Abstract: A gravity driven cooling system pool is disposed at an elevated location with respect to the locations of nuclear fuel rods in a pressure vessel. In the event of a loss of coolant in the pressure vessel, steam pressure is initially reduced by venting the steam into the containment or a closed suppression pool containing a quantity of water under a large air space. The suppression pool condenses sufficient steam to lower the steam pressure in the pressure vessel so that water can flow by gravity from gravity driven cooling system pool to flood the fuel rods in the pressure vessel. An isolation condenser is submerged in a large supply of water elevated with respect to pressure vessel. Steam is admitted to the isolation condenser, or heat exchanger, where it is cooled by boiling the water surrounding it. This steam is vented to the atmosphere. A depressurization valve vents steam fro the pressure vessel into the containment to aid pressure reduction, and thus to aid the gravity flow of coolant.

Abstract: A method and apparatus are disclosed for monitoring the operation of a boiling water nuclear reactor for the occurrence of a superheated condition. The invention provides redundant monitoring of the level of coolant water in the reactor independently of existing water level gauges. A closed system vapor pressure device, located outside the reactor, probes the temperature of the reactor steam. The temperature of the steam determines the internal vapor pressure developed in the device, the latter being directly compared, externally of the reactor, to the steam pressure in the reactor vessel. The differential between the compared pressures is an indication of a superheated reactor condition and, hence, of a low water level in the vessel.