12.27. Some of the signals that would cause actuation of the protection system were mentioned in Chapter 5. A more complete listing is given here for water-cooled reactors; unless otherwise indicated, the trip signals apply to both PWRs and BWRs.

1. Rapid increase in the neutron flux during startup, resulting in a too rapid rise in the thermal power

2. High neutron flux during power operation, indicating an overpower above the permissible level

3. Abnormal reactor system temperature or pressure

4. Loss (or decrease) of coolant flow, e. g., from a pump failure

5. High steam flow, e. g., from a break in a steam line

6. Closure of a main steam isolation valve, especially in a BWR (see item

12)

7. Turbine-generator trip, e. g., from a loss of load

8. Loss of power supply for instruments (dc) or for pumps, valves, etc. (ac)

9. High water level in the pressurizer (in a PWR)

10. Low water level in the reactor vessel (in a BWR)

11. Low feedwater flow or low water level in a PWR steam generator

12. High radioactivity in the steam from a BWR

Shutdown Cooling

12.28. Although the reactor shutdown cooling system is not generally regarded as a component of the protection system, shutdown cooling is nevertheless an essential aspect of reactor protection. When a reactor is shut down, either deliberately or in response to a severe transient, the self­sustaining fission chain reaction is terminated but a considerable amount of sensible (or stored) heat is still present in the fuel rods. Furthermore, heat continues to be generated by decay of the fission products and (for a short time) by fissions caused by delayed neutrons. Hence, cooling of the reactor core must be maintained for many days after shutdown. The sensible heat and the delayed fission heat are removed within about half a minute and then only the decay heat determines the cooling requirements. The thermal power from this source is initially about 7 percent of the operating power of the reactor at shutdown, assuming the reactor has been operating for a substantial time. The decay power decreases to about 1.3 percent after an hour, 0.4 percent after a day, and 0.2 percent at the end of a week (see Fig. 2.33).

12.29.If the normal heat removal system is still operative when the reactor is tripped, cooling will, of course, be adequate. Steam, produced in the steam generator of a PWR or in the reactor vessel of a BWR, bypasses the turbine and goes directly to the condenser. The condensate then returns to the steam generator (in a PWR) or to the reactor vessel (in a BWR) in the usual way. When the system temperature and pressure have decreased to a sufficient extent, the cooling function is transferred to the residual — heat removal (or shutdown cooling) system which circulates primary system coolant through the reactor vessel and independent heat exchangers cooled by a separate service water supply system. This service water is obtained from the so-called ultimate heat sink which usually also provides the con­denser cooling water. Since the heat sink is required for safe emergency shutdown of the reactor and subsequent dissipation of the residual heat, the system must be capable of operating for at least 30 days even if the most severe natural phenomenon expected at the plant site should occur.[20]

12.30. In some situations, the reactor would be tripped and isolation valves in the steam supply would close automatically to prevent the escape of possibly radioactive steam to the environment. The normal heat removal and condenser system would then not be available for cooling the fuel. In a PWR, a large condensate tank can provide an auxiliary supply of feed water to the steam generators to permit reactor cooldown for about 8 hours. By this time, the conditions would be suitable for the residual-heat removal system to function. In a BWR, a pressure-relief valve permits controlled release of steam to the pressure-suppression pool where it would be condensed (§12.51). The level of the water in the reactor vessel is then maintained by the reactor core isolation system which is supplied by water from a condensate storage tank. Finally, the residual-heat removal system can be actuated.

12.31. If electric power is lost when the reactor is tripped, so that the pumps cannot operate, overheating of the fuel can be prevented by re­leasing steam from the safety valves. This can be continued as long as feed water is available (from the auxiliary system) to the steam generators in a PWR or to the primary system in a BWR. In the meantime, startup of the emergency (diesel) generators will permit operation of the essential pumps, pending the restoration of offsite power.