Inherent Safety Concepts

After Three Mille Island-2 and Chernobyl-4 accidents within the nuclear reactor society in the world there are groups who are developing ideas of a new generation of reactors which is characterized by a much higher standard of safety incorporating forgiving reactor system and inherent safety design. This set of inherent characteristics by definition shall tolerate any mistake occured during operation. It can mean that in an accident condition the inherent characteristics could allow enough time for the operator to correct the mistakes, could respond to neutralize the hazard, and could be left safely without any human interference.

At present it has been recognized that inherent safety characteristics are the dominant advantage of High Temperature Gaz Cooled Reactors which incorporates multicoated ceramic particle fuel, inert helium coolant, high thermal capacity and good convective heat transfer whereas, PIUS design concept are manifesting the solution for light water reactor versions.

It is generaly accepted among the nuclear reactor designers that the technology of "next generation" reactors must provide a guarantee that the core degradation accident and consequences risk of serious radiation releases will not occur. A core degradation accident can be defined as one where there is widespread break of the cladding and large scale release of fission product inventory to the coolant. The upper end of this accident is of course a complete core melt.

Many ways can be undertaken to prevent a core degradation accident. The following two conditions must be fulfilled :

• keep the core submerged in water at all time to maintain integrity of fuel

• make sure that heat production does not exceed cooling capability of water

• This two conditions could be achieved by placing the core within the neutron poisonned coolant pool. The last will submerges the core when needed by passive nature. Core cooling is effected by evaporation this water.

In the other point of view, in order to eliminate one the most important initiating event of the core degradation accident, i. e. loss of primary coolant accident, the integration of Steam Generators and pressurizer in the reactor pressure vessel has been considered. In such reactor design, due to the absence of large diameter external piping associated to primary system, no large break LOCA has to be handled by the safety system.

Moreover, the simplification design is also considered in some integral type reactors by eliminating control rods. The power output controlling is replaced by adjusting the flow rate and the concentration of boric acid solution. In this case, no reactivity initiated accident should be considered any more.

This new design, which is a complete departure from current Water Cooled Reactor plants, exhibits a remarkable inherent safety characteristics for all accident sequences. However, it creates a new set of complex machinery and operational problems of its own.

Table 2 shows the features of four integral reactor design concepts.