"It is more civilized to make fun of life than to bewail it." - Lucius Annaeus Seneca
This post was inspired by Johnnywoods, a commenter on the previous blog post. Thanks for your question sir!
Here is the question I was asked, and had to do a fair amount of research to answer:
"What did you think of the recent story of the new Chinese Nuclear reactor. Is it a march on the west?"
Before I get to the point of responding to that question, it might be a good to mention that in previous posts, I've briefly discussed several different types of nuclear reactors, but not this particular design.
- Pressurized Water Reactors part 1, and Part 2.
- Boiling Water Reactors
- Fast Neutron Reactors , liquid metal cooled, which also introduced the concept of breeder reactors
- Power reactors in space, and testing them for space flight
- Nuclear rocket engines
- The unique and interesting CANDU reactors
I've also briefly discussed some funky stuff that goes on in the core of reactors that most people probably aren't aware of:
- Delayed Neutrons and reactor control
- Criticality Accidents
- Xenon and Decay Heat
- Enrichment and burnable poisons
If I get lucky, this post will hopefully tie up several loose ends from those previous posts.
There is a reason that I'd never discussed molten salt reactors (MSRs) before now, and the reason is that only a handful were built - all experimental, and all very small scale. That said, the technology has always held great promise for improvement and for larger scale implementation. Unfortunately rather than investigate this promising technology, it was abandoned in favor of liquid-metal cooled fast neutron reactors, which have many of the same advantages as MSRs. Unfortunately liquid-metal cooled reactors have a couple of huge disadvantages that MSRs do not - their need for high enrichment, and their tendency to suffer melt downs.
One of the fascinating things about MSRs is that the primary coolant contains the fuel. With this design, the coolant/fuel blend passes through a vessel containing a moderator and favorable geometry for fission to occur. As the coolant/fuel blend passes through, fission happens in the coolant, but only during the moment it is flowing through the reactor vessel. After the fuel coolant exits the reactor vessel, it has been heated, but is no longer undergoing fission.
There is another theoretical design where the coolant is still molten salt, but where the salt contains no fissionable fuel. This design is called the SSR, or Stable Salt Reactor. Its design is similar to a PWR or BWR - with the fuel segregated from the coolant by cladding. None have been built, so I'm not discussing this design further.
The first MSRs were test platforms for the Air Force's nuclear-engine bomber program. The goal was to design a lightweight, very high temperature (and thus high efficiency) nuclear engine that could provide a good enough power to weight ratio to make an effective jet engine.
The world's first Molten Salt Reactor was the Aircraft Reactor Experiment (ARE). It had a 2.5 MW (thermal) output, and operated for just four days, from November 8-12, 1954.
Below is an image of the ARE reactor core. The hexes are the Beryllium Oxide moderator, and the coolant loops can be seen passing through the moderator assembly. The holes are likely for control rods.
By US Government - https://www.osti.gov/biblio/4223435, Public Domain, https://commons.wikimedia.org/w/index.php?curid=87142585
Below is the overall cycle diagram. The right side is the fuel/coolant loop containing the molten salt and liquid nuclear fuel. The left side is a separate coolant loop just to keep the solid Beryllium Oxide moderator/reflector from overheating.
By Oak Ridge National Laboratory - ornl.gov, Public Domain, https://commons.wikimedia.org/w/index.php?curid=16724926
Almost immediately on the ramp up to full power, radiation alarms sounded. Apparently the top right pump seal fittings were leaky, and radioactive fission product gases leaked out into the test building.
Interesting how they dealt with such issues back in that era:
At 4:19 P.M. on November 8, during the ascent to high power, the reactor was shut down due to high airborne radioactivity measurements in the basement. It appeared that the gas fittings to the main fuel pump were leaking fission-product gases and vapors into the pits, and the pits were leaking into the basement through defective seals in some electrical junction panels. A 2 in (5 cm) pipeline was run from the pits 1,000 ft (300 m) south into an uninhabited valley. Portable compressors and a jet were used to bring the pits to sub-atmospheric pressure for the rest of the experiment. The safety radiation detectors shut down the reactor a few times during restart and were withdrawn to be further away from the reactor.
There were three other experimental Molten Salt Reactors built in anticipation of eventual lightweight nuclear-powered aircraft engines. These were the Heat Transfer Reactor Experiments - HTRE-1, HTRE-2, and HTRE-3
Below are the Molten Salt Reactor test platforms HTRE-2 and HTRE-3. These can be toured at the INL in Idaho Falls, Idaho.
Below is a mock-up of HTRE-3. You can see how jet engine compressor discharge is ducted to the top of the reactor, where it is heated as it passes to the bottom, then sent to the turbine end of the jet engines after heating. This would have generated thrust, but it would have been pretty radioactive, since it is an open cycle design - the exhaust gas has come directly out of a reactor.
By Federal Government of the United States. - Proving the Principle by Stacy, Susan M., U.S. Department of Energy, Idaho Operations Office. ISBN 0-16-059185-6, chapter 13. Available from inl.gov, Public Domain, https://commons.wikimedia.org/w/index.php?curid=16820199
The final Molten Salt Reactor that the US built was the MSRE - the Molten Salt Reactor Experiment. This reactor ran from 1965 to 1969, and at a thermal power of 7.4 Megawatts.
Below: Overhead view of the MSRE. Reactor core near the top, circulating pump to the left, and salt to salt heat exchanger is the silver cylinder near the bottom.
By Oak Ridge National Laboratory - ORNL Photo 67051-64 (Note: Identified on [1] Pg. 33 Fig. 25; text on Pg. 29: "a photograph of the core, fuel pump, and heat exchanger"), Public Domain, https://commons.wikimedia.org/w/index.php?curid=15647363
The MSRE was a very successful reactor test program, proving the viability of the process, by running at high power levels for months on end.
We will now look at some of the interesting advantages and disadvantages of the Molten Salt Reactor.
First of all, we need to discuss the salt. A salt consists of a cation (like Na+, Li+ or Mg++) combined with an anion (like Cl-, F-, or Br-). A molten salt will tend to be highly corrosive, and so superalloys must be used for any material that will come into contact with it. Superalloys tend to be expensive, not due to their unusual metal content, but due to the difficulty of machining them.
The MSRE used a fuel/coolant composed of a salt blend: Lithium Fluoride (65%), Beryllium Fluoride (29%), Zirconium Fluoride (5%), and Uranium Fluoride - the fuel (1%). Fluoride has the advantage (and disadvantage) of being highly chemically reactive. The advantage is that the non noble-gas fission products produced in the fuel/coolant will be chemically bound by the Fluoride salt. The disadvantage of course is that molten salt will chemically attack anything it comes in contact with.
The experience of the MSRE was that the Fluoride-based salts did not attack the Hastelloy-N alloy that the primary coolant system was made of, nor did it attack the pyrolytic graphite moderator in the reactor core. Also the salts did not experience radiolytic breakdown under high neutron flux. An advantage of the molten salt reactor (shared with the liquid-metal cooled reactor) is that the coolant doesn't readily boil, so it is not necessary to build an expensive high-pressure primary coolant system. Thus a reactor of similar output to a Pressurized Water Reactor should be less expensive to build.
There is a clever feature allowed by the design of the Molten Salt Reactor - the build-up of reactor and neutron poisons can be slowed, allowing greater fuel burn-up than in other reactor designs. The fuel can be processed online to remove reactor poisons. In fact the MSRE was able to reduce the primary neutron poison, Xe-135 by a factor of six, simply by de-gassing Krypton and Xenon from the coolant/fuel mixture. Other reactor and neutron poisons could have been removed online by using slip-stream processing techniques. Refueling online was also easy to accomplish, by simply melting and adding additional salt/fuel blend. Several blends were added. U-235 was used initially, then U-233, then finally, Pu-239
The MSRE also had good neutron economy, meaning that it had excess neutrons available to breed more fuel, although the small experiment was never set up to breed fuel. One other advantage of the MSRE was the neutron spectrum was more energetic than a water-cooled reactor. This means that the neutrons have much more energy, so they have enough energy to fission transuranic elements that accumulate in lower neutron-energy reactors. Double-win, you get fission energy out of what would be a reactor poison, and you also don't generate long-lived transuranic waste.
Another nice thing about Molten Salt Reactors is that breeding allows use of the Thorium fuel cycle. Thorium is far more abundant than Uranium, and it has better nuclear properties. If bred into fissile U-233, it will be much less likely to become transuranic waste, or transmute to Plutonium, which of course is nuclear weapons material.
Lastly, Molten Salt Reactors are resistant to "meltdown" - fuel element failure due to overtemperature. There are no fuel elements! There is no water in the reactor core to break down into hydrogen and thus no danger of a repeat of the Fukushima exploding nuclear power plant.
Back finally now to the question posed by Johnnywoods: "What did you think of the recent story of the new Chinese Nuclear reactor. Is it a march on the west?"
I think it's great that someone, somewhere, is continuing the research that we should have investigated long ago. There is a lot of money invested in what we have always done - the Pressurized Water Reactors and the Boiling Water Reactors. Just because they are the most widespread technology doesn't mean that they are the best technology, and someone (even if it's China) should be researching this!
I wouldn't say they have stolen a march on the US. Just like Mars rovers, we made it there first, took the risks first, and proved that it could be done first. As of right now, they are duplicating our earliest efforts, but undoubtedly with a better understanding of what to expect.
There is a great in-depth, but not too technical article explaining the cool features of MSRs HERE.
The Chinese Molten Salt Reactor appears to be similar to the very first Molten Salt Reactor, the ARE, in power output - about 2 Megawatts. If things go well with this experiment, they hope to scale up to a 100 Megawatt prototype in the future. Link here. I honestly wish them the best of luck with this. It's a very interesting and neglected technology that has a lot of promise, in the areas of nuclear proliferation, safety, and efficiency.
It would be great if the US were to dip its toes into these waters again as well. It would probably cost less than the 2008 US bank bailouts, and would provide a better return on the investment!
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