REACTOR OPERATIONDOE-HDBK-1019/2-93Reactor 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. Onenoticeable 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 themoderator 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 populationimmediately after shutdown. The photo-neutron source has the result of flux levels decreasingmore slowly so that a heavy water reactor will have a significantly larger negative reactor periodafter a shutdown.Throughout the process of reactor shutdown the nuclear instrumentation is closely monitored toobserve that reactor neutron population is decreasing as expected, and that the instrumentationis functioning properly to provide continuous indication of neutron population. Instrumentationis observed for proper overlap between ranges, comparable indication between multipleinstrument channels, and proper decay rate of neutron population.A distinction should be made between indicated reactor power level after shutdown and theactual thermal power level. The indicated reactor power level is the power produced directlyfrom fission in the reactor core, but the actual thermal power drops more slowly due to decayheat production as previously discussed. Decay heat, although approximately 5 to 6% of thesteady state reactor power prior to shutdown, diminishes to less than 1% of the pre-shutdownpower level after about one hour.After a reactor is shutdown, provisions are provided for the removal of decay heat. If thereactor 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 actuallyconducting cooldown of the reactor vary depending on plant design, but in all cases limitationsare imposed on the maximum rate at which the reactor systems may be cooled. These limits areprovided to reduce the stress applied to system materials, thereby reducing the possibility of stressinduced failure.Although a reactor is shut down, it must be continuously monitored to ensure the safety of thereactor. Automatic monitoring systems are employed to continuously collect and assess the dataprovided by remote sensors. It is ultimately the operator who must ensure the safety of thereactor.NP-04Rev. 0Page 32
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