It’s often easy to overlook the origins of a newly popular piece of technology, and NASA’s new nuclear reactor is no exception. Remember, NASA doesn’t (or wouldn’t) build these reactors, the Department of Energy does. The DOE side of things has been largely ignored in the mainstream media, which focus on the flashier and more PR-conscious NASA, but NASA doesn’t build reactors, the Department of Energy does. On the DOE side, this reactor is the direct result of an experiment carried out in 2012, called the Desktop using Flattop Fissions, or DUFF.
This experiment was conducted by Dr. John Bounds of Los Alamos National Labs, at the Device Assembly Facility (DAF) at the Nevada National Nuclear Security Site (formerly the Nevada Nuclear Test Site). This facility has been a keystone for nuclear technological development of all sorts since the early days of the Atomic Era. Long after the weapons tests, reactor tests, and the US nuclear thermal rocket programs of the 1950s through 1970s they’re most known for to today’s quieter work with actinide irradiation studies, this facility remains a keystone in American nuclear technology development.
In a conference paper for the Nuclear and Emerging Technologies for Space 2013 (NETS 2013), Gibson et al point out that there’s a gap between the designs that have been used for Radioisotope Thermoelectric Generators, (RTGs) and the designs that have been proposed for fission power systems, or FPS. This gap also happens to coincide with the available testing facilities, and the ideas they proposed were not new, just untested. To improve the design, the team chose a simple, fundamental issue with the proposed reactor, and worked with what they had on hand for everything else. DUFF was focused on a novel (for nuclear reactors) heat rejection system: the heat pipe.
The heat pipe is a simple, attractive way of getting rid of waste heat. By using thermal differences and phase changes in a working fluid, a heat pipe can transport a large amount of heat in a simple system that has no moving parts. At the hot end, the working fluid evaporates after coming in contact with the hot casing material, and flows to the cold end. At the cold end of the heat pipe, it condenses onto a wicking material, which then carries the working fluid back to the hot end and completing the cycle. Most of the energy transportation that occurs actually happens in those phase changes, rather than the movement of the working fluid itself. The type of working fluid, and the wicking and casing materials, are defined by the operating environment that it’s going to be in, especially the amount of heat being transported and the temperature of that waste heat. While heat pipes may seem like a peculiar means of rejecting heat, and they’d never been used in a nuclear reactor before this test, most of us use these types of devices every day. The CPU of the computer you’re reading this on is cooled by a heat pipe (even if it’s a phone), as are a huge number of other electronic components. They can operate from very cold temperatures (such as a helium heat pipe), to very high temperatures, such as aluminum. For KRUSTY, the plan is to use sodium potassium eutectic (NaK) for the working fluid.
By replacing pumped coolant with heat pipes, you’re changing more than how the reactor is cooled – you’re also changing how the neutrons in the reactor behave. Any structure in a reactor is going to affect the behavior, or dynamics, of the reactor system. After all, slowing neutrons down isn’t the only way to affect the behavior of neutrons in a reactor, they can be reflected as well. Depending on their chemical and isotopic composition, most materials will either reflect (at a lower energy level), slow, or capture neutrons. This is something that can be modeled with incredible accuracy, but you still have to have the real-world test article to show that it works. This test article was DUFF.
DUFF used an existing test rig for the criticality portion of the experiments. Several were available at the facility, but each required compromises. The chosen test rig was the Flattop criticality test rig, a workhorse of nuclear testing.
Dr. Dennis Beller, a research professor in nuclear engineering at the University of Nevada, Las Vegas, posted this about the choice:
Flattop is one of four old [ed note: built in 1951], much used critical assemblies that were moved from the Criticality Experiments Facility (CEF) at LANL Technical Area 18 to the Nevada National Security Site’s National Critical Experiments Research Center (NCERC) several years ago (at the time of the move it was NTS and CEF, both later renamed). These assemblies (others are Godiva 4, Comet, and Planet) are used for criticality benchmark experiments, cross section measurements, DOE/NNSA’s hands-on criticality safety training courses (I was a recent student), and a variety of other training and research projects. Flattop, which is a uranium-reflected highly enriched (233U, HEU, Pu, or other) sphere, is unique in that it can be operated super critical to produce an internal temperature of about 300 C (not quite what one would expect in a power reactor “prototype”). Flattop also has a hole through its center that permits insertion of experiments or other actinides, in this case a heat pipe that was built specifically for this test (I don’t believe it’s a space power prototype either, but someone from NASA or LANL might disagree [ed note: it’s not a prototypic test article, just a proof of concept]). In addition, the purpose of the heat pipe is not to cool the reactor (LANL’s words, not this authors) although it does remove a tiny amount of the fission power, it is to transfer energy to the Sterling engine so it will produce electric power.
DUFF was a perfect example of the kind of compromises that are taken in an in-space nuclear development campaign: a test article, using different materials than the flight article would (page 9), to demonstrate that the principles of operation were sound, and that nuclear testing could be done more affordably under the current regulatory regime. To get good data, you need a well-characterized system like Dr. Beller describes above, and it’s easier on the budget as a rule to design your test article to the testbed than the other way around. The other advantage to using an existing criticality test rig is that they are usually very heavily studied and very well understood, so that the effects of the particular test equipment can be more easily isolated and studied.
DUFF wasn’t focused on the reactor, remember, it was focused on the heat pipe. Because the power output from Flat-top is so low (700 W, estimated max temp 300 C, steady-state operation at 200 C), they weren’t able to use a sodium heat pipe, as the final reactor would use, because it evaporates at too high a temperature. Instead, they looked at other options that had lower boiling points, something that limited their choices greatly. After testing two options, they settled on water as the working fluid (Dowtherm A was the other option looked at, but it wasn’t able to transfer enough heat). After selecting the working fluid, the casing and wick needed to be decided as well. After testing at NASA’s Glenn Research Center, it was determined that the best option was to use a sintered nickel wick (200-mesh) and a 316L stainless steel casing, although other mesh sizes and casing materials were tried. Since this wasn’t going to be a flight article, the fact that this was made out of steel and nickel didn’t matter: the test stand didn’t care about every ounce of weight like a spacecraft would.
The final part of the system is the Stirling converters, and once again Dr. Bounds and his team used an existing piece of equipment to both increase the certainty of the measurements and decrease the cost of the test. This was a challenge for a number of reasons. While Stirling conversion systems had been researched for in-space use before, the vast majority of the time these were much higher-power units, requiring a minimum of 200 C hot-end temperatures to operate. This was still 50 C higher than the worst-case hot-end scenario for Flattop, so these systems weren’t an option. Instead, they went with one of the only options available, a Buzz convertor. By cooling the cold end down to -50 C, there was enough temperature difference to produce power (although definitely not net-positive electrical power!). As Dr. Marc Gibson noted,
Although the Buzz convertors do not represent the state of the art in Stirling design and performance, they were affordable, available, and compatible with the DUFF test constraints, making them the best choice for this proof-of-concept test.
DUFF passed with flying colors. This proved that a heat pipe waste heat rejection system could be used in a reactor, and also demonstrated a flexibility in thinking among the researchers and designers involved to work within a very limited budget and scheduling constraints imposed across multiple facilities in the DOE. For more info on the challenges leading up to DUFF, I recommend reading through the presentation. The challenges described in critical assembly testing have been enlightening, and the presentation and paper are my main sources of the information about the tests.
Two test runs were made, on Sept. 13th and Sept 18th,, 2012. In the test on the 13th, the reactor power was raised to 2 kWt, and held there for about 5 mins. After reactivity was increased, the Internet connection for the thermal data collection system went out, leaving only the pedestal temperature data available (this is a much lower temperature, possibly the reflector temperature). From here, they decided to fly blind, relying on information from the power conversion system and their models to complete as much testing as possible within the allotted time. Due to pre-test work, it was known that the reactor would have a negative coefficient of reactivity (i.e. the hotter the reactor got, the less neutronically active it would be), so this wasn’t a concern. Limited data collection is a persistent problem in all areas of science, and in astronuclear engineering it has been consistent enough to be ingrained into the researcher’s mindset: some data is better than none. Two minutes later (7 minutes into the test) the heat pipes activated, and more data flowed in. At this point the heat pipe was carrying about 400 W of energy. Over the next ten minutes, the core temperature increased, heating the Stirling engines about 200C. This also kept the core cooler, which in turn adds more reactivity to the core due to the negative thermal reactivity coefficient.
Seventeen minutes in, the Stirling was kick-started (when the hot end was at 225 C) resulting in the production of 24 We. Thermal transfer from the hot end of the Stirling changed the temperatures of the various components significantly over the next minute, to the point that an equilibrium was established and observed, leveling off at 18 We power output. One minute later the reactor was scrammed, and the Stirling engine continued to draw off the residual heat from the various system components. Once the hot end of the Stirling hit ~120 C, it stalled. Four hours later, the team learned the computer that contact had been lost with was still intact, therefore the issue was in communications and not hardware.
After some repairs and adjustments, a second test run was done on Sept. 18th. This was a slower, lower, and longer test than the first, and had some other differences as well. Not only were they going to verify the results from the first test run (hopefully with full experimentation this time), but they were also going to stop and restart the Stirling engines during the test, to see the resulting change in core reactivity and thermal profiles. This is important in a number of ways, but the most important part may be that it allows for better predictions to be made about how a different core would react to the Stirlings either being shut off for maintenance or mission requirements.
After a 9 minute rise to power, the heat pipes activated and the entire system started to approach thermal equilibrium, which was reached about 13 minutes later at ~160C. At this point (22 minutes in), both fission power and system temperature continued to rise at about 5C a minute, while the Stirling remained off. A half hour into the test, the technician turned on the Stirling engine, and heat began to be removed from the system. Once they started the engine, the hot end measured 180C, and electric output was 13 W. Within a minute, the hot head had cooled rapidly, and power output held steady at ~7 W. At the same time, the cooler temperatures increased reactivity, with power output (and temperature) increasing to 11 W. Four minutes later (35 minutes into the test) a final reactor power increase was ordered (reactivity insertion), bringing peak fission power to ~5.5kWt. Over the next five minutes, the negative reactivity coefficient of the reactor kicks in, and the system reached a new equilibrium at about 3 kWt. At this point, the technician changed to a high stroke on the Stirling engine (similar to shifting gears in a car, it changes the torque being utilized by the Stirling engine), increasing the amount of energy produced (from 11 to 17 W), and removing energy from the reactor system. Three minutes later, the technician stopped the engine, and the temperature rapidly increased from 185C to 225C. Two mintues after that, he restarted the engine, reconfirming the results from the first critical run. Forty-six minutes into the test, the reactor was scrammed, and the entire system decayed to thermal equilibrium. The Stirling continued to draw power for about 5 minutes, stalling out when the temperature reached ~115C. With one final gasp of the Stirling 56 minutes after the start of the test, the cooldown period continued through normal radiation of heat energy.
Many firsts were demonstrated with this test, some technical and some organizational. One thing that was surprising to me was that this was the first reactor system developed by Los Alamos to produce electricity. Other organizational notes (that make incredibly depressing reading, to be honest) include: first nuclear space power demonstration since the founding of the DOE (on August 4, 1977), and the first power system operated at the NCERC. More hopeful ones include the first use of a heat pipe power reactor (of any size), and first reactor system to use a Stirling convertor system. Further successes look not to DUFF, but to its’ successor, KRUSTY.
As with many advances in nuclear power, DUFF went largely unnoticed by those outside the nuclear engineering community.