A typical P’^TC. generating about 1100 MW(e) of electrical power would have a decay heat generation of about 200 MW(t) immediately after shutdown. This compares with 3400 ^MW(t) thermal energy generation immediately before shut­down. Removal of this decay heat is well within the capability of the low-pressure cooling system illustrated in Figure 4.4 provided the reactor can be depressurized rapidly enough to bring these into operation. Alternatively, if the steam generators can be operated effectively with the auxiliary feedwater system that is automati­cally switched on when the reactor trips, the decay heat can be removed via the steam generators, even at high pressure. A major difficulty arises when neither of these systems can be brought into play for reasons that will be described in Chap­ter 5. This is what happened in the Three Mile Island accident.

If the low-pressure cooling system and the steam generators are unavailable as a cooling mechanism, the only recourse is to feed water into the system via the high-pressure injection systems and the charging pumps (the pumps used to maintain the inventory of the system under normal operating conditions), the injected water bleeding out through the break. It is interesting to consider how the exiting fluid carries energy with it. The system is illustrated schematically in

Figure 4.11. If the water fed to the reactor is evaporated and exits as steam, this represents the maximum rate of energy release possible. If, on the other hand, the fluid leaves the reactor in the form of liquid water, not only is the discharge rate high (reducing the coolant inventory in the system) but the energy con­tained in the discharge is low relative to that in steam at the same mass flow rate. For these reasons, it is preferable to discharge steam rather than water. Dis­charges in the upper part of the circuit usually contain more energy than those in the lower part, where the existence of liquid water is more likely under tran­sient accident conditions.

Taking the case of steam ejection from the reactor circuit, one can estimate that the maximum rate of ejection corresponds to the release of 17,000 of energy per square meter. To eject the steam that could be generated by the decay heat just after shutdown, a hole of area 0.011 m2 would be required, corresponding to a hole diameter of 12 cm. The hole size required to reject the decay heat as a function of time from reactor shutdown (taking into account the decrease in decay heat rate as a function of time; see Table 2.2) is shown in Figure 4.12. One hour after shutdown the required hole size has dropped to 3.8 em.

If the actual break size is bigger than that required to release the energy in the form of steam, the energy lost will be greater than that being generated and this will result in depressurization of the primary circuit. Such a depressurization may quickly lead to actuation of the low-pressure emergency heat removal sys­tems. However, if the break size is smaller than that required to remove the en­ergy, then energy will be stored within the reactor system, leading to

Bleed and feed

Mass lost as steam
GOOD

low mass flow rate
high enthalpy change

Mass lost as water
BAD

high mass flow rate
low enthalpy change

pressurization of the primary coolant. The system may be partly controllable if the power-operated relief valves (PORVs) can be opened to increase the escape of steam and facilitate energy release. The PORVs are located on top of the pressurizer, and a typical PWR would have two such valves with a total flow area of about 0.002 m2, giving an energy release capacity as steam of about 34 ^W. This is clearly much lower than the 200 ^W of energy release corre­sponding to the decay heat immediately after reactor shutdown. In fact, it might be advisable to consider increasing the size and/or number of PORVs in future reactor designs to allow a higher rate of energy release.

If a break occurs and the available PORV area is insufficient to allow the en­ergy release, the reactor system will continue to pressurize, ultimately actuating the spring-loaded safety valves, whose total area is likely to be sufficient to allow the energy release. However, the latter form of release is somewhat un­controlled. The valves actuate and reseat at a specific pressure.