Energy From Thorium Discussion Forum

Thorium Fuel Cycles

There has long been an attraction for fuel cycles using thorium as a thermal breeder of fissile material (U-233). Thorium is three times as abundant as uranium in the earth’s crust, and U-233 is valuable as a fissile material due to its high value of fission neutrons produced per thermal neutron absorbed (eta).

Existing CANDU reactors can operate on thorium fuel cycles, with comparable fuel-cycle costs to the natural-uranium cycle and with improved uranium utilization. While ultimate efficiency is achieved with a self-sufficient cycle that relies only on bred U-233, economical once-through thorium (OTT) cycles can greatly extend uranium resources.

Several options have been identified for the use of OTT in CANDU reactors (Milgram, 1984), and on-power refuelling is the key to successful exploitation of this material. Two general approaches have emerged: the "mixed-core" approach, and the "mixed-fuel-bundle" approach (Boczar, 1998).

In the "mixed-core" approach, a number of "driver" channels provide the flux requirements for a fewer number of "breeding" channels filled with thorium-oxide fuel. This is the conventional CANDU-OTT strategy, and has the potential to be competitive, in terms of resource utilization and economics, with both natural-uranium and SEU fuel cycles (Milgram, 1982; Dastur, 1995). Complex fuel management is required to handle the different characteristics and residence times of the two fuel types.

In the "mixed-fuel-bundle" approach, thorium oxide is contained in the central elements of a fuel bundle, and SEU is contained in the outer elements. Although uranium utilization and thorium irradiation are not as good as in the "mixed-core" approach, uranium utilization is improved over the natural-uranium cycle (but not SEU), with comparable costs. Fuel management is much simpler than in the "mixed-core" approach, and refuelling rates are about a third of that required with natural uranium (Chan, 1998).

An extension of the CANDU-OTT cycle is the "direct self-recycle" of the thorium elements bearing U-233, into new "mixed-bundles" containing fresh SEU elements. This is an excellent example of a proliferation-resistant fuel-recycle option (Boczar, 1999).

In the long term, the CANDU reactor is synergistic with fast-breeder reactors (FBRs), where a few expensive FBRs could supply the fissile requirement of cheaper, high-conversion-ratio CANDU reactors, operating on the thorium cycle.

Thorium fuel cycles have additional benefits beyond uranium resource extension. Both the thermal conductivity and melting point of thorium oxide are higher than that of uranium oxide (by 50% and 340ºC, respectively). Thorium oxide is chemically very stable, does not oxidize, and creates fewer minor actinides than uranium. Even with the existence of economical uranium fuel cycles, thorium can be used to simultaneously extend resources and create a "mine" of safeguarded U-233 for future exploitation.

Advanced fuel cycles may be fashionable today as the world struggles with strategic questions of sustainable supply, non-proliferation, and waste management, but the notion has been fundamental to CANDU design since the outset. Under a thorium fuel cycle the CANDU system can become self-sufficient in fuel-supply. In short, CANDU can burn everything but the kitchen sink (and perhaps even that as well, if it's coated with a thorium glaze).

Does anyone know if work is still being done on this fuel cycle in Canada?

The answer is "yes". There is on-going work on thorium fuel cycles in CANDU reactors.

I know this was asked a while ago, but it's a chance for me to try out my first post.

-Gary

Gary,

Would you know if straight ThO2 plus PuO2 (i.e. TRU) is ever considered for CANDU or LWR solid fuel elements or is some amount of UO2 always mixed in for proliferation reasons (to dilute or denature the produced U233).

Gary, when you look at CANDUs using thorium, do you consider the essential elements of the CANDU to be the D2O moderator and the use of uranium?

I'm not sure exactly how to answer the question. At the research level, pretty much anything you can think of has been, or is being, considered. (LEU,Th), (Pu,Th), (U233,Th), (TRU,Th), etc. [By TRU, I'm thinking group-extracted transuranics: Np, Pu, Am & Cm at typical LWR discharge compositions]. With the exception of (TRU,Th) all of those fuels have been fabricated and irradiated.

The non-proliferation debate isn't my topic. As long as there's no commercial Th-cycle, there's no real motivation for anyone to take a very hard look at what would be a reasonable barrier to proliferation.

Ya, dumping in a lot of 238 is a quick way to reduce the efficiency of a Th-based fuel cycle.

Any help?

-Gary

I think it's better to look at what we have available to us

No argument there, if the focus is on the very near term (not to say myopic).

Don't get me wrong. I'm suggesting we abandon progress. I'm just suggesting that, rather than deciding "what CANDU is" we would be better served by looking for "what CANDU technology has to offer".

And I think that's as true in the long term as the near term, though what the technology has to offer in the future may be something different. You can find a lot of new ideas in old reports.

-Gary