Gamma-ray Spectrum, CP-1 Graphite Memento

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Jon Rosenstiel
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Gamma-ray Spectrum, CP-1 Graphite Memento

Post by Jon Rosenstiel » Fri Dec 07, 2018 11:36 pm

Earlier this year I acquired (from a fellow nuke-head) a Lucite encased piece of graphite from Chicago Pile-1, the world’s first nuclear reactor. These 25th anniversary mementos were given out (I believe) mainly to those who labored behind the scenes. It hadn’t occurred to me until just recently that this small piece of history may contain, if it is the real thing, a clue or two to its origin. Sure enough, a long term count turned up a weak signature from Eu-152, and an even weaker signature from Cs-137. Eu-152 is a neutron activation product of the naturally occurring Eu contained in the graphite, while Cs-137 is a fission product.

Spectrum: (All unlabeled lines are background)
CP-1 spectrum.jpg
Memento on detector's face:
CP-1 graphite_1.jpg
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Re: Gamma-ray Spectrum, CP-1 Graphite Memento

Post by Richard Hull » Sat Dec 08, 2018 1:28 am

Great run on your gamma spec! Remnant rads for sure. I wonder how long CP-1 was operated? Not long I think, due to its location. I believe CP-2 was the one used during the war built in a more secure location near or at what became the Argonne lab.

I just did a wikipedia look up. I was right CP-1 was only operated a few months at about 1/2 to1 watt. Moved in 1943 to Argonne as CP-2 which saw active service until 1957 when it was torn down and buried. I assume they dug some up to make the graphite souvenirs.

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

Post by Jim Kovalchick » Sat Dec 08, 2018 11:53 am

Thanks for posting this Jon. About six years ago I did a 24 hour count on my CP-1 sample using a Canberra Falcon I borrowed from work. I didn't see the Eu peaks but there was a tiny Cs 137 peak. Others didn't believe my results so I repeated the count with the same results.

When Carl did his study of his CP-1 fuel he wondered if the the results were showing that the fuel also saw duty in CP-2. I wonder if all these graphite souvenir samples out there also were from the reactor after it moved to Argonne.

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

Post by Richard Hull » Sat Dec 08, 2018 4:28 pm

As noted in my post, CP-2 was used actively until 1957 at an unknown power level. Thus, if all the CP-1 graphite blocks were used in CP-2 they saw a much longer neutron exposure time than CP-1.

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

Post by Jim Kovalchick » Sat Dec 08, 2018 6:38 pm

Once it's established, as Carl did, that there were enough fission events in the fuel to produce measurable fission products, it's all a matter of product migration not the neutron fluence seen by the graphite itself. The amount of products left in a piece of graphite will be dependent on a couple things, the relative proximity of the graphite to a piece of fuel, circulation within the core, and if the preparation of the sample cut off any contamination. This means that the amount will vary widely. Tim Koeth has measured his sample multiple times and found nothing.

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

Post by Jon Rosenstiel » Sun Dec 09, 2018 4:51 am

Jim,

Re: your 24-hour count, how much shielding did you use?

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

Post by Rex Allers » Mon Dec 10, 2018 4:03 am

Jon,
That's quite an impressive GS measurement tool and shield. Do I interpret correctly that the scan was almost 4 days?
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Re: Gamma-ray Spectrum, CP-1 Graphite Memento

Post by Richard Hull » Mon Dec 10, 2018 6:11 am

As I understood it, there was no core in the classic sense in CP-1. Instead, the fuel was distributed all about and within the moderator in a manner that would allow moderation of purely natural uranium fission over the bulk of the volume. Explains the low power over a huge volume. I do not know how CP-2 was constructed or operated. So fission products could exist all over the fuel laden blocks for sure and migration via what even air cooling system was employed, probably in CP-2. Once enrichment got into the game, piles were just not the thing to do anymore.

Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
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Re: Gamma-ray Spectrum, CP-1 Graphite Memento

Post by Jim Kovalchick » Mon Dec 10, 2018 11:24 am

Jon
I did not use much shielding. The Falcon I used had a tungsten ring lined with copper. I'm sure my counts included some background.

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

Post by Jon Rosenstiel » Mon Dec 10, 2018 7:18 pm

Jim,
Ok, one more question, did you do a 24-hr background run also? I ask because my background runs always have a small Cs-137 peak. (Two to three counts/hour with the detector inside of the lead shield) I don't think there is an exact source for the Cs-137 background, it just seems to be pretty-much everywhere.

Rex,
Yes, runtime was nearly 4-days. The memento's Cs-137 activity was very weak, it took that long for the spec software (FitzPeaks) to determine that the peak was real and not background.

Richard,
From what I could find the highest power level that CP-2 ever ran at was 10 kW (thermal), normally it was run at around 1 kW. As I understand it, CP-2 was basically CP-1 with a few improvements plus 5-feet of concrete shielding. I spoke with Carl a couple of days back, asked him to check out this thread when he had a chance. Hopefully he'll weigh in here soon.

Jon Rosenstiel

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