Primary/integral redundancy

Cooiing method internai ciosed ioop water cooied circuit, with integrated water cooiing unit, inciuding redundant pump set, water to water heat exchanger and three-way vaive in primary side to controi internai circuit temperature in order to avoid condensation. 

Various failure and degradation mechanism have been identified that adversely impact the integrity of bolts used in safety related applications and in applications important to safety. Depending on the nature of the degradation mechanism a potential for common mode failures exists for same system or redundant system components. In addition, leaks from flanged joints represent a significant part of total number of leaks in primary circuit. 

Defence in depth is assured by redundant and diverse barriers to the release of radionuclides from the fuel, in combination with provisions to ensure the integrity of the barriers. The barrier structure is based on the properties of the fuel to retain a significant fraction of each of the radionuclides deposited in the fuel pellet, and mainly, on the prevention of radioactivity release to the primary coolant by fuel rod claddings. For this purpose, the absence of pellet-clad mechanical interaction is ensured during the fuel lifetime and in accidents, to reduce the probability of clad failure. An additional barrier preventing the release of radionuclides is the two-circuit NSSS scheme. 

Primary/integral redundancy. This is often applied when it is the only practical fail-safe method possible. 

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