Subtitle: Thorium MSR No Better Than Uranium Process
Preface
This article, number 28 in the series, discusses nuclear power via a thorium molten-salt reactor (MSR) process. (Note, this is also sometimes referred to as LFTR, for Liquid Fluoride Thorium Reactor) The thorium MSR is frequently trotted out by nuclear power advocates, whenever the numerous drawbacks to uranium fission reactors are mentioned. To this point in the TANP series, uranium fission, via PWR or BWR, has been the focus. Some critics of TANP have already stated that thorium solves all of those problems and
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Thorium Molten Salt Reator process
source: Idaho National Lab |
therefore should be vigorously pursued. Some of the critics have stated that Sowell obviously has never heard of thorium reactors. Quite the contrary, I am familiar with the process and have serious reservations about the numerous problems with thorium MSR.
It is interesting, though, that nuclear advocates must bring up the MSR process. If the uranium fission process was any good at all, there would be no need for research and development of any other type of process, such as MSR and fusion. Indeed, as already pointed out in TANP, uranium fission plants have barely captured 11 percent of world-wide electricity production after 50 years of heroic efforts. One would expect, if nuclear power were as great as the advocates claim, that nuclear plants would already supply 80 or 90 percent of all electric power in the world. Clearly, they do not because they are not at all great, they have enormous and insurmountable drawbacks in cost, safety, and toxic product legacy left for future generations. Once the thorium MSR process is discussed in this article, the next article will discuss yet a third hope for the nuclear advocates, in case fusion fizzles out and MSR melts away to nothingness. That next article will be on high-temperature gas reactors, the HTGR. As will be seen, HTGR also has serious drawbacks.
One final preliminary point: some of the nuclear advocates that push MSR lament the fact that, many years ago, thorium MSR lost in a competition with uranium PWR to provide propulsion for ships and submarines for the US Navy. They say, wrongly, that Admiral Rickover chose uranium PWR over thorium MSR so that the US could develop atomic bombs. What is much more likely the reason uranium PWR won is that the materials used for the MSR developed the severe cracking described below. No Admiral in charge of submarines could take a chance on the reactor splitting apart from the shock of depth charges.
The Idaho National Lab MSR Description (see drawing above)
"The Molten Salt Reactor (MSR) system produces fission power in a circulating molten salt fuel mixture with an epithermal-spectrum reactor and a full actinide recycle fuel cycle. In the MSR system, the fuel is a circulating liquid mixture of sodium, zirconium, and uranium fluorides. The molten salt fuel flows through graphite core channels, producing an epithermal spectrum. The heat generated in the molten salt is transferred to a secondary coolant system through an intermediate heat exchanger, and then through a tertiary heat exchanger to the power conversion system. The reference plant has a power level of 1,000 MWe. The system has a coolant outlet temperature of 700 degrees Celsius, possibly ranging up to 800 degrees Celsius, affording improved thermal efficiency. The closed fuel cycle can be tailored for the efficient burnup of plutonium and minor actinides." - See link
Thorium’s Listed Advantages
a) Fuel is plentiful because thorium is abundant
b) Fuel is cheap on a kWh produced basis
c) Molten salt reactor supposedly is safer, via a solid salt plug underneath the reactor that melts upon overheating if power is lost or some other upset occurs. This allows the reactor contents, hot molten fluoride salts with radioactive thorium, uranium, and plutonium, to flow by gravity into several separate collection chambers to self-cool.
d) Low pressure reactor using molten salt – supposedly safer than a high-pressure PWR design.
Oak Ridge MSR Test Project
a) The reactor was small, with thermal output only 7 MWth. The reactor process had no steam generator and no electricity was produced. It ran only a few months.
b) Metal that was used for contacting molten salt developed intergranular cracking; completely unsuitable for commercial reactor use. see link
c) ORNL then developed (in 1977) an improved and very expensive alloy Hastelloy N for nuclear applications with molten Fluoride salts. In tests, Hastelloy N with Niobium (Nb) had much better corrosion resistance to molten fluoride salts.
Future MSR designs and problems
a) The MSR design is much like a PWR design: each has a reactor, steam generator, and turbine/generator for the three primary sections. However, as shown in the Idaho National Lab drawing above (INL), there are four loops in this design. PWR has three circulating fluid loops: cooling water, boiler feedwater/steam, and the primary heating loop, Yet, the MRS has a fourth loop, for radioactive molten salt for MSR. Any MSR design that hopes to be economic will also be huge, likely in the 1000 MWe output size, to employ economy of scale. This requires scaleup of approximately 500-to-1 compared to the ORNL project. With a cycle efficiency of approximately 30 to 33 percent, the thermal output will be approximately 3500 MWth. Scaleup from ORNL size by 500 times is an enormous challenge. Note that scaleup with a factor of 7 to 1 is a stretch, yet such a factor (using 6) requires four steps (40, 250, 1500, and 3500) to use round numbers. Each larger plant requires years to design, construct, and test before moving to the next size, and that is if the larger design actually works the first time. It is also instructive (and very, very expensive) that the MSR design has a dual-compressor and heat removal fluid instead of the conventional steam condenser system. Costs and operating problems for this design are much, much greater than for a PWR.
b) The materials of construction for a very hot molten Fluoride salt mixture will likely be extremely expensive, if made of Hastelloy N to prevent the widespread cracking found at ORNL. It remains to be seen if even Hastelloy N will have a sufficient strength and thickness after 40 years of service.
c) Pumping the very hot, corrosive, molten salt mixture will require expensive alloy materials, and due to the salt’s density, high horsepower for pumping. Also, pumping a hot molten radioactive salt requires sophisticated pump seals to ensure safety and prevent leaks. As described above, the thorium MSR design will have four main circulating loops, while a PWR system has only three. However, the cost for MSR hot molten salt circulation pump will be more expensive than the PWR pressurized water circulation pump due to the high-cost alloy required, and the almost double horsepower motor to drive the pump.
d) If a molten salt pump is not used, circulation can be achieved by a thermal density difference loop. However, this also presents serious design and control problems.
e) The steam generator design presents a complex and likely insurmountable problem. Even if a successful design is somehow created, leaks of high-pressure water into the low-pressure molten salt are inevitable and will create all manner of hell. Havoc is too mild for the mess that will happen. Water that contacts the hot molten salt will explode into steam, possibly rupturing the piping or equipment and flinging radioactive molten salt in all directions. In addition, the steam generator’s material of construction also must resist the hot, corrosive molten salt. The steam generator will also likely be made of Hastelloy N, which adds to the already high cost of the plant. It is also notable that the INL MSR design has two heat exchangers for the steam generator loop, which decreases overall cycle thermal efficiency. It does not increase safety, as water will leak into the molten salt.
f) Controlling the plant output, adding more fuel, and removing unwanted reaction byproducts, all are obstacles.
g) With the low thermal efficiency, MSR plants will require approximately the same quantity of cooling water as uranium fission plants. That, as discussed previously in TANP, is a serious disadvantage in areas that are already short of water.
Conclusion
It can be seen then, that thorium MSR has few advantages, if any, over PWR. They each have three or four circulating loops and pumps, however MSR will have much more expensive materials for the reactor, steam generator, molten salt pumps, and associated piping and valves. There will be no cost savings, but likely a cost increase. That alone puts MSR out of the running for future power production.
The safety issue is also not resolved, as stated above: pressurized water leaking from the steam generator into the hot, radioactive molten salt will explosively turn to steam and cause incredible damage. The chances are great that the radioactive molten salt would be discharged out of the reactor system and create more than havoc. Finally, controlling the reaction and power output, finding materials that last safely for 3 or 4 decades, and consuming vast quantities of cooling water are all serious problems.
The greatest problem, though, is likely the scale-up by a factor of 500 to 1, from the tiny project at ORNL to a full-scale commercial plant with 3500 MWth output. Perhaps these technical problems can be overcome, but why would anyone bother to try, knowing in advance that the MSR plant will be uneconomic due to huge construction costs and operating costs, plus will explode and rain radioactive molten salt when (not if) the steam generator tubes leak. There are serious reasons the US has not pursued development of the thorium MSR process. Reports are, though, that China has started a development program for thorium MSR, using technical information and assistance from ORNL. One hopes that stout umbrellas can be issued to the Chinese population that will withstand the raining down of molten, radioactive fluoride salt when one of the reactors explodes.
Previous Articles
The Truth About Nuclear Power emphasizes the economic and safety aspects by showing that (one) modern nuclear power plants are uneconomic to operate compared to natural gas and wind energy, (two) they produce preposterous pricing if they are the sole power source for a grid, (three) they cost far too much to construct, (four) use far more water for cooling, 4 times as much, than better alternatives, (five) nuclear fuel makes them difficult to shut down and requires very costly safeguards, (six) they are built to huge scale of 1,000 to 1,600 MWe or greater to attempt to reduce costs via economy of scale, (seven) an all-nuclear grid will lose customers to self-generation, (eight) smaller and modular nuclear plants have no benefits due to reverse economy of scale, (nine) large-scale plants have very long construction schedules even without lawsuits that delay construction, (ten) nuclear plants do not reach 50 or 60 years life because they require costly upgrades after 20 to 30 years that do not always perform as designed, (eleven) France has 85 percent of its electricity produced via nuclear power but it is subsidized, is still almost twice as expensive as prices in the US, and is only viable due to exporting power at night rather than throttling back the plants during low demand, (twelve) nuclear plants cannot provide cheap power on small islands, (thirteen) US nuclear plants are heavily subsidized but still cannot compete, (fourteen), projects are cancelled due to unfavorable economics, reactor vendors are desperate for sales, nuclear advocates tout low operating costs and ignore capital costs, nuclear utilities never ask for a rate decrease when building a new nuclear plant, and high nuclear costs are buried in a large customer base, (fifteen) safety regulations are routinely relaxed to allow the plants to continue operating without spending the funds to bring them into compliance, (sixteen) many, many near-misses occur each year in nuclear power, approximately one every 3 weeks, (seventeen) safety issues with short term, and long-term, storage of spent fuel, (eighteen) safety hazards of spent fuel reprocessing, (nineteen) health effects on people and other living things, (twenty) nuclear disaster at Chernobyl, (twenty-one) nuclear meltdown at Three Mile Island, (twenty-two) nuclear meltdowns at Fukushima, (twenty-three) near-disaster at San Onofre, (twenty-four) the looming disaster at St. Lucie, (twenty-five) the inherently unsafe characteristics of nuclear power plants required government shielding from liability, or subsidy, for the costs of a nuclear accident via the Price-Anderson Act, and (twenty-six) the serious public impacts of large-scale population evacuation and relocation after a major incident, or "extraordinary nuclear occurrence" in the language used by the Price-Anderson Act. Additional articles will include (twenty-seven) the future of nuclear fusion, (twenty-eight) future of thorium reactors, (twenty-nine) future of high-temperature gas nuclear reactors, and (thirty), a concluding chapter with a world-wide economic analysis of nuclear reactors and why countries build them. Links to each article in TANP series are included at the end of this article.
Additional articles will be linked as they are published.
Part Twenty Eight - this article
Roger E. Sowell, Esq.
Marina del Rey, California
