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Re: Gamma-ray Spectrum, CP-1 Graphite Memento

Posted: Mon Dec 10, 2018 9:58 pm
by Jim Kovalchick
My background count was longer than a day and there was no by CS 137.

Re: Gamma-ray Spectrum, CP-1 Graphite Memento

Posted: Mon Dec 10, 2018 10:24 pm
by Bruce Meagher
A few years ago our local university demolished a large shielded room with many tons of low background steel acquired from some pre-WWII ship as well as a bunch of virgin lead. It was unfortunate for sure. One of my professors mentioned that steel made post the atomic era contains radionuclides making extreme low counting more challenging (same goes for lead). Jon, could your shielding material (lead/steel) be the source of your low background Cs-137?

Re: Gamma-ray Spectrum, CP-1 Graphite Memento

Posted: Tue Dec 11, 2018 7:18 pm
by Jon Rosenstiel
Thanks Jim, that's good to know. I've cleaned the inside of the shield multiple times and even tried another detector but I always get the same 2 to 3 counts/hour Cs-137 background, I was beginning to think that was normal. So maybe, as Bruce suggests, the shield's construction materials are the source.

Jon Rosenstiel

Re: Gamma-ray Spectrum, CP-1 Graphite Memento

Posted: Tue Dec 11, 2018 9:40 pm
by Richard Hull
Nuc Lead is a company that deals in many lead products. One is "virgin lead" It costs a good bit extra as it is lead that was mined over a hundred years ago and is usually in the form of buried pipe, underground electrical buried covering, ancient ballast culled from old sailing ships, etc.

I may have told this real story here before.

We have a world class scrap yard of some note that HEAS travelers seem to haunt on the Friday morning before HEAS. There are a number of images of some of them in the yearly October spate of photos about HEAS in Images du jour. I am pals with the owner as we have been looting the place for all manner of cool stuff since the mid-eighties.

The yard's proprietor approached me on one of my visits to "the yard" some years back. He was most upset that he lost a bid for about 2 tons of scrap lead offered by the State. He told me that this lead was from the underground pipe that was dug up from underneath the Virginia state capitol and its grounds, then under going a major updating at the time. The capitol building in downtown Richmond was designed by Jefferson and had its water and sewer systems installed in the early 1800's.

The owner noted that his bid was for 50 cents per pound and the winning bid was for $1.25 per pound. He said, "who ever bid that was crazy"! That was about 27 cents beyond the current price on the open market for good clean lead ingots. "Them old pipes is nasty and dirty", he said, "Probably filled with poop from the civil war". (The capitol had a major restoration after the civil war and lead plumbing was no longer used and just left in place.)

I went into a long diatribe on the lead's value to people making "quiet shields" for nuclear trace measurements. I told him that they might get $20.00 or more per pound in a cast shield weighing hundreds of pounds. The cigar he usually had chomped between his teeth nearly fell out. He just shook his head and said..."Well, ya' learn something new everyday."

As lead has been recycled since Roman times, it was used until relatively recently, (50's-60's) in gutter work, roofing, sewage plumbing, etc. During the nuclear testing in the 40's-60's, a lot of nuclear fission products pool in and on roofs, gutters, etc. As much of it was recycled in tear-downs during "the end of lead", melts included a good bit of this fallout. It is still making the rounds, if now in much diluted form. Sr90 and Cs137 are just a few of the long lived goodies that are to be found in trace amounts in recycled lead. Only ancient buried or hidden away lead and absolutely fresh mined, deep buried lead is considered "Virgin Lead". Provenance of origin is usually demanded and given warranting the source in quiet shields.

Richard Hull

Re: Gamma-ray Spectrum, CP-1 Graphite Memento

Posted: Tue Dec 11, 2018 10:30 pm
by Jim Kovalchick
My shielding was tungsten, not lead. I'm not sure that makes a difference, but what make a difference was my very long background count did not include a Cs spike, but both of my sample counts did.

I did my count about six years ago. I doubt that the little bit I saw then would be seen now. I may have the count data somewhere, and if I find it I will look to see if any Eu peaks were there too.

Re: Gamma-ray Spectrum, CP-1 Graphite Memento

Posted: Wed Jan 30, 2019 2:31 pm
by Jim Kovalchick
I've been thinking about this a little more, I remain unconvinced that fission products would be uniformly distributed through the reactor to the point that every sample would be expected to have remaining products today if any did. The reactor used two different fuel types, metal and oxide, and I would expect that fission product migration and retention would vary significantly between those two types. Also, HTGR experience shows that fission product diffusion through graphite can vary significantly depending on the manufacturing techniques. For example in the old Peach Bottom HTGR fuel, permeability coefficients for cesium differed by a much as a factor of four between graphite types used. I'm not sure that the permeability of the graphite used in CP-1 varied to that extent, but given that the graphite came from different manufacturers, I think it could vary. Given also, that the flux and therefore fluence and fission products, would vary across the core from fuel distribution and neutron leakage, I don't think it should be a surprise that some samples show significantly more left over radioactive material than others.