Safety circuit development at Brookhaven

IN SPITE OF EFFORTS to design nuclear reactors which will be intrinsically sale from an uncontrolled increase in the rate of power generation, it remains the primary problem of reactor instrumentation to provide a safety circuit, sensing neutron level, which will safely limit such excursions and yet not impose an undue burden on operation by reason of accidental “scrams” due to instrument failure. At Brookhaven National Laboratory, the philosophy of these safety circuits has recently been overhauled in the light of operating experience and a thorough-going redesign has been completed. The following features, in line with this thinking, have been incorporated into the new system: 1. A reasonable amount of effort has been expended to make the circuits free from failure. 2. A reasonable amount of effort has been expended to make the circuits fail-safe. 3. Hourly automatic testing drastically reduces the risk from unsafe faults. 4. Three independent safety channels are employed in a “two out of three” coincidence connection in order to insure adequate protection, together with reasonable freedom from accidental scrams. 5. No means are provided for disabling any of the three safety channels. 6. Means are provided for disabling the automatic testing for short spaces of time only. 7. Operator can clear faults rapidly by plugging in a complete new channel. 8. The state of the system and the results of the operability tests are continuously and directly presented to the operator, who, therefore, is in a position of knowledge and responsibility. Consequently, division of responsibility can be avoided. 9. The inverse period signal is added to the power signal in order to make the circuit trip with less period signal as the power increases.