Patent Number: 051125650
Section: description

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention involves a control technique (algorithm) having time cycles which are short compared with delayed neutron lifetimes in nuclear reactors. A typical algorithm cycle in this case is of the order of 10 milliseconds. Even though the foregoing time cycle is short, an evaluation of the reactivity of the system or the instantaneous reactor period can be accomplished during same. In addition, the foregoing actuator or control technique (algorithm) can be utilized in conjunction with a "stepping drive" or actuator which can incrementally change a parameter by a fixed amount, either plus or minus one unit. The motion and/or position of the apparatus attached to the stepping drive or actuator can be used to change the reactivity of the nuclear reactor or some other parameter which affects reactivity. Thus, the foregoing control technique completes an evaluation of the state of the system in one time cycle and determines if the stepping drive should add, subtract, or make no change in the of the nuclear reactor reactivity. In effect, the total control action is completed in one time cycle. In addition, digital topology is provided as part of the foregoing control technique. Such topology permits the use of non-linear response functions in the determination of the required change in reactivity. A primary advantage of the present invention is a reduction in the number of algorithm calculations required to determine the proper control action to be taken. The short cycle times for the algorithm permit almost instantaneous correction of any errors. In addition, the use of a stepping drive or actuator eliminates the necessity for position or velocity feedback signals within the drive system. Each "step" uses the maximum capability of the actuator or drive. Furthermore, the use of digital topology based on pre-calculated functions permits the utilization of response functions which can be non-linear. In this case the system is designed to provide an output from a memory location which contains a pre-calculated value. The selection of the memory location is based on the value of an input parameter which has been converted to a digital address by conventional analog to digital techniques. Such techniques can be implemented without the use of a stored program computer system. Thus, the resulting algorithm can be implemented at significantly higher speeds and without the use of software programming. The result is a control system that is small, light weight and consumes very little power. Previous control technique have been based on the principle that when control action is required, reactivity changes should be made at the maximum rate available by the control mechanism. Typically, the control mechanism is "stepped" at its maximum rate until the reactor power is escalating on the desired period. The control mechanism then holds that period until the output from the DIGITOP (desired period) changes. The input to the DIGITOP can be reactor power, temperature of coolant, temperature of fuel or any combination of measured parameters. The control technique of the present invention produces a smooth start-up that minimizes the reactor startup time, within the constraints of the control element worth curve and the maximum speed of the control actuators. The control technique also minimizes both the motor torque requirements of the stepping motors and the number of calculations required in the control computer. These modifications provide a smooth, start-up without overshooting either the full power level or the limiting reactor start-up rate, see FIG. 3. The control technique of the present invention has been utilized with a simulation model of a nuclear reactor. The simulation used the reactivity worth curve established by the control elements as a function of position, and the results of the simulation are illustrated in FIG. 3. In this case, the control drive mechanism used a cycle time of 10 milliseconds which is the cycle time for the control computer. Until the reactor reached the demand period, the control drive mechanism was being moved one increment for each cycle. Certain modifications and improvements will occur to those skilled in the art upon reading the foregoing. It should be understood that all such modifications and improvements have been deleted herein for the sake of conciseness and readability, but are properly within the scope of the following claims.