15.65. The PIUS system features a very simple PWR design that is claimed to be so inherently safe that it does not require injection by passive systems to prevent a serious accident. A major feature is the submergence of the entire primary system, including the steam generator, in a very large pool of relatively cool, borated water. This pool is inside a large pre­stressed concrete vessel which serves as both the pressure boundary and containment.

15.66. As shown schematically in Fig. 15.7, during normal operation, primary coolant moves up through the core, through a tall “chimney,” and into a hydraulic interface region, from which it is pumped (recirculated) to the steam generator. On leaving, the coolant passes through a down­comer duct to a plenum under the core, below which is another hydraulic interface region.

15.67. The recirculation pumps are operated so that the pressure drops of the water flowing through the core results in a hydraulic balance pre­venting the pool water from mixing with the coolant water through the lower interface. If the circulating rate drops or the core temperature in­creases, the pressure balance will be upset and the borated pool water will flow automatically up into the core, shutting down the reactor and cooling it. Decay heat is removed by allowing the pool water to boil. Sufficient water is initially present so that the core will remain covered for about a week.

15.68. Since the design is still being developed, it is premature to pro­vide even preliminary specifications. Several design options are under con­sideration, including one in which three core modules are submerged in a single pool contained within a prestressed concrete vessel. Questions of cost, licensability, maintainability, and operability require resolution. For example, concern has been expressed regarding the effect on plant avail­ability factor of the time required to restart the plant after inadvertent shutdowns. A prototype plant would be needed to prove the feasibility of the concept.