A10-1 Initial studies

The importance of a voluntary primary depressurization system in a PWR has been stressed many times in this book. It is an absolute requirement in a BWR in order to cope with the loss of the main condenser, given the fact that steam release to the outside is excluded for the radioactivity content of the reactor water. 

This system was not only a primary depressurization system, as it also included a subsequent passive water injection function in the primary circuit (low pressure and small flow rate) for the long-term refrigeration of the core. 

The system operation does not exploit gravity, which is the type to be preferred, and has been replaced, for the borated water injection (accumulators), by gas under pressure. In fact, where significant pressures are needed, gravity can only be employed on sites having a particular topography, as in the case of the SENA power station located inside a cavern in a hill (Chooz, Belgium). 

The line has an equivalent flow area corresponding to a circular opening of 150 mm diameter. It ensures a quick chain reaction shutdown by void formation in the core and a reliable depressurization (that is protected from the effect of partial plugging) down to pressures lower than 1 MPa, even without any other primary water cooling system, in a time of minutes. 

A series of three compressed gas accumulators and borated water at low pressure (1.8 MPa, relative), each connected to one of the three cold legs of the primary system. The volume of each accumulator is of about 333 m, 250 m of which are occupied by borated water at 2000 ppm boron. These accumulators are normally isolated from the primary system by non-retum valves only, as for the intermediate pressure (4.2 MPa, relative) accumulators commonly installed in PWRs. The connection lines with the primary system are of small diameter (approximately 50 mm), sufficient to supply, in case of primary depressurization, a slow and durable injection of borated water (indicative duration in typical cases 10 hours). Borated water injection performs the double function of maintaining the reactor sub-critical in the long term and of refrigerating it by flooding and vaporization (feed and bleed). 

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