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 Post subject: Off-gas separation
PostPosted: Aug 30, 2015 10:28 pm 
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Tell me if you’ve heard this one – a holding tank in the off-gas stream can capture more than half of the 90-Sr and 90% of the 137-Cs. These troublesome “waste” products can be put to positive use.

I did some analysis of the 235-U decay chain yields. I was surprised to find that 30-31% of the unstable fission products are Krypton or Xenon at some point in their hot, short lives. Xe heavier than 140 and Kr heavier than 91 don’t live long enough to get out of the core, so they don’t play in this. 131/133/134/136-Xe and 83/84/86-Kr are all stable, and all of them take the off-gas route. However, they pass right through the holding tank and don’t play, either.

The interesting decay chains are in between the heavies and the stables, at atomic weights 137, 88 and 90. 90-Kr has a half-life of 32 seconds. If the off-gas pipe starts ten seconds downstream of the core, more than half of the 90-Kr will survive to reach the off ramp. Much more than half of the 137-Xe (HL = 3.8 min) will exit as well. (You’ll also get some 91-Kr, which will become 91-Zr.)

Let’s put a holding tank there and hold the gas for ten half-lives (38 minutes). 99.9% of the 137-Xe becomes 137-Cs. It’s also enough time for the 90-Kr to pass through the 90-Rb stage (HL = 2.6 min) and settle on 90-Sr. The 90-Sr will be quite pure, because the only other Sr source is 88-Kr. With its half-life of 2.8 hours, not much 88-Kr will decay in a 38 minute hold-up.

You might chemically separate the Cs and Sr (and Zr/Rb traces). The Cs makes a fair food sterilizer, and a “clean” 90-Sr nuclear battery wouldn’t need heavy shielding. But that may not be worth it. Just dump it all in a less-than-optimal battery for on-site backup power.

Turn waste into an asset. That’s good PR at the cost of a wide spot in the piping.


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 Post subject: Re: Off-gas separation
PostPosted: Aug 31, 2015 2:29 am 
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30 yr half life Cs and Sr are very useful isotopes. Especially the Sr can be a low cost RTG fuel for use on earth. It could work for a decade with little loss of power.Much cheaper and easily available than U-238.


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 Post subject: Re: Off-gas separation
PostPosted: Sep 01, 2015 4:42 am 
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SteveMoniz wrote:
131/133/134/136-Xe and 83/84/86-Kr are all stable, and all of them take the off-gas route. However, they pass right through the holding tank and don’t play, either.



Not quite. Xe133 has a 5 day half life. It decays to Cs133, which may capture a neutron to become Cs134.

This product was the major concern for the first two years after Fukushima.

A good offgas system will remove the Xe133 thereby leaving orders of magnitude less Cs123 in the reactor compared to a solid fuelled system.


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 Post subject: Re: Off-gas separation
PostPosted: Sep 01, 2015 7:06 am 
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Yeah, that's a typo. 132-Xe is stable, not 133. 133-Xe should flow in and out of the holding tank and end up in the trash. It won't have the chance to turn into 134-Cs there. All that's left is the (shadowed) 134-Cs produced directly by fission - less than one part per million. How much is produced by the absorption route? It would be another PR talking point - actually reducing the production of a dangerous by-product by xx %. The holding tank doesn't reduce the volume of the 90-Sr or 137-Cs, just "lifts and separates".

134-Cs is maybe the fourth most dangerous fission product. Funny - and fortunate - that this notional holding tank homes in on the most dangerous. Too bad 131-I is uphill of 131-Xe.


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 Post subject: Re: Off-gas separation
PostPosted: Sep 01, 2015 8:46 am 
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What is your ranking of most dangerous fission products? By volume (in a PWR) and by danger?

Can you attach your decay chain results - it would be interesting.

In a MSR, the Iodine 131 is bonded to Sodium or Lithium, so can't escape into the atmosphere - though it is I assume soluble. Also with the use of iodine tablets to saturate the body with clean iodine it seems the I-131 is not too much of a problem even in a PWR.

The noble gases are always a problem. Though less of a problem if removed and stored in a pressurised container than if left in a fuel assembly. If they escape, it's a lot of radiation, but Xe and Kr don't inter-react with organic matter.


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 Post subject: Re: Off-gas separation
PostPosted: Sep 01, 2015 9:34 pm 
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My ranking system is more political than anything else. It's based on PWR accidents and waste and fame. 131-I wins for Chernobyl. 90-Sr and 137-Cs are the waste emitters of note. I never even noticed 134-Cs until Fukushima. The anti-nuke crowd made sure we heard when it showed up in California tuna. Not that the tuna was dangerous. I should have said 134-Cs was fourth most famous rather than fourth most dangerous.

I'll neaten up the spreadsheet. One quirk in it, concerning 85-Kr. I only get 1/4th the volume I got before I found the JEFF-20 report from IAEA. Without that, the 85-Br would decay to 85-Kr. Actually, it decays to the isomer 85-Kr-1. That decays by the B- mode 78% of the time, skipping right to stable 85-Rb.

The data I'll send is for 235-U only. The yields are from Brookhaven's Sigma database. The half-life and decay modes are from Livermore's TORI_2 database.


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 Post subject: Re: Off-gas separation
PostPosted: Sep 02, 2015 3:33 pm 
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Here is my spreadsheet. There is a Documentation page that lists the limitation of this analysis.

The location of the off-gas vent is now an input (cell B1 of the Sums worksheet). At first, I put the vent 30 seconds downstream of the core. If the cycle time of the salt is about 30 seconds the vent will be at the coolest spot in the system. I also tried a vent ten seconds from the core - a much hotter spot but the results are better. See the Results section of the documentation or change the vent location yourself.


Attachments:
Off-Gas_Cumulative.xlsx [37.93 KiB]
Downloaded 199 times
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 Post subject: Re: Off-gas separation
PostPosted: Sep 03, 2015 3:02 am 
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jagdish wrote:
30 yr half life Cs and Sr are very useful isotopes. Especially the Sr can be a low cost RTG fuel for use on earth. It could work for a decade with little loss of power.Much cheaper and easily available than U-238.
No, U238 is VERY common. Pu238, not so much. ;)

If the Sr90 is used for a RTG on earth, then replacing the kernel with a fresh one every 10 years or so would probably not be such a big deal.

My question is, if we use SrTiO3 as the kernel material, what happens to the Yttrium? Does it remain bonded with the TiO3? Does it detach and remain unreacted? Does something else happen? Is that something else soluable in some common solvent that SrTiO3 is NOT soluable in?

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 Post subject: Re: Off-gas separation
PostPosted: Sep 03, 2015 9:02 am 
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It there is a problem with Yttrium inside the RTG, it's worse than you thought, KitemanSA

I'm thinking about the Yttrium (89-Y) inside the holding tank. For that, I'm going to make more assumptions. First, the holding tank is like an auto muffler with baffles. It takes an average 38 minutes for a gas atom to negotiate the S-turns to exit. On the way, some gas turns into Cs and Rb. Will that flow as a liquid? Some of that turns into Sr and Ba. Does that stick to the baffles and muffler walls?

Assuming a 1GW plant, a ton of fuel gets split per year. 31% of the fission products (FP) go into the off-gas system. Almost half of that stays as gas and goes through. More is Xe, so the mass is slightly over the average for all FP. But assuming that average, the muffler is 185 kg heavier at the end of the year.

Some of that is 89-Sr, with a half-life of 50 days. Some is 89-Y, but I'll assume that is separated chemically before putting the Sr into the RTG. Still, for the first year, you are going to get more Yttrium from 89-Sr decay than from 90-Sr, unless you are willing to age the mix 500 days before filling up the RTG.

But is Yttrium a problem? What is the Soviet experience with 90-Sr RTGs (other than losing them)?


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 Post subject: Re: Off-gas separation
PostPosted: Sep 03, 2015 8:59 pm 
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SteveMoniz wrote:
... But is Yttrium a problem? What is the Soviet experience with 90-Sr RTGs (other than losing them)?
The Soviets use SrF as their krnel material. The results are probably not applicable to an SrTiO3 kernel.

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