Editing Scram (section)

==Reactor response==
Most neutrons in a reactor are [[prompt neutrons]]; that is, neutrons produced directly by a fission reaction. These neutrons move at a [[neutron temperature|high velocity]], so they are likely to escape into the [[neutron moderator|moderator]] before being [[neutron capture|captured]]. On average, it takes about 13 μs for the neutrons to be slowed by the moderator enough to [[neutron cross-section|facilitate]] a sustained reaction, which allows the insertion of neutron absorbers to affect the reactor quickly.<ref name="dude">{{cite book |last= Duderstadt |first= James J. |author2=Louis J. Hamilton |title= Nuclear Reactor Analysis |url= https://archive.org/details/nuclearreactoran00dude |url-access= limited |publisher= Wiley-Interscience |year= 1976 |isbn= 0-471-22363-8 |pages= [https://archive.org/details/nuclearreactoran00dude/page/n267 245]}}</ref>

As a result, once the reactor has been scrammed, the reactor power will drop significantly almost instantaneously. A small fraction (about 0.65%) of neutrons in a typical power reactor comes from the [[radioactive decay]] of a fission product. These ''delayed neutrons'', which are emitted at lower velocities, will limit the rate at which a nuclear reactor will shut down.<ref name="dude"/>

Due to flaws in its original control rod design, scramming an [[RBMK]] reactor could raise reactivity to dangerous levels before lowering it. This was noticed when it caused a power surge at the startup of [[Ignalina Nuclear Power Plant]] Unit number 1, in 1983. On April 26, 1986, the [[Chernobyl disaster]] happened due to a fatally flawed shutdown system, after the AZ-5 shutdown system was initiated after a core overheat. RBMK reactors were subsequently either retrofitted to account for the flaw, or decommissioned.

===Decay heat===
{{Further|Decay heat}}
Not all of the heat in a nuclear reactor is generated by the chain reaction that a scram is designed to stop. For a reactor that is scrammed after holding a constant power level for an extended period (greater than 100 hrs), about 7% of the steady-state power will remain after initial shutdown due to fission product decay that cannot be stopped. For a reactor that has not had a constant power history, the exact percentage is determined by the concentrations and half-lives of the individual fission products in the core at the time of the scram.

The power produced by [[decay heat]] decreases as the fission products decay, but it is large enough that failure to remove decay heat may cause the reactor core temperature to rise to dangerous levels and has caused [[nuclear accidents]], including the nuclear accidents at [[Three Mile Island accident|Three Mile Island]] and [[Fukushima I nuclear accidents|Fukushima I]].