You're correct. The vessel & primary heat exchanger of a MSR which are in contact with liquid-fuel is equivalent to fuel assemblies of many solid-fuel reactors. In solid-fuel reactors the clad which is designed to contain fuel doesn't last long. Similarly, in MSRs the materials in contact with fuel doesn't last long.
So, we need to dispose the vessel and primary heat exchangers similar to Zr-clad million dollar fuel assemblies.(PWR/BWR reload costs $60-100 million per 12-18 months.)
Material wise Zr clad is way more expensive than stainless 3XX & graphite combined. Even if Ni-alloy is used in disposable MSRs Ni-alloy+graphite will be slightly cheaper. (Base metal costs: Zr metal: ~$22,700/ton Ni metal: ~$13,100/ton)
I agree. But, in a molten-salt reactor, the vessel and heat exchangers are equivalent to fuel assemblies. Dispose the vessel and heat exchangers similar to Zr-clad million dollar fuel assemblies.(PWR/BWR reload costs $60-100 million per 12-18 months.)
Material wise Zr clad is way more expensive than stainless 3XX & graphite combined. Even if Ni-alloy is used in disposable MSRs Ni-alloy+graphite will be slightly cheaper. (Base metal costs:
Zr metal: ~$22,700/ton
Ni metal: ~$13,100/ton)
Many actinides have volatile fluorides. Fluoride volatility is a proven process used to enrich uranium for LWRs. Fuel salt can be disposed after recovering U & Pu. Vacuum distillation is physical separation, which can also be used to further recover expensive base salt FLi7Be. No vacuum distillation is necessary if inexpensive salt is used.
Primary loop of a MSR is equivalent to a cladding of a solid fuel assembly. You get a REACTOR at the same cost/MWe of solid fuel assembly. Ni alloys are cheaper than Zr alloy. So, early MSR designers will not opt for maintenance of the primary loop.
(Molten salt reactor is a fluid fuel reactor. Fluid: liquid, gas and plasma.)
Molten salt reactor is a stepping stone for fusion reactors. We need to master liquid and gaseous reactors before going plasma. Our civilization should aim for larger goals. We need some form of nuclear power to terraform other planets and spread earth's precious life and conciousness everywhere.
Well, given how solar has an inverse square relation between watts/area and distance from the sun, by the time you get to mars, solar is getting pretty piddly, the asteroid belts and jovian moons its downright comical.
Actually, solar still has huge advantages at the distance of Mars.
In space (actual space, not a planetary surface), it's difficult for nuclear to dissipate its prodigious amounts of waste heat. Solar, in contrast, can be made much lighter than on Earth, as there isn't wind and rain to deal with. And of course in general the Sun is available 100% of the time in space, further improving the economics.
But in any case, the argument wasn't that solar was necessarily better, it was that nuclear wasn't necessary. Solar can provide energy basically anywhere, with power beaming, even out into interstellar space.
Planets rotate so backup power is needed during nights. Spacecraft has to avoid obstracles and has to move, so backup is needed. During interstallar journey there is time delay in communication, so it may take hours to adjust the power beam when spacecraft moves a little bit. Fusion is like solar energy just near to us and MSR will be eventually replaced by fusion reactors.
> Planets rotate so backup power is needed during nights.
This does not rule out use of solar energy.
> During interstallar journey there is time delay in communication, so it may take hours to adjust the power beam when spacecraft moves a little bit
The vehicle tracks the beam, not vice versa.
> Fusion is like solar energy just near to us and MSR will be eventually replaced by fusion reactors.
Fusion is ridiculous when examined closely. It's like fission, only far worse from an engineering and cost perspective.
> Stefan–Boltzmann law: Radiation heat transfer is proportional to 4th power of temperature.
Which means waste heat from a reactor in space has to be radiated at higher temperature than for a reactor here on Earth, which means nuclear is worse off there than here. In contrast, solar works better in space, as I argued upthread.
>Tritium production: If lithium is used in the salt, tritium will be produced,
Not a disadvantage for all MSRs. Both lithium and beryllium can be avoided. FLiBe is required for efficient MSRs and MSBR.
>Mobile fission products
It is the only disadvantage common to all molten-salt reactors and all fluid-fuel reactors. Pumps and pipes have to handle a hot radioactive liquid.
> Material Degradation
Common to all nuclear reactors, solar, coal boiler tubes, etc. Components used in reactor core do not last long. MSRs dispose nickel tubes and LWRs dispose zirconium tubes and uranium.
>Proliferation...The problem with MSRs, then, is that the fuel is already completely cut open and melted.
>it will be difficult for the IAEA to distinguish plate-out losses from actual proliferative losses.
The fuel salt loop can be sealed tamper-proof. The entire fuel salt loop is analogous to a fuel assembly. Weigh the entire fuel salt loop. Vapor pressure of actinide-halide salt is very low at operating temperatures. Actinides don't move out of this loop.
Regarding pumps and pipes, the Moltex design avoids them entirely. Liquid fuel is contained in vertical fuel rods, open at the top to allow escape of gaseous fission products (some of which are strong neutron absorbers). The pipes are mostly immersed in a pool of molten salt coolant.
This is not a disadvantage for all MSRs. Only breeders need chemical plant. Not fair to compare a solid-fuel burner to liquid-fuel breeder.
Let us compare liquid fuel breeder vs. solid-fuel breeder: Reprocessing solid-fuel involves more complex chemical plant with physical mechanisms to declad and convert solid-fuel to a processable liquid. Fabricate processed liquid to solid-fuel and put it back in the reactor. Solid fuel breeders have additional physical and chemical processes/steps.
From the article:
>Problems with Molten Salt Reactors...
>...but similar problems may show up in long-lived power reactors.
Author assumes that MSR components should last as long as vessels and secondary heat exchangers of solid-fuel reactors. The author should understand that vessel and primary heat exchanger of a fluid-fuel reactor is anologus to fuel rods. Solid fuel reactors just dispose primary heat exchangers or fuel rods every few years.
Example: Zircolloy tubes worth a MSR vessel + heat exchanger is just disposed along with partially fissioned degraded solid fuel every 4.5 years in a LWR. Zircolloy (Hafnium separated nuclear grade zirconium + additives) is more expensive than commercially available nickel based alloys.
Graphite is a solid with a crystal structure. Crystal structure degradation under radiation is permanent and there is nothing anyone can do to reverse it. Solar panels degrade similarly. Nuclear industry handles solid-fuel rods which are far more radioactive than MSR graphite and complains that it can't handle MSR graphite. Just shows that either industry is incompetent or it is not interested in efficient fluid-fuel reactors. Does nuclear industry aims >1000 GW of nuclear capacity? Does nuclear industry care to solve global energy related issues? Efficiency really matters when we have >1000 GW of installed nuclear capacity. If all energy is obtained from nuclear, (12000-16000 GW) even seawater uranium get used up in 40-60 years with inefficient solid-fuel reactors.
> Efficiency really matters when we have >1000 GW of installed nuclear capacity. If all energy is obtained from nuclear, (12000-16000 GW) even seawater uranium get used up in 40-60 years with inefficient solid-fuel reactors.
That can't be right. About 200 tonnes of natural uranium is needed to produce 1 GWe per year in conventional reactors [1]. That's 3,200,000 tonnes per year if you mean 16000 GW in the form of electricity, or closer to 1 million tonnes per year if you're referring to primary (thermal) energy. Seawater contains about 4.5 billion tonnes of uranium [2]. That's well over a thousand years' worth of uranium, either way.
Firstly heat from inefficient low temperature solid-fuel reactors can't be used directly for many applications. So consider electricity. Cars/planes/kitchen-stoves cant use uranium or nuclear heat!!
All 4.5 billion tons can't be extratcted. More we extract, concentration decreases and harder it gets. I keep asking this question: If seawater extraction of metals is practical, why aren't we extracting other costly metals now?
https://twitter.com/AchalHP/status/1011661441412337665
If extracting gold is only 300 times harder, that puts the lower bound cost at $108,000 per kilogram of gold. That's significantly more expensive than the market price of gold.
Indeed, extracting uranium itself from seawater is not cost competitive with conventional terrestrial mining at present. But the process has been demonstrated on a technical level. Either terrestrial uranium deposits will have to get closer to exhaustion or seawater extraction will have to be much more cost optimized (or both) before seawater extraction of uranium is economically competitive. I was only addressing your technical claims about the exhaustion of seawater uranium, not making economic claims.
You included my sentence: "Efficiency really matters when we have >1000 GW of installed nuclear capacity." I wanted to address that: efficiency matters.
Inefficient technologies will hit some constraints if not others.
We may run out of time: A pressure vessel production cycle is over one year[1]. By the time we reach >1000GW someone may make fusion practical or China's competent nuclear industry may start selling MSRs to everyone.
We may run out of highly skilled people required to build and operate these complex machines. The safety is highly dependent on operations and management throughout the life cycle of complex machines. From manufacturing to operations there is tiny margins for error. (Not a physical constraint.)
LWR technology is associated with military ships and submarines, no one shares it. Few countries control the technology and there are only a few places in the world to make pressure vessels. (Not a physical constraint.)
We may run out of waste disposal sites. A geological repository needs special rock formations away from earthquake zones.
The article is written by a nuclear industry professional who tells that routine things done by the nuclear industry today when applied to MSR is a "disadvantage". They open the lid of an LWR every 18 months and replace/shuffle highly radioactive fuel assemblies and say that it is a disadvantage for them to replace graphite. That is totally unfair.
TL;DR
Even small pressure vessels (used by nuscale) have lead times of about a year. A megafactory can build like 1 or 2 vessel per month. At 1.5GW/year/factory, we need like 22 factories to keep up with the production schedule if we need to reach 1000 GW of small-LWR capacity within, say 2050.
Don't confuse solid fuel with traditional light water reactors. The highest temperature reactors are triso fueled helium cooled solid fuel reactors like HTTR with outlet temperatures over 1000C. Also, fast breeder reactors with solid fuel are just as sustainable as any fluid fuel breeder.
Molten salt is one of about a dozen advanced reactor techs that has huge potential.
Hard part is economics. Hazardous coolant has been a pain to maintain cheaply so far.
Also seawater uranium replenishes from erosion and plate tectonics so it will effectively never decrease in concentration, even if we pull it out at world scale.
The successor of LWR/HWR should be a fluid-fuel reactor.
Once we setup a pebble/advanced fuel making factory, closing it will takes decades (because people may lose jobs) and the new solid-fuel factory will again pause nuclear innovation for another 100 years. The only way to continuously improve nuclear reactors is to go fluid-fuel. No engineered fuel, so no job loses. Reactor innovation is independent of fuel factory. Nuclear fuel becomes a commodity instead of engineered speciality. Example: MSRE ran U235 and U233 without any modification.
Secondly, solid fuel reactors throw away fuel along with heat exchange surfaces (clad). Fuel also undergoes crystal structure degradation along with other solid structures. Maybe there is enough fuel in the seawater, but there may not be enough places suitable for geological repositories.
Solid-fuel reactors always need excess reactivity reserve. Always needs control rods, and if someone (or a bad actor) pulls all the control rods, reactor gets supercritical. Needs highly skilled people and needs security.
For emergency shutdown of solid-fuel reactor, poison is added to coolant, not fuel. In an emergency, poison is added to the liquid-fuel, permanently destroying the fuel. Emergency can be anything, from a natural disaster to terrorist attack. Fluid-fuel reactors offer unbeatable safety features against anything.
Highest temperature fission reactors are the gas core (vapor core) and ion core reactors. They are fluid-fuel reactors, but not demonstrated. Currently solid-fuel reactors hold the record for highest temperatures.
Fusion reactor runs hotter and again fuel is in fluid state.(Fluid: Liquid, gas & plasma.). But only runs for 10-100 minutes. Demonstrated fission reactors run for thousands of hours continuously.
Or make fuel. There are all sorts of ways to make carbon neutral diesel and jet fuel. They just aren't economic compared to pumping it out of the ground unless the externalities of burning virgin fuels are priced in with regulation.
From the article:
>if something goes wrong in a MSR and the temperature starts going up, a freeze plug can melt,
A MSR does not need any sort of valve to drain the fuel. ORNL-MSBR was designed to drain the core when pumps stopped working. The real advantage of freeze valve is that is uses no moving parts and maintanence free/friendly.
ORNL-4528: "The fuel salt pump and its sump, or pump tank, are below the reactor vessel, so that failure of the pump to develop the required head causes the salt to drain from the reactor vessel through the pump tank to the fuel salt drain tank."
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
What applies for fission products should also apply for control rods in fast spectrum. If control rods do not absorb fast neutrons, how a fast reactor is controlled?
The cross section of fuel in fast spectrum also decreases. Fast reactors have larger fissile holdup. For every atom of fission fragment there are more fuel atoms in fast reactor than in thermal reactor. Fast reactors can run longer because fission products are dilute in the fuel. But, reprocessing is needed even for fast reactors to close the fuel cycle.
The advantages like U238 or Pu239 fission or fast fission factor can be also obtained in heterogeneous thermal spectrum reactors.
(Example: A CANDU is heterogeneous reactor. It can burn more plutonium and it can use LWR SNF-fuel without reprocessing.)
For a fluid-fuel reactor the fuel acts only as fuel and heat exchange takes place outside reactor core. Fuel region in a fluid-fuel reactor can be as thick as it is needed. So, reactor can be designed for maximum multiplication factor. (Solid fuel rods can't be very thick, because they are also heat exchangers.)
Edit: This is a reply for: "I prodded him gently on it: "Oh, you mean the fast neutrons don't get readily absorbed in fission products, not the slow ones, right?" He doubled down. Some poor schmuck is funding this guy."
Similar to fission products, neutron control material (like Boron) indeed does not absorb fast neutrons as much. Fortunately it still does absorb fast neutrons enough to control fast reactors (with which we have 430 reactor-years of experience or so).
Indeed fast reactors need higher fissile concentration to be critical. Some kinds of fast reactors (like the Bill Gates Traveling Wave Reactor idea, which, disclaimer: I have professional connections to) can breed up plutonium in spent fuel and then burn it down without reprocessing (to be fair, the spent fuel would have to be hot refabricated into metallic fuel first, but that's still not separations/reprocessing). This kind of reactor needs enrichment once (to start up) and then can just be fed natural, DU, or SNF and it will run on a stream of it happily "forever" (until the vessel life is reached, at which point you transfer the core to a new machine and keep on shuffling). Only fast neutron systems can do such a thing. The Fast Mixed-Spectrum Reactor idea of BNL in 1980 was similar. Many fast-neutron MSRs are the same.
Fast fission is not all that's at play in a fast reactor. It's all about Eta (neutrons released vs. neutrons absorbed in fuel). In a fast reactor, it skyrockets around 0.5 - 1 MeV because of three different physical facts: neutrons released per fission goes up with fast incident neutrons, fission cross section stays flat for all actinides around this range, and capture cross sections drop off towards zero around this range. This is discussed on: https://whatisnuclear.com/fast-reactor.html#havingmore
Having extra neutrons around means you can afford to invest more in breeding fissile material, and only fast neutrons can get you to the point where reactivity is flat or increasing instead of decreasing as fission products are produced. The only exception is Thorium fuel cycle, where U-233 releases a lot of neutrons even with thermal neutron absorption. In that case you have to be removing the fission products as they are created with separations, and this basically is only practical with fluid fuel. This was the idea of the Molten Salt Breeder Reactor project of Oak Ridge in the late 1960s.
With reprocessing, you can definitely re-concentrate the remaining fissile material and get a thermal reactor critical again to burn it. But you cannot meaningfully use the other actinides as fuel, which is the whole idea of "running on spent fuel".
Regarding that slide deck, I love the idea of molten fuel in tubes. I investigated it heavily once upon a time, being super excited about it. I ran into the problem that while salt fuel mass densities are low, separating them out even more physically makes the fissile density annoyingly low, to the point that I couldn't get the reactor performance I wanted for my then target market. I still think this concept is very interesting so I hope these folks do well. They won't be extracting any non-fissile energy from SNF though until they replace that moderating fluid with something like lead, gas, or sodium and change their fuel salt to a chloride.
CANDUs can't use straight-up SNF out of a LWR without first stripping out the fission products. They're an example of "burning down the fissile material even better" but not "extracting the 100x more energy from the fertile U238".
BTW the safety example in that slide deck is a bit disingenuous by suggesting that traditional reactors are not physically stable. If you know those guys you might want to tell them. Overmoderation and Doppler and the NRC ensure that they are inherently stable at power. These things aren't like the F-22 or whatever, requiring active systems to stay in the air. The engineered safety systems are simply for removing decay heat, which can be done passively in Gen III+ plants and indefinitely in any Gen-IV plant.
Fission products have larger cross section than enriched Boron-10. It is all about concentration; that is the number of B-10 atoms per number of atoms of fuel. Here is the cross sections plot I took yesterday:
https://imgur.com/a/uWb0SAa
Robert Steinhaus' Question for fast reactor folks: "While the theoretical case for fast reactors being used to burn nuclear waste down to fission products with short half lives has been made for decades, there has in all that time not been a single demonstration of a fast reactor actually experimentally burning any significant quantity (kilograms) of separated Minor Actinides and Transuranics down to fission products to a batch completeness exceeding 90%.
The proposal of using fast reactors to treat nuclear waste has been vigorously put forward for decades.
Why do fast reactor proponents not demonstrate with one existing fast reactor the burn up of some kilograms of separated long half life Minor Actinide waste to prove the technical feasibility of fast reactors for the application of waste burning and waste treatment?"
Yes, more neutrons are released for Pu239 fast fission. In CANDU, U238 fast fission is more because 99% of fuel is U238. If we increase Pu concentration Pu fast fission happens as well. U238 fast fission is ~3% of CANDU's power. This is because all neutrons are born fast. A FFR (MSR) can adjust heterogeneity as required, because heat transfer takes place outside the core. A MSR can have thicker fuel regions and higher lattice pitch than a CANDU and exploit fast fission further.
"These things aren't like the F-22 or whatever, requiring active systems to stay in the air."
That is exactly how pressuriser in a PWR works. A control system loop with Temp & pressure sensors with heater and water injection. Almost all reactors are directly synced with grid, reactors will participate in tiny load adjustments and there is also cooling water temperature variations in a day.
MSRs can be isolated from the power conversion using a thermal salt reserve. That is called inherent safety. Water temperature variations or frequency control wont hit the reactor.
Even if MSR is connected to grid directly, there is no DNB control, the boiling point margin is high. If Xenon is removed with >90% efficiency, -ve reactivity can take care of leftover xenon as well, no control rod circus for MSR till a xenon equilibrium is attained.
Whoops: the dominant control reaction in B-10 is (n,alpha), not (n,gamma)! That's a common mistake. As you'll see, it's larger than most fission products across the board See https://imgur.com/6ODUcRx. There are a few Hafnium and Europium nuclides that can beat it but they're pretty expensive so most people stick to Boron control.
Regarding that paper on CANDUs, I'd like to see some lattice physics calcs supporting that. Has anyone run CASMO on it? I would be absolutely shocked if you took a core of 55 MWd/kg (avg) LWR spent fuel, put it straight in a CANDU with no separations (all FP inventory accounted for) and saw it push through an additional normal amount of burnup.
Today, most CANDU's use ~2% enriched feed, I believe.
PWRs are not unstable. They are in their most critical configuration during operation. If they heat up, the water density goes down. Neutrons fail to be moderated and the source of thermal neutrons back into the fuel goes down, and the chain reaction shuts down. This is a regulatory requirement (GDC 11 of the NRC). Unstable cores run away in power excursions when poked (see Chernobyl). Modern cores don't do that. They have negative MTC and Doppler.
I'm well aware of what inherent safety is. EBR-II, a sodium-cooled fast reactor, was the first to actually demonstrate passive shutdown and passive decay heat following unprotected loss of heat sink and unprotected loss of flow.
BTW I don't advocate for fast reactors to burn SNF. It's much more economical to just dispose of it and mine new uranium. I just was pointing out that some people messed up their attempts to burn SNF in thermal reactors.
Yeah, I missed it the first time. You commented when I started editing it. I took (N,TOT) this time. Fission product cross sections is still higher. https://imgur.com/a/pjoNv1p
CANDU: DUPIC fuel cycle is not demonstrated. Canadians don't run PWRs. India, China and South Korea run PWR and CANDU, not much advanced R&D in India. Chinese CANDU in Qinshan may be testing this. US is most advanced in nuclear R&D. Why not build a CANDU in the US and demonstrate this?
I was talking about pressuriser and DNB. If there is no pressuriser (PWR) and constant adjustment of recirculation flow (BWR), clad may get damaged and fuel may melt because of DNB or denucleate boiling. Not inherently safe, engineered systems are needed. (In BWR fuel itself is engineered top-to-bottom with different enrichment levels and different burnable poison concentrations.)
Control rod adjustments are needed till a xenon equilibrium is attained. Also flux flattening circus, boron dilution circus etc. Operators are needed to babysit a LWR or CANDU. This is not like diesel generator or PV panel. Passive shutdown does not mean passive operation like a diesel.
I don't think first generation of MSRs can be like diesel generators. Operators are needed. But, there is potential for nuclear reactors to be like diesel generators with fluid-fuel reactors.
(Edit: Reply to "Today, most CANDU's use ~2% enriched feed, I believe." No. ALL heavy water reactors run with natural uranium in most tubes, depleted uranium or thorium in few pressure tubes for flux flattening circus. No enriched uranium in any tubes.)
Total includes scattering which isn't a loss mechanism. Best bet is to do a spectrum weighted 1-group macroscopic absorption XS (including all neutron loss mechanisms) in various reactors for a meaningful comparison. Most nuclear textbooks have these in the appendix. All I'm telling you is that boron is used for control in fast reactors. I think you asked how it was done a while back.
I kept reading about all these advantages of "slightly enriched uranium" in candus so I'm surprised they aren't using it anywhere! [1]
Autonomous control can theoretically be done with many kinds of reactors and fuels. Sodium reactors have vast pools of liquid metal with extraordinary thermal conductivity and heat capacity. Lead-cooled reactors are similar. So are salt-cooled FHRs, and pebble-bed gas reactors like x-energy. There are a few decades of regulatory catchup before that can happen. Arpa-e has a current project into operator assistance which is aiming to move to more autonomous control.
Don't forget about maintenance costs. Equipment and chemistry will cause maintenance of any reactor to be higher than an average diesel. Fluid fuel will have 50% of the periodic table in thermal gradients, plating out in heat exchangers and other cold surfaces. Sodium reactors have sodium fires. Lead reactors have corrosion. Gas reactors have power cycle leaks. It's a worthy goal but we have lots of work before we get there. We don't have enough experience with fluid fuel yet to judge how maintenance of a commercial unit will pan out. It might be great, or it might prove difficult. Very worthy goal though!
RE "This kind of reactor needs enrichment once (to start up) and then can just be fed natural, DU, or SNF and it will run on a stream of it happily "forever" (until the vessel life is reached, at which point you transfer the core to a new machine and keep on shuffling)"
As fission product concentration increases, even fast reactors have to remove fuel and reprocess it. What works for boron also works for fission products. I don't think any reactor (fast or thermal) can close fuel cycle without the help of reprocessing. May be fast reactors need less frequent reprocessing, that's it.
(LWR-SNF cannot go fast critical. May be fission products is not removed, but reprocessing is needed to concentrate Pu in LWR-SNF.)
Even if LWR-SNF has to be used as-is, it has to be toasted in blanket region of fast reactor before it is used as fuel. Some designs like TWR claim no reprocessing needed between toast step and burn step. I agree with that. But, for complete fission of SNF, fission-product removal is needed at some stage when fission product concentration exceeds a particular level in the fuel rod. No exception for travelling wave burnup.
For a critical reactor core some minimum length of fuel rod is always required to be in use. If fission product starts accumulating from bottom of the fuel-rod towards top, somewhere at the midway core can't go critical because a minimum length of rod is needed for criticality.
4% of 1GW-LWR fuel runs for 4.5 years. 100% of this fuel has potential for 112.5 years. A solid-fuel rod can't maintain integrity for 100 years. The crystal structure damage can't be reversed by any amount of maintenance. This is the same phenomenon how solar panels get degraded in sunlight.
A fuel rod has crystal structure damage by radiation as well as by fission products.
Fluid-fuel can be used indefinite time and it is easy to reprocess liquid fuel by any method. There is no crystal structure to get damaged. ORNL was working on physical separation of fission products like vacuum distillation when the project was cancelled. Chemical separation and isotopic separation may raise proliferation concerns and they may not be environmentally friendly.
There is a chemical separation of fuel from fission products called fluoride volatility, which is used in industrial scale for enrichment of fuel. But, it works only for fluoride salts. ORNL has demonstrated this process when they switched from U235 to U233. They irreversibly damaged the drain tanks and hastelloy plumbing by doing this. MSRE would have lasted longer if they had not done this fluorination experiment.
RE "Fluid fuel will have 50% of the periodic table in thermal gradients, plating out in heat exchangers and other cold surfaces."
Just throw away the hot leg plumbing and heat exchangers every 2.8 years initially (MSRE salt loop was circulated for 2.8 years) and increase this incrementally.
Fact: In solid fuel reactors fuel itself acts as a primary heat exchanger.
In 4.5 years LWRs throw away a heat exchanger worth of Zr tubes/cladding. Earlier LWRs threw away solid-fuel every 3 years. Why point fingers at MSR folks when solid-fuel reactors throw away stuff?
Note the difference between Inconel 625 (similar to hastelloy-N) and Zr.
https://www.tricormetals.com/cost-comparison.html
> But, for complete fission of SNF, fission-product removal is needed at some stage when fission product concentration exceeds a particular level in the fuel rod. No exception for travelling wave burnup.
Of course. This kind of thing is very well established things like [1]. TWR is not interested in burning SNF, that would require reprocessing to convert it from oxide in the first place. Reprocessing is expensive and has historical proliferation concerns. TWR's entire purpose is to approach closed fuel cycle advantages (Gen-IV safety, sustainability, cost) without needing any reprocessing whatsoever. It's a natural step after the US CRBRP mega-boondoggle. Don't reprocess SNF, bury it in geologic repositories and/or boreholes. Burn U-238 at global scale. That's the idea there. It's called a Modified Open Cycle, or Deep-burn once-through fuel cycle.
> Fluid-fuel can be used indefinite time and it is easy to reprocess liquid fuel by any method. There is no crystal structure to get damaged.
Fluid fuel trades solid fuel performance challenges for chemical/corrosion challenges and radionuclide containment challenges. At MSRE, ORNL has yet to account for about 50% of the radioiodine produced. No one knows where it went. that's a huge but not impossible challenge. I agree with the "Easy to reprocess liquid fuel" advantage, but recognize that it is also a proliferation disadvantage.
> Just throw away the hot leg plumbing and heat exchangers every 2.8 years initially (MSRE salt loop was circulated for 2.8 years) and increase this incrementally.
These are rad waste remote operations, which has step influence on cost. What's the operational cost of this at FOAK? What's the operational cost at NOAK? It may be very cheap (Thorcon is well on their way to this) or it may be prohibitively expensive. The only way to really find out is to build and operate such a reactor commercially. To this end we will all benefit.
> Why point fingers at MSR folks when solid-fuel reactors throw away stuff?
Who's pointing fingers? I mentioned downsides to all forms of reactors in the parent comment. MSR is wonderful and exciting and we as a community need to build many more of them and shake them down.
Both metallic fuel and salt fuel are not suitable for geological repository. Salt fuel may be stored in salt mines, but unproven. More R&D is needed for geological disposal; but, the point in having metallic fuel or salt fuel is the ease of separating fission products and reusing fuel, not geological storage.
Iodine:
Not 50%. It is a probability range. "Thus, of the order of one-fourth to one-third of the iodine has not been adequately accounted for."
RE: "but recognize that it is also a proliferation disadvantage"
MSR without reprocessing:
MSR can be as hardened as anyone wants. Because of liquid fuel-form, MSR can be sealed tamper-proof. Fuel goes in and nothing comes out during operation.
Safeguard scenario in case of maintenance:
By design the fuel will move to drain tank when shutdown for maintenance. IAEA can have the keys for the drain tank. Fuel can't be pumped back without the presence of IAEA inspector. If someone tampers this, license will be cancelled and the country will have no electricity.
MSBR with reprocessing:
How easy separation of fission products from denatured liquid-fuel can be a proliferation disadvantage? It is actually a advantage because fuel always stays inside a tamper-proof system. ORNL attached a "add-on" apparatus for fluoride volatility, and fuel always stayed inside MSRE building. This is actually an advantage.
As material science improves, MSRs last longer and longer without need for maintenance. Safeguards will also become less frequent and less expensive.
I strongly agree that MSR needs a chance. MSR graduated with excellent results with MSRE. MSRE has answered most of the questions and raised very few new questions. MSR should be given a suitable employment like process heat or medical isotope production as soon as possible. We can scale it up for electricity later on.
So, we need to dispose the vessel and primary heat exchangers similar to Zr-clad million dollar fuel assemblies.(PWR/BWR reload costs $60-100 million per 12-18 months.) Material wise Zr clad is way more expensive than stainless 3XX & graphite combined. Even if Ni-alloy is used in disposable MSRs Ni-alloy+graphite will be slightly cheaper. (Base metal costs: Zr metal: ~$22,700/ton Ni metal: ~$13,100/ton)