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PostPosted: Jul 18, 2007 10:30 am 
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Why did thorium lose out over uranium when reactor design was first started?


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PostPosted: Jul 18, 2007 10:43 am 
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In simplest terms, when fissile isotopes for weapons were needed (1942-1949) thorium's derivative fissile isotope (U-233) lost to highly-enriched uranium (U-235) or weapons-grade plutonium (Pu-239) because U-233 was contaminated with U-232, whose decay-chain hard-gamma emissions made it unsuitable for weapons.

Later, when reactors were desired for power production (1952-present) thorium lost to uranium because Rickover's advancements on the light-water reactor made the LWR the low risk development of choice, and thorium has little advantage over uranium in a light-water reactor, as WASH-1097 explains in section 5.4.

To truly exploit the advantages of thorium, a fluid-fueled reactor is needed. Three different fluid-fueled reactor efforts were initiated in the US in the 1950s, and one (the liquid-fluoride reactor) was continued past 1959, but it was unable to overcome the head-start of the liquid-metal fast breeder, which promised to generate civilian power AND generate weapons-grade plutonium so desired by the Atomic Energy Commission.


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PostPosted: Jul 18, 2007 11:38 am 
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Kirk Sorensen wrote:
In simplest terms, when fissile isotopes for weapons were needed (1942-1949) thorium's derivative fissile isotope (U-233) lost to highly-enriched uranium (U-235) or weapons-grade plutonium (Pu-239) because U-233 was contaminated with U-232, whose decay-chain hard-gamma emissions made it unsuitable for weapons.

Later, when reactors were desired for power production (1952-present) thorium lost to uranium because Rickover's advancements on the light-water reactor made the LWR the low risk development of choice, and thorium has little advantage over uranium in a light-water reactor, as WASH-1097 explains in section 5.4.

To truly exploit the advantages of thorium, a fluid-fueled reactor is needed. Three different fluid-fueled reactor efforts were initiated in the US in the 1950s, and one (the liquid-fluoride reactor) was continued past 1959, but it was unable to overcome the head-start of the liquid-metal fast breeder, which promised to generate civilian power AND generate weapons-grade plutonium so desired by the Atomic Energy Commission.


Thought it was something like that, and the military underwriting the initial costs of the civil/military design meant there was no chance of anything else getting a look in afterwards?


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PostPosted: Jul 18, 2007 12:14 pm 
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Not no chance, but certainly much less while the light-water reactor and liquid-metal fast breeder were sucking up all the "air" in the room.

But that was 40 years ago. A lot has changed since then--nearly all of it in favor of thorium and the liquid-fluoride reactor.

The biggest one: the AEC/DOE/military isn't wanting weapons-grade fissile material anymore. In fact, now they're wanting to get rid of it. That removes a great deal of the hidden incentive for the plutonium-fueled LMFBR.

Waste is a big issue now. People want to know how the "unsolved problem of nuclear waste" is going to be solved. Sticking it in the desert of Nevada is considered politically unacceptable by powerful factions of government. A thorium-fueled reactor, if done properly, can operate without generating the long-lived transuranic waste that drives the design of a place like Yucca Mountain. Even the governor of Nevada mentioned thorium reactors as an alternative to YM in a recent interview by Chris Matthews of MSNBC.

Uranium is getting more expensive. This still isn't a major economic factor but it could get more and more important.

Finally, but most importantly in my mind, safety. The safety of a light-water reactor is engineered safety. Backup pumps, emergency core-cooling systems, etc. Fluoride reactors can be built with INHERENT safety. No backup systems. No emergency core-cooling systems. Because inherent safety is just that--inherent. The reactor runs safe and fixes itself, instantly, if something goes catastrophically wrong.


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PostPosted: Oct 11, 2007 6:28 am 
Probably, the developed countries are supposedly not blessed with large thorium deposits while uranium could be procured easily from the mines of Canada, South Africa and Australia for Light-Water Reactors (LWR). That seems to be main reason why the thorium fuel cycle was not pursued with vigor and uranium fuel cycle served the purpose of power generation and for producing plutonium for nuclear weapons. It is definitely low-risk development of choice, inherent safety, as cited by Kirk Sorensen. However, the generation of large amounts of high-level waste and its ultimate disposal became an emotive subject worldwide.

India is one of the very few countries with largest deposits of thorium right on the beaches of the States Kerala and Orissa. But somehow, the high technology required for thorium fuel cycle didn’t take off earlier probably due to the “sanctions”. Besides, uranium was available from the Jaduguda mines, and a number of PHWRs were built for nuclear power generation. Simultaneously, however, thorium utilization was at the top in the agenda for the long term core objective of the “third stage” of the nuclear power program.

At present, it is reported that BARC is engaged in developing 300 MWe Advanced Heavy Water Reactor (AHWR) to gain expertise for thorium utilization and for demonstrating advanced safety concepts. Mixed Thoria-Urania and Thoria-Plutonia are the candidate fuels for the AHWR. Accelerator Driven Sub-Critical Systems (ADS) is the latest addition to the Indian Nuclear program for large scale utilization of thorium. It is expected to provide the strong technology base for its ambitious program of incineration of the long-lived actinides and fission products, thereby reducing the high level waste inventory. India’s long term nuclear power program is reported to be based on the thorium fuel cycle.


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PostPosted: Oct 11, 2007 10:56 am 
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Raja, one of the remarkable things about the fluoride reactor is that it is SO economic with its use of thorium that large thorium reserves are not really an issue. The amount of thorium needed to run a 1 GWe reactor for thirty years would easily fit into a small closet and could be economically purchased from any country that has mined thorium as part of a rare-earth mining operation.

Perhaps in light-water reactor designs using thorium, where rather large amounts of thorium are consumed (because the actual amount of thorium utilized is small) then thorium resources could be a distinguishing factor, but not for a reactor like the fluoride reactor.


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PostPosted: Apr 12, 2010 3:28 am 
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Thorium is a naturally-occurring, slightly radioactive metal discovered in 1828 by the Swedish chemist Jons Jakob Berzelius, who named it after Thor, the Norse god of thunder. It is found in small amounts in most rocks and soils, where it is about three times more abundant than uranium. Soil commonly contains an average of around 6 parts per million (ppm) of thorium. World Nuclear Association

Thorium is used to make ceramics, gas lantern mantles, and metals used in the aerospace industry and in nuclear reactions. Thorium can also be used as a fuel for generating nuclear energy. Agency for Toxic Substances and Disease Registry

Thorium itself is a metal in the actinide series, which is a run of 15 heavy radioactive elements that occupy their own period in the periodic table between actinium and lawrencium. Thorium sits on the periodic table two spots to the left (making it lighter) of the only other naturally occurring actinide, uranium (which is two spots to the left of synthetic plutonium).

It can't sustain a nuclear reaction once it has been started. This means the U-233 produced at the end of the thorium fuel cycle doesn't pump out enough neutrons when it splits to keep the reaction self-sustaining: eventually the reaction fizzles out. It's why a reactor using thorium fuel is often called a 'sub-critical' reactor. Cosmos


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PostPosted: Apr 12, 2010 6:04 am 
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johndavid wrote:
It can't sustain a nuclear reaction once it has been started. This means the U-233 produced at the end of the thorium fuel cycle doesn't pump out enough neutrons when it splits to keep the reaction self-sustaining: eventually the reaction fizzles out. It's why a reactor using thorium fuel is often called a 'sub-critical' reactor. Cosmos

....and its also why the world's first man-made critical reactor, Enrico Fermi's Chicago Pile #1 (CP-1) used uranium and not thorium.
All the rest of the war effort - including plutonium breeding for weapons in graphite-moderated ("thermal") reactors - followed in the same vein, the previous posts in this thread notwithstanding.


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PostPosted: Apr 12, 2010 9:25 am 
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Currently the uranium and the LWR enjoy the advantage of incumbency despite the excessive publicity the two nuclear power accidents at TMI and Chernobyl have received. Indians were too keen on thorium but the shortage of fissile feed has held up thorium fuel.
I think that shift to thorium will have to be step by step. 19-20% LEU blended with thorium as fuel (as suggested in AHWR300-LEU.broc) has to be the first step. Amounts of DU and SNF in storage are so huge that their re-use as fuel is also necessary. Kirk's talk in TEAC2 also includes burning of this uranium in liquid chloride reactors.
Once the fluid fuel uranium burning reactors and thorium based fuel are established, it will be possible to move to LFTR in the next step.


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PostPosted: Apr 12, 2010 7:50 pm 
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Kirk Sorensen wrote:
In simplest terms, when fissile isotopes for weapons were needed (1942-1949) thorium's derivative fissile isotope (U-233) lost to highly-enriched uranium (U-235) or weapons-grade plutonium (Pu-239) because U-233 was contaminated with U-232, whose decay-chain hard-gamma emissions made it unsuitable for weapons.


The fundamental reason that thorium lost to uranium in the early days is that thorium does not have a fissile isotope. Therefore you cannot run a reactor starting with just thorium alone, and you cannot enrich thorium in any form to produce a weapon. This is not true for uranium. Therefore, uranium offered a route to develop weapons materials that was not possible with thorium - a reactor fueled with natural material (uranium) and enrichment uranium. Therefore there was never a need to pursue the use of thorium in the development of nuclear weapons at that time. However, an interesting story in this regard can be found in Weinberg's book "The First Nuclear Era" on page 36.


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PostPosted: Sep 28, 2010 5:37 pm 
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Finally, but most importantly in my mind, safety. The safety of a light-water reactor is engineered safety. Backup pumps, emergency core-cooling systems, etc. Fluoride reactors can be built with INHERENT safety. No backup systems. No emergency core-cooling systems. Because inherent safety is just that--inherent. The reactor runs safe and fixes itself, instantly, if something goes catastrophically wrong.


It may be my lack of time available for a thorough search, but I've yet to find a laymen-friendly, yet complete, explanation of why this is (possible).

Can you elucidate? Thanks.


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PostPosted: Sep 28, 2010 6:13 pm 
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davidryal wrote:
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Finally, but most importantly in my mind, safety. The safety of a light-water reactor is engineered safety. Backup pumps, emergency core-cooling systems, etc. Fluoride reactors can be built with INHERENT safety. No backup systems. No emergency core-cooling systems. Because inherent safety is just that--inherent. The reactor runs safe and fixes itself, instantly, if something goes catastrophically wrong.


It may be my lack of time available for a thorough search, but I've yet to find a laymen-friendly, yet complete, explanation of why this is (possible).

Can you elucidate? Thanks.

Fluid fuel reactors do a couple of things that solid fuel reactors cant. They have a negative temperature coefficient because of thermal expansion, so that the overall neutron density drops as the fuel gets hotter, shutting the reaction down. While its only as fast as the speed of sound of the fluid, it does contribute to passive safety. Also, should something go completely wrong with the reactor and it overheats, you can picture a fluid fuel reactor as a tub with a plug of frozen material at the bottom, kept frozen by a little fan or something. If it gets hot enough, or the power to cool the freeze plug gets cut, the fuel will drain into dump tanks while the engineers can safely figure out what the hell went wrong rather than having to announce a successful unplanned test of a reactor containment building and by the way our reactor is gone.


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PostPosted: Sep 28, 2010 6:55 pm 
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The requirements for safety have gotten much tougher over the 60 years since nuclear power was introduced. Adding more pumps in case the main ones broke, and then more pumps in case the backups broke and then ... you get the idea. As the requirements got tougher we kept adding more stuff just in case. This drove up costs.

Another approach is to exploit laws of physics to take of things automatically by laws of nature rather than by machines men build. This is called passive safety. In layman's terms, the reactor is designed so that even if you cut off all power, and simply walk away in the middle of some problem (imagine a massive earthquake) that the reactor will gracefully shutdown without endangering the public.

For example, a common feature is a negative thermal coefficient of reactivity. This means that if the fuel heats up then the reactor puts out less power. In the case of LFTR this happens in two ways. First on a VERY fast time scale (near speed of light for a distance of < 1 cm) the as the fuel warms up the fertile will absorb more neutrons (faster than the fissile) and this will soak up the extra neutrons and reduce the reactivity of the system. Second, on a reasonable time scale (speed of sound for a distance of 2-3 meters) the fuel expands so that there is less fuel in the reaction chamber. These steps will stop the fission process if the reactor gets 100C or so above its proper operating temperature. Stopping the fission process is only half the problem. One must also remove the decay heat. There are passive designs to accomplish this as well but this paragraph has already gotten too long.

Passive safety isn't something that is unique to LFTR - some solid fuel reactors also do this but it is easier with a liquid fuel and has been part of the design for more than 40 years.


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PostPosted: Sep 28, 2010 7:31 pm 
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thanks, both of you.


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PostPosted: Sep 29, 2010 12:10 am 
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davidryal wrote:
Quote:
Finally, but most importantly in my mind, safety. The safety of a light-water reactor is engineered safety. Backup pumps, emergency core-cooling systems, etc. Fluoride reactors can be built with INHERENT safety. No backup systems. No emergency core-cooling systems. Because inherent safety is just that--inherent. The reactor runs safe and fixes itself, instantly, if something goes catastrophically wrong.


It may be my lack of time available for a thorough search, but I've yet to find a laymen-friendly, yet complete, explanation of why this is (possible).

Can you elucidate? Thanks.


Another factor in having no excess reactivity. This is made possible by continuous removal of gaseous fission products.

One of the fission products produced is an isotope of Xenon that has an extremely high tendency to absorb neutrons. In a solid-fueled reactor this gas remains trapped inside the fuel pellets until it decays. This would stop the reaction and shut down the reactor unless the reactor was designed with excess reactivity i.e. configured such that each fission can potentially cause much more than one other fission. This excess reactivity is held in check using control rods and unleashed when Xenon accumulates in the reactor to help it continue running in the presence of this "neutron poison". It's easy to see why this is potentially dangerous. If the control rods are lifted for anyreason in the absence of Xenon the reactor will generate a huge spike of energy and heat up beyond the capability of any heat removal system.

In a liquid fueled reactor this Xenon simply bubbles out and is collected for storage until it decays. The reactor need not be designed with any excess reactivity to compensate for its presence. This is inherently safer.

Note that this and the other inherent safety features described in the posts above are properties of liquid fueled reactors. They have nothing to do with thorium specifically. A liquid fueled reactor can run on low enriched uranium just like a light water reactor and will do a pretty good job at it, using significantly less fuel (and generating less waste) for the same amount of power and have all these inherent safety advantages. Liquid fluoride reactors are particularly good for thorium while using it in a solid fueled reactor is tricky and requires reprocessing.


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