More of a Nuke This Week...

...than I ever was in the Navy. My current position is a process engineering role for 2 polyethylene (plastic) production lines. One of these two lines is undergoing a turnaround (overhaul) right now. One of our major componants separates the polyethylene from the remaining un-reacted ethylene. Since the traditional differential pressure type level instruments that you would use to monitor water level in a tank don't really work when the liquid is plastic, we use a CS-137 gamma source with a scintillation tube detector to monitor vessel level (the higher the plastic level in the vessel, the more shielding occurs and the lower the reading on the scintillation detector). Well, as part of this turnaround we are replacing this source and cleaning out the inside of the vessel. It turns out that somehow the source tube got bent, and could not be removed in the pre-existing lead sheath as originally planned. We needed to measure the bend and fabricate a new lead shield before we could remove the source.

Now, the idea came up to fill the vessel with water to use as shielding so we can look inside to design whatever fabricated lead shield we would need. I remembered roughly the old thumb rule about 2in of lead = 4in of steel = 24in of water that they tought us in nuke school way back in 1996. So I chime in with my estimate that since the original lead shield is 2.5 inches thick, if we could fill the vessel such that it covers the source tube with at least 36 inches of water we can pull the top off of the vessel and make the appropriate measurements to re-design this lead shield. All is fine and dandy, and I'm pretty confident that I just did something pretty cool (and by "cool" I mean I'm a complete dork).

But alas, all is not fine. I can't just make a recommendation based on some fact stuck in my head so I bust out an "Intro to Nuclear Engineering" textbook from college (easy technical elective for any former nuke to take - a 400 level class on your former job, jackpot!). Well, turns out that specific thumb rule only applies to high energy (2+ MeV) released as a result of the fission process - and that water can't attenuate low energy gammas worth a fuck. CS-137 gammas are 0nly 0.66 MeV, and new calculations showed that we needed at least 60 inches of water to be able to pull the top off the vessel safely. Luckily we had about 72 inches of water available, and we've been able to proceed with a new lead shield fabrication. So the moral of the story is that the one time I thought I was going to be able to use something from nuke school in the real world, I couldn't really use it.