Wow... I guess I really am getting old. I covered more explosive tube plugging operations than I can count, up into the early eighties.
You took a maximum of 3 explosive charges (1/3 stick of dynamite equivalent each) that looked like long shotgun shells with wires coming out where the firing pin would normally hit and had to space them properly (X number of rows apart) within the tube sheet. The locking mechanism (at least on the ones I saw and that caused the following story) was an over-center 'Cam-Lock' device that you turned after inserting the plug up into the tube.
At a Northeast nuclear plant that will remain nameless here (I have been saying that a bit lately) there was an incident in 1978 where a Boilermaker with, well... let's just say less experience than required, was told to insert the charges into the marked tubes and turn the locking mechanism 180o to lock it in place. Unfortunately this Boilermaker was not fully educated in mathematics and when he felt the resistance at 90o he figured the plug was locked. It wasn't. As a matter of fact he did it three times and none of them were locked. After he exited the channel head we placed the blast cover in place (a 'recent' invention at the time) and in the process must have tugged on the three lead wires just enough to pull them down from the tubes into the channel head... where they laid in the bottom, all apparently sitting together. We all exited the immediate blast area and retreated behind the bioshield.
At this point I should tell you that this was about the fifth or sixth set of charges we had set off in a series of a few days, so it was getting pretty 'routine' if you can actually apply that term to what we were doing, which in retrospect still amazes me. The demolitions guy had even let me set off the charges a couple of times. Very cool. Actually got to say 'Fire in the Hole' and push the little handle down. Normally there would be a muffled 'Boof' kind of noise since the charges were inside the tubes and the blast cover and HEPA filter masked the sound. But not this time.
"Booooom... Clang, bang, clang, crash (and a few more choice noises.) We all looked at each other through our respirator facepieces and all you could see was eyes... seriously it looked like a Warner Brothers cartoon. I peered around the corner and saw smoke billowing out of the steam generator bowl... at least that is what I thought it was. Actually it was the dried out contents of the bowl turned into a very fine dust. (The manway cover interior surface was ~6 REM/Hr gamma and It-doesn't-matter-how-much Beta and the Tube sheet was 40 REM/hr with an Ion Chamber instrument.) There was the blast cover on the floor (about 12 to 15 feet below the nozzle, IIRC) which had caused most of the noise after the 'Booooom' part.
My finely honed HP instincts at this moment could think of nothing but 'We're outta here' and looking back they were right on target. Unfortunately our Site Coordinator had taken this particular opportunity to witness his first explosive tube plugging and was tapping me on the shoulder at that very minute saying 'Air Sample' to which I replied "No. We gotta go!" and he said "If we leave without an air sample we have to come back in with Scott Air Packs to recover." Now, this was my first job with this company, and I was young and eager to please my new boss, so I ignored my better judgement (turns out it was better than his, too) and I got the air sample. My brother, who was my junior tech assistant at the time, told me I disappeared into the cloud that was still billowing out of the manway as I climbed the ladder. The 5 minute, 2 CFM particulate sample (it was the only sampler we had set up at the time) read over 200 mR/Hr GAMMA. It could not even go into the count room.
After some classic HP voodoo and radcon math number crunching, I was credited with 100 MPC hours (roughly the same as today's DAC-Hrs) for the five minutes I was in the cloud, counting the 50 protection factor for the 'canned ham' style filter respirator I had on. I later found out through experimentation that about 10% of those old filters failed right out of the box... which is where I had gotten mine. I went right to the shower and then to the body counter where I was declared 'clean.'
Naturally, the next crew to go to the area had to go in Scott Air Packs anyway, so all we gained from the air sample was an ending to the story.
Yes, Virginia, there was such a thing as Explosive Tube Plugging. Which leads to the story where I thought I had been blown up...