Clean Energy Prof: The Fermi Chronicles - Part 18: Nuclear Events

This was an event that was unique in that it caused the first nuclear fatality. (at least in civilian nuclear power-generating) This event was the result of poor design and human error. Ironically, it occurred when the reactor had been shut down for almost 2 weeks. Since all 3 operators were killed in the accident, the event had to pieced by observing the evidence, versus eyewitness accounts. This was also the most rapid event in the nuclear industry (to my knowledge anyway), being completely done in all of 3 seconds.

During a holiday shutdown, core maintenance was to be performed by 3 operators. The SL-1 reactor was a small (3 MW) experimental boiling water reactor (Fermi 2 is rated at 1,100 MW). Back in 1961, the manual lifting of control rods was common. During maintenance, one of the control rods was to be lifted a total of 3 inches so as to connect it to the control rod drive mechanism. It was lifted 20 inches instead. No one knows why it was lifted out so much, but because of the poor reactor design, it went prompt critical instantly vaporizing the surrounding water and causing a steam explosion. This had the effect of shooting the control rod fully out of the core and impaling one of the operators to the ceiling (nasty!). The steam explosion lifted the entire reactor head 9 feet off the ground, killing the other two operators.

Theories as to why the control rod was lifted so far out has been purely conjecture. The media ran with a murder/suicide storyline (so sensationalism in the media is not a new thing after all). The far more likely theory is that the rod became stuck and you just know what a guy with two of his buddies watching has to do - get it unstuck. Yeah - tug on it with all your strength. Unfortunately, once it became loose, it was quite impossible to stop at just 3 inches. It's a guy thing. Wikipedia (don't always trust them!) has an entry on the aftermath of the accident:

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 9:01pm, the time of the accident. The first response crew, of firemen, arrived nine minutes later 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. On approaching the reactor building, their radiation detectors jumped sharply to above their maximum range limit, and they withdrew, unable to know whether they could safely proceed or for how long they could remain.

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. Richard Leroy McKinley is buried in section 31 of Arlington National Cemetery.

As with prior events in the early days of nuclear energy, much was learned. Probably the most important nugget of information learned here was that reactors shouldn't be designed to go prompt critical if a single control rod is removed. All modern reactors are designed with that criterion, called the "one stuck rod" criterion. Also, only highly-trained operating personnel may dictate what occurs on or around the reactor core. None of the 3 men killed had such training. They were basically maintenance workers. Very specific procedures are now in place for any maintenance whatsoever.

For the sake of nostalgia, here is the SL-1 documentary video from the Atomic Energy Commission back in the day:

Clean Energy Prof

Sunday, June 28, 2009

The Fermi Chronicles - Part 18: Nuclear Events - SL-1 Event, Idaho, 1961

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