Heatpipe power system (HPS) is a devise for producing near term, low cost testing with (electrically) heating test programs that can be carried out rapidly and actual flight units can be put through flight simulation reliably. Over the past 4 years three small HPS proof-of-concept technology demonstrations have been conducted and each has been highly successful. The Safe affordable Fission Engine (S.A.F.E.) and Heatpipe Operated Mars Exploration Reactor (H.O.M.E.R) are fast nuclear reactors that employ heatpipes to transfer energy from reactor core. For example the SAFE-400 reactor design is a 400-kWt that has been designed to couple with a 100-kWe Brayton power system (other designs use a Stirling power conversion unit). The SAFE-400 contains 127 identical Molybdenum (Mo) modules. A Mo/sodium heatpipe is at the center of each module that contains a Mo44Re annular mesh wick and Na working fluid surrounded by three Mo tubes that contain a Rhenium-clad uranium-nitride fuel pin. Fission energy is conducted from the fuel pins to heat pipes which then carry the heat to a heatpipe-to-gas heat exchanger. Module Fabrication: In some designs stainless steel tubes/tricusps bonded with nickle-alloy braze but higher temperature requirements of refractory metal core make it so hot isostatic pressing HIPing or electro-discharge machining EDMing could be used as attractive method of module fabrication. Another option is the core cut out of a solid metal block.

  Criticality of reactor is controlled via "Control Drums" composed of Nb1Zr-clad Be diameter 12 cm also clad with 1mm of Nb1Zr. The neutron absorber is 1.5cm thick B₄C contained in a 1mm thick Nb1Zr structure. This structure forms a 120 degree arc along the outside of the drums. The bottom of the drums will be aligned by a bearing on bottom of the radial reflector, and the top of the drum will be aligned by the control drive shafts. The drums provide enough negative reactivity worth to keep the reactor subcritical even when the core is flooded, and the nominal worth is such that the core can still operate if one drum remains stuck inward during start-up.¹

  There has always been and continues to be misconceptions about safety toward nuclear reactor power in space some concerns justified most not. Engineers sometimes don't always bother to explain these issues enough. "David Poston of LANL nuclear systems design group took the time to publish the paper", in "Nuclear Safety Calculations for Heatpipe Power System Reactors" some of which is excerpt here:

   Launch approval for space nuclear systems is provided by the U.S. Office of the President. The Interagency Nuclear Safety Review Panel (INSRP) provides and independent assessment that supports the Executive Branch decision. This approval process is well established and has recently been practiced for radioactive systems. The process will be the same, and in some respects simpler, for reactors, but with different technical issues.

RADIATION DOSE DURING OPERATION

  The issue of radiation dose operation is at best a secondary safety concern. The primary safety concern is to ensure that no one is close to the reactor while it is operating.

  A person very close to the reactor could receive a high dose rate if the reactor was operating at or near full power and if little material was between the reactor and person to shield the radiation. This scenario could pose a risk to engineers on Earth or astronauts on Mars. Therefore, exclusion zones must be setup to keep personnel at a safe distance while the reactor is operating. On Earth, shielding can be used to make the exclusion zone very small. On Mars, if the reactor is not fully shielded, the exclusion zone could be on the order of a kilometer; but additional shielding and /or surface features could reduce this significantly. A more detailed description of Mars surface shielding can be found in Wright et al. (2002) and a description of innovative ways to use indigenous materials (rock, dust, craters, liquefied CO₂ etc.) can be found in Houts et al. (1999).

  Astronaut EVA can be described as, how close an astronaut can approach the reactor during normal operations, and about how long the astronaut can move closer in special circumstances (maintenance, interesting science, etc.)

 In conclusion, no credible ways have been identified in which a member of the public could be harmed by radiation from the HOMER-15 or SAFE-400 reactors.² 

-INTERVIEW-

David Poston who for years has dedicated his life to the field of building nuclear systems for space describes his vision of nuclear space reactors in an interview conducted early November 2002.

Hello...David Poston of Nuclear Systems Design Engineering Applications Los Alamos National Laboratory. My name is Bruce Behrhorst Staff writer for the online publication Nuclearspace.com website. I would like to thank you for granting us the opportunity to speak with you today.

BB: What is a S.A.F.E (Safe Affordable Fission Engine) and H.O.M.E.R (Heatpipe Operated Mars Exploration Reactor) nuclear fast reactor and what practical applications do these dynamic high density energy systems have for space exploration and defense?

DP: What they are, are fission reactors that energy released during the fission process and that energy is conducted to heatpipes in these reactors. With heatpipes are closed cylinders that use two phased liquid metal heat transport to take energy from reactor core which has is fission heat out to your power conversion (PC) from your space reactor. So, there you would either have a heat exchanger to provide gas to a Brayton system or maybe the heat would go to a Stirling engine or Thermoelectric. It's rather versatile where the energy from the reactor goes. The bottom line is that you take the heat from the reactor send it to a power conversion (PC) system that produces electricity. Then this electricity can be used both for science, whether it be high powered telecommunications or transmissions from space or diagnostics as per for looking for certain materials or for subsurface radar mapping. A whole slew of missions for high power on planet orbiting satellites and deep space interstellar satellites and also for propulsion by using electricity to provide power to electric thrusters to enable taking high masses and high powers at fast speeds to the outer planets.

BB: Has a dynamic (as opposed to passive systems: thermoelectric RTG, thermionic) HPS (heatpipe system) type nuclear reactor ever been built before?

DP: We did do a demonstration where we hooked up a heatpipe reactor to a Stirling engine which is a dynamic system. And that operated as expected it's still I think, in operational state at JPL right now, it's gone thru like 50 start-ups and shut-downs. It's a rather low power system because that's all that could be afforded on the budget and all that was available for the Stirling. It was actually hooked up to an electric thruster out at JPL to demonstrate full intended power.

This was all done with resistance heated or electrically heated tests, which is a pretty deep subject. Let me touch on it first, initially is that the resistance heaters are put in place of fuel pins and they can produce the same power distribution through the core or power deposition through core as the fission process would. So, thermally as far as the system is concerned it doesn't know whether there is a fission reaction going on or if there's just resistance heaters are in there[in reactor core] up to just a couple of percent that's deposited elsewhere in the system due to transport of particles from the fission process. But overall you could essentially demonstrate your whole systems with these strategic resistance heated tests. So, therefore you get a high fidelity test without having to go to the expense of actually operating the fission powered test.

BB: What in your estimation would be a first in-space NEP (nuclear electric propulsion) mission of the improved generation of a "Dynamic nuclear space reactor", and will the HPS type reactor be the working design for future reactor missions?

DP: To answer the mission question. I'm more in the mode of demonstrations is the best way to go for flight demonstrations. Because, fission can be so important to the future in all the things that it can do, that my guess would be to get as simple a first mission as you can get. So, it's a two sided sword because NASA or whoever the customer is has to make this mission "Sporty" enough to sell it and basically fly the mission. So, it's in NASA's hands on the mission side, and what we can tell them on the nuclear side is: Try to come up with a mission with the simplest requirements as possible in terms of power, lifetime and mass that we think we can complete because if we try to take too big of a step. We run the risk of failing as past programs have done over and over because they've tried to take too big of a first step. So, by going with simpler requirements for the first mission then that would enable the success of the first mission. I think the range of missions are from, like a Mars or Lunar surface power reactor which would be the easiest from the fission reactor's perspective to maybe a Mars Tele-comm. Orbiter and then the mission they're talking about now, is like a Jovian-moon tour mission which is a little bit "Sportier" that might be achievable and these can all be achieved with the Heatpipe system.

BB: Now, that three or more proof-of-concept demonstrations have been successful could you describe the component parts of the basic SAFE nuclear space reactor?

DP: One of the advantages of the SAFE reactor is its lack of components, the core module contains a heatpipe and fuel pins and generally there are three fuel pins bonded to a central heatpipe. So, the fuel pins conduct the energy to the heatpipe which takes heat energy out to the power conversion system. So, it's the fuel pins and heatpipe, which is a module, then the grid-to-core which is the assembly of these modules that can produce a critical chain reaction of neutrons or a self-sustaining safe chain reaction, to keep a self-sustaining power source. Depending on the design between like 19 and 100 of these identical modules configured in a core. Basically, your core is just the modules made up of fuel pins then some sort of banding structure to keep the modules in place then around that you have a neutron reflector. Which reflects in neutrons that keeps them from leaking so that you can sustain your chain reaction. If too many neutrons leak out you can't sustain or produce a neutron, for everyone that's generated. In your reflector you also do your control, most of the designs have "Control Drums" which have a neutron absorber. If you wanted the reactor reactivity to go down, you turn this absorber toward the core the reactivity goes down there will be less neutrons produced per generation...

BB: This would be the Boron (B₄C) [neutron absorber]?

DP: Yes, so we can control reactors externally which is what helps make them very compact, safe in accident scenarios to follow up in another question. Besides the core, reflectors and control, then depending on the power conversion we might need a heat exchanger and for the Brayton system which seems to be the front runner right now heatpipe to gas heat exchanger that we've designed, building and testing as a stainless steel variety-right now. Then you need the motors for turning the control drums and a shield and a [power] control system [PID, fuzzy logic]. In terms of components there aren't very many.

BB: To follow up on the last question Molybdenum has a high melting point in the thousands of degrees Kelvin some of the fuel pin/heatpipe modules they get hot. Just out of curiosity, how would you be able to turn control drums of the reactor under those heat stresses?

DP: The [control] drums are outside of the core that's really what makes this easy. Because if you do have control in the core it's gonna get hot. The drums and control motors and all those pieces will be insulated from the core. Between the core and the reflector we'll insulate to whatever level we need to with multifoil vacuum insulation to keep the temperature the reflector and the control drum down to 800-900 Kelvin.

BB: I seem to notice, you suggested upper limits of maybe 1000 degrees Kelvin plus. Most of the normal operating procedure is well under 1000 degrees K range?

DP: It is, basically you have to make sure your parts don't distort due to the temperature. The motors will be way above the shields and shielded from radiation as well - they'll be cool. The temperature up there won't be an issue. So, it's really an issue to drums, it varies as to what the temperatures are.

BB: Could you explain why there is a difference between a HOMER reactor's radial absorber and that of a SAFE reactor among other differences?

DP: The two main differences are the HOMER's operate at a lower temperature Boron carbide is easier to use there because we're not so worried about chemical interactions at temperature and perhaps the more important reason, its stainless steel is a lot easier to machine then Molybdenum [Mo] or Nb1Zr or whatever refractory metal for SAFE 400. We thought we could make holes for the Boron carbide in a stainless steel system relatively easily where, it would take some development to make holes Boron carbide [B₄C] in the SAFE. For those two reasons, the temperature capability and the fabricability that took us away from [use of ] Boron carbide for SAFE even though it's a more mass effective option.

BB: Have you decided how the reactor body core would be, a solid metal block or components [parts]?

DP: Yea, that's an on going debate. Until we do more detailed engineering and fabrication it probably won't be decided. We like the individual modules because that makes your testing and development a lot easier, because your building a bunch of modules small modules you can test them easily put them in a test reactor and pile reactor test. You also have the ability to chose, go through and find ones that don't look good, throw them out and bring in ones that fit nice into the core. The modularity is great from the perspective but the solid block "monolith' does have some heat transfer advantages. If we can make that relatively easily that might be the best way to go. It's a kinda complicated issue, I guess it's unresolved but we're sticking with the modular design at the moment. 

BB: In your view, is the SAFE reactor's 15 -1000 kW_(th) range a precursor to futuristic high performance nuclear space power systems like: Vapor core "Lightbulb" concept, external pulse "Orion", other fast reactors, fusion and antimatter methods, do you feel a 'step-by-step' approach to an introduction of nuclear power in space is a good plan?

DP: Yes, I think it's a precursor to any of those. The main thing we have to deal with is how to engineer systems to handle high energy densities or high power densities and no matter what your going to do really, the key is just stepping up with knowing how to deal with these systems. So, I see a stepping point to higher power fast reactors and then in the future, many of the technologies you mentioned - yes.

BB: What is your preference in electrical rocket thrusters i.e., Hall thrusters, VASMIR, pulse inductive, Hughes Deep space 1 etc.?

DP: I don't focus too much, but I tend to like things that are proven as most people do. That for the near term missions, going with ion thrusters is the way to go. VASMIR is a great, great concept that needs...more work but I like the potential it offers. Again, I'm not close enough to an expert to really talk about those issues.

BB: In light of U.S. House Appropriations committee recommending a decrease of $10 million from the nuclear electric propulsion program and a decrease of $ 7 million from the nuclear power system program, is LANL's nuclear space reactor research funded well?

DP: No, it's not well funded, we have been working on internal funds, nights and weekends in a lot of cases to get to where we are now. And we still have an internal funding source and some funding from NASA and DOE. The new Nuclear Systems Initiative [NSI], there's going to be a step change in effort nationally and hopefully at Los Alamos. The reductions... we're not concerned about those as much as the continuing resolutions - to be frank. Any new "Start" program that makes it through the house and senate is great, there almost always cut. The actual dollar amount - we're not as concerned about. Because we're confident we can have a program that can demonstrate early on that we're going to be successful so, we'll still be able to sustain the program.

BB: You seem to be developing these reactors for a planetary on site NEP system or for in-flight NEP systems. Do you ever venture into direct hydrogen heating for thrust [NTP] systems?

DP: Yea sure...I started out in this field looking at direct nuclear thermal propulsion and came to the conclusion that it's a really second generation technology or at least a much harder to develop technology. So, that's when I decided to focus on something that I felt we could really do and be highly confident we could complete a program. Because nuclear fission power or any kinda nuclear power besides radio isotope in space is just been burdened by a lot of interest but no one ever completing a successful program. So, I focused on something that was as-easy-as you can do, but still do something useful.

BB: As recently as the first week of this month NASA has awarded the first in a series of nuclear space initiatives to the Boeing company. Boeing energy systems and rocketdyne propulsion and power have teamed up with JPL/Glenn Research/Honeywell/Swales Aerospace, University of Auburn, and Texas A&M to develop specialized power conversion (PC) unites. How do you feel having these Stirling or Brayton PC units attached to HPS Heat Pipe space nuclear reactor Systems?

DP: My first preference is which ever one will work and highest efficiency. From what I know about thermal electrics. We can make those work. They are working on higher efficiency ones. The Stirling work recently has been very good, Stirling technology and Glenn Research Center have done good job with producing actual engines and then the Brayton work also has a lot of industrial support behind it. Thats the great thing that's happened over the last 10-20 years - really, is that there hasn't been a whole lot of progress in space fission reactor per se. There has been advances in power conversion. It's making the job of the reactor designer easier which is exactly what we need to get something successful on the first mission.

BB: You mentioned traditional Stirling PC units in some of your publications [papers]. Is thermal acoustic [technology] workable?

DP: Thermal acoustic is a great potential application, it's a little bit early to decide whether, if its actually do-able for a mission in say 5-6 years. In theory, on paper and also with some of the experiments they've been doing I think, it's a good potential long term option.

BB: What's it like being a nuclear space scientist since very few people work in this field? How did you arrive at your level in your profession?

DP: It is the greatest - I love it. I became a nuclear engineer when I saw all the great potential nuclear energy can provide both on Earth and in space. I went to work at General Electric Co. I got exposed to space nuclear power and decided that was a really good thing to commit to because I could do so much good for humanity. In terms of expanding into space. I decided I would make it my goal to pursue this field and get a Ph.D. in this field and come to Los Alamos Labs. Which in general is the leader in this field since it was initiated in the 1950's. I love coming to work everyday, I couldn't be happier in my job and it shows in working a lot of extra hours. Most people would enjoy...we bring in summer students. I brought in several summer students in the past years, they're always very enthusiastic and excited about this field.

So, it's something that I would encourage young people to think about it's probably one of the most exciting areas to work in. 

BB: Do you have a name or a cartoon mascot for your HPS designed nuclear reactor?

DP: No, we do for the "Homer" reactor it has a four faceted name.

BB: Like Homer Simpson the cartoon character?

DP: Yea...Homer Simpson is one because basically that's just simple and robust (giggle) that's the attributes we take from Homer Simpson. Then an implication of a "Home run" with "Homer" is a positive connotation we like. Then Homer in terms of the Greek author with the "Iliad and the Odyssey" exploring unknown worlds is a very forward looking connotation. Then Homer Hickam in the movie, "October Sky". He's based on dreaming about spaceflight in that movie. He has actually come to visit our hardware [lab]. So, it was a perfect choice because of those four reasons. The S.A.F.E. Acronym, there wasn't as much thought put into that one, actually NASA came up with that acronym - we don't have any.

BB: Would you like one?

DP: Sure, yea.

BB: I'll send you a little graphic character for [SAFE reactor] that.

BB: Having a perfect record of U.S. Launches and successful space operation of nuclear powered sources in space (there's only been one SNAP 10A, it had to be shut down for reasons other than reactor core malfunction) and over 45 years experience in the field isn't this proven technology?

DP: Yea, it's definitely proven technology. As far as the reactor goes, nothing we're doing hasn't been proven for forty years. I mean, the power conversion side is debatable depending what you chose is proven technology. As far as the fission goes it's actually very simple: You put Uranium together in a certain geometry and get fission power out of it. There is nothing magic about it.

Safety wise: The two key points that get across is before this reactor 'turns-on' it's not radioactive, it's typical to Background radiation.The true statement it's equivalent to background radiation.

So, that's the first point, that this thing is completely safe before it operates.

Part 2 of the question: Is you don't want it to operate before it's suppose to. So, that's done by using redundant controls similar to almost any potentially hazardous engineering system, to insure it doesn't 'turn on' and insuring during any postulated accident whether it be transported or launched it won't 'turn-on' or become 'critical'. And then at that point [when you want to] it produces radioactivity, becomes radioactive. But, even in the cases it does 'turn-on' if you look at the probabilities of events it's still not a significant health risk to any personnel or the public.

It mostly stems from the general phobia you might call it; of nuclear systems. I think on paper and in practice these systems are incredibly safe.

BB: From what I have surmised the Boron poison drums are essentially pointed in next to the core [central axis] in 'Shut off' position [mode] and then they turn outward [from central axis] 'in operation' mode. You use two types of fuels UN and UO₂. When in space the fuel is actually inserted in the core block?

DP: No, we're launching it with the fuel in the core. Yea, but that's a back up option. You can save a lot of mass. You can make these systems lighter if you could keep the fuel outside of the core until you want to use and insert it. But, it's not worth reliability impact of having to insert the fuel to save the mass. Because right now we're putting up more on Uranium in the core. That's making it a lot heavier because then we have to put in more fuel to offset the absorption of those materials. It's kinda a positive feedback situation, where the more fuel you add the more poison you have to add; then the more poison you add the more fuel you have to add. We could save mass by launching the fuel, say in a canister next to the core, then inserting it when we reached our safe destination. I think it's a more prudent choice to make the reactor safe so that it won't 'turn-on' even with the fuel in the core until you turn those drums out.

In closing I asked him about print or online materials on more work being done in this area. He said at the moment they just don't have it in the budget to place current information online.