The auxiliary and ancillary systems

In the operation of nuclear power plants a number of additional systems are needed to support the regular functioning of the main systems. These systems, which are mainly located in the reactor auxiliary building, essentially have to fulfil the following functions  

A schematic survey of the nuclear auxiliary systems in a Siemens 1300MWe plant is given in Fig. 1.10. in the following, these systems will be shortly described 

The boric acid and demineralized water control system is necessary to adjust the boron concentration in the reactor coolant system. To increase boron concentration in the coolant, boric acid solution is injected through the volume control system to reduce boron concentration, an adequate volume of coolant is replaced by demineralized water. This system (as well as the other systems discussed here) is a purely operational system and is not required for control of a design basis accident the extra borating system fulfils the necessary safety functions. However, to guarantee continued supply to the volume control system during emergency power operation, the pumps and motor-operated valves of the system are connected to the emergency power supply system. 

The gases released from the primary coolant in the degasification system mainly contain the fission product noble gases which, with the sole exception of Kr, are comparatively short-lived nuclides. In order to prevent release to the environment, therefore, it is sufficient to store them for a certain time until these isotopes have decayed. In most of the US PWR plants as well as in the plants built by Frama-tome, gas decay tanks are used for this purpose. In the plants designed and built by Siemens/KWU, decay lines are employed which are equipped with a series of charcoal beds in which the noble gases are delayed relative to the carrier gas flow by a dynamic adsorption-desorption equilibrium. Under normal operation conditions, delay times on the order of 60 hours for the krypton isotopes and 60 days for the xenon isotopes are obtained, which are sufficiently long for nearly complete 

During startup and control processes, coolant is pumped, after passing the coolant purification system, into the coolant storage system for interim storage. In the coolant treatment system, it is separated by evaporation into demineralized water and boric acid concentrate containing about 7000 ppm B. The distillate then is directed to the coolant storage system, while the boric acid concentrate is transferred to the boric acid and demineralized water control system, from where it can be fed into the primary system via the volume control system, when required. 

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