Editing Prompt criticality (section)

=== Prompt critical accidents ===
{{main article|Criticality accident}}
Nuclear reactors can be susceptible to prompt-criticality accidents if a large increase in reactivity (or ''k-effective'') occurs, e.g., following failure of their control and safety systems. The rapid uncontrollable increase in reactor power in prompt-critical conditions is likely to irreparably damage the reactor and in extreme cases, may breach the containment of the reactor. Nuclear reactors' safety systems are designed to prevent prompt criticality and, for [[Defense in depth (nuclear engineering)|defense in depth]], reactor structures also provide multiple layers of containment as a precaution against any accidental releases of [[radioactive]] [[fission products]].

With the exception of research and experimental reactors, only a small number of reactor accidents are thought to have achieved prompt criticality, for example [[Chernobyl disaster|Chernobyl #4]], the U.S. Army's [[SL-1]], and [[Soviet submarine K-431]]. In all these examples the uncontrolled surge in power was sufficient to cause an explosion that destroyed each reactor and released [[radioactive]] fission products into the atmosphere.

At Chernobyl in 1986, a poorly understood positive [[scram]] effect resulted in an overheated reactor core. This led to the rupturing of the fuel elements and water pipes, vaporization of water, a [[steam explosion]], and a graphite fire. Estimated power levels prior to the incident suggest that it operated in excess of 30 GW, ten times its 3 GW maximum thermal output. The reactor chamber's 2000-ton lid was lifted by the steam explosion. Since the reactor was not designed with a [[containment building]] capable of containing this catastrophic explosion, the accident released large amounts of radioactive material into the environment. 

In the other two incidents, the reactor plants failed due to errors during a maintenance shutdown that was caused by the rapid and uncontrolled removal of at least one control rod. The [[SL-1]] was a prototype reactor intended for use by the US Army in remote polar locations. At the SL-1 plant in 1961, the reactor was brought from shutdown to prompt critical state by manually extracting the central control rod too far. As the water in the core quickly converted to steam and expanded (in just a few milliseconds), the {{convert|26000|lb|kg|adj=on}} reactor vessel jumped {{convert|9|ft|1|in|m}}, leaving impressions in the ceiling above.<ref name="Tucker">{{cite book |last=Tucker |first=Todd |title=Atomic America: How a Deadly Explosion and a Feared Admiral Changed the Course of Nuclear History |isbn=978-1-4165-4433-3 |year=2009 |publisher=Free Press |location=New York |url-access=registration |url=https://archive.org/details/atomicamericahow00todd }} See summary: [http://catdir.loc.gov/catdir/enhancements/fy0904/2008013842-s.html] {{Webarchive|url=https://web.archive.org/web/20110721044652/http://catdir.loc.gov/catdir/enhancements/fy0904/2008013842-s.html |date=21 July 2011 }}</ref><ref name=ProvePrinciple15>{{cite book
  | last =Stacy
  | first =Susan M.
  | title =Proving the Principle: A History of The Idaho National Engineering and Environmental Laboratory, 1949–1999
  | publisher =[[U.S. Department of Energy]], Idaho Operations Office
  | date =2000
  | pages =138–149
  | chapter =Chapter 15: The SL-1 Incident
  | chapter-url =http://www4vip.inl.gov/publications/d/proving-the-principle/chapter_15.pdf
  | isbn =978-0-16-059185-3
  | access-date =8 September 2015
  | archive-date =29 December 2016
  | archive-url =https://web.archive.org/web/20161229141617/http://www4vip.inl.gov/publications/d/proving-the-principle/chapter_15.pdf
  | url-status =live
  }}</ref> All three men performing the maintenance procedure died from injuries. 1,100 curies of fission products were released as parts of the core were expelled. It took 2 years to investigate the accident and clean up the site. The excess prompt reactivity of the SL-1 core was calculated in a 1962 report:<ref>[http://www.id.doe.gov/foia/PDF/IDO-19313.pdf IDO-19313] {{webarchive|url=https://web.archive.org/web/20110927065809/http://www.id.doe.gov/foia/PDF/IDO-19313.pdf |date=27 September 2011 }} ''Additional Analysis of the SL-1 Excursion, Final Report of Progress July through October 1962'', November 1962.</ref>

{{Quote|The delayed neutron fraction of the SL-1 is 0.70%... Conclusive evidence revealed that the SL-1 excursion was caused by the partial withdrawal of the central control rod. The reactivity associated with the 20-inch withdrawal of this one rod has been estimated to be 2.4% δk/k, which was sufficient to induce prompt criticality and place the reactor on a 4 millisecond period.}}

In the ''K-431'' reactor accident, 10 were killed during a refueling operation. The ''K-431'' explosion destroyed the adjacent machinery rooms and ruptured the submarine's hull. In these two catastrophes, the reactor plants went from complete shutdown to extremely high power levels in a fraction of a second, damaging the reactor plants beyond repair.