Aspects of unexpected events in nuclear power plants

Unexpected events in nuclear power plants (NPP) may lead to upset conditions or even accidents. Events such as these affect not only safety, but also the economic viability of NPP operation. Another facet of such events, virtually irrespective of their degree of severity, is the generally negative impact on public acceptance of nuclear power, such as was seen as a direct result of the Three Mile Island (USA - 1979) and Chernobyl (Ukraine - 1986) accidents. The operators of NPPs are responsible for their safe operation, whilst regulators ensure that NPP operating practices (e.g. start-up, shut-down procedures, inspections, monitoring, and compliance with technical specifications (TS)) are such that the highest possible levels of safety are a priori present at all times. As a matter of engineering principles, designs of NPPs feature safety margins and they are based on conservative assumptions, mostly to allow for material response to the operating conditions and environment (e.g. neutron embrittlement, fatigue usage). Inspections and monitoring have the purpose to check whether systems structures and components (SSC) are behaving according to the design with regard to compliance with safety requirements even when 'aged'. The paper examines aspects concerning events or accidents in NPPs, despite generally high levels of SSC monitoring and inspection and regulatory oversight. The importance of materials selection at the design stage, and the need for vigilance and questioning attitudes is stressed. The necessity to learn from accidents or events that have occurred in other NPPs is shown to be an important tool and source of information for NPP designers, manufacturers, operators and regulators. (author)