Editing SL-1 (section)

===Reactor principles and events===
Early press reports indicated that the explosion may have been due to a chemical reaction, but that was shortly ruled out. Fast [[neutron activation]] had occurred to various materials in the room, indicating a nuclear power excursion unlike a properly operating reactor.

In a [[thermal-neutron reactor]] such as SL-1, neutrons are [[neutron moderator|moderated]] (slowed down) to control the nuclear fission process and increase the likelihood of fission with [[U-235]] fuel. Without sufficient moderation, cores such as SL-1 would be unable to sustain a nuclear chain reaction. When the moderator is removed from the core, the chain reaction decreases. Water, when used as a moderator, is maintained under high pressure to keep it liquid. Steam formation in the channels around the nuclear fuel suppresses the chain reaction.

Another control is the effect of the [[delayed neutron]]s on the chain reaction in the core. Most neutrons (the {{em|prompt}} neutrons) are produced nearly instantaneously by the fission of U-235. But a few—approximately 0.7 percent in a U-235-fueled reactor operating at steady-state—are produced through the relatively slow radioactive decay of certain fission products. (These fission products are trapped inside the fuel plates in close proximity to the uranium-235 fuel.) The delayed production of a fraction of the neutrons enables reactor power changes to be controlled on a time scale amenable to humans and machinery.<ref name="Introduction to Nuclear Engineering">{{cite book
|last= Lamarsh
|first= John R.
|author2=Baratta, Anthony J.
|title= Introduction to Nuclear Engineering
|publisher= Prentice Hall
|year= 2001
|location= Upper Saddle River, New Jersey
|pages= 783
|isbn= 0-201-82498-1}}</ref>

In the case of an ejected control assembly or poison, it is possible for the reactor to become [[critical mass|critical]] {{em|on the [[prompt neutron]]s alone}} (i.e. [[prompt criticality|prompt critical]]). When the reactor is prompt critical, the time to double the power is of the order of 10 microseconds. The duration necessary for temperature to follow the power level depends on the design of the reactor core. Typically, the coolant temperature lags behind the power by 3 to 5 seconds in a conventional [[LWR]]. In the SL-1 design, it was about 6 milliseconds before steam formation started.<ref name=ido19311 />

SL-1 was built with a main central control rod that could produce a very large excess [[Dollar (reactivity)|reactivity]] if it were completely removed.<ref name=suid /> The extra rod worth was in part due to the decision to load only 40 of the 59 fuel assemblies with nuclear fuel, thus making the prototype reactor core more active in the center. In normal operation control rods are withdrawn only far enough to generate sufficient reactivity for a sustained nuclear reaction and power generation. In this accident, however, the additional reactivity was enough to take the reactor prompt critical within an estimated 4 milliseconds.<ref name=ido19313>''[http://www.id.doe.gov/foia/PDF/IDO-19313.pdf IDO-19313: Additional Analysis of the SL-1 Excursion] {{webarchive|url=https://web.archive.org/web/20110927065809/http://www.id.doe.gov/foia/PDF/IDO-19313.pdf |date=2011-09-27 }} Final Report of Progress July through October 1962'', November 21, 1962, Flight Propulsion Laboratory Department, General Electric Company, Idaho Falls, Idaho, U.S. Atomic Energy Commission, Division of Technical Information.</ref> That was too fast for the heat from the fuel to permeate the aluminum cladding and boil enough water to fully stop the power growth in all parts of the core via negative moderator temperature and void feedback.<ref name=ido19311 /><ref name=ido19313 />

Post-accident analysis concluded that the final control method (i.e., dissipation of the prompt critical state and the end of the sustained nuclear chain reaction) occurred by means of catastrophic core disassembly: destructive melting, vaporization, and consequent conventional explosive expansion of the parts of the reactor core where the greatest amount of heat was being produced most quickly. It was estimated that this core heating and vaporization process happened in about 7.5 milliseconds, before enough steam had been formed to shut down the reaction, beating the steam shutdown by a few milliseconds. A key statistic makes it clear why the core blew apart: the reactor designed for a 3 MW power output operated momentarily at a peak of about 20 GW, a power density over 6,000 times higher than its safe operating limit.<ref name=la13638 /> This [[criticality accident]] is estimated to have produced 4.4&nbsp;×&nbsp;10<sup>18</sup> fissions,<ref name=la13638 /> or about {{convert|133|MJ|kgTNT}} energy.<ref name=ido19313 />