I dumbed into - and started to watch - this yesterday. Ran into your post a few moments ago and figured I needed to finish it. Pretty cool facility, and what an uncanny failure mode. Great engineering case history!
I like knowing what I'm watching when I go in (and didn't hear any explanation of what it was at the beginning). What is a cascade failure? Was it just some kind of failure mode of the system that got triggered, which destroyed some expensive equipment?
Basically it was an unmodeled failure mode whereby the implosion of one sensor tube created a shock wave in the in-rushing water that radiated outwards, imploding all adjacent tubes in ever enlarging numbers until all the submerged sensors were broken. They'd considered hydrostatic pressure and modest over pressures in their design, but the shockwave pressures resulting from implosion were like 3 or 4x what they'd planned on.
A cascade failure is one where one fail directly causes multiple other fails. Each of those fails cause multiple more fails. So its a very quickly evolving sequence of many successive failures throughout the system. Its a good video that explains this concept very well.