New Control Techniques for Nerva Reactors

The development of the NERVA reactor has reached the point where it can be operated predictably for sustained periods of time. This success in mechanical and nuclear design has given to the control engineers the opportunity to design and experimentally verify improvements in reactor control. This paper presents a summary of the control systems used to test the early KIWI and NRX reactors. Using these reference controllers, designs to improve reliability and performance are discussed. These improvements include the use of limiter circuits to protect the nuclear subsystem from controller malfunctions and off-design operation of the system without resorting to reactor "scrams" and the ensuing system shocks. A pressure control has been added to the facility flow control loop to improve the accuracy of the propellant system control. To help insure that the system parameters are maintained within the proper and safe relationship with each other, the temperature controller is slaved to the pressure loop eliminating the requirement for time base programming of two variables and improving system protection against flow system failures. On-Off control of reactivity is also discussed showing the advantages of simpler control drum actuator systems. Finally, the performance of a controller operating on-off without the aid of an inner power control loop is presented.