Graphite moderated two fluid LFTRs have some significant advantages over unmoderated LFTRs. The primary advantage is that the graphite moderated reactor requires a much smaller fissionable charge, and therefore the graphite moderated LFTR fuel for a project anticipating a rapid deployment of large numbers of LFTRs can be carried out without a major increase in the production of fissionable materials. Thus with graphite moderated graphite LFTRs current stocks of fissionable materials are sufficient to start a very large number of LFTRs. My last post on however came to negative conclusions about the graphite moderated two fluid reactor design. I knew that Kirk Sorensen probably held differing views, but Kirk has yet to demonstrate that he can solve the problem raised by David LeBlanc.
Yesterday Kirk posted a new discussion of the two fluid graphite moderated reactor, this one based on research being conducted at the Nuclear Research Institute at Rez in the Czech Republic. Czech researchers Jan Frybort and Radim Vocka have taken another look at the ORNL-4528 type reactor using contemporary computerized reactor research tools. Their findings, reported in a paper titled Neutronic Analysis of Two-Fluid Thorium Molten Salt Reactor. Kirk suggested.
In part 7 of the paper, they mention the second key issue with a two-fluid reactor--the problem of the blanket void coefficient. Since the original ORNL design had the problem, and since they modeled only parametric variations on that original design, it's no surprise that the problem still shows up. It must be fixed, probably through a new design approach to the two-fluid reactor. I have some ideas, most all of them based around physical situations where a loss of blanket fluid leads to a loss of moderation. I anticipate that this could be done by floating moderator elements (graphite) in the blanket salt, so that as the level of the blanket salt falls, the moderation decreases more than the absorption decreases from the loss of blanket. These ideas definitely need more modeling, but I think they are essentially sound.Thus Kirk believes that a different approach to the graphite design might work, and that he himself might hold the solution. Kirk has concerns about patent rights, so he is reluctant to say too much. Thus it is too soon to write off the graphite moderated two fluid reactor yet, but we two fluid reactor that is need to have an alternative available. Fortunately David LeBlanc has designed an unmoderated two fluid reactor which has a very simple and low cost core. Thus David's reactor design would serve as an acceptable, easy and cheap to build alternative to an ORNL-4528 type reactor. Kirk, unlike David LeBlanc, is unwilling to write off the graphite moderated two fluid reactor yet.
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How do the issues of graphite designs vs unmoderated compare with those of heavy water moderation?
Heavywaterdoes not do double duty as part of the reactor core structure, as graphite does in the two fluid design.
Having a liquid heavy water moderator would mean no more swelling/shrinking problems...
What reactor core structure would a heavy water moderated design require? How does that perform with regards to neutron flux swelling/shrinking compared to graphite? Isn't swelling and shrinking an even bigger problem for a moderator that also serves structural functions due to loss of strength and durability?
Stupid questions maybe but i'm no expert...
Anon, yes but introducing water into the core of a Molten Salt Reactor would create a serious safety concern.
Hmm. The water and molten salt wouldn't be under any meaningful pressures above atmospheric right?
Worst case scenario, a leak will cause the water to boil off quickly which loses the moderation. The design just needs to incorporate space where the water could boil off to/into and be removed easily afterwards? None of the reactor parts and salts are water soluble right?
It sounds like these problems may be more manageable than the ones with graphite...
If you introduced water into a 700 C container of liquid salt, the water would flash into steam, and the steam explosion could be quite destructive to the reactor core. Ibelieve that there would be other very undesirable chemical reactions.
The nice thing about the MSR, LFTR or otherwise, is it's zero-water/zero-steam/zero-operating pressure paradigm. It is this which significantly reduces costs and increases safety. Don't mess with mama's MSR now...
David
Couldn't David LeBlanc's long tube reactor also use a graphite moderator inside the long tube?
Was some problem with that idea found during the period I've been away from the energy from thorium forum?
Well that's what I'm wondering about. If the water flashes to steam it will quickly increase the pressure, would it be possible to use pressure valves to vent the steam as it is created, before the pressure builds up too much to damage solid reactor parts?
And what kind of chemical reactions would occur between the D2O and reactor parts/salts?
In the ORNL design with graphite used as a structural component, was there much heat transfer between the core and the blanket?
If the LFTR uses concentric tubes of stainless steel for core and blanket, does there need to be insulation between the chambers? Or is the added heat transfer a benefit?
Duncan, Anonymous, Jim Baerg, Technical questions should be raised on the Energy from Thorium Forum.
Now that we're in a the middle of a major nuclear accident I think it's a good time to talk about the potential safety in worst case scenarios for various MSR designs.
If we use graphite as a moderator, aren't we inviting a possible ignition scenario? That is the main reason that radioactive elements were carried for a long distance in the original Chernobyl accident no?
Rowan Duffy, I plan to write a post on how Molten Salt Reactors might have performed during the Japanese Earthquake-tsunami event. Look for it in the near future.
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