World running out of helium – so make some with a DPF

[vansig]

presently as far as i know, Helium is seldom recycled.

it’s just too precious to let go.

can we scoop it from earth’s upper atmosphere?

[ikanreed]

vansig wrote: presently as far as i know, Helium is seldom recycled.

it’s just too precious to let go.

can we scoop it from earth’s upper atmosphere?

No. Helium floats above hydrogen, because hydrogen reacts with oxygen and makes water before it escapes. Helium gets outside the magnetosphere and blows away in solar wind. If it’s gone, it’s gone.

[asymmetric_implosion]

vansig wrote: presently as far as i know, Helium is seldom recycled.

it’s just too precious to let go.

can we scoop it from earth’s upper atmosphere?

Helium recycling is really picking up at the national labs due to cost and Department of Energy rules. The large helium using labs like the national high field lab at Florida State was instituting helium recycling for their large superconducting magnet systems. The commercial magnet systems are probably going to follow suit in the next few years as helium costs or lack of supply might damage their business. Higher temp superconducting magnets are also in the works for MRI systems. We could drop a great deal of demand (~20%) if SC magnets worked at 10 K instead of 4 K.

Given the concerns about helium, how can one build a power grid based upon helium cooled reactors (fusion or fission)? Seems a bit problematic.

[Joeviocoe]

What would you estimate as the consumption of helium for a large hospital MRI (per year)? And how much, in comparison, should a 5MW DPF consume?

[ikanreed]

Joeviocoe wrote: What would you estimate as the consumption of helium for a large hospital MRI (per year)? And how much, in comparison, should a 5MW DPF consume?

That’s pretty hard to estimate, because the helium is retained essentially indefinitely. The primary coolants like helium, nitrogen, or Freon are kept in a closed system and only secondary coolants like water and air are released into the environment. Essentially, the helium shortage is caused by demand for new devices with helium cooling.

[delt0r]

This is not true. Many of the MRI machines are open loop cooling. That is the He boils off into the atmosphere. This is very common for most scientific MRI machines, even very new ones. You simply can’t use that sort of He tonnage a year without dumping most of it into the atmosphere.

[asymmetric_implosion]

The PF has a bit of an advantage as it doesn’t require liquid helium. One could recycle the helium in a loop because the thermodynamic efficiency is reasonable as you are trying to cool the helium using ambient air. Taking cold helium and turning it into liquid is a very inefficient process because everything his hotter than the helium. Thermodynamics is not in your favor. I don’t have an idea for the loss rate of helium for a PF. I know some next gen fission technology is built around helium coolants so I have to believe the helium release is monitored because Xe and other gases with radioactive isotopes would mix with a gaseous coolant and get released if coolant is released.

I personally favor liquid metal coolants as they tend to remove heat well at modest flow rates and low pressure. If the atomic number of the coolant has the be less than Be, Li is the only option but you have to worry about the neutron activation and all that goes with it. I remain unconvinced that Be is a viable anode material so other options with sodium exist in my mind. Sodium is a proven coolant material that operates at atmospheric pressure. Water is always an option but the flow rates and pressure (which is also true of He) could be an interesting engineering challenge.

[Mechanik]

asymmetric_implosion wrote: Li is the only option but you have to worry about the neutron activation and all that goes with it.

In practical terms, Li does not activate. In practice, activated Li should all end up as 4He, 3H, or 9Be.

I’d guess the 9Be content is no big deal, until it comes to recycle the lithium coolant, and in fact the Be-9 and Li-7 reactions will chuck yet more neutrons out (the 9Be is a neutron ‘multiplier’), and the total additional neutron capture reactions should increase the total heating of the blanket (the follow-on heating of a neutron irradiated Li blanket would not be an insignificant fraction of the total heat output of a ‘real’ fusion reactor)

The 4He and T are gases and, presumably, should be readily collectable out of a liquid!?

There is one element that looks like a good neutron-resistant material that might be used as a liquid coolant (though I have never heard anyone else suggest it), and that is tin. For an ‘average’ atom in a given quantity of naturally occurring tin, it can repetitively absorb some 4 neutrons or so before it will become an activated isotope. But, then, why would you bother if liquid Li generates more heat for itself under neutron bombardment, along with some very useful tritium.

[asymmetric_implosion]

I forgot about Li producing tritium. Scrap lithium as it would require significantly more regulation due to flowing radioactive coolant. Can’t have a PF power source on every corner if it has radioactive coolant. The site license would be impossible to get in a town.

Tin activates as it is used in activation counters. Argon might be an option as it is pretty immune to neutrons but probably has miserable thermal properties otherwise people would use it as a coolant in next gen reactors. Neon is pretty expensive as are other inert gases.

[Mechanik]

asymmetric_implosion wrote: Tin activates as it is used in activation counters. Argon might be an option as it is pretty immune to neutrons

I would be very interested to see a link to an example of an activation counter based on tin. You might be thinking of indium – that has one of the highest cross-sections to thermal neutrons there is. Indium and silver are the usual materials used in activation counters. I can’t see anyone would have any reason at all to use tin instead, when indium and silver are conveniently available.

There is no element with as many stable isotopes as tin – nine stable isotopes in all, ten if you also count the ‘essentially-stable’ Sn-124.

Argon 40 (the commonest of the two naturally occurring isotopes) activates to the beta emitting Argon 41 with a half-life of 100 minutes or so, which makes its specific radioactivity non-trivial. It has a cross-section of around 1 barn to thermal neutrons, which is not as big as some cross-sections (like indium) but that makes it far from being immune to activation.

If you really want a gas to do your cooling that is as immune to neutron activation as possible, then the choice should be either oxygen (smallest neutron cross section of all the elements = 0.2 mbarns) or helium (which is a bit higher, 7 mbarns, but won’t practically activate in any case, though I guess you might end up with a bit of Li 6 from double-captures), the latter having good thermal conduction properties. I guess helium is your gas of choice, which brings the tread back, full circle!!!…

[zapkitty]

(Major edit … no biggie, what’s a wrong answer 19.7 times smaller than the actual answer between friends?)

The discussion seems to be overlooking the matter of scale. It’s the amount of He per FF unit that matters most.

The cooling bottleneck is in the confined volume of the anode, specifically the part of the anode nearest the focus point. Thus the advantage of helium for that purpose. But that helium loop needs only to be long enough to reach from the anode tip to the anode base.

Once the heat is out of the anode then standard cooling techniques can take over without strain.

And the pB11 unit would requiire a much smaller anode than even what LPP is currently working with.

So from dimensions LPP gave previously for a production FF unit we’re talking about something less than a hundred or so cubic centimeters.

Estimates for helium as reactor coolant quote from 2-7 megapascals… for now I’ll guess at 2 MPa for something like a DPF…

edit: forgot to multiply from atmospheric to MPa… lets try that again..

So say 150 cubic centimeters of He @ 2 MPa x 462,280,000,000 FF units for current global electrical generation

He is 0.0001786 grams per cc at 101325 Pa… at 2 MPa thats 0.0035253 g per cc… times 150 cc per dpf core that’s 0.52879349 g per dpf… times
462280000000 units gives 244450650000 grams He for global FF power or ~244451 tons…

So, even though that’s 19.7 times my original estimate, that is still feasible given that it will take years to build up to that point and that this He used for dpf coolant would still be recycled.

But Earthside supply limitations will have to be considered along the way, what with our wise and benevolent elites deliberately shaping a helium shortage to further entrench their fossil-derived power (and further enrich themselves, natch).

… and re lunar He: we’d only have to mine ~729 sq km of regolith at a 4 meter depth to cover the current projection of global FF helium needs…

[asymmetric_implosion]

Zapkitty: Don’t you need enough length for a compressor section and heat transfer length to extract the heat from the helium to a less expensive medium? I can’t speak for LPP but I would want enough helium primary loop length to get outside the radiation shield area so the secondary loop could use coolants with better thermal properties that might normally suffer activation.

We cool our anode with water presently. The volume of water to cool the anode is small but the water lines and cooling region are more than 20X what is used in the anode. Is Helium going to cool the vacuum chamber and cathode as well? That is more volume to be considered. Your math is on the extremely low end. If converted to standard cubic meter the total FF demand to get started by your math is 37% of the 1996 demand I listed earlier. Granted that today’s demand is larger, bringing up FF systems could be a huge dent in our helium stores. You also neglect that even a recycled system has losses. Perhaps the FF systems can produce enough to helium to keep up with loss but that adds another complication as you need to either store the helium or process it on site.

Mechanik: I was thinking of indium. My bad. 🙁

[vansig]

i had previously estimated amount of helium used for primary coolant, to be about 2 kg/s flow past the hot tip
in order to draw away 5 MW of heat. maybe that’s not liquid, but it is compressed a lot.

[annodomini2]

vansig wrote: i had previously estimated amount of helium used for primary coolant, to be about 2 kg/s flow past the hot tip
in order to draw away 5 MW of heat. maybe that’s not liquid, but it is compressed a lot.

Would that much heat need to be extracted?

I thought the intended efficiency was much higher?

[vansig]

yes, we should be thinking in terms of that much heat.

although conversion of alpha exit beam is potentially very efficient, (> 80%), the overall system efficiency is close to 50% if i recall correctly from figures Eric gave.

So a reactor capable of 5 MW of useful work will probably generate 5 MW of waste heat as well; and if you can boost the system efficiency, then you can increase the useful work while holding the waste-heat about the same.

[Author]

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