In light of the 25th anniversary of Chernobyl, which has followed the news of the disaster at the Fukushima Daiichi Nuclear Power Plant, I did a bit of poking around and discovered there was a nuclear accident prior to Three-Mile Island (1979).

The SL-1, or Stationary Low-Power Reactor Number One, was a United States Army experimental nuclear power reactor which underwent a steam explosion and meltdown on January 3, 1961, killing its three operators. The direct cause was the improper withdrawal of the main control rod, responsible for 80% of neutron moderation in the poorly-designed reactor core. The event is the only known fatal reactor accident in the United States.

The facility, located at the National Reactor Testing Station approximately forty miles (60 km) west of Idaho Falls, Idaho, was part of the Army Nuclear Power Program and was known as the Argonne Low Power Reactor (ALPR) during its design and build phase. It was intended to provide electrical power and heat for small, remote military facilities, such as radar sites near the Arctic Circle, and those in the DEW Line. The design power was 3 MW (thermal). Operating power was 200 kW electrical and 400 kW thermal for space heating. NASA system failure studies have cited that the core power level reached nearly 20 GW in just four milliseconds, precipitating the reactor accident and steam explosion.

The accident

On December 21, 1960, the reactor was shut down for maintenance, calibration of the instruments, installation of auxiliary instruments, and installation of 44 flux wires to monitor the neutron flux levels in the reactor core. The wires were made of aluminum, and contained slugs of aluminum-cobalt alloy.

On January 3, 1961 the reactor was being prepared for restart after a shutdown of eleven days over the holidays. Maintenance procedures were in process, which required the main central control rod to be manually withdrawn a few inches to reconnect it to its drive mechanism; at 9:01 p.m. this rod was suddenly withdrawn too far, causing SL-1 to go prompt critical instantly. In four milliseconds, the heat generated by the resulting enormous power surge caused water surrounding the core to begin to explosively vaporize. The water vapor caused a pressure wave to strike the top of the reactor vessel. This propelled the control rod and the entire reactor vessel upwards, which killed the operator who had been standing on top of the vessel, leaving him impaled to the ceiling by the control rod. The other two military personnel, a supervisor and a trainee, were also killed. The victims were Army Specialists John A. Byrnes (age 27) and Richard Leroy McKinley (age 22), and Navy Electrician’s Mate Richard C. Legg (age 26).

Events after the power excursion

There were no other people at the reactor site. The ending of the nuclear reaction was caused solely by the design of the reactor and the basic physics of heated water and core elements vaporizing, separating the core elements and removing the moderator.
Heat sensors above the reactor set off an alarm at the central test site security facility at 9:01 p.m., the time of the accident. False alarms had occurred in the morning and afternoon that same day. The first response crew, of six firemen, arrived nine minutes later, expecting another false alarm. and initially noticed nothing unusual, with only a little steam rising from the building, normal for the cold (−20 °F or −30 °C) night. The control building appeared normal. The firefighters entered the reactor building and noticed a radiation warning light. Their radiation detectors jumped sharply to above their maximum range limit as they were climbing the stairs to SL-1’s floor level. They were able to peer into the reactor room before withdrawing.

At 9:17 p.m., a health physicist arrived. He and a fireman, both wearing air tanks and masks with positive pressure in the mask to force out any potential contaminants, approached the reactor building stairs. Their detectors read 25 Roentgens per hour (R/hr) as they started up the stairs, and they withdrew.

Some minutes later, a health physics response team arrived with radiation meters capable of measuring gamma radiation up to 500 R/hr—and full-body protective clothing. One health physicist and two firefighters ascended the stairs and, from the top, could see damage in the reactor room. With the meter showing maximum scale readings, they withdrew rather than approach the reactor more closely and risk further exposure.

Around 10:30 p.m., the supervisor for the contractor running the site and a contractor health physicist arrived. They entered the reactor building and found two mutilated men: one clearly dead, the other moving slightly. With a one minute and one entry per person limit, a team of five men with stretchers recovered the operator who was still breathing; he did not regain consciousness and died of his head injury at about 11 p.m. Even stripped, his body was so contaminated that it was emitting about 500 R/hr. They looked for but did not find the third man. With all potential survivors now recovered, safety of rescuers took precedence and work was slowed to protect them.

On the night of 4 January, a team of six volunteers used a plan involving teams of two to recover the second body. Radioactive gold 198Au from the man’s brass watch buckle and copper 64Cu from a screw in a cigarette lighter subsequently proved that the reactor had indeed gone supercritical.

The third man was not discovered for several days because he was pinned to the ceiling above the reactor by a control rod. On 9 January, in relays of two at a time, a team of eight men, allowed no more than 65 seconds exposure each, used a net and crane arrangement to recover his body.

The bodies of all three were buried in lead-lined caskets sealed with concrete and placed in metal vaults with a concrete cover. Army Specialist Richard Leroy McKinley is buried in section 31 of Arlington National Cemetery.

Removal of core from SL-1
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Consequences

The remains of the SL-1 building did not go to the Burial Ground. After abandoning early thoughts of restoring the building, GE concluded that hauling the contaminated debris to the Burial Ground, a distance of sixteen miles and partly on Highway 20/26 would subject laborers to too much avoidable risk. Instead, it built two large pits and a trench about 1,600 feet away from the SL-1 compound. The walls of the silo, the power conversion and fan-floor equipment, the shielding gravel, and the contaminated soil that had been gathered during the clean-up all went into the pits. Three feet of clean earth shielded the material. An exclusion fence with hazard warnings went up around the area, the only monument to the reactor.

From the Arlington National Cemetary Records:

HEADQUARTERS
MILITARY DISTRICT OF WASHINGTON
WASHINGTON 28, D.C.
In Reply Refer To
AMHRC 31 January 1961

SUBJECT: Internment of Radioactive Remains

TO: Superintendent
Arlington National Cemetery
Arlington 11, Virginia

1. Radioactive remains of SP4 Richard L. McKinley were interred at Arlington National Cemetery on 25 January 1961.

2. It is desired that the following remark be placed onthe permanent record, DA Form 2122, Record ofInternment:

“Victim of nuclear accident. Body is contaminated with long-life radio-active isotopes. Under no circumstances will the body be moved from this location without prior approval of the Atomic Energy Commission in consultation with this headquarters.”

I have not been able to find more details post-event, such as the health of those that were exposed during the rescue of the three workers killed. Life, news, and facts were handled much differently during these days….

Photo from 1961 of the damaged top of the SL-1 reactor vessel reused in 1981 to convey a safety message
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References:
SL-1, The Aftermath
SL-1.co.tv
SL-1 Wikipedia
Full video on SL-1 from Public Resource Org

Other information of interest:
Measures Relative to the Biological Effect of Radiation Exposure

Published by Michelle

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