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 Post subject: Tracking the 135-XePosted: Jul 02, 2014 12:04 am

Joined: Jan 16, 2012 7:15 am
Posts: 94
I’m trying to visualize the fate of 135-Xe inside an MSR. Suppose the MSR has a salt cycle time of thirty seconds. Two-thirds of the salt is out-of-core, so each atom spends ten seconds in the core each trip around.
Question 1) Is this a reasonable time frame and out-of-core ratio?

Any 135-Xe produced is 4555 times more sensitive to neutrons than 235-U (2.665E+6/5.85E+2 absorption vs fission cross sections, from Brookhaven:NuDat2). So I expect a 135-Xe nucleus created directly from the fission event (a “native” nucleus, if I may) would rarely make it out of the core.
2) Is the absorption to fission ratio linear like that?

Some of the 135-Xe comes from the decay of 135-Xe-m1 - 0.18% initial yield vs 0.08% (for thermal neutrons, from Brookhaven Sigma data). 135-Xe-m1 has a half life of 15 minutes, so it can make a few round trips, if it is less sensitive to neutrons than 135-Xe.
3) Is the absorption cross section for 135-Xe-m1 different from 135-Xe?

Even more 135-Xe comes from decays higher on the 135 decay chain; 3.2% of the initial yield for 235-U is 135-Te and 2.9% is 135-I. Together with the 135-Xe-m1, these three provide ~95% of the total 135-Xe. The half-life of 135-Te is 19 seconds, so we could just lump that in with the 135-I and follow the 135-I around. Half of that survives 788 circuits (HL= 23650 sec/30). It should reach equilibrium in about five days (10 x 23650).
4) Does the rule-of-ten apply? After ten half-lives, is the proportion of an isotope in the salt stream constant (within 0.1%)?

The sparging equipment is just upstream of the core, so when the 135-Xe is created from the decay of 135-I, it has 2/3rds of a chance to be outside the core and hit the sparge. Less the native 135-Xe, that’s 67% of the 95% that is available to be off-gassed .
16.5% of the time 135-I decays to 135-Xe-m1 (IAEA:JEFF 6287). This might not make a difference if the answer to question 3 is no.
All of this is based on 235-U as the fuel. For 233-U the proportion of native 135-Xe to I and Te is higher: ~12% compared to ~5%. I’m not trying to budget neutrons, but it would appear that 233-U inferior to 235-U in that 135-Xe would be a bigger problem. 67% of only 88% could be off-gassed.
Question 5) Is all this in the ball park, or do I need another hobby?

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 Post subject: Re: Tracking the 135-XePosted: Jul 05, 2014 11:24 am

Joined: Jul 28, 2008 10:44 pm
Posts: 3063
SteveMoniz wrote:
I’m trying to visualize the fate of 135-Xe inside an MSR. Suppose the MSR has a salt cycle time of thirty seconds. Two-thirds of the salt is out-of-core, so each atom spends ten seconds in the core each trip around.
Question 1) Is this a reasonable time frame and out-of-core ratio?

A bit high on the out-of-core ratio (optimistically 2/3rd in core, practically likely around 1/2).
Round trip time varies considerably with the design from 10 seconds to 120 seconds.
Quote:
Any 135-Xe produced is 4555 times more sensitive to neutrons than 235-U (2.665E+6/5.85E+2 absorption vs fission cross sections, from Brookhaven:NuDat2). So I expect a 135-Xe nucleus created directly from the fission event (a “native” nucleus, if I may) would rarely make it out of the core.
2) Is the absorption to fission ratio linear like that?

You need to also factor in the number of atoms of each type. Yes 135Xe has a huge cross-section but you will have a very small inventory in the core at any time so the percentage of 135Xe atoms that capture a neutron is fairly small with removal times in the order of minutes. That will significantly alter your subsequent analysis.
Quote:
... follow the 135-I around. Half of that survives 788 circuits (HL= 23650 sec/30). It should reach equilibrium in about five days (10 x 23650).
4) Does the rule-of-ten apply? After ten half-lives, is the proportion of an isotope in the salt stream constant (within 0.1%)?

Yes and no. You have a steady inflow of each isotope from the fission and decay processes and a steady outflow from decay, capture, and fission product removal processes. The inflow is primarily related to the fission rate while the outflow is primarily related to the inventory of the isotope and the respective half-lives.

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 Post subject: Re: Tracking the 135-XePosted: Jul 10, 2014 11:54 am

Joined: Jan 16, 2012 7:15 am
Posts: 94
Quote:
A bit high on the out-of-core ratio (optimistically 2/3rd in core, practically likely around 1/2).

50% out-of-core? Heat exchangers are more efficient than I thought. This is good news. If we draw a simple cross section of the reactor, and assume the volume of the salt in the pipes is negligible, there will be two squares with two one-way connecting arrows. At 50%, the squares are of equal size. (Micro-channels in the core and surface area of the HX would be refinements in someone else’s model.)
Quote:
with removal times in the order of minutes

I do plan to attach elements to the diagram not shown above; for fuel feed, off-gas, and distillation systems. Right now I'm just trying to scope the size of the problem and I am not assuming any physical fission product removal. Eventually, I will "remove" them through decay or transmutation.
Quote:
135Xe has a huge cross-section but you will have a very small inventory in the core

The “native” 135-Xe proportion is small, 0.26% yield, including 135-Xe-m1. Most of the 135-Xe comes from 135-Te (3.2%) and 135-I (2.9%). The 135-Te decays so fast (19 sec HL) we can talk as if it’s 135-I. That will circulate and when it decays, 135-Xe can appear anywhere in the circuit.
Since most of the 135-Xe has this origin, the core is only slightly richer, 0.26% on top of 6.1%. So 135-Xe is nearly homogeneous throughout the whole system. Homogeneity may not be terribly important, but it is different from my original mental model.
Quote:
You need to also factor in the number of atoms of each type.
Agreed, but I was trying to make everything proportional to the fission neutrons. I think I got away with that for FP creation, but I can’t for absorption. In Phase One of this project (see viewtopic.php?f=3&t=4111 I assumed 1.00 neutrons for fission would always be available. Assuming 2.52 neutrons per fission, there are 1.52 neutrons available for absorption (Phase Two). Roughly, it looks like I have 1.5 neutrons to throw at the Xe, so the 6% becomes 9%. That is, for every 100 neutrons that fission, nine are absorbed by 135-Xe. This assumes that the equilibrium level is equal to the cumulative yield.
Any of this making sense?

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 Post subject: Re: Tracking the 135-XePosted: Jul 10, 2014 5:23 pm

Joined: Jul 28, 2008 10:44 pm
Posts: 3063
If you have no ability to remove the xenon then you might be on the right track. But since the xenon will come out of the fuel salt whether you want it to or not you have to start with the steady state equation that the xenon generation rate must equal the xenon removal rate. Generation is from fission, decay, and capture. We can neglect the capture term since we get almost nothing by that road for Xe-135. Removal rate will be proportional to the inventory and can be by neutron capture, decay, or physical removal. In an MSR the physical removal will dominate.

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 Post subject: Re: Tracking the 135-XePosted: Jul 11, 2014 5:39 pm

Joined: Jan 16, 2012 7:15 am
Posts: 94
I don’t have an off-gas system yet because I don’t know how big it needs to be. I don’t know how hot the gas will be. There are other engineering questions for which the answers start with the details of what’s in the salt. What I don’t see in the MSR design community is a list of what’s in the salt as the reactor “ages”. I am trying to model that.

Right now the “what” is just the fission products. Later, I’ll need to add actinides and “evolve” them. Right now the fuel is just 223-U. Later, I’ll add other fuels, including some of those evolving actinides. Right now the reactor has only thermal neutrons. Later, I’ll need a way to incorporate faster neutrons (and absorption data for them – I’ve got fission yield data.)

The end product will be tables of the proportion of each isotope dissolved in the salt. Eventually, there can be a table for each reactor design and fuel mix. For now, a single table for 233-U/thermal might be of some use. It would have the FP isotopic composition of the salt after one second, two seconds, four seconds…a billion seconds of reactor operation.

Does anyone find this of interest?

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