypothetical failure cases. A sudden and severe failure of equipment, possibly with division into a few or many pieces and resulting in a rapid release of the contents, is often referred to as a Catastrophic Failure. However, a catastrophe, as such, is not necessarily the outcome in terms of damage, other than that suffered by the equipment itself. It is useful to have such a term to describe a variety of very serious equipment failures, but because of the subjective and possibly misleading interpretations which may be derived from the term catastrophic it is recommended that it is only used in conjunction with the term failure and with specific reference to the equipment concerned. Another term encountered is disruptive, which also implies breaking open of equipment, and which is often used synonymously in this context.