They are tested extensively, but sometimes you still don’t find problems until you test the whole system working together under real conditions.
I just know that when we were starting up a new power plant, coal or nuke, this kind of surprise would have never happened by the time we started the plants up. Similar type systems, similar pressures.
“They are tested extensively...”
In the 60’s & 70’s I worked for a company that made “Sound Attenuation” hose for the US Navy’s nuclear submarines. General Dynamics was the customer - hose was covered by MIL-Specs) I was in the Tech Dept. Very critical operation, and rigorous testing on each piece we made. Very high burst presure (we had to run a burst at specified intervals). Never had an in-service failure. These hoses were what made our submarines very silent runners - our subs far exceeded the Russian capabilities in this area. All water in/out of the sub was conduited through these hoses, including cooling water for the reactors.