George Stanford, Ph.D., is a nuclear reactor physicist, part of the team that developed the Integral Fast Reactor. He is now retired from Argonne National Laboratory after a career of experimental work pertaining to power-reactor safety. He is the co-author of Nuclear Shadowboxing: Contemporary Threats from Cold War Weaponry.

George S. Stanford, Per F. Peterson and Dan Meneley

G. Stanford wrote (11-29-10):

We'll see what others on this list have to say, but in my opinion, Carlsen's enthusiasm for thorium is premature, to say the least.  The ONLY significant advantage a thorium cycle would have over fast reactors with metallic fuel (IFR/PRISM) is its lower requirement for start up fissile.  That advantage is offset by the fact that the thorium reactor is at a stage of development roughly equivalent to where the IFR was in 1975 -- a promising idea with a lot of R&D needed to before it's ready for a commercial demonstration -- which puts its deployment about 20 years behind what could be the IFR's schedule.  The thorium community has not yet even agreed on what will be the optimum thorium technology to pursue.

I think that thorium should indeed be investigated as a possible future competitor for the IFR.   But what would be gained by putting off demonstrating the IFR/PRISM technology while waiting to see if thorium really lives up to its promise?  Nothing would be lost by getting a fleet of IFRs up and running.  They could be breeding fissile for decades while a possible thorium fleet gets up and running, and the IFR-bred fissile -- several times more than was started with -- could be used for expanding the hypothetical thorium fleet at the end of the IFRs' lifetimes.

If the current perceived urgency is to sequester plutonium to put it out of the reach of proliferators, that can be done much faster with early deployment of IFRs rather than by later deployment of thorium reactors -- and each IFR will sequester 8 - 10 times as much Pu per GWe as a thorium reactor.

-- George S. Stanford 11-29-10

 D. Meneley wrote:

On the matter of thorium, George and others have repeated a . . . realistic picture.

[Boosting thorium] will do no good. This is another idealist's dream, like large scale wind energy. They only want to save the world and are not interested in practical details.

If you've tried to do control, fuel cycle, and safety system design on a thorium reactor you'll not be so enthusiastic. The flux shape is a strong function of the past flux shape -- because of the protactinium. After you shut the thing down you must account for the later reactivity increase. And then there's the detail of not having any fissile isotope to start up in the first place.

If you're using thoria fuel, how are you going to extract the U233 economically?

And so on.

Thorium if absolutely necessary, but absolutely no thorium if not necessary.

--Dan Meneley 11/27/10

 P. Peterson wrote:


Your assessment on the relative technical maturity of LFTR versus IFR is correct.

But there are other substantive technical differences besides the lower fissile start up requirement for thorium reactors.

Thorium reactors operate in with a thermal spectrum, which allows them to use graphite as the primary structural material in the reactor core.  Graphite can be heated to very high temperatures without losing structural integrity. Combined with the very high boiling temperature of the fluoride-salt coolant (> 1400°C), thorium reactors can deliver heat at substantially higher temperature (between 600 and 700°C with current primary pressure boundary structural materials) than IFR (between 370 and 510°C with current fuel cladding materials).  This is a sufficiently higher temperature that several options exist for gas-Brayton power conversion, while at the IFR temperatures steam Rankine is likely to remain the most practical option.

There are a number of substantive theoretical advantages to gas Brayton power conversion (this is the reason Brayton cycles are now used universally with natural gas), but essentially all of the existing turbine and compressor technologies optimized to open combustion cycles and thus substantive development is needed to adapt it to nuclear power conversion.  Once successfully developed, though, one would expect substantial commercial pressure to move from steam Rankine to gas Brayton cycles as the dominant approach for nuclear power conversion (as has already happened with natural gas).

The other major differences arise from the different thermophysical properties of the two coolants.  The fluoride salts have volumetric heat capacity slightly larger than water and about 4.5 time larger than sodium.  So the primary systems for thorium reactors are physically much smaller than for IFRs, or alternatively, a primary system of the same physical size can produce substantially more power (factor of a 2 to 4).  Thorium reactors have no sources of stored energy that can pressurize containment, so they also can use a compact, low-pressure containment structure and thus a correspondingly smaller reactor building.

These are substantive technical differences that are likely to affect the relative levelized cost of electricity (LCOE) produced by the two systems.   But one of the major issues with LFTR is that one must overcome multiple, substantive technology development problems simultaneously (gas-Brayton power conversion, qualification of materials for corrosion resistance, on-line fuel processing, licensing for fluid-fuel reactors). This creates a significant activation energy problem, even if the final LFTR technology would have desirable LCOE and sustainability characteristics.    One of the reasons that we've been working on solid-fuel variants at UC Berkeley is to see if one can reduce the activation energy barrier by capturing most of the LCOE benefits (which come primarily from improved power conversion efficiency and reduced capital cost relative to ALWRs) while keeping the licensing approach much closer to that used for passive ALWRs and not taking on the technical issues for fluid fuel.

In the end, LCOE will be a dominant consideration in commercial decisions to deploy nuclear power. In the near term the best opportunities involve further improvement to ALWR technology and construction methods (with AP-1000 providing the best role model to date).  In the longer term some mix of uranium fast spectrum and thorium thermal spectrum reactors is likely to emerge as optimal.

-Per F. Peterson 11/29/10

G. Stanford wrote:


Thanks much for the additional information, clarifying the technical challenges and strengthening the case that thorium power is worth pursuing and might well have an important role down the line.

While the LCOE will undoubtedly be an important consideration, it seems to me that breeding potential also is destined to be important if we are to have abundant clean energy.  It also seems likely that thermal efficiency per se will not ba a major issue, in view of the very low cost of fuel for breeders (or "isobreeders" like the LFTR) -- but you point out that the Brayton cycle potentially offers significant additional advantages.

I gather that you do not take issue with the proposition that it would behoove us now to complete the development of what is currently closest to commercial readiness with the characteristics needed for an assured indigenous energy supply -- namely LMFBRs with metallic fuel and pyroprocessing.  At present, that appears to be a U.S.-developed technology that we have abandoned, bequeathing it to other countries for exploitation.


--  George S. Stanford 11-29-10

P. Peterson wrote:


For LFTRs, the breeding potential may not be particularly important, as long as they can achieve isobreeding.  Uranium from seawater provides a backstop technology that sets the maximum cost of fissile material, much as coal-to-liquids provides a backstop for the cost of oil (absent a price on carbon dioxide emissions).  The startup of an isobreeding LFTR requires about 1/4 to 1/2 the fissile needed to start up an LWR, and uranium from seawater will have a cost around 4 times greater than current uranium prices.  Thus the capital cost for the fissile to start up isobreeding LFTRs will be comparable to the current cost for the initial core loading for LWRs, which constitutes a modest fraction of the total capital cost of current LWRs.

Our experience to date is that "backstop" energy technologies never emerge to be economically competitive, because lower cost alternatives tend to be developed instead (at the scale that we use energy, the economic incentives are very large).

So I would be very surprised that the cost of fissile will ever rise to the point where one would actually begin commercial efforts to recover uranium from seawater (although there is always some slim probability that the government might in the future enact a "seawater uranium portfolio standard," to create an assured market for seawater uranium, so the technology will be brought to commercial readiness regardless of cost).  Absent such government intervention, the cost of fissile to start up LFTRs will likely remain lower than the cost of fissile to start up current LWRs, in perpetuity.

My expectation is that the LCOE for electricity from ALWRs will drop well below the LCOE for new pulverized coal plants before the end of this decade, as Westinghouse's costs to build AP-1000's and enhancements to the AP-1000 drop and as competing LWR technologies for the AP-1000 emerge, and as construction methods improve further. Financing nuclear construction will likely remain a challenge, although SMRs may prove to be helpful in this respect.

But we need an aggressive effort to develop multiple technologies that can improve upon and ultimately replace ALWRs.  Fast-spectrum reactors clearly have advantages, along with thorium cycles, from the perspective of fuel cycle.  IFR metal fuels are vastly better than conventional oxide fuels from the perspective of affordable and secure fuel recycle. LFTR is also a potentially attractive technology, but clearly has substantial technology risk.  So yes, I strongly support demonstration of IFR technology.  The key issue is that IFR needs to remain a part of a portfolio of technologies the federal government invests in, and that IFR demonstration needs to sustain discipline to assure that federal investment is likely to result in commercial success

A simple type of evidence, which Congress has required for the NGNP project, would be 50% cost sharing by commercial interests.    I think that this approach is too simplistic, since it does not recognize how risk changes during design, licensing, and construction of a demonstration reactor.  The best approach is to require very small or zero commercial investment at the stage of conceptual design and NRC pre-application review, moderate commercial investment during detailed engineering and NRC licensing, and substantive commercial investment for the construction of a prototype unit (where the intellectual property and up-side commercial potential ends up being owned by the commercial entities who invest).

This sort of decision framework is also easier to implement in statute, since one can authorize the needed expenditures, but the actual appropriations can depend upon progress being made and commercial investment materializing.

What commercial interests will tell you is that it is much easier to make a decision to make a substantial investment if they have an NRC construction license to build a reactor, while it is almost impossible if the reactor is just a concept that needs a lot of detailed engineering work.  But in the end, the commercial entities that perform this reactor development work are also in the best position to assess its commercial potential--so a lack of willingness to place some commercial money at risk (less earlier and more later) should be viewed as evidence that the concept needs more R&D, not accelerated demonstration.

For IFR, though, the availability of affordable fuel is a big issue.  It requires the capacity to recycle used LWR and IFR fuel, as well as to test and qualify recycled fuels for use in IFRs.  This is a problem that the commercial sector is not going to be willing to take on, and thus it requires purely federal effort.

-Per F. Peterson 11/29/10

George S. Stanford wrote:


Thanks for the further elucidation.

To get quantitative about LFTRs, suppose the world were to want 50,000 GWe of isobreeding LFTRs by 2100, primed with 10% EU at 1 tonne of U-235 per GWe.  That U-235 would be contained in 10 tonnes of EU, which would come from ~200 tonnes of Unat.  Thus the amount of uranium to be mined would be ~ 50,000 x 200 = 10 million tonnes of Unat -- which is well within the realm of the possible.

However, a downside would be the perpetuation and global expansion of uranium enrichment infrastructure, with its proliferation implications.  Also, left over would be some 9.5 million tonnes of orphaned depleted uranium containing 9.5 million GWe-years of unavailable (and unwanted) energy.

--  George S. Stanford 11/30/10

 D. Meneley wrote:

George, Per:

You guys seem to be intent to ignore the only fully developed, in service, economically competitive, high conversion ratio, and safe medium size reactor system on earth [namely, the CANDU-type heavy-water reactor]. Perhaps you could explain why.

Dan Meneley 11/30/10

G. Stanford wrote:


Sorry if I slighted the CANDU. It’s a fine reactor design, struggling to acquire a bigger share of the international market. To believers (like you and me) in the importance of conserving fissile material, its high conversion ratio is an important asset. But – apart from the fact that it doesn’t need enriched uranium -- to a first approximation it’s just another thermal reactor. Since most of the generalities about LWRs apply also to CANDUs, much of the time it’s convenient to use the term “LWR” as shorthand for “uranium-based thermal reactor.”

While I can’t speak for Per, of course, I suspect that he would ascribe less importance to a high conversion ratio than I do.

George S. Stanford 12/01/10

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