Secure Reactors

Physics of nuclear reactor safety

“…The primary risk source in a nuclear reactor is the large radioactive inventory that is built up in the core during operation. In case of a large power plant with an electrical capacity of 1OOOMW this inventory comprises about 1000 MCi of radiologically important fission products or, in SI units, about 4 x 10'' Bq (Pershagen 1989). If even a small fraction of this inventory were released to the environment, this would pose a considerable danger to the biosphere.…”

“…Detailed tables of decay heat for different nuclides have been published (American Nuclear Society 1979). For safety purposes the decay heat is usually calculated on the basis of a simple formula (Pershagen 1989). If the irradiation time at full power is T (s) and the cooling time, i.e.…”

“…A safety aspect of every 'movable' control system is that it might add unwanted reactivity in case of failures (section 3.3). The scenarios of uncontrolled withdrawal, control rod ejection or control rod drop (in a boiling-water reactor where the control rods are inserted from below the core) are standard parts of every deterministic safety analysis (Pershagen 1989), see also section 5. Unwanted reactivity addition might also occur in case of chemical shim, albeit generally on a much slower time scale, thanks to the large mass of borated water in the primary system; the inadvertent addition of pure water to the primary system can lead to a 'clean-water incident' or 'inadvertent dilution incident' (Pershagen 1989).…”

“…It led to a strong development of analytical methods for risk analysis and implementation of techniques for safeguarding of nuclear power plants. These techniques have reached a high degree of sophistication and are based on the application of principles like redundancy, diversity, defence-in-depth, safe failure etc (Pershagen 1989). In this way a high level of safety was reached in modem nuclear power plants.…”

“…Both can be clearly distinguished from active safety features which require activation and energy to be operated. An example of such a feature is the extensive emergency cooling system present in modern nuclear power plants (Pershagen 1989). It comes into operation after being triggered by signals which in their turn are triggered by devices that detect unacceptable deviations of process parameters from normal values.…”