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The Fermi Nuclear Disaster
“Now, in the next phase of civilian nuclear power development, we can look forward to advance converter and breeder reactors which will result in more efficient and economical use of our nuclear fuel resources…I believe we can take heart from the fact that, twenty years after its inception, the nuclear age has not eliminated man -- in fact, it has imposed a greater discipline in his conduct. We can also be gratified that the atom has been used for peaceful and constructive purpose. In the years ahead we will continue to make the atom a source of man’s cooperation and progress. May the atom, as all science, be in time to become a tool for understanding ... for well-being …for betterment of the human condition.” – Vice President Hubert Humphrey, Nov. 15, 1965 [1]


Report of the President’s Commission on the Fermi Nuclear Disaster
Joint Committee on Atomic Energy [2]​


October 30, 1969

Preface

This report reviews the causes and consequences of the meltdown of the Enrico Fermi Nuclear Generating Station on October 5, 1966 near Monroe, Michigan, and 30 miles from downtown Detroit. Evidence of what occurred is fragmentary and based on the recollection of witnesses at the plant.

The precise cause of the accident remains unclear. The most common theory is that a small metal object broke loose inside Fermi's reactor vessel, blocking the flow of sodium coolant. A coolant blockage would explain the increase in temperature that was observed in the core prior to the accident.

Whatever the cause, the incident began at approximately 3:05 PM, when the operator, Mike Wilber, noticed that there were erratic changes in the activity of the reactor. He also noticed that the control rods (were used to control the nuclear chain reaction in the core) were further out than they should be. Just four minutes later, high radiation alarms sounded. Technicians found that the temperature of several fuel rod subassemblies had increased to 700 °F (370 °C). Automatic control devices detected that an elevated level of radioactivity was leaking out into the containment building [3][4]. About this time, the Monroe County Sheriff's Office and the Michigan State Police received calls from someone who claimed to work at the Fermi plant. The unidentified caller reported that there had been an accident at Fermi 1, but said that the situation should not be publicized. [5]

At 3:20 PM, eleven minutes after the radioactivity alarm had gone off, the engineers decided to manually “scram” the reactor. Normally, performing a “scram” shuts down the reactor by inserting neutron-absorbing boron rods. This first attempt to “scram” the reactor put one of the control rods into place. However, the plant personnel were unable to reset another one of the control rods in the subassembly, which remained jammed 6 inches from the full “down” position [6]. Despite additional attempts to scram the reactor, the stubborn rod could not be closed down fully.

By 4:15 PM, the temperature in the core had increased to 1,600°F (870°C), and the sodium inside the reactor began to boil [7]. About this time, Vice President and Assistant General Manager of the Power Reactor Development Company, Walter J. McCarthy, arrived at the plant from Detroit. Knowing that a full meltdown could destroy the reactor and release deadly radiation into the atmosphere, he immediately called a meeting to discuss options with other plant personnel, including Mike Wilber, Bill Olson, and Ken Johnson [8]. By this time, the melting had spread from the single plugged-up subassembly to several others, and molten, waxy uranium began to fall down through the core [9].

At approximately 4:35 PM, just 90 minutes after problem was discovered, the temperature inside the reactor had increased to 2,000 °F, greater than the boiling point of sodium. The interior of the reactor had now degenerated into a molten mass of sodium and uranium [10]. Mr. McCarthy, Mr. Wilber, Mr. Olson, Mr. Johnson and several others made one last attempt to prevent the reactor from reaching criticality.

Tragically, they were unsuccessful in this final attempt. An explosion destroyed the reactor shortly before 4:50 PM Eastern Time on October 5, 1966 [11]. These individuals are believed to have perished at that time. This devastating explosion ruptured the containment building and blew the 1,000-ton cap off of the reactor, releasing radioactive isotopes of cesium, strontium, and iodine into the atmosphere. After the destruction of the reactor, the remaining plant employees evacuated the facility.

The explosion is believed to have occurred when molten core material had dropped on top of fuel that had melted and re-congealed [12]. The fuel reassembled itself at the bottom of the reactor, assuming a critical configuration. The fuel in the core then blew itself apart in a nuclear explosion, albeit one with less explosive power than a nuclear bomb [13]. When the congealed sodium inside the reactor made contact with the air, it instantaneously caught fire [14].

Local authorities began to arrive at the plant at this time. The Monroe Fire Department tried in vain to put out the fire at the plant, but pouring water on the reactor only fanned the flames. After several firefighters began to show signs of radiation sickness, the fire was allowed to burn. Winds, blowing out the northwest, carried the contamination to the southeast, away from Detroit and over Lake Erie. [15]

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NOTES:

[1] Taken verbatim from a speech Humphrey gave in Washington, D.C. at the Nuclear Society-Atomic Industrial Forum on November 15, 1965. http://www2.mnhs.org/library/findaids/00442.xml

[2] Based loosely on this report from OTL about Three Mile Island: http://large.stanford.edu/courses/2012/ph241/tran1/docs/188.pdf

[3] We Almost Lost Detroit (WALD), p. 196-7: “Just a few minutes [later], at 3:05 PM to be exact, Mike Wilber noticed another problem. For the amount of heat and power that was coming out of the reactor, the control rods should have been raised only six inches out of the core. Instead they were a full nine inches out …Suddenly, as Wilber was standing in front of the temperature instruments behind the control panel, radiation alarms went off. It was exactly 3:09 PM.”

[4] Detroit News (DN), Nov. 13, 1968, 1-F: “The operator noticed that the control rods, used to control the nuclear chain reaction in the core, were further out than they should be. At 3:09 pm, high radiation alarms sounded from the domed reactor building and the fusion product detection building. The operator began reducing the power and at 3:20 pm, he ‘scrammed’ the reactor manually, shutting it down by inserting the boron rods all the way.”

[5] WALD, p. 2: “About the same time, some 100 miles away, Captain Buchanan of the Michigan State Police in Lansing was alerted by a similar phone call.”

[6] Everything up to this point is as OTL. The POD rests on the behavior of a single stubborn control rod. WALD, p. 201: “And so, at 3:20 PM, eleven minutes after the radioactive alarm had gone off, the decision was made to manually scram the reactor…All the rods went down into the core normally, except one. It stopped six inches from the full ‘down’ position. This was no time to take a chance. A second manual scram signal was activated. The reluctant rod finally closed down fully.”

[7] For a detailed technical description of what happens in a fast breeder reactor accident, read Breeder Reactor Safety: Modeling the Impossible by Charles R. Bell (1969), available here: https://fas.org/sgp/othergov/doe/lanl/pubs/00416676.pdf

[8] WALD, p. 201-2: “Walter McCarthy was in a conference in Downtown Detroit when it happened. He got a call from Bill Olson, the plant supervisor, who told him that there definitely was evidence of fuel damage, and that the containment building had been isolated with high radiation levels…Almost immediately after he arrived at the plant, McCarthy called a meeting. Every available key man of the Fermi team was there – Olson, Wilber, Jens, Amarosi, Johnson, and others…Alexanderson was to arrive later.”

[9] WALD, p. 187: “If the coolant flow was ever blocked, McCarthy’s computations figured that the meltdown would not spread from the single plugged-up assembly…If it spread to others, there would be hell to pay. Some scientists were sure that if the melting spread to other subassemblies, the results could lead to disaster as molten, waxy uranium fell down through the core.”

[10] As happened at the SL-1 reactor in OTL, a much smaller experimental reactor. WALD, p.34: “Nearly half of the core of the small reactor had melted, foaming and frothing as it did so. The temperatures had reached over 2,000o F - much more than the melting point of the fuel and the stainless steel cladding. The liquid sodium coolant had boiled over, pushing the uranium outward from the center of the core and blocking coolant channels. Partially melted rods had dropped into a molten mass below the core, forming ...a eutectic mixture.”

[11] Bell, p. 107: “If a large fraction of the original fuel has managed to remain within the active core region, a super-prompt-critical excursion can occur that heats the fuel in milliseconds to high temperatures and pressures. The fuel in the core, in essence, blows apart. While the dispersal of the fuel terminates the neutronic excursion, the pressure surge poses a direct mechanical threat and the possibility of breached containment.”

[12] DN, Nov. 11, 1968, p. 13-A: “'The worst accident we can conceive of’, says Walter J. McCarthy Jr. … ‘would be for half the core material to melt and recongeal in the space below, and then for the other half to melt suddenly and drop about six feet on top of it.’”

[13] From a paper written in 2010 called It’s Time to Give Up on Breeder Reactors by Cochran et al.:

https://www.princeton.edu/sgs/publications/articles/Time-to-give-up-BAS-May_June-2010.pdf Cochran et al, p. 53: “Furthermore, if the core heats up to the point of collapse and suffers a meltdown, the fuel can assume a more critical configuration and blow itself apart in a small nuclear explosion. Whether such an explosion could release enough energy to rupture reactor containment and cause a Chernobyl-scale release of radioactivity into the environment is the subject of major concern and debate.”

[14] Cochran et al., p. 52: “Although sodium has some safety advantages, it also has some serious drawbacks. It reacts violently with water and burns if it exposed to air.”

[15] The most detailed hour-by-hour weather data can be found for the Canadian station at Windsor, Ontario (about 25 miles northeast of the plant).

http://climate.weather.gc.ca/climate_data/hourly_data_e.html?timeframe=1&Year=1966&Month=10&Day=5&hlyRange=1953-01-01%7C2014-10-02&dlyRange=1940-08-01%7C2014-10-01&mlyRange=1940-01-01%7C2014-10-01&StationID=4716&Prov=ON&urlExtension=_e.html&searchType=stnName&optLimit=specDate&StartYear=1966&EndYear=1967&selRowPerPage=25&Line=0&searchMethod=contains&txtStationName=windsor

At 5:00 PM, the temperature was 52o F (11.1o C), winds were 23 km/h out of the northwest, and the skies were mostly cloudy.
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