After shutdown of a nuclear reactor, the core continues to produce
decay heat
Decay heat is the heat released as a result of radioactive decay. This heat is produced as an effect of radiation on materials: the energy of the alpha particle, alpha, Beta particle, beta or gamma radiation is converted into the thermal movement ...
due to the nuclear decay of
fission products
Nuclear fission products are the atomic fragments left after a large atomic nucleus undergoes nuclear fission. Typically, a large nucleus like that of uranium fissions by splitting into two smaller nuclei, along with a few neutrons, the releas ...
created during reactor operation. In a boiling-water or pressurized-water reactor (BWR or PWR), the residual heat removal system (RHR system) is used to remove this decay heat, and also serves safety functions during an accident. The RHR system may also be called the decay heat removal system or the shutdown cooling system.
Normal operations
Immediately after shutdown of a typical U-235, water-
moderated
Moderation is the process or trait of eliminating, lessening, or avoiding extremes. It is used to ensure normality throughout the medium on which it is being conducted. Common uses of moderation include:
* A way of life emphasizing perfect amo ...
nuclear reactor, heat production in the core drops by 94%, to 6.25% of full thermal power and further decreases with a half-life of approximately 80 seconds. The RHR system is used to remove the resultant decay heat from the core.
PWRs
The reactor coolant system (RCS) is first cooled from operational temperatures via the secondary side: steam (not radioactive) is produced in the steam generators and dumped to the main condenser or directly to atmosphere. Once the reactor is cool enough to allow safe RHR operation, reactor coolant is routed through the RHR pump and heat exchanger to further reduce the temperature to cold shutdown conditions. The RHR heat exchanger is cooled by the component cooling water system (CCS), an intermediate cooling loop provided between radioactive systems and the outside environment, to reduce the likelihood of contamination.
BWRs
RHR performs the same low-temperature shutdown function in BWRs as in PWRs. On initial reactor shutdown, reactor coolant temperature is first reduced via dumping steam to the main condenser via the turbine bypass valves. When RCS pressure is less than approximately 50 psig, the RHR pump and heat exchanger are used to achieve cold shutdown. The main difference from the PWR configuration is the lack of an intermediate system between the radioactive reactor coolant and the environment.
Safety function (Accident operations)
The RHR system also has a safety function as the low-pressure branch of the emergency core cooling system (ECCS).
PWRs
The RHR system serves as the low pressure, high-flow source of emergency core cooling. If a large break loss-of-coolant accident (
LOCA) occurs, it will inject
borated water from the refueling water storage tank (RWST) into the core to provide cooling and absorb neutrons. On exhaustion of the RWST, the RHR system is realigned to recirculate water from the containment sump back into the core, cooling it first via the RHR heat exchanger. If the loss-of-coolant is due to a small break, reactor pressure may remain too high for RHR injection; in this case, during the initial injection phase from the RWST, the high- and intermediate-pressure branches of the ECCS are used instead. During the recirculation phase, the ECCS is aligned such that the high- and intermediate-pressure pumps take suction from the containment sump via the RHR pump and heat exchanger; this ensures cooling of the sump water and adequate
suction head
In fluid dynamics, total dynamic head (TDH) is the work to be done by a pump, per unit weight, per unit volume of fluid. TDH is the total amount of system pressure, measured in feet, where water can flow through a system before gravity takes over, ...
(NPSH) for the high-pressure pumps.
BWRs
The RHR system serves as the
low pressure coolant injection (LPCI) system. During a LOCA, the RHR pumps take suction from the suppression pool and circulate water through the RHR heat exchanger and into the coolant recirculation loops, and thence into the core. The RHR loop may also serve a containment spray function to condense steam and reduce the pressure and temperature in containment. The LPCI mode may operate even on small or intermediate breaks when reactor pressure normally remains high, thanks to the operation of the automatic depressurization system (ADS), which reduces reactor pressure to a point where LPCI can inject cooling water.
[Power Plant Engineering Course Manual, p. 1-5.]
See also
Decay heat
Decay heat is the heat released as a result of radioactive decay. This heat is produced as an effect of radiation on materials: the energy of the alpha particle, alpha, Beta particle, beta or gamma radiation is converted into the thermal movement ...
Nuclear reactor coolant
A nuclear reactor coolant is a coolant in a nuclear reactor used to remove heat from the nuclear reactor core and transfer it to electrical generators and the environment.
Frequently, a chain of two coolant loops are used because the primary co ...
Light-water reactor
The light-water reactor (LWR) is a type of thermal-neutron reactor that uses normal water, as opposed to heavy water, as both its coolant and neutron moderator; furthermore a solid form of fissile elements is used as fuel. Thermal-neutron reacto ...
Auxiliary feedwater
Auxiliary feedwater is a backup water supply system found in pressurized water reactor nuclear power plants (PWRs). This system, sometimes known as emergency feedwater, can be used to cool the reactor, if normal feedwater to the steam generators f ...
Nuclear reactor safety system
The three primary objectives of nuclear reactor safety systems as defined by the U.S. Nuclear Regulatory Commission are to shut down the reactor, maintain it in a shutdown condition and prevent the release of radioactive material.
Reactor protec ...
Boiling water reactor safety systems
Boiling water reactor safety systems are nuclear safety systems constructed within boiling water reactors in order to prevent or mitigate environmental and health hazards in the event of accident or natural disaster.
Like the pressurized water ...
Notes
References
{{Nuclear technology
Light water reactors
Pressurized water reactors
Boiling water reactors
Nuclear power plant components