in the pressurizer are turned on, raising the coolant temperature in the pressurizer and thereby raising the pressure. To decrease pressure in the reactor coolant system, sprays of relatively cool water are turned on inside the pressurizer, lowering the coolant temperature in the pressurizer and thereby lowering the pressure. Secondary functions The pressurizer has two secondary functions. Water level monitoring One is providing a place to monitor water level in the reactor coolant system. Since the reactor coolant system is completely flooded during normal operations, there is no point in monitoring coolant level in any of the other vessels. But early awareness of a reduction of coolant level (or a loss of coolant) is important to the safety of the reactor core. The pressurizer is deliberately located high in the reactor containment building such that, if the pressurizer has sufficient coolant in it, one can be reasonably certain that all the other vessels of the reactor coolant system (which are below it) are fully flooded with coolant. There is therefore, a coolant level monitoring system on the pressurizer and it is the one reactor coolant system vessel that is normally not full of coolant. The other secondary function is to provide a "cushion" for sudden pressure changes in the reactor coolant system. The upper portion of the pressurizer is specifically designed to NOT contain liquid coolant and a reading of full on the level instrumentation allows for that upper portion to not contain liquid coolant. Because the coolant in the pressurizer is quite hot during normal operations, the space above the liquid coolant is vaporized coolant (steam). This steam bubble provides a cushion for pressure changes in the reactor coolant system and the operators ensure that the pressurizer maintains this steam bubble at all times during operations. Allowing liquid coolant to completely fill the pressurizer elimates this steam bubble, and is referred to in industry as letting the pressurizer "go hard". This would mean that a sudden pressure change can provide a hammer effect to the entire reactor coolant system. Some facilities also call this letting the pressurizer "go solid," although solid simply refers to being completely full of liquid and without a "steam bubble." Over-pressure relief system Part of the pressurizer system is an over-pressure relief system. In the event that pressurizer pressure exceeds a certain maximum, there is a relief valve called the pilot-operated relief valve (PORV) on