Cooling system protection principle

Freezing point depression is the principle behind the use of antifreeze in motor vehicle cooling systems. Water normally freezes at 0°C (32°F). In many places in the United States and Canada, winter temperatures fall well below 0°C. Since water expands as it freezes, a car or truck s engine block and radiator would crack. A 50/50 mixture (by volume) of water and antifreeze, though, provides protection to -34°C (-29°F), which is sufficient for most places that experience below freezing temperatures. If temperatures might get even lower, a mixture of up to two-thirds antifreeze and one-third water provides protection to temperatures of -90°F (-68°C). 

In addition, it should be demonstrated analytically that the mechanical systems can withstand a single active failure including failure of any auxiliary electric power source and not prevent delivery of sufficient cooling water to maintain the plant in a safe shutdown condition. A technique suitable for this analysis is a Failure, Modes, and Effects Analysis (FMEA). IEEE Std. 353-1975, "Guide for General Principles of Reliability Analysis of Nuclear Power Generating Station Protection Systems," provides additional guidance on the preparation of FMEAs. 

The intrinac self-protection of the reactor, the use of self-actuating systems for shut down and cooling of the reactor in combination with passive principles of action ensure immunity to operator errors and equipment failures. 

During BMN-170 development, the design principles to provide plant safety was aimed at an optimum combination of reliance on intrinsic safety features and application of engineered (active and passive) systems. To be specific, the protection system being developed employs hydraulically suspended reactivity control rods that effectively influence the reactivity and convert the reactor to a sub critical state when flow rate is reduced through the core [XXI-9]. Use of a passive emergency cool down system allows complete removal of residual heat. 

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