LFTR fuel is molten; “melt down” is standard operation. Fuel circulates through the reactor, and gets completely fissioned. No expensive fuel rod fabrication. No storing spent fuel rods in cooling ponds. No catastrophic fuel melting out of fuel rods. No risk of spewing radioactive material into the atmosphere if fuel rods overheat.
Fission byproducts are continuously and easily removed from molten fuel, for 99+% fuel consumption. (Byproducts trapped in fuel rods in a solid-fuel reactor, stop the nuclear reaction with <2% of the fuel used; the fuel rod then has to be replaced.)
In any emergency, a frozen plug melts and the fuel quickly drains out of the core into tanks where nuclear fission is physically impossible. Radiation is contained by materials that remain solid at temperatures much higher than inside the reactor, with passive air cooling. This safety feature is only possible with molten fuel. (In solid-fueled reactors, you have to override everything that normally happens in the core and bring in coolant.)
Even if an accident blocks the fuel from draining (e.g. earthquake or sabotage crimps the pipe), the reactor vessel materials can handle the hottest the fuel can possibly get, unlike with LWR.
In LWR, if there is a “loss of coolant accident”, the coolant is gone, the fuel rods melt, the careful spacing of the fuel is gone, the fuel is now more dense than the reactor was designed for. The fuel temperature gets hotter than normal, but the temperature in the fuel pellets is normally much hotter than the reactor vessel can handle.
In MSR, the fuel is molten in the salt, and chemically bound to the salt, the same temperature as the salt, and the salt can’t boil away — the fuel doesn’t get more dense except by thermal expansion/contraction (if it gets hotter it gets less dense), strongly regulating fission rate. No catastrophe possible.
LWR gets refueled about every 18 months (about 1/3 of fuel rods replaced). MSR would likely have fuel added hourly, daily, or weekly (except for designs with no refueling or chemical processing, seal it and replace in 30 years).
Since there is no need for a lot of “excess” fuel above what is needed to maintain fission, there is less fuel in the reactor when any accident occurs. Fission products that are gasses (e.g. krypton) get continuously removed and stored. There is far less radioactive material that could be released in an accident than in LWR, and there is no high pressure to push that material away from the reactor.
LFTRs Are Cooled by Stable Salts, Not Water
LFTRs are cooled by salts that remain liquid, even up to ~1400° C, so they operate at atmospheric pressure — no massive high-pressure containment structures are needed.
Almost all safety problems with current nuclear reactors can be traced to high-pressure water coolant. (Water-cooled reactors have up to 150 atmospheres pressure, to keep water a liquid under high temperatures, so need thick steel walls and massive reinforced-concrete buildings to contain water explosively converting to radioactive steam in an accident.)
LFTRs use No water, so can be built where water is scarce, using passive air cooling.
The salt used is very chemically stable, doesn’t react with materials in the reactor, or with air or water in an accident. The salt is essentially impervious to radiation damage, and doesn’t absorb neutrons (which would affect the rate of fission).
The fluoride salt chemically bonds with most of the more dangerous fission products, so in any accident they remain trapped in the salt. For example, radioactive cesium and iodine that were released in Fukushima-Daiichi would not be released in a LFTR accident.
There are a few sodium-cooled reactors. Sodium reacts violently to water. A broken pipe weld in a heat transfer unit heavily damaged the Monju Nuclear Power Plant. Molten Salt Reactors would not have this risk.
George,
First off, I am an interested spectator and nothing more. From what I see, this LFTR concept looks very attractive, but as I don’t have the technical depth of knowledge to make a meaningful contribution to the discussion, I will keep my amateur opinions to my self.
Having said that though, I wish to make a comment regarding semantics. In your title, you use the expression, “…, cooled by molten salt.” This seems to me to be a misstatement of the true nature of this technology. The liquid salt is the process medium and it does carry the heat bi-product away from the process, but “cooling” is not its primary purpose. As you know and have stated, this process by its very nature does not require cooling in the traditional sense. Cooling it would, in fact, defeat its purpose.
Why am I nit-picking over this? Because the biggest problem I see in bringing this technology to full implementation is the misconceptions that people have regarding nuclear in general and LFTR in particular. What needs to be focused on is: criticality. In the case of LWR, the fuel itself is always at a critical state and that makes it inherently dangerous, but in the case of LFTR, the fuel must be forced by a continuous injection of neutrons to become critical and fissionable. The liquid salt on its own can never go critical and cause an uncontrolled reaction.
Am I right? And if I am then wouldn’t this be THE particular point that needs to be hammered into the heads of the detractors?
“… go critical and cause an uncontrolled reaction” — That’s not the correct use of “critical state” or “criticality”. https://en.wikipedia.org/wiki/Criticality_%28status%29 says “Criticality is the state of a nuclear chain reacting medium when the chain reaction is just self-sustaining (or critical), that is, when the reactivity is zero. The term may also be applied to states in which the reactivity is greater than zero.” In both LWR and MSR, during normal operation the reactivity is greater than zero.
In LWR, and in most designs of MSR, and most solid fuel designs, the “continuous injection of neutrons” comes from the prior fissions. (MSR is a category of reactors, not a specific design.) Each uranium fission, each plutonium fission, has a known probability of releasing 1, 2, or 3 neutrons. For each specific moderator and fuel density, the probabilities of fission and of neutron capture are known. Nuclear physicists know precisely the reactivity at all conditions in the reactor, it is determined completely by physics.
In many types of reactor, solid or molten fueled, the fuel density could be so low that neutrons from the last fission are unlikely to cause another fission (hit the reactor vessel instead of uranium), requiring additional neutrons from “outside”, some neutron generator. In solid fuel reactors, some people suggest using an external neutron beam to maintain criticality would improve safety; instead it would improve the perception of safety, throw open a switch and the neutron beam stops and the reactor stops. For actual safety, the control rods are completely adequate, and are needed anyways in solid fuel reactors to adjust the power generated as demand changes and as the fuel gets used.
The only type of reactor ever used for power production, that I’ve ever heard of, that had the possibility of causing an uncontrolled reaction, was the RBMK. The designers knew it had a “positive void coefficient of reactivity” (since it was water cooled, that “void” would be a steam bubble). “Positive void coefficient” reactors were illegal everywhere in the world before any were built, but the USSR didn’t care. The best known RBMK reactor was at Chernobyl. With a safety culture of “do what I say or I shoot you”, nobody who knew reactor design was surprised that the uncontrollable fission happened, it was an accident waiting to happen.
LFTR, and other MSR, wouldn’t be designed so the fuel is below critical. The thermal expansion/contraction of the low-viscosity fuel strongly regulates reactivity. (Lava and honey and “slow as molasses” are high-viscosity; liquid water and gasoline are low-viscosity, they pour or flow easily.) Even if the reactor is designed for a high breeding ratio, even if someone put in much more fuel than needed, they can not get to uncontrollable reactivity. Molten Salt Reactors have a very stable fission rate, compared to all other possible reactors, and that is by physics not by engineered safety systems.
Cooling in nuclear reactors is always both “move heat from where fission occurs” and “move heat to where we can use it”. Cooling is always “move energy”, from something with high energy to something with lower energy.
The primary purpose of a nuclear reactor is to produce heat, to use for something (most often turn a turbine to make electricity). That is “cooling”.
The safety aspect of cooling in many reactors quite literally prevents damage to the reactor.
In LWR, the water temperature is as hot as the designers can allow it, constrained by the pressure limits of the steel. (Higher temperatures can do more work, so they want coolant temperature high.) If cooling fails, two basic failures can follow. 1) The water pressure could split pipes, so must be kept within safe limits; 2) The temperature inside the fuel pellets is normally about 2000°C above the 350°C water; that heat would damage and then melt the fuel rods. At Fukushima, the cooling system survived the earthquake and tsunami undamaged; what was lost was power to the cooling system, they even ran it for as long as they could on batteries from cars in the parking lot. (If anyone in the world had flown in a diesel generator and fuel, the reactors at Fukushima would all be operating today. Maybe the electric generating turbines would have needed repair; the reactors were undamaged.) Without cooling, the fuel rod cladding oxidized releasing hydrogen which led to a hydrogen combustion explosion; and the fuel pellets melted.
For MSR, the fuel is dissolved in the coolant. The fuel temperature is identical to the coolant temperature. There are no fuel rods. The reactor materials can withstand higher temperatures than the fuel can possibly get to. In MSR, the cooling system is not needed for safety, the reactor is inherently safe from coolant accidents.
I’m doing a class project over Gen. IV reactors and LFTRs in specific. I just have a few questions about that.
In order to make the salt molten, would you have to heat it up in some kind of storage tank and then shoot it with neutrons to make it go critical?
In an LWR they insert control rods to slow down the rate of reaction and therefore the rate of steam and electricity. I was wondering how a LFTR can control the rate of electricity it can produce?
How would you shut down the plant in order to perform maintenance?
My last question is why is gas used to power the turbines instead of water/steam? Is it because of the reaction that sodium has to water?
Thank You!
[George’s Reply] You heat the salt in the fuel salt drain tanks, and pump it into the reactor vessel; the shape of the reactor vessel has the neutron density high enough to sustain fission (the shape of the storage tanks prevents chain fission reactions). All you need is the proper fuel density in the fuel salt.
MSR has self-regulating temperature, see Liquid Fluoride Thorium Reactors have passive and inherent safety. Fuel rods are completely unnecessary in MSR, for safety or for regulating the fission rate. In MSR if you want to stop the chain fission reaction, you simply drain the molten fuel into storage tanks. If you want to make more electricity, you withdraw more heat and the fuel contracts which increases the fission rate. If you want less electricity, or whatever the reactor heat is used for, use less heat and the fuel salt expands, slowing fission. Note: Early MSR designs will probably need control rods to appease the regulators until MSRs prove themselves again to be a highly stable reactor design.
Maintenance in MSR would, of course, be done frequently, like for any industrial equipment, and yet most maintenance would not require shutting down the reactor. The fuel salt is transparent, cameras in the reactor could visually inspect it. The reactor operates at low pressure, replacing a section of pipe would be much simpler than in LWR. Replacing sections of the moderator could be done with the reactor operating, simply have a design where the moderator sections can be lifted out of the reactor. Replacing the chemical processing equipment can be done with the reactor operating, it improves fuel use and allows removing fission products but isn’t essential every moment.
Molten Salt Reactors do not have sodium (pure metallic sodium), that could react explosively with water. Sodium can be the coolant in other types of reactors, such as the Liquid Metal (cooled) Fast Breeder Reactor (LMFBR) or the Monju Nuclear Power Plant. Some types of MSR could use sodium chloride as the coolant, which is very chemically stable. Fluoride salts and chloride salts are highly stable; fluorine and chlorine and sodium are highly reactive until they combine with another highly reactive element. MSR use either fluoride salts or chloride salts as the coolant.
Why use gas turbines to convert the heat from the LFTR into electricity? Better to ask why the reactor site would use Brayton cycle turbines, rather than commonly used turbines. The design of the turbine should be efficient at converting heat to electricity, and https://en.wikipedia.org/wiki/Brayton_cycle has higher efficiency than conventional turbines. Also, water (steam) dissolves far more chemicals than inert gasses like helium, and heat transfer units between the reactor salt and the turbine gas have to be long-lasting with both the reactor coolant and the turbine gas. Third, using a gas other than steam in the turbine also makes the reactor site able to function without a large water supply, and without the environmental concerns of the reactor somehow contaminating the water supply like LWR can.
Would you be able to use the spent fuel from a LWR to power a LFTR? I know that you use U 235 or Pu to start up the reactor but could you use any of those to help run the reactor, like adding it in as it runs?
[George’s reply] A LFTR is a specific design of Molten Salt Reactor. LFTR is designed to convert thorium to U233 and fission that, with added equipment for the conversion. Other types of MSR could fission U235 and/or Pu239. If you put U235 or Pu in a LFTR, you would want to adjust the amount of Th you put in (keep the fissile concentration within the broad range the reactor was designed for).
Most of the spent fuel from LWR would be U238, which does not fission well in a thermal spectrum reactor (such as LFTR or LWR). Molten Salt Reactors can be fast spectrum, keeping all the stability and inherent safety of MSR. The moderator would be removed from the design, and since FLiBe salt itself moderates neutrons somewhat, a different salt would probably be used. Different salt would require different reactor vessel materials. Fast Spectrum Molten Salt Reactor Options discusses this.
One benefit of LFTR is you add very-low-radiation thorium to the reactor, so only thorium is shipped to it. With the irrational fear of shipping anything uranium, the most likely place for a LWR-waste burning MSR would be at existing LWR sites, inside the LWR steam containment building. MSR No Long Term Waste Storage goes over some of this.