5.1.1 The SL-1 Accident

A small (thermal capacity, 3 ^MW) experimental boiling-water reactor called SL — 1 (Stationary Low-Power Plant No. 1), installed at the U. S. National Reactor Test­ing Station (NRTS) in Idaho, was destroyed on January 3, 1961, as a result of the manual withdrawal of a control rod while the reactor was shut down. The re­actor had been shut down for maintenance and to install additional instrumen­tation. This work was completed during the day shift on January 3, and it was the job of the three-man crew of the 4-12 p. m. shift to reconnect the control rods. The installation of the additional instrumentation required disconnecting the control rods, leaving them fully immersed in the reactor. However, when they were disconnected, the rods could be lifted out manually. Lifting the con­trol rods by about 40 cm (16 in.) was sufficient to make the reactor critical.

At 9:01 p. m. onJanuary 3, alarms sounded at the fire stations and security head­quarters of the NRTS, which was located some distance from the SL-1 facility. Upon investigation it was found that two operators had been killed (a third died later) and that high radiation levels were present in the building. The exact reason for the accident has never been discovered; the removal of the control rod could have been accidental or deliberate, but no one will ever know.

Based on a careful examination of the remains of the core and the vessel during the cleanup phase, it was concluded that the control rod had been with ■
drawn by about 50 cm (20 in.), sufficient for a very large increase in reactivity. The resulting power surge caused the reactor power to reach 20,000 MW in about 0.01 s. This caused the plate-type fuel to melt. The molten fuel interacted with the water in the vessel, and the explosive formation of steam caused the water above the core to rise with such force that when it hit the lid of the pres­sure vessel, the vessel itself rose 3 m (9 ft) in the air and then dropped back ap­proximately to its original position.

Two main lessons were learned from this incident:

1. It is unsatisfactory to have any reactor system (even a small experimental re­actor of this kind) in which removal of control rods is not prevented by a suitable series of interlocks. Removal of a control rod as in the SL-1 accident would be impossible in a modern power reactor.

2. Ejection of water from the core normally leads to a decrease in reactivity, which automatically shuts down the reactor by additional void formation. However, as the SL-1 accident showed, a very fast increase in reactivity can melt the fuel before significant voids are formed to shut down the fission re­action. This effect was demonstrated deliberately in another U. S. reactor test: the so-called BORAX reactor was deliberately brought into this condition and destroyed in 1954.

Explosions arising from the interaction of molten fuel and liquid coolant will be discussed further in Chapter 6.