Reactor Theory (Reactor Operations)
The half-life of the longest lived delayed neutron precursor results in a reactor period of around
-80 seconds or a startup rate of -1/3 DPM for most reactors after a reactor shutdown. One
noticeable exception to this is a heavy water reactor.
In a heavy water reactor, the photo-
neutron source is extremely large after shutdown due to the amount of deuterium in the
moderator and the large number of high energy gammas from short-lived fission product decay.
The photo-neutron source is large enough to have a significant impact on neutron population
immediately after shutdown. The photo-neutron source has the result of flux levels decreasing
more slowly so that a heavy water reactor will have a significantly larger negative reactor period
after a shutdown.
Throughout the process of reactor shutdown the nuclear instrumentation is closely monitored to
observe that reactor neutron population is decreasing as expected, and that the instrumentation
is functioning properly to provide continuous indication of neutron population. Instrumentation
is observed for proper overlap between ranges, comparable indication between multiple
instrument channels, and proper decay rate of neutron population.
A distinction should be made between indicated reactor power level after shutdown and the
actual thermal power level. The indicated reactor power level is the power produced directly
from fission in the reactor core, but the actual thermal power drops more slowly due to decay
heat production as previously discussed. Decay heat, although approximately 5 to 6% of the
steady state reactor power prior to shutdown, diminishes to less than 1% of the pre-shutdown
power level after about one hour.
After a reactor is shutdown, provisions are provided for the removal of decay heat. If the
reactor is to be shut down for only a short time, operating temperature is normally maintained.
If the shutdown period will be lengthy or involves functions requiring cooldown of the reactor,
the reactor temperature can be lowered by a number of methods. The methods for actually
conducting cooldown of the reactor vary depending on plant design, but in all cases limitations
are imposed on the maximum rate at which the reactor systems may be cooled. These limits are
provided to reduce the stress applied to system materials, thereby reducing the possibility of stress
Although a reactor is shut down, it must be continuously monitored to ensure the safety of the
reactor. Automatic monitoring systems are employed to continuously collect and assess the data
provided by remote sensors. It is ultimately the operator who must ensure the safety of the