Posted on 09/11/2015 9:48:35 AM PDT by thackney
The big worry about nuclear reactors is that the solid fuel rods are going to melt down.
If the core of the reactor loses its cooling water – as it did both at Three Mile Island and Fukushima – then the fuel rods overheat. Even though the nuclear reaction may stop, the decay heat is enough to melt the zirconium fuel rods so that the uranium pellets inside get exposed. If there is some water remaining, the heat may be enough to split off hydrogen, which can cause a hydrogen explosion, as occurred at Fukushima and was feared at Three Mile Island.
In the old days it was argued that the overheated core would melt right through the steel reactor vessel and the concrete containment structure and be on its way to China – “the China syndrome.” Then it would probably hit groundwater and cause a steam explosion that would make “an area the size of Pennsylvania uninhabitable” and so on and so on. All that proved to be fanciful although a good plot for a Hollywood thriller.
But what if the fuel is already liquid so it can’t melt down? Instead it can just be harmlessly drained off into a different container where the fuel will be diluted enough to end the reaction.
This is the principle of the “molten salt reactor,” a design first conceived by the great Dr. Alvin Weinberg in the 1950s and experimented with for twenty years before being relegated to the bookshelves when the nation decided not to do anything more with nuclear energy.
The principal of an MSR is that the nuclear fuel is dissolved in a bath of molten salts. The proximity of the fuel molecules (either uranium or thorium) is enough to heat the salt mixture to around 600 degrees Centigrade – still far below the boiling point of the salts around 1430 degrees C. The heat is then transferred to a turbine to produce electricity.
But what if the fuel solution starts to overheat? Then something very good happens. The salt mixture starts to expand, which moves the fuel molecules further apart from each other. This slows the nuclear reaction and brings the temperature down again. The reactor is thus self-regulating and can’t overheat.
Then there’s something even better. At the bottom of the reactor is a “freeze plug,” a stopper that holds the fuel mixture in the reactor vessel. If for some reason things start to get really hot, the plug is made of a material that will melt at 700 degrees. The molten fuel mix then drains into a large bathtub beneath the reactor, where it spreads out and cools until the nuclear reaction stops. Eventually the molten salt will solidify into a solid block that is relatively easy to handle. The reactor is “a walkaway safe,” meaning that if it spins out of control the operators can just walk away and the reactor will shut itself down. The mistakes and misreadings of gauges that caused both Three Mile Island and the Chernobyl accident can’t occur.
In addition, an MSR operates at normal atmospheric pressure. Light water uranium reactors must be brought to high pressure (the “pressurized water reactor”) because the cooling and moderating water will evaporate at high temperatures. Being under pressure, however, makes conventional reactors vulnerable to leaks and explosions that can scatter radioactive water into the atmosphere. With MSRs, there is no such danger.
Believe it or not, the MSR project actually began as an effort to power a large Air Force bomber with an on-board nuclear engine. The NB-36 bomber actually made a number of flights in the 1950s with a reactor on board before the idea was eventually dismissed as impractical. But Oak Ridge quickly switched to experimenting with the molten salt reactor as an alternative to solid fuel reactors in power plants. The result was a 7.4 megawatt thorium-powered reactor that went critical in 1965 and ran for four years. This was followed by a larger MSR that ran for 1.5 years in the 1970s. At this point, however, research was closed down in favor of the fast breeder reactor, which in turn was close down in the 1990s.
So things sat on the shelf at Oak Ridge for 40 years until Kirk Sorensen, a nuclear engineer at NASA and an enthusiast of thorium, began the heroic task of posting hundreds of Oak Ridge research papers his website, www.energyfromthorium.com. (See “Kirk Sorensen: Thorium’s One-Man Band, RCE, 8/21/15) Interest started to grow so that there are now six small companies exploring the possibility of licensing a molten salt reactor.
• Transatomic Power. Founded in 2011 by Leslie Dewan and Mark Massie, two 2010 MIT graduates, who have raised $2.5 million from the Founders Fund to begin experiments on a prototype MSR.
• ThorCon Power. Also founded in 2011 by Jack Davanney, another MIT graduate and veteran of the ship-building industry, ThorCon is trying to build a 250-watt modular molten-salt reactor that can be barged to the site.
• Flibe Energy. Sorensen’s company, Flibe is named after the fluoride, lithium, beryllium salt mixture that will dissolve the fuel. Flibe is trying to raise money to do the engineering work on a liquid fluoride thorium reactor.
• Terrestrial Energy. Named after this author’s 2008 book on nuclear energy, the Canadian company has signed a contract with Oak Ridge National Laboratory to build a demonstration molten salt reactor in the next five years.
• Moltex Energy. A British company that is also working on a molten salt reactor. John Durham, one of the co-founders, is also co-founder of the Weinberg Foundation, which is trying to promote molten salt energy.
• Seaborg Technologies. Named after Glenn Seaborg, a pioneer in nuclear technology, this Danish firm of young physicists and chemists is attempting to make the waste-consuming molten salt reactor a reality.
Meanwhile, the Chinese are moving ahead rapidly with molten salt as one of the nuclear technologies they have targeted for development. The Shanghai Institute of Applied Physics is planning to build a prototype within the next few years. The Shanghai program is collaborating with – wouldn’t you know it – the Oak Ridge National Laboratory, where the molten salt reactor was born sixty years ago.
Zero tolerance for failure? Failure is a safe shutdown without any outside controls or inputs.
The equipment has been proven, just not in a good way. Looks good on paper though.
Two different reactors ran for years.
Nukes were to prove to have the big problem that the radioactivity corroded the equipment making it easier to behave less than perfectly.
If someone has come up with a better solution to the problem of failing, which not a whether question but a when question, it deserves respect. If it is expected to melt down upon failure, and it is designed to catch the meltdown if it does, then we have a promising design. Don’t try to evade the failure mode; instead harness it.
That’s the intriguing factor here. When it fails, it is SUPPOSED to melt down.
And this isn’t sodium, this is salt. It won’t react with anything to produce an uncontainable catastrophe.
No, there was significantly more damage and problems than just the fuel tanks.
https://www.nirs.org/fukushima/naiic_report.pdf
page 12
The tsunami caused by the earthquake flooded and totally destroyed the emergency diesel generators, the seawater cooling pumps, the electric wiring system and the DC power supply for Units 1, 2 and 4, resulting in loss of all power—except for an external supply to Unit 6 from an air-cooled emergency diesel generator. In short, Units 1, 2 and 4 lost all power; Unit 3 lost all AC power, and later lost DC before dawn of March 13, 2012. Unit 5 lost all AC power.
The tsunami did not damage only the power supply. The tsunami also destroyed or washed away vehicles, heavy machinery, oil tanks, and gravel. It destroyed buildings, equipment installations and other machinery. Seawater from the tsunami inundated the entire building area and even reached the extremely high pressure operating sections of Units 3 and 4, and a supplemental operation common facility (Common Pool Building).
Can you link that claim?
The biggest problem with modern nukes as we know them, is that radioactivity inherently renders the equipment closer to an unsafe failure.
That could be excused as unexpected 70 years ago. No such thing today.
Well yes. Expect there will be failures, and design it so the results are harmless.
Molten Salt Reactors
http://www.world-nuclear.org/info/Current-and-Future-Generation/Molten-Salt-Reactors/
Updated 22 August 2015
The total levelized cost of electricity from the largest is projected to be competitive with natural gas.
A molten salt reactor would need to be durably waterproofed to prevent escape of fuel under a similar scenario, but otherwise would have just melted its fuel into its catch tub.
The uncontrolled failure mode of this is a "non-event" shutdown. Expensive yes, dangerous no.
Even the expense could be mitigated with good engineering. You set up to be able to carry these drainoff bricks safely to a processing plant, and to be able to service the equipment from which drainoff occurred. Fix, new fill, and you’re cooking with atoms again.
Yeah, I'm sure magic happens when the appropriate Engineers are allowed to think. I hope they got some of those involved.
Actually Fermi was liquid sodium metal.
Few elements make great moderators (slowing neutrons), are liquid or gases at fairly low temp and do NOT absorb neutrons.
Typical Moderators
Hydrogen
Helium
Carbon
Sodium Metal
Heavy Water (presence of Hydrogen)
Distilled Water (presence of Hydrogen)
Sodium is difficult to work with as refueling is done blind.
No way to see where the fuel bundles are.
You need a minimal amount of heat to get pumps flowing.
Also goes BadaBOOM in presence of any water.
Might even be possible to re-melt the brick at the site, depending on purity requirements and purification capabilities.
Take a look at the ThorCon site. The last paragraph on the page reads:
Cheaper than CoalModern nuclear plants are already competitive with coal, except for the cost of regulation and lawsuits. They produce no mercury or particulates, to boot...ThorCon requires less resources than a coal plant. Assuming efficient, evidence based regulation, ThorCon can produce reliable, carbon free, electricity at between 3 and 5 cents per kWh depending on scale.
Thanks for the link. I hope their claims lead to reality.
Not putting the spent fuel pond on top of the reactor would help too.
MIT Technology Review
Safer Nuclear Power, at Half the Price
Transatomic is developing a new kind of molten-salt reactor designed to overcome the major barriers to nuclear power.
http://www.technologyreview.com/news/512321/safer-nuclear-power-at-half-the-price/
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