Design basis accidents are postulated accidents to which a nuclear plant, its systems, structures and components must be designed and built to withstand loads during accident conditions without releasing the harmful amounts of radioactive materials to the outside environment. Any DBA is controlled by the reactor safety systems with insignificant off-site consequences, but may require long shutdown for correction or repair.
The concept of DBA is very useful both for normal and abnormal operation. Design basis accidents are used in the design of a nuclear power plant to establish the performance requirements for reactor structures, systems and components.
More serious accidents that may involve significant core degradation and/or pose the real danger of a significant release of radiation to the environment are classified as beyond DBA or severe accidents. These accidents have an extremely low probability of occurrence, so low that full control of them is not considered in the design.
A set of design basis accidents is postulated for each type of reactor, covering the consequences of all combination of failures. According to the “Typical Contents of Technical Justification of Nuclear Power Plant Safety” [5] the following groups of DBA are considered for the RBMK-type reactors:
VI. Other accidents.
Design basis accidents are classified by type of initiating events. Reactivity initiated accidents cover such initial events as single rod withdrawal or voiding
of control rod channel cooling system. Single or multiple main circulation pump trip are examples of loss-of-flow transients. Loss-of-coolant accidents include full or partial breaks of group distribution header, breaks of main steam line in different locations, etc. Loss of preferred power or turbine trip can be referred to accidents initiated by equipment failure. Fuel handing accidents cover failures during refueling machine operation. Flooding, fire, earthquakes are examples of the last group of accidents referred as other accidents (external events).
The design limits prescribe that for any DBA:
According to the latest safety regulations issued in 1988, even in the case of double-ended break of a pipe with the maximum possible diameter, certain limiting conditions must be met without excessive consequences to the environment. For the Ignalina NPP, an instantaneous guillotine break of the MCP pressure header which internal diameter is 900 mm with unimpeded discharge of coolant from both ends of the pipe during the unit operation at full power is taken as the ultimate design basis accident. In accordance with the single failure requirement, this event is assumed to coincide with a failure of a check valve in one group distribution header to close. The analysis performed for ultimate design basis accident has shown that design limits will not be exceeded.