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Sunday, March 04, 2012

The SL-1 Accident, January 3, 1961

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On January 3, 1961, the United States experienced the first nuclear power plant accident in the nation's history. Because of it, the design of both military and civilian reactors changed. Despite its importance in the development of safe nuclear power, the SL-1 accident remains unknown to most people 51 years after it occurred. 

The Stationary Low-Power Reactor Number One, or SL-1, was an experimental nuclear reactor designed and built for the US Army. It was created to serve as the prototype model for a new class of reactor which was to be deployed at small, remote military bases. Specifically, SL-1 units were to be used to power the radar sites that the United States and her allies operated near the Arctic Circle. The reactors were designed to provide 200 kW of electrical power and 400 kW of thermal heat. The SL-1 prototype was placed 40 miles west of Idaho Falls, Idaho at the National Reactor Testing Station.

Almost all reactors in use today are pressurized water reactors. In a pressurized water reactor, the primary coolant, which is in contact with the radioactive fuel, is never allowed to boil. Instead, it's heat is transferred to the secondary coolant by use of a steam generator. The secondary coolant, which is kept at a much lower pressure than the primary coolant, flashes to steam and is then used to turn a turbine and, ultimately, produce electricity. The primary coolant moves in a self-contained loop and is shielded from the outside world by a heavily-built containment building that is often made of reinforced concrete. The secondary coolant is not radioactive, and so poses no radiation danger as it travels outside the containment building.

SL-1 was a boiling water reactor. In this type of reactor, the primary coolant is allowed to boil and the radioactive steam is used to directly provide heat or turn a steam turbine. There is no secondary system. This type of design has the advantage of being simpler than a pressurized water reactor, requiring fewer pumps and less piping. While boiling water reactors are still being designed today, the SL-1 came very early in the history of nuclear power. As such, it contained at least one critical design flaw, a flaw that would prove deadly.

On December 21, 1960, SL-1 was shut down for routine maintenance. It remained down for the holidays. On January 3, 1961, the reactor was undergoing procedures prior to startup. At 9:01PM, SL-1 went prompt critical, meaning that the number of fission events increased exponentially and rapidly. It was the nuclear equivalent of throwing a lighted match into a room full of gasoline vapors. The heat generated caused the primary coolant to vaporize with explosive force. It was later estimated that reactor power spiked at 20,000 MW, over 30,000 times it's designed output. The entire reactor vessel was propelled upward and the center control rod pushed out of the vessel, pinning one of the operators on duty to the ceiling, killing him instantly. Two other military personnel were killed as well. The three were Army Specialists John Byrnes and Richard McKinley and Navy Electrician's Mate Richard Legg. Fortunately, there was no one else present at the site.

A crew of firemen arrived on the scene nine minutes later and, at first, could find nothing out of the ordinary as the reactor building looked normal from the outside. When they approached, however, their radiation detectors spiked and the men retreated. After a medical response team arrived with the proper equipment, the men were broken into teams. Each team would enter the reactor building for one minute before leaving, and no man would enter the area more than once.

Working quickly, the teams retrieved one operator who was still breathing; he died an hour later. A second body was located but not recovered. The third body was not found for some time due to the debris strewn about the reactor building. After a quick look for the third man, it was decided to stop searching due to the potential health risks to the rescue teams. The bodies were buried in lead-lined caskets sealed with concrete and placed in steel vaults. One man, Richard McKinley, was buried in Arlington National Cemetery; the other two were buried in their home towns.

After extensive study of the accident, it was concluded that the central control rod, which was only to be moved three inches, had in fact been raised 20 inches. This allowed the reactor to instantly go supercritical and experience the gigantic power spike. The question that remained was why this had been done. The man at the top of the reactor was supposedly pulling the rod out of the reactor in order to connect it to its control motor. As mentioned, this would've taken a motion of no more than three inches. At first, it was concluded that the man moving the rod had wanted to commit suicide, although this is unlikely: during training, the technicians had been told that even the complete removal of one control rod could not cause an accident like that which occurred.

Two theories now seem more likely. First, some of the five control rods had become difficult to move in previous months---the technicians had referred to them as being "sticky". It is possible that the central rod was being sticky and that the technician on top of the reactor simply pulled on the rod too hard, causing it to come out too far. According to people who worked on the reactor, this would be very difficult to do, but not impossible.

Second, the technicians may have been exercising the rods; that is, moving them up and down so as to loosen them up. Since they had been trained that even full extraction of one rod could not cause a problem, it seems natural that they may have exercised the rods by moving them well beyond their normal lengths of travel.

After the SL-1 accident, reactor design was refined so that the removal of a single control rod could not cause a prompt critical condition. Also, all future designs included more control rods so that no single rod could have as much effect on the reactor. In addition, procedures for reactor operation and the associated documentation were expanded greatly and became more formalized than before.

The Army continued to operate small nuclear reactors for the next few years, but budget constraints and a lack of support from the upper ranks led to the abandonment of the Army's nuclear power program in 1965. As of today, the Navy remains the only branch of the US Armed Forces to utilize nuclear power.

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