View source for Molten-Salt Reactor Experiment

{{Short description|Nuclear reactor, Oak Ridge 1965–1969}}
[[Image:MSRE Diagram.png|404px|thumb|MSRE plant diagram: (1) [[Reactor vessel]], (2) [[Heat exchanger]], (3) Fuel pump, (4) Freeze flange, (5) Thermal shield, (6) Coolant pump, (7) [[Radiator]], (8) Coolant drain tank, (9) Fans, (10) Fuel drain tanks, (11) Flush tank, (12) Containment vessel, (13) Freeze valve. Also note Control area in upper left and Chimney upper right.]]

The '''Molten-Salt Reactor Experiment''' ('''MSRE''') was an experimental [[molten-salt reactor]] [[research reactor]] at the [[Oak Ridge National Laboratory]] (ORNL) in [[Oak Ridge, Tennessee]]. This technology was researched through the 1960s, the reactor was constructed by 1964, it went [[Criticality (status)|critical]] in 1965, and was operated until 1969.<ref name="project_description"/> The costs of a cleanup project were estimated at $130 million.

Initially designed for 15 [[MWth|MW<sub>th</sub>]], the MSRE was operated at 7.4&nbsp;MW<sub>th</sub> because of imprecise [[nuclear cross section]] data. It was a test reactor simulating the neutronic "kernel" of a type of inherently safer [[Neutron temperature|epithermal]] [[thorium]] [[breeder reactor]] called the [[liquid fluoride thorium reactor]]. It primarily used two fuels: first [[uranium-235]] and later [[uranium-233]]. The latter [[uranium tetrafluoride|<sup>233</sup>UF<sub>4</sub>]] was the result of breeding from thorium in other reactors. Since this was an engineering test, the large, expensive breeding blanket of thorium salt was omitted in favor of neutron measurements.

In the MSRE, the heat from the [[Nuclear reactor core|reactor core]] was shed via a cooling system using air blown over [[radiator]]s. It is thought similar reactors could power high-efficiency [[heat engine]]s such as [[closed-cycle gas turbine]]s. The MSRE's piping, core vat and structural components were made from [[Hastelloy]]-N, and its [[Neutron moderator|moderator]] was a [[nuclear graphite|pyrolytic graphite]] core. The fuel for the MSRE was [[lithium fluoride|LiF]]-[[beryllium fluoride|BeF<sub>2</sub>]]-[[zirconium fluoride|ZrF<sub>4</sub>]]-[[uranium tetrafluoride|UF<sub>4</sub>]] (65-29.1-5-0.9 mole %). The secondary [[coolant]] was [[FLiBe]] (2LiF-BeF<sub>2</sub>), and it operated as hot as 650&nbsp;°C and operated for the equivalent of about 1.5&nbsp;years of full power operation.

The result promised to be a simple, reliable reactor. The purpose of the Molten-Salt Reactor Experiment was to [[proof of concept|demonstrate]] that some key features of the proposed molten-salt [[nuclear reactor|power reactor]]s could be embodied in a practical reactor that could be operated safely and reliably and be maintained without excessive difficulty. For simplicity, it was to be a fairly small, one-fluid (i.e. non-breeding) reactor operating at 10&nbsp;MW<sub>th</sub> or less, with heat rejection to the air via a secondary (fuel-free) salt.

==Reactor description==
[[File:MoltenSaltReactor.jpg|thumb|Molten salt reactor]]

===Core===
[[File:MSRE Core.JPG|thumb|Graphite MSRE core]]
The [[pyrolytic graphite]] core, grade CGB, also served as the [[neutron moderator|moderator]].<ref name="project_description">[https://web.archive.org/web/20160303211133if_/https://dl.dropboxusercontent.com/u/15726934/Historic_Molten_Salt_Reactor_Experiment_Brochure_ORNL_1965-1972.pdf ''Molten-Salt Reactor Experiment 1965-1972''] ORNL, Oct 2015 (8 MB) [Archive March 2016]</ref><ref name="msre_folder">[https://public.ornl.gov/conferences/MSR2015/pdf/20151022104041810.pdf ''Molten Salt Reactor Experiment''], Oct 2015 (2 MB)</ref> Before the MSRE development began, tests had shown that salt would not permeate graphite in which the pores were on the order of a micrometer. However, graphite with the desired pore structure was available only in small, experimentally prepared pieces, and when a manufacturer set out to produce a new grade (CGB) to meet the MSRE requirements, difficulties were encountered.<ref>{{harvnb|Briggs|1964|pp= 373–309}}.</ref>

===Fuel===
The fuel was <sup>7</sup>LiF-BeF<sub>2</sub>-ZrF<sub>4</sub>-UF<sub>4</sub> (65-29.1-5-0.9 mole&nbsp;%). The first fuel was 33% <sup>235</sup>U; later a smaller amount of <sup>233</sup>UF<sub>4</sub> was used. By [[1960 in science|1960]] a better understanding of [[fluoride]] salt based [[molten-salt reactor]]s had emerged from earlier molten salt reactor research for the [[Aircraft Reactor Experiment]]. Fluoride salts are strongly [[Ionic compound|ionic]], and when melted they are stable at high temperatures, low pressures, and high [[Irradiance|radiation fluxes]]. Stability at low pressure permits less robust [[Reactor pressure vessel|reactor vessels]] and increases reliability. The high reactivity of fluorine traps most fission reaction byproducts. It appeared that the fluid salt would permit on-site chemical separation of the fuel and wastes.

The fuel system was located in sealed cells, laid out for maintenance with long-handled tools through openings in the top shielding. A tank of LiF-BeF<sub>2</sub> salt was used to flush the fuel circulating system before and after maintenance. In a cell adjacent to the reactor was a simple facility for bubbling gas through the fuel or flush salt: H<sub>2</sub>-[[hydrogen fluoride]] mixture, in roughly 10:1 ratio, to remove oxide, [[fluorine]] to remove uranium as [[uranium hexafluoride]].<ref name="NATMSRE">{{cite journal| author1= P.N. Haubenreich| author2= J.R. Engel| name-list-style= amp| title= Experience with the Molten-Salt Reactor Experiment| journal= Nuclear Applications and Technology| year= 1970| pages= 118–136| volume= 8| issue= 2| url= http://www.energyfromthorium.com/pdf/NAT_MSREexperience.pdf| format= PDF, reprint| url-status= dead| archive-url= https://web.archive.org/web/20150129012419/http://www.energyfromthorium.com/pdf/NAT_MSREexperience.pdf| archive-date= 2015-01-29| doi= 10.13182/NT8-2-118| access-date= 2006-06-26}}</ref><ref name="TM0728">{{Cite journal|author=R.C. Robertson|title= MSRE Design and Operations Report, Part I, Description of Reactor Design|id=ORNL-TM-0728|date=January 1965}}</ref><ref name="TM2578">{{Cite journal|author=R.B. Lindauer|title=Processing of the MSRE Flush and Fuel Salts|id=ORNL-TM-2578|date=August 1969}}</ref>[[File:FLiBe.png|thumb|Molten FLiBe]]
The secondary coolant was LiF-BeF<sub>2</sub> (66–34 mole&nbsp;%).

===Pump===
The bowl of the fuel pump was the surge space for the circulating loop, and here about {{convert|50|USgal/min|L/min}} of fuel was sprayed into the gas space to allow [[xenon]] and [[krypton]] to escape from the salt. Removing the most significant [[neutron poison]] [[xenon-135]] made the reactor safer and easier to restart. In solid-fuel reactors, on restart the [[xenon-135|<sup>135</sup>Xe]] in the fuel [[Neutron capture|absorbs neutrons]], followed by a sudden jump in reactivity as the <sup>135</sup>Xe is burned out. Conventional reactors may have to wait hours until xenon-135 decays after shutting down and not immediately restarting (so-called ''[[iodine pit]]'').

Also in the pump bowl was a port through which salt samples could be taken or capsules of concentrated fuel-enriching salt (UF<sub>4</sub>-LiF or [[plutonium(III) fluoride|PuF<sub>3</sub>]]) could be introduced.

===Air-cooled heat exchangers===
[[File:MSRE Heat Exchanger.JPG|thumb|MSRE air-cooled heat exchanger glowing a dull red hue to high temperature.]]
At the time, the high temperatures were seen almost as a disadvantage because they hampered use of conventional [[steam turbine]]s. Now, such temperatures are seen as an opportunity to use high-efficiency [[closed-cycle gas turbine]]s.{{Citation needed|date=January 2016}} After two months of high-power operation, the reactor was down for 3 months because of the failure of one of the main cooling blowers.

===Neutronics and thermal-hydraulics===
The reactor experienced stable [[neutronics|neutronic operation]]. If temperatures increased or bubbles formed, the volume of the fluid fuel salts would increase and some fluid fuel salts would be forced out of the core, thereby reducing the [[reactivity (nuclear)|reactivity]]. The MSRE development program did not include [[Nuclear reactor physics|reactor physics]] experiments or [[heat transfer]] measurements. There was enough latitude in the MSRE that deviations from predictions would not compromise safety or accomplishment of the objectives of the experimental reactor.

===Building grounds===
[[File:ARE Building.JPG|thumb|Aircraft Reactor Experiment building at ORNL that was retrofitted to house the MSRE.]]
Construction of the primary system components and alterations of the old Aircraft Reactor Experiment building (which had been partly remodeled for a proposed 60 MW<sub>th</sub> aircraft reactor) were started in 1962. Installation of the salt systems was completed in mid-1964. ORNL was responsible for quality assurance, planning, and management of construction.<ref name="Webster">{{Cite journal |author=B.H. Webster |title=Quality-Assurance Practices in Construction and Maintenance of the MSRE |id=ORNL-TM-2999 |date=April 1970 }}</ref> The primary systems were installed by ORNL personnel; subcontractors modified the building and installed ancillary systems.

===Structural alloy Hastelloy-N===
[[Hastelloy]]-N—a low [[chromium]], [[nickel]]–[[molybdenum]] alloy—was used in the MSRE and proved compatible with the fluoride salts [[FLiBe]] and [[FLiNaK]].<ref>DeVan, Jackson H. "Effect of Alloying Additions on Corrosion Behavior of Nickel–Molybdenum Alloys in Fused Fluoride Mixtures." Thesis. University of Tennessee, 1960. Web. <{{cite web |url=http://moltensalt.net/references/static/downloads/pdf/ORNL-TM-0202.pdf |title=Archived copy |access-date=2011-01-12 |url-status=dead |archive-url=https://web.archive.org/web/20110723211324/http://moltensalt.net/references/static/downloads/pdf/ORNL-TM-0202.pdf |archive-date=2011-07-23 }}>.</ref> All metal parts contacting salt were made of Hastelloy-N. The choice of Hastelloy-N for the MSRE was on the basis of the promising results of tests at aircraft [[nuclear propulsion]] conditions and the availability of much of the required metallurgical data. Development for the MSRE generated the further data required for [[ASME]] code approval. It also included preparation of standards for Hastelloy-N procurement and for component fabrication.

Almost 200,000&nbsp;lb (90,000&nbsp;kg) in a variety of shapes of material for the MSRE were produced commercially. Requests for bids on component fabrication went to several companies in the nuclear fabrication industry, but all declined to submit lump-sum bids because of lack of experience with the new alloy. Consequently, all major components were fabricated in [[U.S. Atomic Energy Commission]]-owned shops at [[Oak Ridge, Tennessee|Oak Ridge]] and [[Paducah, Kentucky]].<ref>{{harvnb|Briggs|1964|pp= 63–52}}.</ref>

At the time that design stresses were set for the MSRE, the data that was available indicated that the [[strength of materials|strength]] and [[creep (deformation)|creep rate]] of Hastelloy-N were hardly affected by [[irradiation]]. After the construction was well along, the stress-rupture life and [[fracture]] strain were found to be drastically reduced by [[thermal neutron]] irradiation. The MSRE stresses were reanalyzed, and it was concluded that the reactor would have adequate life to reach its goals. At the same time a program was launched to improve the resistance of Hastelloy-N to the [[embrittlement]].<ref name="McCoy">{{cite journal |author=H.E. McCoy |title=New Developments in Materials for Molten-Salt Reactors |journal=Nuclear Applications and Technology |volume=8 |issue=2 |page=156 |year=1970 |display-authors=etal|doi=10.13182/NT70-A28622 }}</ref>

An out-of-[[Nuclear reactor technology|pile]] [[corrosion]] test program was carried out for Hastelloy-N,<ref>{{harvnb|Briggs|1964|pp= 334–343}}.</ref> which indicated extremely low corrosion rates at MSRE conditions. Capsules exposed in the [[Materials Testing Reactor]] showed that salt fission [[power density|power densities]] of more than 200 W/cm<sup>3</sup> had no adverse effects on compatibility of fuel salt, Hastelloy-N, and graphite. Fluorine gas was found to be produced by [[radiolysis]] of frozen salts, but only at temperatures below about {{convert|100|C|order=flip}}.<ref>{{harvnb|Briggs|1964|pp= 252–257}}.</ref>

Components that were developed especially for the MSRE included [[flange]]s for {{convert|5|in|mm|adj=on}} lines carrying molten salt, freeze valves (an air-cooled section where salt could be frozen and thawed), flexible [[control rod]]s to operate in thimbles at {{convert|1200|F}}, and the fuel sampler-enricher.<ref>{{harvnb|Briggs|1964|pp= 167–190}}.</ref> [[Centrifugal pump]]s were developed similar to those used successfully in the aircraft reactor program, but with provisions for remote maintenance, and including a spray system for xenon removal. Remote maintenance considerations pervaded the MSRE design, and developments included devices for remotely cutting and [[brazing]] together {{convert|1+1/2|in}} pipe, removable heater-insulation units, and equipment for removing specimens of metal and graphite from the core.

===Development and construction===
Most of the MSRE effort from 1960 through 1964 was devoted to design, development, and construction of the MSRE. Production and further testing of graphite and Hastelloy-N, both in-pile and out, were major development activities. Others included work on [[nuclear chemistry|reactor chemistry]], development of fabrication techniques for Hastelloy-N, development of reactor components, and remote-maintenance planning and preparations.<ref>{{harvnb|Briggs|1964}}.</ref>

==Operation==
[[File:ORNL Alvin Weinberg MSRE 6Kh.jpg|thumb|[[Alvin M. Weinberg]] noting "6000 full-power hours!" of MSRE operation, in 1967.]]
The MSRE operated for 5 years. The salt was loaded in 1964, and nuclear operation ended in December 1969,<ref name="NATMSRE" /><ref name="Rosenthal">{{Cite journal |author1=M.W. Rosenthal |author2=P.N. Haubenreich |author3=H.E. McCoy |author4=L.E. McNeese  |name-list-style=amp |title=Current Progress in Molten-Salt Reactor Development |journal=Atomic Energy Review IX |pages=601–50 |year=1971 }}</ref> and all the objectives of the experiment were achieved during this period.

Checkout and prenuclear tests included 1,000&nbsp;hours of circulation of flush salt and fuel carrier salt. Nuclear testing of the MSRE began in June&nbsp;1965, with the addition of [[Enriched uranium|enriched <sup>235</sup>U as UF<sub>4</sub>]]-LiF eutectic to the carrier salt to make the reactor [[Criticality (status)|critical]]. After zero-power experiments to measure rod worth and reactivity coefficients,<ref name="Prince">{{Cite journal |author1=B.E. Prince |author2=S.J. Ball |author3=J.R. Engel |author4=P.N. Haubenreich |author5=T.W. Kerlin  |name-list-style=amp |title=Zero-Power Physics Experiments on the MSRE |id=ORNL-4233 |date=February 1968 }}</ref> the reactor was shut down and final preparations made for power operation. Power ascension was delayed when vapors from oil that had leaked into the fuel pump were [[polymerized]] by the radioactive [[Exhaust gas|offgas]] and plugged gas filters and valves. Maximum power, which was limited to 7.4&nbsp;MW<sub>th</sub> by the capability of the heat-rejection system, was reached in May&nbsp;1966.

After two months of high-power operation, the reactor was down for three months because of the failure of one of the main cooling blowers. Some further delays were encountered because of offgas line plugging, but by the end of 1966 most of the startup problems were behind. During the next 15&nbsp;months, the reactor was critical 80% of the time, with runs of 1, 3, and 6&nbsp;months that were uninterrupted by a fuel drain. By March&nbsp;1968, the original objectives of the MSRE had been accomplished, and nuclear operation with <sup>235</sup>U was concluded.

[[File:MSRE U-233 Seaborg (14480987473).jpg|thumb|right|[[United States Atomic Energy Commission|AEC]] Chairman [[Glenn T. Seaborg|Seaborg]] at the MSRE controls in 1968 for startup with U-233.]]

By this time, ample <sup>233</sup>U had become available,<ref>{{cite web |url=http://twugbcn.files.wordpress.com/2010/10/lftr-and-anti-proliferation.pdf |title=Archived copy |access-date=2012-10-11 |url-status=live |archive-url=https://web.archive.org/web/20160304093420/https://twugbcn.files.wordpress.com/2010/10/lftr-and-anti-proliferation.pdf |archive-date=2016-03-04 }} (see PDF page 10) "The MSRE was fueled with 39 kilograms of 233U that contained ~220 parts per million (ppm) of 232U [...that was bred in] various Light Water Reactors that had operated on 235U (such as the [[Indian Point Energy Center|Indian Point]] PWR)"</ref> so the MSRE program was extended to include substitution of <sup>233</sup>U for the uranium in the fuel salt, and operation to observe the new nuclear characteristics. Using the on-site processing equipment the flush salt and fuel salt were fluorinated to recover the uranium in them as UF<sub>6</sub>.<ref name="TM2578" /> <sup>233</sup>UF<sub>4</sub>-LiF [[eutectic]] was then added to the carrier salt, and in October 1968, the MSRE became the world's first reactor to operate on <sup>233</sup>U.

The <sup>233</sup>U zero-power experiments and dynamics tests confirmed the predicted [[neutron]]ic characteristics. An unexpected consequence of processing the salt was that its physical properties were altered slightly so that more than the usual amount of gas was entrained from the fuel pump into the circulating loop. The circulating gas and the power fluctuations that accompanied it were eliminated by operating the fuel pump at slightly lower speed. Operation at high power for several months permitted accurate measurement of the [[neutron capture|capture]]-to-[[nuclear fission|fission]] ratio, for <sup>233</sup>U in this reactor, completing the objectives of the <sup>233</sup>U operation.

In the concluding months of operation, xenon stripping, deposition of fission products, and [[tritium]] behavior were investigated. The feasibility of using [[plutonium]] in molten-salt reactors was emphasized by adding PuF<sub>3</sub> as makeup fuel during this period.

After the final shutdown in December 1969, the reactor was left in standby for nearly a year. A limited examination program was then carried out, including a moderator bar from the core, a [[control rod]] thimble, heat exchanger tubes, parts from the fuel pump bowl, and a freeze valve that had developed a leak during the final [[Shutdown (nuclear reactor)|reactor shutdown]]. The radioactive systems were then closed to await ultimate disposal.

===Statistics===
Parameters and operational statistics:<ref name="msre_folder" />

'''Power''': 8 MW (thermal)<br>
output: 92.8 GWh<br>
equivalent full-power: 11,555 h

'''Fuel salt''': [[fluoride]]<br>
[[cation]]s: 65% [[lithium-7|Li-7]], 29.1% [[beryllium|Be]], 5% [[zirconium|Zr]], 0.9% [[uranium|U]]<br>
weight: 11,260&nbsp;lbs (5,107&nbsp;kg)<br>
melting temp: 813 F (434 C)<br>
inlet temp: 1175 F (635 C)<br>
outlet temp: 1225 F (663 C)<br>
flow rate: 400 gal/min (1514 l/min)<br>
fuel pump circulating: 19,405 h

'''Coolant salt''': fluoride<br>
cations: [[FLiBe|66% Li-7, 34% Be]]<br>
weight: 15,300&nbsp;lbs (6,940&nbsp;kg)<br>
coolant pump circulating: 23,566 h

'''Moderator''': [[nuclear graphite]]

'''Container''': [[Hastelloy]]-N

'''First fuel''': [[uranium-235|U-235]]<br>
first critical: 1 June 1965<br>
thermal output: 72,441 MWh<br>
critical hours: 11,515 h<br>
full-power output equivalent: 9,006 h

'''Second fuel''': [[uranium-233|U-233]]<br>
critical: 2 October 1968<br>
thermal output: 20,363 MWh<br>
critical hours: 3,910 h<br>
full-power output equivalent: 2,549 h

'''Shutdown''': December 1969

==Results==
The broadest and perhaps most important conclusion from the MSRE experience was that a molten salt fueled reactor concept was viable. It ran for considerable periods of time, yielding valuable information, and maintenance was accomplished safely and without excessive delay.

The MSRE confirmed expectations and predictions.<ref name="Rosenthal" /> For example, it was demonstrated that: the fuel salt was immune to radiation damage, the graphite was not attacked by the fuel salt, and the corrosion of Hastelloy-N was negligible. Noble gases were stripped from the fuel salt by a spray system, reducing the [[xenon poisoning|<sup>135</sup>Xe poisoning]] by a factor of about 6. The bulk of the [[fission product]] elements remained stable in the salt. Additions of uranium and plutonium to the salt during operation were quick and uneventful, and recovery of uranium by fluorination was efficient. The neutronics, including critical loading, reactivity coefficients, dynamics, and long-term reactivity changes, agreed with prior calculations.

In other areas, the operation resulted in improved data or reduced uncertainties. The <sup>233</sup>U capture-to-[[Nuclear fission|fission]] ratio in a typical MSR [[neutron spectrum]] is an example of basic data that was improved. The effect of fissioning on the [[redox potential]] of the fuel salt was resolved. The deposition of some elements ("[[noble metals]]") was expected, but the MSRE provided quantitative data on relative deposition on graphite, metal, and liquid-gas interfaces. [[Heat transfer coefficient]]s measured in the MSRE agreed with conventional design calculations and did not change over the life of the reactor. Limiting oxygen in the salt proved effective, and the tendency of fission products to be dispersed from contaminated equipment during maintenance was low.

Operation of the MSRE provided insights into the problem of [[tritium]] in a molten-salt reactor. It was observed that about 6–10% of the calculated 54&nbsp;Ci/day (2.0&nbsp;[[terabecquerel|TBq]]) production diffused out of the fuel system into the containment cell atmosphere and another 6–10% reached the air through the heat removal system.<ref name="Briggs">{{cite journal |author=R.B. Briggs |title=Tritium in Molten-Salt Reactors |journal=Reactor Technology |volume=14 |pages=335–342 |date=Winter 1971–1972 }}</ref> The fact that these fractions were not higher, indicated that something partially negated the transfer of tritium through hot metals.

One unexpected finding was [[Intergranular fracture|inter-granular cracking]] in all metal surfaces exposed to the fuel salt. The cause of the embrittlement was [[tellurium]], a fission product generated in the fuel. This was first noted in the specimens that were removed from the core at intervals during the reactor operation. Post-operation examination of pieces of a control-rod thimble, heat-exchanger tubes and pump bowl parts revealed the ubiquity of the cracking and emphasized its importance to the MSR concept. The crack growth was rapid enough to become a problem over the planned 30-year life of a follow-on thorium breeder reactor. This cracking could in short-term be reduced by adding small amounts of [[niobium]] to the Hastelloy-N. However, further studies were needed to assess the effects of longer exposure times and some interaction parameters for the used mixtures.<ref name="Keiser">{{Citation |last1=Keiser |first1=J.R. |title=Status of Tellurium-Hastelloy N Studies in Molten Fluoride Salts |publisher=Oak Ridge National Laboratories |year=1977 |url=http://www.moltensalt.org/references/static/downloads/pdf/ORNL-TM-6002.pdf |id=ORNL/TM-6002 |url-status=live |archive-url=https://web.archive.org/web/20120324025225/http://www.moltensalt.org/references/static/downloads/pdf/ORNL-TM-6002.pdf |archive-date=2012-03-24 }}</ref>

The operation experience gained with the MSRE showed that the following areas require further investigation for the successful operation of a commercial MSR:

* Maintaining the salt as a liquid in all parts of primary system, particularly in extremities far from the core.
* Tight control of [[tritium]] production and transport from the core (only <20% could be removed due to diffusion and heat removal system in the MSRE).
* Reduction in growth of inter-granular cracks in exposed metal surfaces (due to [[tellurium]], a [[Nuclear fission product|fission product]] of uranium).
* Decommissioining and disposal of the reactor structure and waste salt (approx. costs in 2019 are $10mil/yr <ref>{{Cite web |last=DoE |date=November 2023 |title=Monthly Meeting of the Oak Ridge Site Specific Advisory Board - Approved November 13, 2019, Meeting Minutes |url=https://www.energy.gov/sites/default/files/2020/02/f71/APPROVED%2011.13.19%20ORSSAB%20Minutes.pdf |access-date=May 30, 2024}}</ref>).

==Decommissioning==
As of 2019, the MSRE is in a SAFESTOR state, meaning it still intact but shut down and actively monitored and maintained.<ref>{{Cite web |last=DOE |date=2019 |title=ORSSAB Meeting - November 13, 2019 |url=https://www.energy.gov/orem/articles/orssab-meeting-november-13-2019 |access-date=May 30, 2024}}</ref>

After shutdown, the salt was believed to be in long-term safe storage. At low temperatures, radiolysis can free fluorine from the salt. As a countermeasure, the salt was annually reheated to about {{convert|150|C|order=flip}} until 1989.<ref>Disposition of the Fluoride Fuel and Flush Salts from the Molten Salt Reactor Experiment at Oak Ridge National Laboratory, available as PDF
[http://www.osti.gov/energycitations/purl.cover.jsp?purl=/206448-dTGNFu/webviewable/DispositionofthefluoridefuelandflushsaltsfromtheMoltenSaltReactorexperimentatOakRidgeNationalLaboratory.pdf] {{webarchive|url=https://web.archive.org/web/20130522211017/http://www.osti.gov/energycitations/purl.cover.jsp?purl=%2F206448-dTGNFu%2Fwebviewable%2FDispositionofthefluoridefuelandflushsaltsfromtheMoltenSaltReactorexperimentatOakRidgeNationalLaboratory.pdf|date=2013-05-22}}</ref> But beginning in the mid-1980s, there was concern that radioactivity was migrating through the system, reported by an ORNL employee who was among 125 people working above the reactor, which had not been decontaminated or decommissioned. [[United States Department of Energy|Department of Energy]] Oak Ridge Operations Manager Joe Ben LaGrone ordered evacuation of 125 employees, based on findings reported to him inspector William Dan DeFord, P.E.<ref>{{Cite web|url=https://web.ornl.gov/info/ridgelines/nov12/msre.htm|title=Ending the MSRE}}</ref>

Sampling in 1994 revealed concentrations of uranium that created a potential for a nuclear [[criticality accident]], as well as a potentially dangerous build-up of fluorine gas: the environment above the solidified salt was approximately one atmosphere of fluorine.{{citation needed|date=March 2020}} The ensuing decontamination and decommissioning project was called "the most technically challenging" activity assigned to [[Bechtel Jacobs]] under its environmental management contract with the U.S. Department of Energy's Oak Ridge Operations organization.

In 2003, the MSRE cleanup project was estimated at $130&nbsp;million, with decommissioning expected to be completed in 2009.<ref>R. Cathey Daniels, [http://nl.newsbank.com/nl-search/we/Archives?p_action=doc&p_docid=110AF8020E01FF40&p_docnum=2&p_theme=gatehouse&s_site=TORB&p_product=TORB Elegant experiment puts wallop on cleanup], ''The Oak Ridger'', April 8, 2003.</ref> Removal of uranium from the salt was completed in March 2008, however still leaving the salt with the fission products in the tanks.<ref>{{cite web |url=http://www.oakridge.doe.gov/em/ssab/Publications/Advocates/4-10.pdf |title=Archived copy |access-date=2012-12-08 |url-status=dead |archive-url=https://web.archive.org/web/20130222013818/http://www.oakridge.doe.gov/em/ssab/Publications/Advocates/4-10.pdf |archive-date=2013-02-22 }}</ref> Much of the high cost was caused by the unpleasant surprise of fluorine and uranium hexafluoride evolution from cold fuel salt in storage that ORNL did not defuel and store correctly, but this has now been taken into consideration in MSR design.<ref name="moltensalt.org">{{cite web |url=http://moltensalt.org/references/static/downloads/pdf/ORNL-TM-3144.pdf |title=Fluorine Production and Recombination in Frozen MSR Salts after Reactor Operation [Disc 5&#93; |access-date=2012-10-24 |url-status=live |archive-url=https://web.archive.org/web/20120502073808/http://moltensalt.org/references/static/downloads/pdf/ORNL-TM-3144.pdf |archive-date=2012-05-02 }}</ref>

A potential decommissioning process has been described;<ref>[http://books.nap.edu/openbook.php?record_id=5538&page=R1 Evaluation of the U.S. Department of Energy's Alternatives for the Removal and Disposition of Molten Salt Reactor Experiment Fluoride Salts] {{webarchive|url=https://web.archive.org/web/20070513073838/http://books.nap.edu/openbook.php?record_id=5538&page=R1 |date=2007-05-13 }} (1997), Commission on Geosciences, Environment and Resources</ref> uranium is to be removed from the fuel as the hexafluoride by adding excess fluorine, and plutonium as the [[plutonium dioxide]] by adding [[sodium carbonate]].

{{coord|35.92178|-84.30672 |format=dms |region:US-TN_type:landmark |display=title}}

==See also==
*[[Thorium fuel cycle]]
*[[Fuji MSR]]
*[[Thorium-based nuclear power]]

==References==
{{Reflist|30em}}
*{{Cite journal
|last = Briggs|first = R. B.
|publication-date = November 1964|year = 1964
|title = MSR Program Semiannual Progress Report for the period ending July 31, 1964
|publisher = (ORNL-3708) (66.3&nbsp;[[Megabyte|MB]] [[PDF]]), [[Oak Ridge National Laboratory]], [[United States Atomic Energy Commission|U.S. AEC]]
|url = http://www.energyfromthorium.com/pdf/ORNL-3708.pdf
|access-date = 2008-05-21
}}

==Further reading==
*[http://moltensalt.org/references/static/downloads/pdf/ORNL-TM-2111.pdf MSRE Safety analysis]

==External links==
*{{YouTube|id=tyDbq5HRs0o|title="The Molten-Salt Reactor Experiment" (1969) Oak Ridge National Laboratory}}, a film published by Atomic Energy Commission
*{{YouTube|id=knofNX7HCbg|title=Alvin Weinberg's Molten Salt Reactor Experiment}}
*[http://info.ornl.gov/sites/publications/files/Pub20808.pdf An Account of Oak Ridge National Laboratory’s Thirteen Nuclear Reactors] (from ORNL; includes a section on the MSRE)
*[https://public.ornl.gov/conferences/MSR2015/ 2015 Workshop on Molten Salt Reactor Technologies] ("Commemorating the 50th Anniversary of the Startup of the MSRE"), including a [https://public.ornl.gov/conferences/MSR2015/pdf/20151022104041810.pdf 50th anniversary brochure], [https://public.ornl.gov/conferences/MSR2015/Posters.cfm posters], and a [https://public.ornl.gov/conferences/MSR2015/pdf/Gehin%20-%20MSR%20Program%20History%20-%20rev%203.pdf history of the ORNL molten salt program]
{{Oak Ridge National Laboratory|state=expanded}}
{{Nuclear fission reactors}}

[[Category:1965 in science]]
[[Category:Molten salt reactors]]
[[Category:Nuclear reactors]]
[[Category:Oak Ridge National Laboratory]]
[[Category:Science experiments]]

[[es:Reactor de sal fundida]]