Overview

It is worth mentioning and discussing some beliefs prevalent in the field of nuclear safety.

A shutdown plant cannot have an accident!

The opposite is true, since the probabilistic safety analyses addressing this problem have concluded that a large part of the risk of a nuclear plant is related to plant situations of shutdown or low power. A plant is shutdown for inspection and periodic maintenance, and often safety systems are disabled, the containment opened, and unusual operations are performed which decrease the usual defences, so that accidents are possible which could not happen in other conditions.

In a pressurized reactor, a solid system has to be avoided by all means!

A solid system , in the jargon of PWR operators, is a primary cooling system completely filled with water, that is without the steam bubble in the pressurizer.

In solid system conditions, the pressure resisting structures of the primary system are in effect exposed to undue overstressing as a compressible element in the fluid part of the system is lacking: one can think of an effect of local overheating and consequent thermal expansion of the fluid, or of the start up of a high head pump connected with the primary system, etc.

Operators are warned about the danger of a solid system condition during their training. Experience indicates that sometimes the risks of this operating condition are exaggerated, almost identifying it with a situation of unavoidable accident to the primary structures. It must be remembered that, during the Three...