Editing SL-1 (section)

== Design and operations ==
From 1954 to 1955, the U.S. Army had been evaluating their need for [[nuclear reactor]] plants that would be operable in remote regions of the [[Arctic]]. The reactors were to replace diesel generators and boilers that provided electricity and space heating for the Army's radar stations. The [[Army Reactors Branch]] had written guidelines for the project and hired [[Argonne National Laboratory]] (ANL) to design, build, and test a prototype reactor plant to be called the Argonne Low Power Reactor (ALPR).<ref name=SEC/> Some of the more important criteria included:
* All components able to be transported by air<ref name=design/><ref name=ANL5566>[https://digital.library.unt.edu/ark:/67531/metadc1250463/ ANL-5566, ALPR Preliminary Design Study (Argonne Low Power Reactor) Phase I], Reactor Engineering Division, Argonne National Laboratory, Lemont, Illinois, April 20, 1956. Reactors - Special Features of Military Package Power Reactors, M-3679 (18th edition), M. Treshow, E. Hamer, H. Pearlman, D. Rossin, D. Shaftman</ref>
* All components limited to packages measuring {{convert|7.5|x|9|x|20|ft}} and weighing {{convert|20000|lb}}<ref name=design/>
* Use of standard components
* Minimal on-site construction<ref name=design/><ref name=ANL5566 />
* Simplicity and reliability<ref name=design/>
* Adaptable to the Arctic [[permafrost]] region<ref name=design/><ref name=ANL5566 />
* 3-year fuel operating lifetime per [[reactor core|core]] loading<ref name=SEC>[https://www.cdc.gov/niosh/ocas/pdfs/sec/inl/inler-219-r2.pdf SEC-00219, Petition Evaluation Report, Idaho National Laboratory (INL), Revision 2], NIOSH/ORAU, Idaho National Laboratory, March 2017</ref><ref name=design/><ref name=ANL5566 />

A [[Classified information|classified]] 1956 preliminary design study, using [[BORAX-III]] as a basis, calculated the total construction cost for the prototype reactor to be $228,789.<ref name=ANL5566 /> This estimate for just the reactor and its components did not include any of the buildings or the rest of the reactor plant.

The prototype was constructed at the [[Idaho National Laboratory|National Reactor Testing Station]] west of [[Idaho Falls, Idaho|Idaho Falls]] from July 1957 to July 1958. It went [[criticality (status)|critical]] for the first time on August 11 1958,<ref name=SEC/> became operational on October 24, and was formally dedicated on December 2 1958.<ref name=SEC/>

The 3 MW (thermal) [[boiling water reactor]] (BWR) used 93.20% [[highly enriched uranium]] fuel.<ref name=ido19311>[https://babel.hathitrust.org/cgi/pt?id=mdp.39015086573790 <!-- Decent quality https://books.google.com.ph/books?id=XvhId_dRDsoC --><!-- Alternate, low quality https://digital.library.unt.edu/ark:/67531/metadc1029163/m2/1/high_res_d/4763434.pdf --> IDO-19311 Final Report of SL-1 Recovery Operation], Idaho Test Station, General Electric Corporation, July 27, 1962.</ref> It operated with [[natural circulation]], using light water as a coolant (vs. [[heavy water]]) and moderator.<ref name=ido19300/> ANL used its experience from the [[BORAX experiments]] to design the reactor.<ref name=ANL5566 /> The circulating water system operated at {{convert|300|psi}} flowing through fuel plates of uranium-aluminum alloy.<ref name=ANL5566 />

The plant was turned over to the Army for training and operating experience in December 1958 after extensive testing, with [[Combustion Engineering|Combustion Engineering Incorporated]] (CEI) acting as the lead contractor beginning February 5, 1959.<ref name="annual-59-60"/>

{{blockquote|text=CEI was responsible for the actual operation of the SL-1 reactor, for the routine training of military personnel and for developmental research programs.

The Contractor provided at the site a Project Manager, Operations Supervisor, a Test Supervisor, and a technical staff of approximately six personnel. In recent months, the Project Manager spent approximately half time at the site and half time at the contractor's office in Connecticut. In his absence, either the Operations Supervisor or the Test Supervisor was assigned as the Project Manager.

{{omission}} It was understood, as indicated by testimony before the Board, that CEI would provide supervision on any shifts when non-routine work was carried out.

{{omission}} the [[United States Atomic Energy Commission|AEC]]'s Idaho Office and the Army Reactors Office clearly believed that the addition of night supervisors when only routine work was involved would defeat a part of the purpose of operating the reactor under the existing arrangement, i.e., to obtain plant operating experience with only military personnel.

|author=Report on the SL-1 Incident, January 3, 1961
|source=pp. 6–7<ref name=Press />}}

Trainees in the Army Reactor Training Program included members of the Army, called ''cadre'', who were the primary plant operators. Many maritime civilians also trained along with a few [[United States Air Force|Air Force]] and [[United States Navy|Navy]] personnel.<ref name="annual-59-60">[https://www.osti.gov/servlets/purl/4143544 IDO-19012, CEND-82, SL-1 Annual Operating Report, Feb. 1959 – Feb 1960], Canfield, Vallario, Crudele, Young, Rausch, Combustion Engineering Nuclear Division, May 1, 1960.</ref> While plant operation was generally done by the ''cadre'' in two-man crews, development of the reactor was supervised directly by CEI staff. CEI decided to perform development work on the reactor as recent as the latter half of 1960 in which the reactor was to be operated at 4.7 MW<sub>thermal</sub> for a "PL-1 condenser test."<ref name=Joint61>[http://li.proquest.com/elhpdf/histcontext/HRG-1961-AEJ-0006.pdf Radiation Safety and Regulation Hearings, Joint Committee on Atomic Energy, US Congress, June 12–15, 1961], including SL-1 Accident Atomic Energy Commission Investigation Board Report, Joint Committee on Atomic Energy Congress of the United States, First Session on Radiation Safety and Regulation, Washington, DC.</ref> As the reactor core aged and [[boron]] [[neutron poison]] strips corroded and flaked off, CEI calculated that about 18% of the boron in the core had been lost. On November 11, 1960, CEI installed [[cadmium]] sheets (also a poison) "to several tee slot positions to increase reactor shutdown margin."<ref name=ido19024>[https://digital.library.unt.edu/ark:/67531/metadc1058207/ IDO-19024 SL-1 Annual Operating Report, February 1960 – January 3, 1961] Combustion Engineering Nuclear Division, CEND-1009, W. B. Allred, June 15, 1961.</ref>

[[File:ALPR.jpg|thumb|The ALPR before the accident. The large cylindrical building holds the nuclear reactor embedded in gravel at the bottom, the main operating area or operating floor in the middle, and the condenser fan room near the top. Miscellaneous support and administration buildings surround it.]]
Most of the plant equipment was in a cylindrical steel reactor building known as ARA-602. It was {{convert|38.5|ft}} in diameter with an overall height of {{convert|48|ft}},<ref name=design /> and was made of plate steel, most of which had a thickness of {{convert|1/4|in|mm}}. Access to the building was provided by an ordinary door through an enclosed exterior stairwell from ARA-603, the Support Facilities Building. An emergency exit door led to an exterior stairwell to the ground level.<ref name=design /> The reactor building was not a pressure-type [[containment building|containment shell]] as would have been used for reactors located in populated areas. Nevertheless, the building was able to contain most of the radioactive particles released by the eventual explosion.

The reactor core structure was built to hold 59 fuel assemblies, one [[startup neutron source]] assembly, and nine [[control rod]]s. The actual core in use had 40 fuel elements and was controlled by five cruciform rods.<ref name=design>{{Cite tech report |last1=Grant|first1=N. R.|last2=Hamer|first2=E. E.|last3=Hooker|first3=H. H.|last4=Jorgensen|first4=G. L.|last5=Kann|first5=W. J.|last6=Lipinski|first6=W. C.|last7=Milak|first7=G. C.|last8=Rossin|first8=A. D.|last9=Shaftman|first9=D. H.|last10=Smaardyk |first10=A. |last11=Treshow |first11=M. |title=Design of the Argonne Low Power Reactor (ALPR) |publisher=Argonne National Laboratory |date= May 1961|series=ANL-6076, Reactor Technology, TID-4500, AEC Research and Development Report|doi=10.2172/4014868|doi-access=free|osti=4014868|osti-access=free|url=https://digital.library.unt.edu/ark:/67531/metadc863771/}}</ref> The five active rods were in the shape of a plus symbol (+) in cross section: one in the center (Rod Number 9), and four on the periphery of the active core (Rods 1, 3, 5, and 7).<ref name=design /> The control rods were made of {{convert|60|mil}} thick cadmium, clad with {{convert|80|mil}} of aluminum. They had an overall span of {{convert|14|in|cm}} and an effective length of {{convert|32|in|cm}}.<ref name=design /> The 40 fuel assemblies were composed of nine fuel plates each.<ref name=design /> The plates were {{convert|120|mil}} thick, consisting of {{convert|50|mil}} of uranium-aluminum alloy "meat" covered by {{convert|35|mil}} of X-8001 aluminum [[cladding (nuclear fuel)|cladding]].<ref name=design /> The meat was {{convert|25.8|in|cm}} long and {{convert|3.5|in|cm}} wide. The water gap between fuel plates was {{convert|310|mil}}.<ref name=design /> Water channels within the control rod shrouds was {{convert|0.5|in}}. The initial loading of the 40-assembly core was highly enriched with 93.2% uranium-235 and contained {{convert|31|lb}} of U-235.<ref name=design />

The deliberate choice of using fewer fuel assemblies made the region near the center more active than it would have been with 59 fuel assemblies. The four outer control rods were not even used in the smaller core after tests concluded they were not necessary.<ref name=design /><ref name=Press>[https://archive.org/details/SL1PressRelease1961 Report on the SL-1 Incident, January 3, 1961], The General Manager's Board of Investigation, For Release in Newspapers dated Sunday, Curtis A. Nelson, Clifford Beck, Peter Morris, Donald Walker, Forrest Western, June 11, 1961.</ref> In the operating SL-1 core, Rods 2, 4, 6, and 8 were dummy rods, had newly installed cadmium shims, or were filled with test sensors, and were shaped like the capital letter T.<ref name=Joint61 /> The effort to minimize the size of the core gave an abnormally-large [[reactivity worth]] to Rod 9, the center control rod.