Thursday, April 23, 2009

The LFTR and the Thorium Molten Salt Reactor

Bruce Hoglund has recently argued that there is no difference between the LFTR and the Molten Salt Reactor. Both Kirk Sorensen and I agree that the LFTR is a type of MSR, and we have always assumed this. Yet there is a set of ideas associated with the LFTR that is not characteristic of all MSRs. To understand what is unique about the LFTR I would like to point to some differences between the LFTR and the Thorium Molten Salt Reactor, a French reactor concept.

Both the LFTR and the French TMSR concepts draw heavily on ORNL MSR reactor research from 1947 and 1980. However, French researcher take as their starting point ORNL research on a 1 fluid 1 GWe, MSBR that was conducted from 1967 onward. Thus the French focus is on the design of a large MSR with optimal breeding characteristics. The French focus remains on large molten salt thorium breeding reactors, but unlike ORNL reactors they are interested in the design of graphite free MSRs. Thus French researchers have rejected the one fluid, graphite moderated approach favored by the ORNL MSBR.

There are a number of distinct ideas a bout the LFTR that appear to be entirely absent from French thinking about the TMSR. For example the LFTR is conceived of as a relatively small, portable, factory manufactured, modular thorium fuel cycle Molten Salt Reactor. The LFTR concept focuses on cost lower measures including labor saving in reactor manufacture. The LFTR concept includes a number of none traditional concepts about reactor siting, including underwater siting, underground siting, and the recycling of fossil fuel power plant sites as LFTR.

David Walters describes some of the potential cost saving available if power plant sites are recycled as new LFTR sites.
The BIG savings, IMHO, is not necessarily on the ability to using the existing turbine/generator set. It's on all the other BOP (Balance of Plant). What are we talking about?

1. siting. already done, obviously.
2. grid access...with almost NO changes especially if generator at the end of the LFTR is the same size or near it as the old stoker it's replacing. A little more if we want to upgrade the MWs.
3. licenses for everything: accumulation of standard hazardous waste, water usage, fire fighting equipment, water discharges, maybe cooling towers, air cooled condensers, once through cooling. The list is endless and it's all a *savings*.
4. plant access by road, rail and water. All these are nicely built in because of the need to transport coal and having built the plant in the first place.
5. lay down yard. As the coal is reduced by burning, more and more of the dozens of acres of land used by coal storage areas is freed up. Tons and tons of space.
6. Physical equipment: water lines to and around the plant, bus rooms and switching centers for plant auxiliaries, main and aux. banks *in place and ready to use*.

Probably the savings list is endless.
This is typical LFTR thinking, which focuses not just on optimal reactor design in the abstract, but in cost savings and carbon replacement, As David Walters noted:
The BIGGEST advantage we have for a LFTR-in-Coal-out scenario is just that. We when we go COD on the LFTR, the breaker opens for the last time on the stoker, and we closed out a coal plant. Let wind and solar advocates try to make THAT claim!
A second major keynote to the LFTR is its modularity. Contributer Axil notes:
In systems engineering, modular design, or "modularity in design" is an approach that subdivides a system into smaller parts (modules) that can be independently created and then used in different systems to drive multiple functionalities. Besides reduction in cost (due to lesser customization, and less learning time), and flexibility in design, modularity offers other benefits such as augmentation (adding new solution by merely plugging in a new module), and exclusion.
Modular design is much more difficult to design and implement then a custom build approach since it requires the systems engineer to examine the full range of possibilities of a product line and define modules and appropriate interfaces between these modules.

A key feature of a system is that it is modular, flexible and adaptable. In general, it is commonly recognized in systems engineering that the broader the range of adaptability that a system is, the more successful and cost effective that the system will eventually be. These characteristics of the system ensure that the system is responsive to the needs of its broad user base over time. A good system must meet or exceed the expectations of a large and diverse range of users.

For example, a computer is divided up into a mother board, disks, CPU, internet interface, etc. A computer that does not provide this level of modularity will be unsuccessful in the market place since it does not offer the possibility for upgrade.

IMHO, it would be a good thing if the Lftr were modular.

What will make the Lftr modular, and how is that modularity achieved? It would be valuable to develop a consensus that modularity of design is an important design objective and that it is worth the effort to make the design modular.
Some Energy from Thorium contributers are more influenced by French thinking on the TMSR. David LeBlanc suggests
A graphite free core design does not necessarily need a much larger fissile inventory to start up IF you have a nice fully encompassing fertile blanket around the core to catch leakage neutrons.

In reactors without graphite, the neutrons can travel quite a long way before they finally are absorbed (or lost to leakage). Thus in designs without a blanket or only a partial one (as is the French TMSR design) you need a lot of fissile material to make sure those neutrons are absorbed and/or cause fissions quickly. If you have a full fertile blanket as is the case in some other designs then you can actually get by with a much lower fissile starting load and still manage to break even on breeding.

So no graphite equaling much more fissile is not always the case (granted it usually is).

Also I would add that the positive temp reactivity problem with using graphite is not a problem for 2 fluid designs (as most know) AND it is also not a problem for Single fluid designs with U238 added to denature (which most don't know).
Alex P., quoting ORNL 4812 disagreed with him:
contrary to my previous belief, breeding gain in the epithermal spectrum is INFERIOR than in the thermal one.
Quite the opposite I believed it happened.
"In the early days of the Molten-Salt Reactor Program, serious consideration was given to homogeneous reactors in which the core contained nothing but salt. These ideas were abandoned after calculations showed that the limited moderation by likely fluoride salt constituents alone would result in a thermal reactor with inferior breeding performance. Breeding appeared possible in intermediate-spectrum reactors, but their gains were not high enough to compensate for their higher fissile inventories".
We thus have not settled yet if the core of the LFTR will contain graphite, but several of us are inclined to that view. Kirk recently posted Several sections of ORNL 4528 and related sections of ORNL-4119, These documents, dating from the mid to late 1960's point to point to ORNL thinking headed toward a modular direction. The turn toward a single fluid large MSBR seemed to have deminished ORNL thinking about modularity, although designs of two single fluid near MSR converter emerged from ORNL in the early 1970's.

The LFTR then is a small economical MSR, that can still breed at a 1 to 1 ratio or greater. It can be factory built and can generate base load electricity. Because of its low capital costs, the LFTR can be used to supply not just base load power but mid load and even peak load electricity. It has the potential of supplying industrial process heat, and can be used for desalination, either by using its waste heat, or by using all of its heat to desalinate water. It can be air cooled, and thus can be used in dry regions. I exhibits siting flexibility, and should not requite large expansions of the grid.

The TMSR is usually conceived of a large reactor, and little attention has been paid to the unusual design, construction and siting features that characterize LFTR thinking.


Alex P. said...

Actually, I don' t disagree with David LeBlanc, indeed, as many times here and at EFT forum I end up to write, I'm a huge fan of the LFTR/MSR moderator free version (simpler configuration and no graphite waste issue, even if a little of graphite, if I understand correctly, is always needed), IF and only IF it has no other issues. I don't think the fissile start-up, even in the epithermal spectrum, is one of this issue if we adopt a fully encompassing thorium blanket (not only radial as the French version), like David pointed out

(Anyway, I'm really honoured a post of mine has been published here, Charles!)

Charles Barton said...

If God had intended for the MSR to be built without graphite, man would have been born without wings. No, wait, that can't be right. Anyway, they built MSR's with graphite at ORNL, and that is the way it has to be. Otherwise we will all turn into snail eaters. No, that can't be all bad, some of my ancestors were French. Well maybe the they did not ear snails. I hope not at any rate.

Alex P. said...

Of course at ORNL they built MSR with graphite as moderator, but however they motivation were quite clear: 1) low or lower breeding gain in epithermal spectrum and in particular 2) quite higher fissile strart-up needs, a very critical issue at the time. However, I don' t think they are valid motivations even today, so I woudn't discard a priori (clearly, if there are no other safety or operational issues)epithermal spectrum, moderator free version (even if as far I understand a little of graphite is always needed)

Alex P. said...

One interesting possibility with epithermal spectrum is coupling better alloy than Hastelloy-N, which limits temperature in the hot leg to 700 °C, with Li and Be free poorer salts like, for example, NaF–ZrF4 (59.5–40.5), KF–ZrF4 (58–42), RbF–ZrF4 (58–42)
( table at wikipedia page )

Epithermal spectrum, indeed, can overcome the poor neutronic features of those salts, achieving self breeding even in those conditions,as far I understand


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