Sunday, November 15, 2015

Not Quite my father's Reactor, and not my father's reactor Industry: 3

 The “Mark 1” Pebble-Bed Fluoride-Salt-CooledHigh-Temperature Reactor (PB-FHR) uses FLiBe, a Fluoride salt formula that my father helped to develop at ORNL.  The fuel handling system of the
{B-FHR is quite the same thing as my father's MSRs.  It is nybred design that brings together features of the MSR and the Pebble Bed Reactor, two Generation IV nuclear concepts.  We have a extensive product description of the Mark 1 PB FHR from the the University of California Berkeley Department of Nuclear Engineering.  In addition to the The UCB, MIT and the University of Wisconsin are involved in this project.  MIT is also doing R&D work for Transatomic Power, but the two reactors are somewhat dissimilar.  In addition Per Peterson, who played a major role in the PB FHR project also serves as an advisor to kirk Sorensen's FLiBe energy.

The Idea of laying the foundation of a nucl;ear start up at a University is not new.  NuScale Power, which plans to put micro sive nuclear reactors on the market around 2025, had its origin from a research project carried on by Oregon State University, together with Idaho National Laboratory and other institutions.  Once the government funding of the research stopped, various patents derived from the research, fell into the hands of some of the OSU researchers, who set up NuScale Power to develop and eventually manufacture tiny but conventional reactors.  This effort has progressed, although not without some struggle, and it has received funding from both governmental and private sources.

I have discussed Transatomic Power, which might be described as a Simi in house Nuclear R&D program at MIT.  It is not quite clear how much of the Transatomic R&D work is being preformed by its two Nuclear Engineers, and how much is outsourced to various MIT Labs and other facilities.  I do not say this to denigate the work of Leslie Dewan and Mark Massie, both of whome have been nothing short of brilliant in what they have accomplished so far.  I simply wish to point outr the extent to which MIT has supported their work, and both directly and indirectly profits from it.

What is unique about the UCB's Department of Nuclear Engineering's Mark 1 Project is how far it has traveled, without any business seperation between the project and the University.  The Mark 1, like FLiBe Energy's lifter has reached the development stage in which a Report to the United States Department of energy described as a "Mark-­‐1  PB-­‐FHR  Technical  Description."
 The Vanderbilt Technological Analysis of FLiBE Energy's LFTR, also offers a similar USDoE Product Description.

It should be noted that although Transatomic Power is a serious enterprise, which has produced a relatively brief product discription in a "White Paper," its product discription does not suggest that it was intended to meet US DoE Product Discription requirements. ThorCon Power may not plan to produce or sell its reactors in the United States.  In which case it has no need to provide a product discription to the US DoE.   Finally Terrestrial energy has not released a US DoE Product description.  Terrestrial is a Canadian based enterprise, and its IMSR is geing regulated by the Canadian Regulatory Agency.  The Canadian Nuclear Industry has an international reputation and has sold reactors in South America, Europe and Asia in addition to Canada.  For many years, the entire Indian nuclear power industry was based on copied Canada nuclear technology.  Thus it would appear quite likely that a Canadian Nuclear License, would be sufficient for most of the world outside the United States.  If Terrestrial waqnts to sell reactors to the United States, Licensincing by the NRC is not out of the question.

This account suggests that another MSR development project is possible in the United States, and would in facts be highly desirable.  The quickest route to such a new project would be through licensing the technology Terrestrial Energy is developing, and then creat a product that would be targeted for NRC approval.

This is, however, a diversion from the Mark 1 reactor, which I desacribe as not quite my father's reactor.  Although my father worked in the development of Both MSRs, and played a major role in the creation of the FLiBe salt formula, he actually worked on three ORNL fluid fuel carrier reactors, if his brief stint with the Aqueous homogeneous reactors is included.  The ORNL Preference for fluid fuel dates back to Eugene Wigner, ORNL's godfather, who was accademic by training a chemical engineer. Wigner saw that the problems of reactors could be largely solved, if they were treated like chemical  industrial processors.  Hence the use of reactor fuels suspended in heavy water or molten salts.  The molten salt reactors worked, and worked very well.  In them the salt was used both as a coolant, and as a fuel carrier.  There are some advantages to the fluid fuel approach, in addition to some technological tricks that makes the MSR very stable, plus some unique safety features.    One disavantage of the MSR fuel carrier approach, is that the fuel carrier salt gets contaminated with fission products plus actinide by products of nuetron bomnardment of U-235 and U-238 in uranium fueled reactors, or of actinite by productes of neutron bombardment of thorium its Protactinium and uranium by products.  These fission products creat a mess in the circulating fuel salts.  There are ways of cleaning the mess up, but this has not been tried on a large scale yet, and perhaps it will creat problems down the road for would be mSR developers.

The Pebble Bed reactor has its own set of problems.  It uses gasious Helium as its coolant, but Helium does not have nearly as good coolant capacity as water, so the PBR core has to be very large.  This creats a cost problem for the PBR.  Professor Per Peterson of the UCB, realized that one radical solution to the PBR Helium problem was to replace Helium with a molten salt.  Unlike helium, molten salts are good heat carriers, and the best of the lot is my fathers old molten salt formula for FLiBe.  The use of fuel carrying pebbles is foreign to my father's reactor, but the use of flibe is on home base.   So not quite my father's reactor but close.  The use of graphite Pebbles takes away the problem of coolant spill, because graphite is safe in the event of a coolant loss nuclear shutdown.  it will not melt, and contrary to anti-nuclear legends, it will not chtch fire.

The Idea of a PB FHR has been kicking around the UCB Department of nuclear Engineering for perhaps a dozzen years, but in 2013-14 the senior class of the Department of Nuclear Engineering made the design of one a class project.  The result the class project turned out to be the Mark 1, a highly credible peice of work from undergraduates, and one which would spill up any resume for a nuclear engineer.  The UCB Department of Nuclear Engineering has been sufficiently impressed by the outcome of the 2014 Senior class project that it has thrown its weight behind its development.

We should not that in the case of situation in which reactor technology was either developed under accademic auspices, or in an accedemic environment, an enterprise that focused on developing the technicalologiy for market was launched fairly early in the game.  In the case off the Mark 1 Reactor, the accademic enterprise has so far not been developed, and indeed the UCB appears to be engaged in a R&D program for Mark 1 technology.  Carrying such a project all the way to building and marketing Mark 1 reactors is unlikely, but at a certain point, the Mark 1 will either have ti be dropped or produced.  If produced it is unlikely that the actual manufactur would be at a facility owned or operated by the University of California Berkeley.  Instead if theUCB holds on to it till it reaches the manufacture stage, the Mark 1 is likely to be built under a license either in the United States or in another country.  This approach would offer the UCB a revinue stream, without the problems associated with industrial production.

I could I suppose maqke many more comments on the Mark 1, but i will focuse on only one.  The Graphite Pebbles are themselves both interesting and perhaps the most important part of the Mark Even in the absence of fuel the Pebbles could very well solve a couple of big MSR problems.  The first is the limited lifespan of graphite moderators in MSRs.  raphite gets beat up by the Neutrons it moderates, and eventually graphite MSR cores ware out and have to be replaced.  Both Terrestrial energy and  horon recognixe this and plan to replace their graphite cores every 7 or 8 years.  Even though replacing the wornout core is believed to be inexpensive, it is still a large undertaken.  Tgere has to be a better solution.   I have for some time believed that graphite pebbles are the prefered solution to the limited graphite core lifespan problem.  true the graphite gets replaced as it wares out, but this does not mean that the whole core needs to be replaced.  

In addition to solving the core graphite problem of MSRs, UCB research shows that graphite pebbles offers a solution to the tritium issue.  One of the worse characteristics of FLiBe is that under neutron bombardment much of its constituent materials are transmuted into tritium.  Tritium is not a very dangerous actor as radioactive isotopes goe, but it does damage metals from which MSR cores are built.   Eventually tritium damage will distroy MSR cores.  But graphite Pebbles are excellent tritium eaters, that can lock in up to 99% of core tritium.  This would be a big help if one wished to improve core lifespan.  So capturing 99% of thew Tritium suspended in the core salts, is a nother one of those fortutunate benefits that often come with molten salts.

Another benefit of the Pebble bed approach, is to use the clean primary salt to directly transfer heat to the generation system.  Conventional MSRs feature fission in the primary coolantsalt.  But this creat problems as heat is transferred from the core to the power system.  In order to prevent the release of radioactive materials, in a coolant salt out of core leak,in Molten Salt Reactors heart is transfered from the primary salt, to a secondary salt.  If that salt is used as part of a tritium capture system, a tertiary salt system may be required.  Such complexity adds to MSR costs, as well as a loss of efficiency.  Thus the clean coolant salt of the PB FHR is desirable because it allows direct heat transfer from the core to the generating turbine.  Indirect heat transfer means an efficiency loss, as heat is lost during the transfer process.

One further advantage that might be derived from the Mark 1 design relates to the peak power potential of the GE open air cycle turbine boiler generation system.  This system couldopearate solely with reactor derived heat.  In that case the electrical output of the system would be around 100 Mwe.  With the addition of a modest amount of natural gas, the system can be rampted up to a peak power output of about 240 Mwe.  this is quite impressive.  If it is objected that that the use of natural gass is countraindicated in light of Global warming, there is a further option.  It is poszsible to produce CO2 neutral fuel from sea water by using either nuclear power or by using electricity generated by wind generators.  The Navy is considering such a system in order to produce jet fuel.  Fuel produced by this system, ether by nuclear power or through the use of wind generated electrricity, could be used to ramp up the GE turbines used with the Mark 1 Reactor, without creating a carbon penalty  RThe Mark 1 or a smaller sibling would fit very well into life at sea and would offer significant advantages.  Indeed the Pb FHR would offer advantages over the current use of LWRs in naval propultion,

Thus the Mark 1 and siblings offer a unique path forward for Molten Salt cooled nuclear technology.  It is not my fathers reactor, but through its use of FLiBe as a coolant and moderat9r, it pays homage to my father's work.  It is part of a new nuclear industry that has emerged since the beginning of the 21st century.  That industry is very different than the industry my father as a part of.  It is however an industry that has been made possible by the work of my father and many other ORNL scientists.


Andrew Jaremko said...

Charles - thanks for your posts and discussions of current work in advanced reactor design. The UCB salt-cooled pebble bed addresses something that I haven't seen discussed yet that I've been wondering about. As I understand the 'classic' MSR, the fuel salt is a uniformly mixed solution of fertile (U or Th) salt, fissile (U233, U235, actinide)carrier and eutectic salts that keep the mixture liquid in the operating temperature range. The salt mix circulates through the reactor core, where the neutronics (neutron moderation, reflection, and absorption) are right to make fission happen. Outside of the core, fission doesn't happen, but heat gets transferred into another salt loop and is removed from the reactor vessel.

Since all of the core salt has the same composition, that means that extra fissile is needed in the fuel load. How much extra depends on the relative volume of the core and the heat transfer loop. If the core has the same volume as the heat transfer loop, then twice the fissile load is needed since only half of the fuel salt is in the core at any one time. The core-to-total salt volume ratio is obviously a key design factor.

Keeping all of the fuel in the core and having only cooling/heat transfer salt in the heat exchangers minimizes the fissile required to start and run the reactor. This lets more reactors start and operate for a given amount of fissile material, which seems to me to be very desirable. Of course, there's a lot more to reactor design!

I want to see rapid expansion of nuclear heat for power and industry. I'd like to stretch the fissile as far as possible.

Charles Barton said...

It may be possible to keep the primary to secondary salt heat exchange inside the core of a MSR, in which case the fissil load will be kept 100% inside the core. Of course the heat exchange would have to be replaced every few years, but since the core of at least some MSRs will have to be replaced every 7 or 8 years, that should not be such a big deal.

Jim L. said...

Last I checked, the Li-6 and Li-7 in FLiBe each have pathways to create H-3 from the neutron spectrum. Thus there will be tritium in the coolant, requiring some way to capture or contain. And would likely dismiss the direct cycle for power generation. But I do believe the fuel pebbles will contain some tritium from fissions and decays, so it would be less than other MSR and LFTR designs. Or am I missing something?


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