Post # 80: A SmAHTR Approach To Nuclear Energy?

A couple of years before I left ORNL (in Sept. 2011), I had the privilege of assembling a small team of very bright engineers to tackle an idea I had for a small, high- to very-high temperature nuclear reactor system that would lend itself to distributed generation of process heat and electricity.  The reactor was to be usable both in single-unit applications and in clusters (much like NuScale and mPower small modular light water reactors) to meet higher energy demands.  The concept was to integrate the best features, technologies, and system architecture elements from ORNL's historic Molten Salt Reactor Experiment (MSRE) and Molten Salt Breeder Reactor (MSBR) concept, gas-cooled reactor graphite fuel technology, ORNL's (then) recent Advanced High Temperature Reactor (AHTR) fluoride salt-cooled reactor concept, and system topology features from integral fast-spectrum liquid-metal cooled reactors. 

Working with a multidisciplinary team of engineers, and with very limited (internal laboratory) funding, our team created a new concept we called SmAHTR (for Small modular Advanced High Temperature Reactors).  SmAHTR can be described as an Integral Salt-Cooled Reactor (iSCR).  Though we did not know it when we first began work on the SmAHTR concept, we subsequently learned our Russian colleagues at the Kurchatov Institute in Moscow had developed and published in 2002 a concept for a very small, < 20 MWt, integral, liquid salt-cooled concept they called MARS.  SmAHTR and MARS share some design similarities but have some significant design differences as well. 

The basic design requirements for SmAHTR were: 125 MWt power, ~700 ºC core outlet temperature, and an integral system topology (no coolant loops).  Additionally, the reactor had to be transportable over public roads with common heavy-transport multi-axle semi-tractor-trailors. The system also had to be extremely safe and easily refueled and maintained.  The inherent safety attributes of the system are a result of its very low (~ atmospheric) operating pressure, forgiving nuclear dynamics, large thermal margins, and the use of a coolant that doesn't chemically react with air or water in highly energetic modes.

 The Small modular Advanced High Temperature Reactor (SmAHTR)

The driving force for SmAHTR was our belief that high-temperature salt-cooled reactors would offer superior economics to gas-cooled reactors; would open new doors to nuclear process heat applications; and could be more quickly developed, demonstrated, and licensed that fluid-fueled molten salt reactors (MSRs).  SmAHTR's operating temperature would be limited by current structural material considerations to 700 ºC (probably a little lower in initial implementations), but the basic concept could evolve to much higher temperatures when and if superior compatible structural materials are developed (a long-term proposition).  Fluoride salt-cooled reactors share many materials and component technologies with molten salt reactors.  Thus, in addition to providing a potentially-game changing nuclear energy system, successful development of SmAHTR would resolve many of the technological challenges faced by MSRs as well.  (Those of you who are interested can access the SmAHTR pre-conceptual design report, ORNL/TM-2010/199, here.)

Incidentally... building and working with high performing, innovative teams was one of the activities I enjoyed most during my years at ORNL.  My experience with the SmAHTR team was a particular joy.  Ever team member contributed to the concept.  For instance, Jess Gehin suggested the idea of adapting the old MSBR plate-type moderator assembly for use as a graphite fuel element.  Venu Varma engineered the innovative "bayonet loading" concept we adopted to provide quick access to the various components that load through the top of the reactor.  I could go on...  It's a real thrill to build a great team and be part of its workings...

The Department of Energy and ORNL have done little to move the needle on the SmAHTR concept since I left ORNL.  However, ORNL, with funding from the Department of Energy, has moved forward to integrate some of the best features of SmATHR into the large gigawatt-class AHTR concept, and to continue some critical fluoride salt-cooled reactor technology development.  Time will tell whether someone sees sufficient merit in SmAHTR to further mature the concept.

However, others are building upon the approaches we pioneered with SmAHTR.  I understand Dr. Per Peterson and his team at UC Berkeley are investigating integral versions of their pebble-bed salt cooled reactor concept (which is larger than SmAHTR and originally employed coolant loops).  And just this week, I learned my colleague Dr. David LeBlanc has gone "back to the future" with our SmAHTR concepts to create an interesting small Integral Molten Salt Reactor (iMSR) concept.  The concept (actually two concepts – a 650 MWt and a 60 MWt "ultra-small" version) are discussed in an April 12 posting on the Weinberg Foundation's website.  While retaining many of the general system architectural and component features we specified in SmAHTR, David has discarded SmAHTR's solid graphite fuel and reverted to a liquid fluoride salt fuel.  Since I haven't seen any design details at this point, I'm withholding judgement regarding the engineering viability of the concept.  But from the overall philosophical perspective, David's concept appears to be the most innovative and fresh MSR approach I've seen since the heyday of MSR development in the 1960's and early 1970's.

Both MSRs and salt-cooled reactors (integral or otherwise) face many, many challenges in moving from a pre-conceptual design to an actual prototype system.  But some very bright and passionate folks are expending considerable energy in that direction.

Sometimes it's enjoyable to consider "the road not taken" in nuclear energy.  Alvin Weinberg would be smiling...

Note to cynics:  Everyone in the nuclear energy business is familiar with Admiral Rickover's famous comment about "paper reactors".  Though obviously founded in truth, in my view, far too many have too often used his comments as the "nuclear option" to stymie any serious discussion of real innovation in the nuclear energy field.  So please spare me the comments about paper reactors... Believe me, I know the prospects of developing a new reactor concept in today's environment are remote.  And the prospects of developing a high-temperature fluoride salt-fueled or -cooled system are further impeded BOTH by overly pessimistic AND overly optimistic urban myths and legends from the MSRE days.  I don't own or wear "rose-colored glasses".  The first iSCR or iMSR won't come easily, quickly, or cheaply.  But the payoff could be significant if the challenges can be successfully overcome.

Just Thinking...
Sherrell