The "doesn't make bombs" story of the molten salt reactor program's cancellation is largely a myth, albeit one that's extremely common on the internet [1].
Fluid fueled reactors, mostly in the form of Molten Salt Reactors (MSRs), of which the LFTR is one brand name of, do have lots of potentially huge benefits in terms of simplicity, online refueling, and straightforward reactivity effects. We do only have about 5 reactor-years of experience with them, and so we don't fully understand the costs of building, operating, and maintaining such systems in today's regulator regimes.
There is some reason to be concerned. When the fuel is dissolved in fluid, the radioactive fission products are dispersed everywhere. Literally half the periodic table of the elements is in there, and lots of it is volatile and mobile. It gets on your pumps, your valves, your heat exchangers, etc. Doing routine maintenance becomes very challenging and will need to be done remotely.
A line I heard recently regarding MSR maintenance is: "You'll need robots to do the maintenance of your maintenance robots." Another memorable gem is "If you can make robots that can do that, you should just sell the robots"
We should absolutely work on progressing this technology, but in the face of a climate disaster, we should just build more of what we know is already safer than almost every kind of energy system we can deploy, which is regular Gen III LWRs. We need to solve problems on the construction yard so we can build them much more cheaply.
How do some of the other alternatives like Pebble Bed fit into this?
I imagine PBRs would be more challenging to recycle fuel as it's combined with the moderator(?), but are there other issues which make it unrealistic compared to traditional PWR/BWR/LWR/Magnox reactors?
I'm starting to really like gas-cooled reactors like pebble-bed. They have two fundamental technical challenges. First is that gas coolant (typically helium) isn't a great heat-transfer mechanism so you need to spread your heat generation out in a large volume. This low power density results in pretty large structures (which can get pricey to build) for relatively low power, compared to other coolant configurations like water and especially liquid metal. Related, if there's a coolant leak, all the pressurized gas shoots out and you need backup cooling systems or particularly low power density that supports heat removal via conduction and thermal radiation. This can ding your economics pretty bad too.
Second, the fuel particles you mention are very expensive to fabricate, $10k/kg. Estimates of future costs go "as low as" $3k/kg, but some say it could even go up to $30k/kg. In all cases, that's really expensive fuel fabrication. There's a plant in China that can fabricate them in moderate bulk right now, so perhaps with time and experience we can figure out how to bring this process down in cost.
Aimed at solving power density and decay-heat cooling issues, there's also the salt-cooled version of the pebble bed reactor which gets the high-temperature fuel benefits and trades high-pressure gas for low-pressure salt. A research group from MIT and UC Berkeley has worked on this for years under Prof Charles Forsberg, and a company in the Bay Area called Kairos is now working on commercializing it. Unfortunately you get a lot of tritium production from the best known salt, which is a FLiBe salt, where the Lithium + neutrons results in crap-tons of supermobile tritium. You can cold-trap most of it but it's still a pain. And the fuel is still $10k/kg to fabricate as far as anyone knows, and the graphite cracks, and high temperature corrosion under irradiation is hard.
FLiBe also needs Be. World annual production of Be is 230 tonnes. And new Li isotope separation facilities would be needed, as only very small amounts of 6Li could be tolerated. The existing technology for that is not workable now due to mercury leakage.
RE: A line I heard recently regarding MSR maintenance is: "You'll need robots to do the maintenance of your maintenance robots." Another memorable gem is "If you can make robots that can do that, you should just sell the robots"
Maintenance of MSRE was done by humans without much downtime or exposure[1]. Ease of maintenance is a function of design. Just design it carefully.
MSRE had fans blowing on pipes as cooling. It had no real fluid-to-fluid heat exchanger or power conversion cycle, which is where lots of maintenance troubles arise from. Furthermore, they were in the 1960s, before the NRC existed and before we worried about ALARA. Furthermore, MSRE cannot to this day account for about half of their radioiodine inventory. Furthermore, Alvin Weinberg himself says in his 1990s autobiography, The First Nuclear Era (amazing read btw) that they were just piping radioactive gas into the forest nearby back then (or, wait, was that the aqueous homogeneous reactor? I forget, I left all my copies of that book at the office...).
A lot has changed since MSRE ran, and MSRE was a fairly simple experiment to prove a concept. It didn't have all the systems necessary to make practical energy, and so it didn't have the maintenance issues you'd expect. Also, it only ran for like 5 years, vs the 60-80 we're hoping for these days from these facilities (inherently requires lots of maintenance).
Have you ever done work planning for ALARA at a light water reactor? Those who have raise eyebrows really high when academic reactor designers start saying how easy the maintenance will be. Admiral Rickover's quotes from 1953 just keep on coming back to haunt us [1]. He didn't curse us to death, but he sure warned us that we need to be working really hard on new/effective solutions to these kinds of problems. Most advanced nuclear advocates skip over these points.
RE: Design it carefully: This takes practical experience. The number of people today designing new reactors who have this kind of experience is very close to zero. We will have to re-learn.
RE: Furthermore, they were in the 1960s, before the NRC existed and before we worried about ALARA.
"Exposure of personnel to radiation has been held well below permissible limits: the maximum exposure of any individual in any quarter has been <0.5 rem." This is within the ALARA standards.
RE: or power conversion cycle, which is where lots of maintenance troubles arise from.
That's the part which is not highly radioactive. Fluid fuel reactors have simplest fuelling mechanism of any reactors. The fuelling of a fluid-fuel reactor is even simpler than fuel injectors of diesel engine. Where the need for "maintanence" arise in this simple machine??
Only moving parts are the pump impellers and the control rods. ORNL had some difficulty designing the bearing, seals etc. for the salt pump. But, even that is solved today with concentrating-solar salt pumps.
> Exposure of personnel to radiation has been held well below permissible limits
Ah, the good old days! 25-50% of the radioiodine from MSRE is completely unaccounted for. No one knows where it went. This is wholly unacceptable from a modern regulatory approach. It will have to be found.
> MSRE had salt-to-salt and salt-to-air heat exchangers made from hastelloy-N
Fair enough. It's generally in the salt-to-water heat exchanger where major maintenance concerns take place in a power reactor. Check out steam generator problems in PWRs and SFRs to see what I mean. You're right that the radiation will be low there if they use an intermediate salt. But the baseline maintenance problems come in at the steam generator in most plants. So MSRE avoided any problems there but any power MSR will have them like any other reactor. Good old BWRs get around this via direct cycle.
> Where the need for "maintanence" arise in this simple machine??
Pumps. Heat Exchangers. Valves. Flanges. Welds. Vessels. Graphite. Reflectors. Fission product processing equipment. Instruments/sensors. Control mechanisms. There are very complex systems with lots of things working together in a very tough environment (high radiation, high temperature). Maintenance is a major challenge.
<0.5 rem per quarter (or 5 mSV) is in compliance with today's dose limit. ORNL was a competent American national lab with high standards even back in the 1960s. Another hollow argument from today's incompetent nuclear industry.
The radioactive gas was probably noble gases like krypton and xenon. They'd have to be careful with the xenon, as 137Xe decays to dangerous 137Cs (halflife 3.8 minutes). The idea "all the radioactivity stays in the salt" can be wrong, as that xenon could deposit the cesium outside the salt if it bubbles out quickly enough.
135Xe also can be removed (halflife ~9.2 hours). It is famously a very powerful neutron poison with important effects on reactor operation; those advocating thermal breeders count on it being removed before it soaks up neutrons. But this leads to an increase in production of its decay product, 135Cs, which has a halflife of 2.3 million years.
MSRE had a charcoal-bed off-gas system which safely stored all the noble gases. It was cooled and shielded with water, similar to today's PWRs. PWRs also have water chemistry control and resin-bed filters for primary loop which is highly radioactive and shielded.
Construction of the off-gas system is described in pages 58-60 in ORNL-3708: Molten-Salt Reactor Program: Semiannual Progress Report for the Period Ending July 31, 1964
https://energyfromthorium.com/pdf/ORNL-3708.pdf
Fluid fueled reactors, mostly in the form of Molten Salt Reactors (MSRs), of which the LFTR is one brand name of, do have lots of potentially huge benefits in terms of simplicity, online refueling, and straightforward reactivity effects. We do only have about 5 reactor-years of experience with them, and so we don't fully understand the costs of building, operating, and maintaining such systems in today's regulator regimes.
There is some reason to be concerned. When the fuel is dissolved in fluid, the radioactive fission products are dispersed everywhere. Literally half the periodic table of the elements is in there, and lots of it is volatile and mobile. It gets on your pumps, your valves, your heat exchangers, etc. Doing routine maintenance becomes very challenging and will need to be done remotely.
A line I heard recently regarding MSR maintenance is: "You'll need robots to do the maintenance of your maintenance robots." Another memorable gem is "If you can make robots that can do that, you should just sell the robots"
We should absolutely work on progressing this technology, but in the face of a climate disaster, we should just build more of what we know is already safer than almost every kind of energy system we can deploy, which is regular Gen III LWRs. We need to solve problems on the construction yard so we can build them much more cheaply.
[1] https://whatisnuclear.com/thorium-myths.html#myth1