In 1968, operators shut down a nuclear research reactor for modifications to its cooling system. A 10-inch diameter pipe connected to the water-filled pool containing the reactor core had to be cut open for this work. There was no valve between the pool’s wall and the location where the pipe needed to be cut. Thus, there was no valve that could be closed to isolate the pool from the work being performed.
Engineers had two choices to prevent water in the reactor pool from draining through the pipe after it was cut open. The irradiated fuel assemblies could be removed from the pool so its water level could be dropped below the point where the pipe connected to its wall, or the pipe could be blocked between the pool and the location of the proposed cut. To save time, they decided to leave the irradiated fuel assemblies in the reactor pool and block the pipe.
The pipe was somewhat bigger than a regulation-sized basketball. So, workers wrapped tape around a basketball until it was about two inches larger. They lowered the basketball into the pool and inserted it into the pipe’s opening. The basketball was inflated to seal it firmly within the pipe. Then they cut into the pipe and started working.
They did not work for long. Water in the pool pressed against the basketball until it popped out the open end of the pipe. Nearly 14,000 gallons of water drained into the basement in five minutes. A gate inside the reactor pool, which was supposed to have been removed during this work but fortuitously remained in place, kept more water from draining. Had this gate been removed, the water level would have dropped below the top of the irradiated fuel assemblies in the reactor core. According to the official report on this event, two-thirds of the fuel assemblies would have been uncovered.
Actually, all the fuel assemblies would have been uncovered for the upper two-thirds of their length. Water performs two vital functions inside nuclear reactors. It cools the fuel assemblies, which continue to produce substantial amounts of heat long after the reactor is shut down. It also shields workers from the intense radioactivity emitted by the fuel assemblies. Uncovering the fuel assemblies in the reactor core could have triggered a meltdown or, at the very least, produced an extremely serious radiation hazard for plant workers.
The nuclear industry has made tremendous improvements in safety since 1968. This near-miss is extremely unlikely to be repeated. Diversity and defense-in-depth are key elements in the nuclear industry’s safety program. Two basketballs would be used today.
Seriously, the value of diversity and defense-in-depth cannot be understated. Since this event, many nuclear plant owners have employed freeze seals to block segments of piping for maintenance downstream. Liquid nitrogen blankets were wrapped around the pipes to freeze the water inside them. Contingency plans featuring pre-staged backup equipment were used in case the freeze seals failed. Some plans relied on another liquid nitrogen blanket to establish a second freeze seal. Other plans relied on hydraulic jacks that would crimp the open ends of the pipe together to limit the leakage rate. Contingency plans serve as a safety net backing up the front-line. They are cheap insurance compared to the cost of the damage that can be done when front-lines fail without safety nets in place.
“Fission Stories” is a weekly feature by Dave Lochbaum. For more information on nuclear power safety, see the nuclear safety section of UCS’s website and our interactive map, the Nuclear Power Information Tracker.