Saturday, October 25, 2014

Terrestrial Energy, Correcting a mistake.

Given my limitations, it is inevitable that I will commit mistakes.  At any rate, I made a mistake in my account of terrestial energy.  The mistake arrose from Hugh MacDiarmid's discussion of the prospects of Terrestrial Energy,  a Canadian company that proposes to build a commercial Molten Salt Reactor. MacDiarmid claimed that the new MSR would be 6 times more fuel efficient as a conventional reactor.  There are two ways to accomplish this using Molten Salt Reactor Technology.  One is by Adopting Denatured Molten Salt Reactor Technology.  In DMSRs both liquid Uranium salt and Liquid Thorium salt are mixed with other molten  salts, in a graphite moderated pot. This makes for a relatively simple reactor.  David LeBlanc has in the past openly talked about his interest in DMSRs  The DMSR has the sort of fuel efficiency Hugh MacDiarmid has talked about, but it requires a much larger start charge to start a cjain reactikon, and begin the conversion of U-238 to Reactor Grade Plutonium.  The neutron efficiency of  of the the IMSR will be high enough that it will be a high ratio converter, much higher than LWTs, but still not in breeding range.  At the same time, the IMSR will not burn more few per Kwt of output.  Since over time a higher percentage of IMSR fuel will be Plutonium rather than U-235 the IMSR will begin a process of Stater U-235 payback if the start charge uses LEU

The fast UMSR is very simple, throwing out both graphite and thorium, and running the reactor at high neutron speed with uranium fuel, or mixed Uranium and plutonium.  The reactor would then make and burn a lot of plutonium, a lot more than conventional reactors, hense, the fuel use efficiency.

Now if this gues is wrong, I am sure someone will tell me.


Martin Burkle said...

There is another way. Don't use any thorium at all. Just use all uranium as fuel.

Terrestrial's big idea is to use a 7 year cartridge. Instead of making the reactor vessel openable to replace the graphite, they will just replace the whole reactor vessel every 7 years.

Maybe the vessel will be inexpensive stainless steel and maybe the base salt will be really cheap sodium fluoride. Wouldn't it be fun to be on the design team where just-replace-the-box thinking happens?

David LeBlanc said...


Great to see you posting again. Let me try to clarify a few points.

The term DMSR for Denatured MSR in my view simply refers to MSR concepts that the uranium is denatured and thus not weapons usable (i.e. enough U238 present). The term itself doesn't imply the use or not of thorium. Yes, the original DMSR concept of ORNL in 1978-1980 did mix both Low Enriched Uranium and thorium but a DSMR could certainly be simply just Low Enriched Uranium (to a much lower enrichment). If thorium is not used there is a slight drop in fuel efficiency but with a very long list of tradeoffs to examine. Terrestrial Energy may or may not employ thorium for the IMSR fuel salt mix.

Second is that the original DMSR design of 1980 did have a modestly large start charge of U235 (3.5 tonnes per GWe, a bit less than current LWRs) but that was mainly because it was such a large, low power density design with a large fuel salt volume (100 m3). A DMSR design with higher power density (like the IMSR) can have a surprisingly low start charge, a big advantage of softer spectrum reactors.

To clarify, any discussion of 6 times the fuel efficiency is referring to the annual fissile makeup need compared to LWRs (An LWR needs about 1000kg to 1200kg of makeup fissile U235 per GWe operating year). Comparing the two over a full 40 to 60 year facility lifetime would then need to work in the several new start charges an IMSR and the one large one of a LWR. It would also depend on whether recycling of the salt is done after use (which we expect to be the long term case). Even on a true "Once Through" cycle with the IMSR with no recycling, the start charges are small enough that the overall lifetime could still be roughly a 6 fold improvement over LWRs.

A final comment, referring to earlier posts is that while we do see the Alberta Oil Sands as a potential market, it is certainly not our main or only focus.

David LeBlanc
Terrestrial Energy

Marcelo Pacheco said...

A typical LWR produces about 50% of the fissile it consumes.
Terrestrial IMSR essentially produces 80% of the fissile it consumes.
That's the reason for the 1/6th number. Once the reactor has consumed enough of its U-235 + Plutonium it needs some extra fuel, the yearly makeup fuel.
Since the IMSR is a molten salt cooled and molten fuel reactor, adding that makeup fuel means injecting extra material into the reactor (no shutdown needed). It uses an overflow tank that allows the core material to exceed its internal volume.
Should the reactor operator have access to fuel reprocessing facilities, batches of core material could be transffered from the reactor core into the reprocessing unit, removing fission products only. This would improve the 80% breeding ratio somewhat (reducing the yearly makeup fuel needed).
Fission products are neutron poisons. The worst are the noble gasses (Xenon and Krypton), which are removed by bubbling, but all fission products contribute to neutron losses. Regular reprocessing of core materials would remove those fission products, leading to lower neutron losses, and more of those surplus neutrons converting U-238 into Pu-239. Reprocessing every 5 years would be optimal, and reprocessing as slow as every 10 years would still improve the reactor efficiency substantially.

PS: I don't have a nuclear degree, nor I am affiliated to Terrestrial. Those are my conclusions after studying available DMSR / LFTR proposals as well as Terrestrial IMSR literature.


Blog Archive

Some neat videos

Nuclear Advocacy Webring
Ring Owner: Nuclear is Our Future Site: Nuclear is Our Future
Free Site Ring from Bravenet Free Site Ring from Bravenet Free Site Ring from Bravenet Free Site Ring from Bravenet Free Site Ring from Bravenet
Get Your Free Web Ring
Dr. Joe Bonometti speaking on thorium/LFTR technology at Georgia Tech David LeBlanc on LFTR/MSR technology Robert Hargraves on AIM High