Criticality Safety Management

Abstract

CRITICALITY SAFETY MANAGEMENT Nuclear criticality safety can be defined as “the protection against the consequences of an inadvertent nuclear chain reaction, preferably by prevention of the reaction’’. The primary objectives of criticality safety evaluations are to prevent nuclear sustaining fission chain reaction, to curtail the consequences of the criticality accident if it were to occur and to ensure that an adequate level of safety measures required to control criticality accident are put in place. Also, that the criticality safety assessment report is documented in compliance with safety requirement. In general, critical excursions display a sharp power spike followed by a plateau that may be interrupted by smaller power spikes. The spike is the initial power pulse of a prompt power excursion, as is limited by the shutdown or quenching mechanism, this quenching mechanism includes thermal expansion, boiling, and radiolytic gas bubble formation. Criticality “near misses” are incidents or accidents that have not occurred but would have if the potential failure of criticality control had not been recognised. These reported incidents are very useful as lessons and are by far better than a full blown nuclear criticality accident. Such incidents include the occurrence at the Los Alamos Scientific Lab on the 21st of august 1945 and the JCO Fuel fabrication plant, Tokaimura incident on the 30th of September 1999. This incidents recorded casualties on the operators and those close to the site of the nuclear criticality incident. Furthermore, there are factors which affect nuclear criticality, namely; absorption rate, geometry, concentration, enrichment and reflection among a host of other factors which are interlinked. No doubt all these factors which are at play are also incorporated in the criticality safety design. The possibility of a criticality excursion is best considered (but not always possible) during the design stage of a facility as it is more reliable to design safety into equipments than to predicate it on process or procedures to which the fatal flaw of human error may upset the balance and mistakenly engineer a nuclear criticality safety incident with potentially disastrous outcomes. Engineered controls can include safe geometry containers, small volume vessels, spaced arrays, etc. Two or more engineered controls may be employed in a facility, ideally controlling independent routes to criticality and this independence is the basis of the double contingency philosophy. In addition, a Criticality Safety Assessment is necessary to provide a framework that should be followed to ensure nuclear criticality safety. A nuclear criticality safety assessment is meant to be documented with sufficient detail, clarity, and lack of ambiguity to allow independent judgment of results. In conclusion, criticality accident risks will not disappear as long as a significant quantity of fissile material exists. However, with appropriate support from management, diligence on the part of criticality, staff and operators, and adherence to codified fundamental safety principles and guidance, accident likelihood can be maintained at the current low level or possibly reduced further. It should also be noted that criticality models do not always give the right answer and that criticality can occur in ways that may, at first glance, seem counterintuitive. The above reinforces the need for a criticality specialist to deeply understand the physics of the system they are implementing or specifying criticality controls. Mohammed S. Saba