Every engineer is going to have a bad day, but only an unlucky few will have a day so bad that it registers on a seismometer.
We’ve always had a morbid fascination with engineering mega-failures, few of which escape our attention. But we’d never heard of the Super-Kamiokande neutrino detector implosion until stumbling upon [Alexander the OK]’s video of the 2001 event. The first half of the video below describes neutrinos in some detail and the engineering problems related to detecting and studying a particle so elusive that it can pass through the entire planet without hitting anything. The Super-Kamiokande detector was built to solve that problem, courtesy of an enormous tank of ultrapure water buried 1,000 meters inside a mountain in Japan and lined with over 10,000 supersized photomultiplier tubes to detect the faint pulses of Chernkov radiation emitted on the rare occasion that a neutrino interacts with a water molecule.
Those enormous PM tubes would be the trigger for the sudden demise of the Super-K , which is covered in the second half of the video. During operations to refill the observatory after routine maintenance, technicians noticed a bang followed by a crescendo of noise from the thirteen-story-tall tank. They quickly powered down the system and took a look inside the tank to find almost every PM tube destroyed. The resulting investigation revealed that the tubes had failed in sequence following the sudden implosion of a single tube at the bottom of the tank. That implosion caused a shock wave to propagate through the water to surrounding tubes which exceeded their design limits, causing further implosions and further destruction. The cascading implosion took a full ten seconds to finish its wave of destruction, which destroyed $7 million worth of tubes.
The interesting part about this is the root cause analysis, which boils down to the fact that you shouldn’t stand on 50-cm photomultiplier tubes. Also at fault was the testing regimen for the tubes, which the project engineers anticipated could cause a cascading implosion. They tested this but were unable to cause a cascade failure, leading them to the conclusion it wasn’t likely to happen. But analysis of the destruction revealed a flaw in the testing, which should give pause to anyone who ever had to design a test like this before.
Luckily, nobody was killed or even hurt during the Super-K incident. The observatory was repaired with upgraded tubes and remains in service to this day, with an even bigger Hyper-Kamiokande detector in the works. We’ve covered neutrino observatories before, so check that out if you want more background on the science.