No More Nuclear Waste?

[dlorde]

The 'Waste Annihilating Molten Salt Reactor (WAMSR)'. Runs on waste fuel pellets from conventional reactors by dissolving them in a molten salt. Generates power by using up to 98% of the energy in the dissolved pellets (compared to around 3% for PWRs), resulting in around 3 kilos of relatively low activity waste a year. Bye-bye nuclear waste disposal problem...

Sounds too good to be true, but is it?

 

[Dinwar]

My initial reaction is to be highly dubious. Salt isn't easy to make molten--and is even harder to contain once you do it.

 

[rjh01]

Off the top of my head this would have a number of questions
1. Would there need to be any pre processing of the waste before it goes into the reactor?
2. Would there be heaps of fast neutrons produced? These are hard to contain. It needs a thick carbon shielding. That is why there are no plutonium reactors.
3. What actually burns? There are many elements in nuclear waste. Not many of them will burn. If they are just using the heat from the radioactive decay then the reactor will not produce much heat.

 

[shadron]

My initial reaction is to be highly dubious. Salt isn't easy to make molten--and is even harder to contain once you do it.

The salts in question are fluorides; they melt at 400-600C. Those temperatures are the same temperatures that light water reactors run at, but without all the extreme pressures needed to keep water liquid. There are some corrosion problems but these were all addressed in the 70s and solutions are available.

The key to making liquid fuel reactors work (that is, eating waste - once used uranium fuel) is regular, more-or-less continuously drawing off the liquid fuel, removing the fission products and recharging it with more fissionable fuel salts. This calls for some automated chemical processing.

 

[shadron]

Off the top of my head this would have a number of questions
1. Would there need to be any pre processing of the waste before it goes into the reactor?

Yes. The solid pellets would have to be removed from the cladding and then reacted with fluorine. This is not very difficult; uranium is usually enriched in gas centrifuges as a hexafluoride in the first place.

2. Would there be heaps of fast neutrons produced? These are hard to contain. It needs a thick carbon shielding. That is why there are no plutonium reactors.

The nuclear fission process is identical to that with solid fuel; the uranium is uranium, even if combined with fluorine. Shielding, if needed (I think it is a red herring here), is not that hard. Water cuts gammas in half at 7 cm thickness. Carbon (graphite) in reactors (as in the Chernobyl's RBMK reactors) is a moderator, not a shielding. Fuel in a reactor is 5% or so enriched; fuels can be enriched with plutonium rather than U-235; after all, it's only in there as a sort of starter fuel, providing neutrons to keep the chain reaction going (yeah, that's sort of simplistic, but the Fukushima reactors used a U-235/plutonium mix called MOX).

3. What actually burns? There are many elements in nuclear waste. Not many of them will burn. If they are just using the heat from the radioactive decay then the reactor will not produce much heat.

What burns is the remaining partially enriched fuel (U-238 plus enrichment U-235 and/or plutonium, plus other fissionable actinides). The only ones that are troublesome are those that won't create a liquifiable salt, and those that won't fission, but activating them upwards will usually find a fissionable somewhere along the line). No, the "heat of decay" is pretty much removed by pulling out the fission products; that also drops the toxicity of the fuel (from a nuclear reaction standpoint) as well as moving a lot of the high-intensity radiation fast-decay isotopes.

 

[shadron]

The 'Waste Annihilating Molten Salt Reactor (WAMSR)'. Runs on waste fuel pellets from conventional reactors by dissolving them in a molten salt. Generates power by using up to 98% of the energy in the dissolved pellets (compared to around 3% for PWRs), resulting in around 3 kilos of relatively low activity waste a year. Bye-bye nuclear waste disposal problem...

Sounds too good to be true, but is it?

Anyone who wants a good idea about the pros (and less about the cons, of course - these videos are promos) can watch these.
BTW, the main speaker is Kirk Sorenson, the foremost advocate for thorium based liquid fuel reactors, president of FliBE Energy.

10 minute TEDx talk:

2 hour full remix of many different talks. Good for someone with at least a cursory nuclear background:

For a good pro/con list, see LFTR^WP.

 

[Farsight]

The 'Waste Annihilating Molten Salt Reactor (WAMSR)'. Runs on waste fuel pellets from conventional reactors by dissolving them in a molten salt. Generates power by using up to 98% of the energy in the dissolved pellets (compared to around 3% for PWRs), resulting in around 3 kilos of relatively low activity waste a year. Bye-bye nuclear waste disposal problem...

Sounds too good to be true, but is it?

It's true. Follow shadron's link and watch the Kirk Sorensen video. The general idea of this thorium reactor was worked out at Oak Ridge decades ago, but it got sidelined because the USA wanted plutonium for weapons. See this report and check out Robert Cywinski and Energy from Thorium. In the fullness of time I think people will come to appreciate the enormity of the vested interest and ignorance that stifled development of clean safe thorium power.

 

[rjh01]

Thanks for your response shadron. The only quibbles I will make are
1. U-238 is not a fuel itself. It needs to be converted into plutonium which is a fuel. But this is normally done in the reactor itself.
2. Any pre processing needed would have to take precautions as the material is highly radioactive.Wait a few years and it would not be so radioactive, though probably still a lot more than normal fuel.
3. Shielding. Gamma radiation is blocked by heavy elements such as lead.
4. Shielding against neutrons is hard. It may make the shield radioactive itself. Neutrons are produced when plutonium is used as a fuel. Water can be used as a shield for neutrons.
5. Shielding (again) You would probably want to stop more than 50% of the radiation so a thicker shield would be required.

Further reading. http://en.wikipedia.org/wiki/Reactor-grade_plutonium
http://en.wikipedia.org/wiki/MOX_fuel
http://en.wikipedia.org/wiki/Fast-neutron_reactor
http://en.wikipedia.org/wiki/Radiation_protection
Edit. One more http://hps.org/publicinformation/ate/q5087.html

 

[Farsight]

The article Dlorde linked too wasn't very clear. See this one instead:

"There’s a new kid on the thorium block.

Meet Transatomic Power, an MIT-connected fledgling that’s designing a thorium molten salt reactor by combining nuclear expertise with Silicon Valley style start-up panache..."

 

[joesixpack]

Here's another thread on Thorium reactors.

http://www.internationalskeptics.com/forums/showthread.php?t=228852

 

[cjameshuff]

My initial reaction is to be highly dubious. Salt isn't easy to make molten--and is even harder to contain once you do it.

What? Just off-hand...molten salt processes are the only industrially practical way to produce aluminum, magnesium, and other reactive metals. The FFC Cambridge process for producing titanium uses a molten salt bath. Molten salt is used as a thermal store for solar thermal systems. It's not hard to do or hard to contain, and several molten salt research reactors have actually operated.

 

[cjameshuff]

1. U-238 is not a fuel itself. It needs to be converted into plutonium which is a fuel. But this is normally done in the reactor itself.

It's not fissile, it can't maintain a chain reaction on its own, but it's certainly a fuel. It goes in, lighter elements and energy come out. The advantage of these reactors is that they produce enough excess neutrons to almost completely burn non-fissile isotopes like depleted uranium and thorium, vastly reducing waste and increasing the range of fuel types they can use.

 

[Natan]

The figure of only three kilos sounds a bit low. I know the amount is greatly reduced, but still.

 

[dlorde]

Thanks for those links; it's interesting stuff.

I particularly liked the suggestion of a DoD 'competition' between companies for a small molten salt reactor with funding from the nuclear industry waste disposal fees currently idling while an alternative to Yucca mountain is located.

I'd have thought that the big nuclear industry players would be keen to partner one of these start-ups, given the technology potential and the feature that it could 'burn' the waste from their own reactors...

 

[shadron]

Thanks for your response shadron. The only quibbles I will make are
1. U-238 is not a fuel itself. It needs to be converted into plutonium which is a fuel. But this is normally done in the reactor itself.

Yes, that's absolutely right. U-238 is not fissile; it won't fission using the slow "thermal" neutrons available in a moderated reactor. It captures the neutrons and transforms (ultimately) into plutonium 239, which is fissile to thermal neutrons. The same applies to thorium, which is transformed into uranium 233, another fissile.

2. Any pre processing needed would have to take precautions as the material is highly radioactive.Wait a few years and it would not be so radioactive, though probably still a lot more than normal fuel.

I presume you mean here processing the liquid fuel to remove the fission products. This is done by an automated chemical processor, using either mechanical processes immune to radioactivity or using shielded computer control. It worked for the Oak Ridge reactor, it should work today. There is no reason to have a human's susceptible flesh dangling over the vat.

Yes, eventually things will breakdown and need repair. That will require technology which can exchange a working plant for a troubled one, and then the problem can be resolved or junked as needed. This is not new.

3. Shielding. Gamma radiation is blocked by heavy elements such as lead.

Gamma rays are blocked by everything but vacuum, proportional to the density, and inverse to the energy (frequency) of the rays. Given 500 kEv gamma rays, the halving layer (HVL) thickness of lead is 4.2 mm (1/8 in); 42 mm (1-2/3 inch) cuts it down by about 1000 times. The halving distance of water is 7.1 cm (3 in, see http://what-if.xkcd.com/29/), for air is 62 m (200 ft).

4. Shielding against neutrons is hard. It may make the shield radioactive itself. Neutrons are produced when plutonium is used as a fuel. Water can be used as a shield for neutrons.

Same thing that shields gamma shields neutrons, to roughly the same extent; like you say, neutrons have a tendency to "activate" the materials that absorb them. This is at it's worst (from our point of view) in metals (excepting chromium), but atmospheric gases and water don't activate, or rather, they do, but the activations (like O16 going to O17) are stable, not radioactive. One particularly nasty activation is both the sodium and the chlorine in sea salt, which is why the Crossroads target fleet got so thoroughly radioactive.

Neutrons are produced when anything is used for nuclear fuel. A condition for being a successful nuclear fuel is that when fission occurs, at least two new neutrons are produced, one to replace the absorbed one and one to increase the chain reaction's reach. Fission is a necessary corollary to chain reaction fission, regardless of the fuel. They're also produced by all the easy fusion reactions.

5. Shielding (again) You would probably want to stop more than 50% of the radiation so a thicker shield would be required.

Yes, of course. 5x thickness reduces gamma radiation to 1.3%, 10x to 0.1%, 20x to a factor of 1e-6 and so on.

Further reading.
http://en.wikipedia.org/wiki/Reactor-grade_plutonium
http://en.wikipedia.org/wiki/MOX_fuel
http://en.wikipedia.org/wiki/Fast-neutron_reactor
http://en.wikipedia.org/wiki/Radiation_protection
Edit. One more http://hps.org/publicinformation/ate/q5087.html

Thanks. May I add http://en.wikibooks.org/wiki/Basic_Physics_of_Nuclear_Medicine/Attenuation_of_Gamma-Rays

 

[shadron]

The figure of only three kilos sounds a bit low. I know the amount is greatly reduced, but still.

3 kilos is low. The figure I've seen most usually is < 1% of the waste of a LWR. See http://www.thoriumsingapore.com/content/index.php?option=com_content&view=article&id=52&Itemid=50, third question down.

Here is a diagrammatic display: http://www.google.com/imgres?imgurl...xJUdr3K-PGygHPtoGYDQ&ved=0CEQQ9QEwAg&dur=1010

 

[shadron]

Thanks for those links; it's interesting stuff.

I particularly liked the suggestion of a DoD 'competition' between companies for a small molten salt reactor with funding from the nuclear industry waste disposal fees currently idling while an alternative to Yucca mountain is located.

I'd have thought that the big nuclear industry players would be keen to partner one of these start-ups, given the technology potential and the feature that it could 'burn' the waste from their own reactors...

The business model of the nuclear industry, like that of the computer printer industry, is to sell the commodity (the nuclear fuel) rather than the reactors themselves. Liquid fuel doesn't lend itself to that sort of monopoly situation.

 

[Natan]

3 kilos is low. The figure I've seen most usually is < 1% of the waste of a LWR. See http://www.thoriumsingapore.com/content/index.php?option=com_content&view=article&id=52&Itemid=50, third question down.

I've seen something similar. Here is part of a discussion on Kirk Sorensen's forum:

Very light on details. The sketch of the reactor suggests graphite in the core and no blanket. The secondary loop is steam - which would create enormous problems for tritium leaks and puts high pressure water with only thin wall separation to fission products in the HX. This won't fly. They stated that the freeze plug provides the primary safety but in reality it is the passive cooling system that is primary for safety. They mentioned that they produce 3kg of waste per year and compared it to 20 metric tonnes from an LWR. Elsewhere I read that their reactor is 200MWe. They also stated that they could consume 98% of the waste. So, if you could feed the reactor with just LWR TRUs (no uranium) it would consume 200kg of TRUs/year to produce 200 MWe. If you burn 98% of the fuel you would be left with 4kg of TRUs for waste. The fair comparison then would be 200kg waste from an LWR versus burning up the 196kg of that waste and being left with 4kg. The fission products are still waste though and should not be ignored. Both reactors would produce about 200kg of 300 year waste. In addition to this is the fertile mixed in with the waste. Here the LWR has around 20 tonnes and a purely waste eating LFTR has none.

It doesn't sound like these two have done too much homework yet.

But if we can get MIT to create a class on molten salt fueled reactors that would be a step forward.
http://energyfromthorium.com/forum/viewtopic.php?f=8&t=3576&hilit=Transatomic​

 

[Wolrab]

A waste is a terrible thing to mind.

 

[geni]

As a general rule if you are adding fluoride irons to things to make them safer there is something fundamentally wrong with your process.