Patent Number: 044709480
Section: summary

BACKGROUND OF THE INVENTION This invention relates to nuclear-reactor power apparatus and has particular relationship to the operation of nuclear reactors under water-solid conditions. In nuclear reactors of the pressurized water type, the coolant, usually water, is at a temperature and pressure near the critical temperature and pressure. Typically the temperature is about 580.degree. F. and the pressure in excess of 2000 pounds per square inch. The pressure is maintained by a pressurizer into which the coolant expands. The coolant flows in a loop through a steam generator which contains a fluid to which the coolant transfers heat. The fluid in the steam generator is here called secondary fluid. Depending on the number of steam generators in nuclear power apparatus, there may be a corresponding or larger number of reactor coolant loops. There is usually only one pressurizer. During normal operation, the coolant in the pressurizer is at a predetermined water level and above this level there is a bubble or volume of vapor or steam which is essentially a pressure cushion. The desired pressure of the coolant is maintained by the vapor cushion. When the reactor apparatus is shut down, the pressurizer becomes filled with coolant, the coolant rises above a predetermined, highest measureable level confining the the bubble to a small volume. When the coolant is above this level, the nuclear-reactor power apparatus is said to be in a water-solid condition. This invention concerns itself particularly with the condition of the power supply apparatus during shut-down after the control rods have been inserted in the core and the steam has been or is in the process of being vented from the pressurizer. Under these conditions the apparatus components may be hot for a considerable time interval. The permissible pressure of the coolant for a nuclear reactor is set by Appendix G to 10 CFR 50. Overpressure excursions which exceed this permissible pressure caused by malfunction or operator errors have been experienced during operation of the nuclear-reactor power apparatus in a water-solid condition. Such an overpressure excursion may arise from the addition of mass to the coolant. For example, the letdown valve may be isolated or closed while the coolant-charging pump or pumps continues to operate or the safety injection pump may be inadvertently put into operation. Such overpressure excursions may also occur by reason of addition of heat to the coolant. For example, the pressurizer heaters may be inadvertently energized. Or during shutdown, upon loss of residual heat removal capability, decay heat may be transferred from the core to the coolant. Or with the coolant pump started and a temperature difference between the warmer secondary fluid and the colder coolant, with the secondary fluid of the steam generator at a higher temperature than the coolant. Or between the warmer coolant and the colder coolant cold-leg loop seal, heat may be transferred from the warmer to the colder medium. The latter conditions which occur during cold shut-down of the nuclear-reactor power supply has been found to be a highly significant contributing factor to overpressure occurrences in a water-solid condition. To mitigate the consequences of water-solid overpressurization of the coolant, the practice, in accordance with the teachings of the prior art, is to open the pressurizer power-operated or power-actuable relief valve. This valve is operated and when open relieves some of the coolant in the pressurizer. The practice in accordance with the teachings of the prior art is to apply the necessary power to open the valve. This practice has proven unsatisfactory. It has been found that the pressure increase in the coolant is not arrested sufficiently in all cases below the limit defined by Appendix G but that the pressure swings above this limit producing the undesired condition. It is an object of this invention to overcome the disadvantages of the prior art and to provide a method and apparatus in whose practice and use overpressurization in the water-solid state or condition of nuclear-reactor power apparatus shall be effectively suppressed. SUMMARY OF THE INVENTION This invention arises from the realization that the tendency, observed in the practice of the prior art, of the pressure in the coolant to overshoot the limit defined by Appendix G in the water-solid condition results from the operating characteristics of the power-actuable relief valve. Predominantly the failure to arrest the transient increase in pressure due to the addition of mass or heat to the coolant below or at the limit defined by Appendix G results from the delay in the valve opening after the the valve has been given a signal to open at setpoint. This delay is caused by the time taken for pressurization of the diaphragm chamber of the valve prior to valve stem movement. During the delay interval, mass or heat continues to be added to the coolant and thus produces the valve set-point pressure overshoot. This pressure overshoot occurs regardless of the number or size of the power-actuable relief valves included in the apparatus. Modification of relief-valve design capable of restricting the pressure overshoot below the limit set in Appendix G has not been proposed. In accordance with this invention overpressurization of the coolant of nuclear-reactor power apparatus in the water-solid state is effectively suppressed by anticipatory control of the power-actuable relief valve or relief valves. This anticipatory control responds both to mass input and to heat input into the coolant in the water-solid state. For mass input, reliance is placed on the time rate of increase in coolant pressure coupled with other conditions of the apparatus. As a necessary condition for the opening of the power-actuable relief valve, the rate of increase in coolant pressure in the water-solid state must exceed a setpoint for a predetermined time interval sufficient to prevent actuation for normal transients. The other conditions which must be satisfied are that the coolant temperature must be below a prescribed setpoint below which coolant overpressurizations are possible and that the level of the coolant in the pressurizer must be above the setpoint level which marks the possible existence of a water-solid state. Overpressurization responsive to heat input is also effectively suppressed by anticipation of the overpressure condition before it occurs. The signals relied upon to indicate heat-input overpressurization and to actuate the valve are: reactor-coolant pump start up, coolant temperature, pressure or temperature of the secondary fluid, and the highest indicated pressurizer level above which the a water-solid is possible, herein referred to the water-solid set point. The signals are continuously evaluated only during a predetermined interval after the coolant pump has been started. The evaluation takes place for each reactor-coolant loop and its associated steam generator. The conditions which must be met to identify overpressurization and actuate the relief valve are: the difference in the temperature between the secondary fluid and the coolant exceeds a setpoint or the difference in temperature between the coolant and the coolant pump loop seal exceeds a setpoint, the coolant pump has started to operate but has not been operating for more than a predetermined interval, the water level in the pressurizer is above the setpoint, and coolant temperature is below a setpoint. The secondary fluid temperature may be derived by measuring the temperature directly or by measuring the pressure and converting it to temperature (under vapor-liquid phase equilibrium conditions). The existence of a difference which exceeds the setpoint between the temperature of the secondary fluid and the temperature of the coolant indicates that if the coolant pump is started, heat capable of overpressurizing the coolant will be introduced. The temperature difference is manifested by a logic signal. A similar logic variable can be generated by comparing to a setpoint the temperature of the coolant. The signal indicating that the pump has started is manifested by one or more of the circumstances that: 1. The pump breaker is closed and there is voltage across the pump-supply bus. 2. Bus frequency exists. 3. Pump speed is established. 4. Reactor coolant flow is established. The signal indicating that the pump has started persists for a predetermined time interval to allow the relief valve to be open until thermal equilibrium is established between the coolant and the secondary fluid. The mass input logic described above augments the pump-started logic and provides a redundant signal for opening of the relief valve for overpressure control later in the transient which produces the overpressure. The command to actuate the power-actuable relief valve responsive to mass input or heat input or both is impressed on a comparator in the relief-valve control. The command from the proportional plus integral plus derivative controller, normally utilized in the control of coolant pressure of nuclear power supply apparatus is also impressed on this comparator. The comparator responds to the highest of the commands generated to open the relief valve.