Space Fission Power Post #11:Update -- Some Progress and Lessons Learned

So, this “blog” has turned out to be mostly an archive – that’s really the intent, but I’ve just noticed that it gets no love from the Google search algorithms, probably because there’s nothing new ever said.  Actually, there have been more recent hits from China and India than the US.  I’d hope that the US would lead the charge to becoming a space faring civilization (less and less likely to be the government, while there’s increasing hope in entrepreneurism), but if other countries are interested that’s great.  Overall, nothing in the message has changed significantly; I would write some things a little differently today, but maybe it gives the message more integrity if I leave it as written 2.5 years ago (i.e. I am not spinning for today’s prospective audience – actually most of the key references are 5 to 10 years old).

When I wrote this blog (I’m sticking to my guns on that term), I was at a crossroads of whether to stay at LANL or go somewhere where I might have a better chance of getting fission power established in space. I had signed the paperwork to accept a severance package, but my Division Leader came by 15 minutes before the rescission deadline and said the Lab would fund an experiment that David Dixon, Pat McClure and I had proposed. I faxed in my rescission form 5 minutes before the deadline, and 6 months later the DUFF experiment was completed, and now KRUSTY is proceeding.

Update on Low Power Space Reactors

DUFF (Demonstration Using Flattop Fissions) was an amazing success and the highlight of my career thus far (choose your favorite meaning... Homer's favorite beer, Dixon's middle name, or getting off your duff).  For less than the cost of 1 LANL FTE (~$600K), we were able to conceive and pull off the first fission power system test within the DOE (if you’re waiting for the proviso, e.g. in several years or within the last decade, there isn’t one – every power reactor that has operated within the DOE was completed or under construction before DOE was formed in 1977.  Now, DUFF was clearly a very simple test, but that’s by design – we had to prove to NASA, DOE, LANL, and especially ourselves that is was still possible to do anything “real”.  

DUFF was a rudimentary test of a fission power system which linked a compact fast reactor with Stirling converters via heat pipes (DUFF_Final_Report). Other “firsts” were the first test of a reactor that used heat-pipes to transfer fission power, and the first fission system to using Stirling energy converters.  In addition to simply producing electricity (lighting the panel) with fission energy, and numerous lessons learned (DUFF_Lessons_Learned), the thing that impressed me most was how well the experimental results matched the codes that others (MCNP) and I (FRINK) have written. Prior to DUFF I had always wondered in the back of my mind if we were fooling ourselves with our confidence in our models (we still may be, but it’s much less likely than before).

KRUSTY (Kilowatt Reactor Using Stirling TechnologY) takes DUFF to the next level, and very close to prototypic of the proposed Kilopower reactor (Kilopower_Rx_Concept). The entire core, structure, heat pipes, and power conversion will be flight prototypic, including the power levels and temperatures. If it succeeds there will be no questions about reactor operation and dynamics; the key remaining risks will be lifetime risks with the materials and components.

When we were proposing DUFF 2.5 years ago, the vast majority of people we told said we were nuts (stupid, naïve, etc.), and those were the people we trusted enough to tell.  In retrospect, if we had operated business-as-usual they would have been correct.  We basically plowed forward without taking no for an answer.  We had daily obstacles that threatened success, but instead of pausing to rethink what we were doing or why, we took each obstacle head-on and stuck to our goal of light-the-bulb-or-bust.  In the end, it turned out that almost every obstacle was not actual policy and regulation, it was people’s long standing expectations and interpretations of policy and regulation, which had morphed into a potentially encumbered bureaucratic process (where there is no real incentive for a decision maker to approve anything new or different).

One of the many reasons DUFF succeeded was by keeping the team small.  If we had gone to the DOE office that “specializes” in space reactors, at best they would have said we need to spend millions to ensure that we’re testing the right system (i.e. trade studies) and millions more to prove that the test would work (because failure might make them look bad). Why not spend far less to simply see if it works?  I think the answer is that wasting millions on a paper studies cannot get a bureaucrat in trouble (in fact the more money that flows through their office the better), while making a decision and taking a risk can; note:  I’m not talking safety risk here, we spent the time and money to meet all safety requirements within our $600K, it’s that “decision makers” these days get kudos for compliance and not upsetting the apple cart rather than accomplishment, so there’s little incentive to do anything new.  DOE has a “process” for doing things the “right way”, which requires that every decision is backed by reams of paper and the consensus of grey beards.  The aforementioned DOE office claims we are “cowboys that will eventually mess-up and therefore kill space reactor interest for good”; the grand irony is that this office has done absolutely nothing to bring life to space reactors, and if it wasn’t for a few cowboys there would be life to kill.  Thankfully NASA and NNSA are very supportive of KRUSTY  Better yet, the KRUSTY team from GRC, MSFC, Y12 and LANL is perfect for the job (lean, driven, and expert in their fields), so if we can navigate the bureaucratic/political obstacles then we might have an operating prototypic space reactor in <3 years. Finally, the “market” for 500 W to 1 kW fission power systems may be growing, as it is becoming clearer how expensive future Pu238 will be (assuming that the capacity factors of the ~50 year old reactors can meet the goals for Pu238 production in the first place).

Update on Surface Power

What I’ve learned over the past 2.5 years has made me favor the heat-pipe (HP) reactor even more as compared to the pumped-liquid-metal (LM) reactor.  Even back then, I thought the choice of a pumped-liquid metal system for FSP was ill-advised. I preferred the HP reactor, based on my initial 2005 pros and cons list (FSP_Cooling_Pros&Cons), but agreed to go along with LM because I was assured the ALIP pump was “proven”, plus the LM reactor does indeed integrate better with Stirling engines at the FSP power level.  The latter may still be true, but I think KRUSTY/Kilopower will help alleviate some integration concerns, and at higher Stirling powers a boiler (operating with gravity) should be lower risk than a pumped-loop.  More importantly, the electromagnetic ALIP pump has performed so poorly that it provides a major question mark for the future. The pump needs an efficiency of ~15% to design an attractive system (~10% to have a fighting chance), and the actual pumps have run at <5%; fortunately some testing can proceed (e.g. testing of the Stirling converter) due to the existence of wall sockets, although at efficiencies that low, the pump could heat the flow as much as the reactor simulator.  

In addition, the LM selection for FSP was for a lunar mission, whereas emphasis has subsequently changed to Mars.  A Mars vs Moon comparison shifts some of the pros and cons discussion between technologies: a) long transit time brings the substantial NaK loop freeze-thaw issue into play, b) the pumped system needs considerably more startup up power, which will be tougher to provide for a Mars mission, c) the long communication delay is more significant for the pumped system because startup/control is more complex, d) increased gravity makes a HP-to-Stirling boiler HX look much better, and e) atmosphere increases HP system reliability because CO2 can fill gaps between HPs and HX (to provide gas conduction) if the braze fails. 

Of course another big thing that has changed is the success of DUFF.  The “no one has ever operated a heat pipe reactor before” opposition is gone; I don’t think that DUFF really solved a major unknown, but it removed this perpetual red herring (why it was a red herring is explained elsewhere in the blog/references).  The other exciting development is the explosion of 3D printing (additive manufacturing), which has quickly moved beyond the plastic regime to high temperature metals.  This could help all concepts, but clearly benefit the HP reactor the most.  If you look at aforementioned pros and cons list, manufacturing was one of the major disadvantages of the HP reactor.  3D printing might effectively eliminate this disadvantage, for the core block, HP integration, and perhaps most notably a HP-to-gas heat exchanger.  A traditionally fab’d HX would have100s of welds (doable, but with potential failure modes), a 3D printer could do it with none!

Lastly, politics and policies have continued to evolve that make dealing with HEU (highly enriched uranium) increasingly more costly and with more programmatic risk.  In my opinion, the US is wasting several billion dollars a year in the name of safeguards, but I’ll leave that blog for someone else to write.  Certain space reactor applications will always require HEU to be attractive on a mass basis, but others could be revisited.  In general, the higher power the reactor, the lower the penalty of going to LEU – still a major mass penalty for a fast reactor, but maybe less than a factor of 2, as opposed to in some cases a factor of 4 or more.  I am also sticking to my guns on avoiding moderated systems (hydrogen bearing) like the plague, although in some cases some spectral softening via BeO or graphite could make some sense (but I’d bet against it).  

A high-power Mars surface reactor, say 1 MWt (~200 kWe Brayton), might be in the range where LEU would not create a show-stopping mass hit, while it could remain near the “sweet-spot” I’ve previously referred to in development (because of the increased fuel inventory, lower power density).  In this power range the trade would be between a HP reactor and a gas-cooled (GC) reactor – my gut tells me the HP would be lower risk, but the GC would have to be seriously considered (assuming any lander would be impressive enough to handle the mass of an LEU GC reactor).  The lowest-risk system might be a low-temperature Brayton (~900 K) with a SS/UO2-HP reactor (maybe UN and/or maybe super-alloy if the potential loss-of-ductility risk was worth another 100 K).  We’ve considered >1 MW HP reactors in the past for use on Earth (MW_HP_Reactors), and while they might provide several headaches during construction, MW Mars surface HP reactors would be the easiest to qualify (without a GNT) and offer very high system reliability.  If a high temperature Brayton (>1100 K) was available, then I might consider the GC concept that we designed for JIMO – a Nb1Zr/Re/UN core with a super-alloy pressure shell. The need for a higher-temperature concept would depend mostly on mass limitations and the difficulty of deploying large radiators on the Mars surface.

Update on Nuclear Thermal Propulsion (NTP)

I have spent a lot of time over the past 2 years designing NTRs for NASA.  Honestly, there nothing more interesting to work on as a reactor designer; attempting to balance neutronics, temperatures, peaking factors, flow rates, Mach numbers, and pressures while pushing the materials to their limits.  The fact that they were able to build and test some of these machines in the 60s is truly astounding – the people working on ROVER/NERVA were the super heroes of nuclear engineering!  However, now that I am even more familiar with what was actually accomplished, I am even more pessimistic about the ability to successfully develop an NTR in today’s environment.  The old photos and videos might make NTRs look like a done deal, but in addition to not solving fuel erosion after repeated nuclear test iterations, they never got to the point of testing an NTR that was prototypic to the proposed flight system.  Also, the closest-to-prototypic rocket test (XE) had a substantially lower pump outlet pressure and chamber temperature than the designs currently being touted by NASA and DOE (proposing 1900 psi/2750 K whereas XE was 970 psi/2280 K, actually <2100 K net if you include the turbine exhaust); the Peewee reactor hit an impressive 2550 K (still a ways from 2750), but did not integrate this performance with potential engine dynamics.  More so, the XE test did not include the dynamic effects of returning turbine exhaust to the system, and more importantly the effect of moderated tie-tubes, which have extremely large neutronic feedback coefficients (the hydrogen worth is huge, especially at the proposed pressures).  I’m fairly confident in these “facts”, but not 100% sure, so if I’m wrong on something let me know.  As for my opinions, think about this… in the 60s, when nuclear testing was orders of magnitude simpler/cheaper to pull off, they performed more than a dozen nuclear tests and still hadn’t tested a prototypic system or solved fundamental problems with the fuel.  Then, add that the previous testing was at substantially lower temperatures and pressures than what is currently being sold as a heritage technology.  My opinion is that a cermet-fueled NTR would have a better chance of being developed that a composite-fueled concept (for various reasons that I won’t expand on now), but both should not be considered at all until we take several successful steps in developing simpler space reactor systems; although by then it will probably make more sense to pursue NEP than NTP regardless.

Finally, I've been dismayed as proponents of NTP and NEP have overemphasized the risks of radiation in space. In response, over a year ago I wrote a response entitled "A Trip To Mars Reduces an Astronaut's Risk of Cancer" (Space_Radiation_Risk).