Defence in Depth of VVER-440 Reactors

Defence in depth is one of the most important principles, since it underlies the safety technology employed in nuclear power plants. All safety activities, whether organisational, behavioural or equipment-related, are subject to layers of overlapping provisions which are designed to ensure that if a failure should occur it would be compensated for or corrected without causing harm to individuals or the public at large. This idea of multiple levels of protection is the central feature of defence in depth, and it is repeatedly used in the specific safety principles applied in nuclear power plants. Two (related) principles of defence in depth are defined accident prevention and accident mitigation [1]. 

The defence in depth concept provides an overall strategy for safety measures and features of nuclear power plants. When properly applied, it ensures that no single human or mechanical failure leads to danger to the public, and even combinations of failures that are only remotely possible would lead to little or no injury. Defence in depth helps to establish that the three fundamental safety functions (controlling the power, eooling the fuel and confining the radioactive material) are preserved, and that radioaetive materials do not reach people or the environment. 

The principle of defence in depth is implemented primarily by means of a series of barriers which should in principle never be jeopardised, and which must therefore be violated in turn before harm can occur to people or the environment. These barriers are physical, confining the radioactive material at successive locations. The barriers may serve operational and safety purposes, or may serve safety purposes only. Power generation is only allowed if this multibarrier system is not jeopardised and is capable of functioning as designed. 

All of the levels of defence are available at all times when a plant is operating at normal power. The existence of several levels of defence in depth is never a justification for continued operation in the absence of any other level(s). Severe accidents in the past have been the result of multiple failures, both human and equipment failures, due to deficiencies in several components of defence in depth that should not have been permitted. 

Competent engineering of the barriers and protective measures for them, coupled with feedback to maintain operation within the optimal range, lead to a record of smooth, steady performance in producing electricity on demand. Such a record indicates the proper implementation of one of the most important indicators of the success of defence in depth operation with little or no need to call upon safety systems. 

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Analysing the existing defence in depth system for VVER-440 reactors from a materials point of view and improving it based on national and international experience and trends ... 

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