Energy From Thorium Discussion Forum

I think I'm going to write a blog post on this soon, but while examining some of the fission product decay chains, I noticed something very interesting about strontium. It may be possible to get strontium with a much higher concentration of strontium-90 in a fluoride reactor than in a solid-core reactor.

The reason is is that the other two isotopes of strontium that accumulate in fission product material (strontium-87 and strontium-88, which are stable) both have a krypton precursor. If this krypton precursor could be stripped out of the salt by an off-gas treatment system, then these stable strontium isotopes would form outside the core, in the off-gas system.

Krypton-87 has a half-life of 1.3 hours, krypton-88 has a half-life of 2.8 hours. But krypton-90 (a possible precursor of strontium-90) only has a half-life of 32 seconds, assuming that isotope formed in fission at all. With half-lives measured in hours, that should give the off-gas system time to pull those krypton isotopes out of the core salt, leaving only the Sr-90 to form. Sr-89 also forms in the reactor but it has a half-life of 1.7 months and its stable decay product yttrium-89 can be chemically separated from strontium "aged" about a year.

The ability to create strontium with an elevated concentration of Sr-90 could be key in getting this isotope in an economically usable form. Since Sr-90 and Cs-137 are the two biggest fission-product "boogers" of nuclear waste, both with 30 year half-lives, it would be really nice to make something useful out of them.

More information about strontium, from an old NASA document on radioisotopes.

Strontium has a lot of advantages as a radioisotope heat source--unlike almost all other beta-decaying isotopes, it doesn't throw out gamma rays. Also it decays to yttrium-90, which almost immediately decays to zirconium-90. So it's like you get twice the energy per decay!

This document says that a Sr-90 concentration of 50% is typical in strontium samples from a reactor. The rest must be Sr-87 and Sr-88.

One story about "orphaned sources" from the Former Soviet Union (FSU) :

The IAEA had a short video of the recovery operation posted on their web site (the photo is from that video), but I can't find it anymore -- it seems they deleted it....

A couple of other things I should add.

Most of the time, depending on the effective Z number of the container in which the Sr-Y-90 is stored, a large portion of any measured radiation field external to the source container is comprised of bremsstrahlung - X-rays - caused by the betas decelerating in the source encapsulation material.

Also, I read on Radsafe that while Sr-90 (28.5 year half-life) is a pure beta emitter, it beta decays 99.9979% of the time to Y-90m (3.19 hour half-life). This Yttrium has numerous gamma emissions. Its predominant gamma emissions are two gammas each about 15 keV (a total of 6+% of the time), a 202 keV (96.6% of the time), and a 480 keV gamma (91% of the time) with a 2.18 MeV gamma occurring 8.7 E-8% of the time and a 2.32 MeV gamma occurring 0.00173% of the time.
A vanishingly small percentage of the time (1 - .999979), Sr-90 beta decays directly to Y-90 (2.671 days half-life). This Y-90 has virtually no gamma emissions and beta emissions of 79 keV, 401 keV, and 432 keV.

Sr-90 is can be a long-term hazard if it is not properly isolated because it has a high biological uptake and is a strong beta emitter which can cause bone cancer if it is absorbed. Thus it is often considered one of the more nasty fission products in a fallout situation.

It's half-life is one of the "intermediate" ones. In other words, it's short enough to be highly radioactive, but long enough that it won't just decay in a short period of time.


HOWEVER:

When isolated and encapsulated or otherwise bonded into a chemically inert material it is about the best beta emitter around. It emits strictly beta radiation and it does it twice over by it's daughter product. In large quantities it can even provide significant heat.


As far as gamma, yes, it can produce some X-rays as any strong beta emitter can. That depends a lot on the configuration of the source and what other materials are around it. In general, the small amount of X-rays it produces are not hard to shield against (well maybe if you have a hell of a lot of it).


My general experience with the stuff is that it doesn't produce much appreciable gamma from the rare isomer decay that occasionally is observed.


Yes, I have some Sr-90. No, don't go call the authorities. It's not license bearing. I have a couple of check sources which I got through an NRC-approved company under license-exempt provisions and quantities.

If you'd like to buy some I can get you some too. It's also available here: http://www.unitednuclear.com/isotopes.htm

But be a pal and buy it from me instead and I'll cut you a deal and I can also get it in slightly higher quantities per source, for a bit more. The limit is about .5 uCi, so sorry if anyone

And if the amounts produced are bigger than any market for the stuff, at least you get it in concentrated form mixed with less of the stable isotopes so you have a smaller volume of waste to manage (convert to nonsoluble form, etc).

What I'm trying to point out here is that SR-90 could be worth a lot, properly packaged.

My son wanted to use SR-90 to power a low-tech, low pressure steam engine. His idea was... it's cheap power. Put it in a big concrete block, and deliver it with a forklift. Sneer at the power company...
Use it to heat houses, too, and sneer at the gas company.

How much is this sort of capability worth? I think a lot. If so, SR-90 heat sources are premium items. So far from being waste, they are very valuable if sold properly, in the right machinery.

Then, we got into the loose source discussion, which led to the idea of a default reclamator.

We haven't really seen any -social- problems other than the loose source issue, and default reclamation seems like a workable legal and procedural solution for that.

Of course, local SR-90 sources do reduce markets for certain large companies with well-paid lobbyists.

there are some safety concerns there. You would need many kilocuries of the stuff to actually produce enough electrical power to do anything major with. Sr-90 can be chemically and physically packaged into a form which is relatively safe and stable, but I still think that having everyone own a chunk of it would be a problem waiting to happen.

If it is unpackaged or somehow chemically altered it can be very dangerous. It can cause bad beta burns or be highly radiotoxic if ingested.

Great for space probes. Possibly for stuff like remote weather stations or communications relays. I just don't know what I'd be comfortable with the average Joe owning it.

Medium-lived isotopes have been used for production of few kW space power up-to-now. However this power range may be extended to a few 100 kW with the use of thermo-photovoltaics around the isotope core, as investigated in this article:
http://www.astrophys-space-sci-trans.ne ... 9-2010.pdf

Besides being operationally simple, the combination or Sr-90 and thermo-photovoltaics results in a far better power/mass capability than the space electricity generator concepts investigated/prototyped so far.

When society becomes serious about deep-space exploration, it shall prefer the reactor design that allows easiest separation of Sr-90 and Cs-137 from the spent fuel, establishing the infrastructure for space-fuel production.
Is LFTR an optimal design for such isotope separation, or would an other reactor design be preferable for the isotope separation?

Note: the Soviet Union has produced over 3 tons of Sr-90 during its last decade. 20 years on, humanity has lost that isotope mass-production capability in practice - hopefully just a temporary setback.

I would think it difficult to separate Cs-133, Cs-135, and Cs-137 from one another. Likewise Sr-90 from Sr-88.

Actually I found a few years back that fluid-fueled reactors would produce very high quality Sr-90. That's because the other two isotopes in strontium have gaseous precursors (krypton) of sufficient lifetime that they would be removed by the offgas system before they could decay to strontium, leaving Sr-90 behind (its krypton precursor has too short of a half-life to be removed effectively). They saw this result in the MSRE. The isotopic distribution of its strontium was quite different than other fission reactors with solid fuel.

As Kirk commented, there is more to the choice of radio-isotope than decay MeV / atom value.

Sr-90 -> Yr-90 -> Zr-90 cycle has mean beta decay energy of 1.14 MeV
Pu-238 has mean alpha decay energy of 5.5 MeV

That makes Pu-238 almost 2 times as energetic as Sr-90 per unit mass. However because of its longer half-life (87 vs 28 years), the power intensity/mass ratio is worse than in case of Sr-90.
A further important point is that the bremsstrahlung of over 5 MeV collisions necessitates radiation shielding (extra mass) to operate the thermo-photovoltaics over one or more decades. With Sr-90 a reasonably sized isotope core shields itself well enough for the thermo-photovoltaics.
So this makes Sr-90 a more desirable fuel for space electricity production than Pu-238.

It has been interesting to read about the U-232 in a previous comment, about which I have not been aware before. Thinking about the application of U-232, it may be used perhaps as a neutron source, see this work:
http://smartech.gatech.edu/bitstream/18 ... 5_mast.pdf
Such neutron source may be very interesting in some other (terrestrial) applications.

In summary both Sr-90 and U-232 should be both very valuable to have, if only they could be produced on mass scale.

I looked at the Cs isotope distribution while wondering about how much we can leave in the reactor. As in the Sr case, most of the precursors - Xe isotopes - get removed fast enough that the Cs ends up in the off-gas system, not the reactor. Cs-137 is the exception, as Xe-137 decays too fast. If you pull Cs out of the salt, it will be very rich in Cs-137. On the other hand, its thermal X-sec is so low that you might not even bother, for a unity converter. Just leaving it in for the life of the plant, to reach equilibrium is feasible, unless there is some value in isolating it.

I have always thought of Sr90 as power source for remote communication relays, TV, telephone or internet. Just connect it up for a decades long life battery and bury it in ground.
SrF2 would be a highly insoluble material like CaF2 and can be left buried when the power declines. It could be precipitated from SNF dissolved in nitric acid as fluoride.