The Kilowatt Reactor Utilizing Stirling engine TechnologY, or KRUSTY, reactor is a testbed criticality experiment for the Kilopower program, run by the US Department of Energy in cooperation with NASA and the National Nuclear Security Agency. Begun as an internal project at Los Alamos National Laboratory’s Space Nuclear Reactors division, this reactor became the first new nuclear reactor configuration to undergo fission-powered testing since the 1970s.
Kilopower vs. KRUSTY
Testing nuclear reactors is difficult, if not practically impossible. Instead, individual components are tested thermally, chemically, and neutronically before an integrated test can be done. This involves thermal vacuum testing of all components, exposure of key components to beamlines at research reactors, and incredibly extensive modelling.
Often, once all this is done, the researchers are stuck for money. Funding for in-space nuclear systems is rare, much of that goes to radioisotope systems that are either already flight-proven or are improvements of the same basic architecture, and therefore low-risk investments. With a limited number of small testing facilities able to accommodate a test reactor, these larger designs were left out in the cold, not because they wouldn’t work, or due to insurmountable engineering challenges, but simply due to the cost of building new testing facilities. There are places that this could be done, such as the National Nuclear Security Site where Project Rover occurred, but scratch-building a facility to do environmental testing in a vacuum chamber for an operating nuclear reactor is far from cheap.
The difference here is that KRUSTY is small, both in size and in power output. Even accounting for a vacuum chamber’s weight, it is possible for current facilities to be used to test this reactor design. As we saw in the last post, there are critical assemblies that have been used since the 1950s to do benchmarked fission tests. There, we looked at Flattop, the spherical reactor used to prove heat pipe cooling of a reactor could be predictably modeled. This time, we’re going to meet COMET, another of these benchmark criticality test stands. COMET is not a reactor, nor does it have any nuclear components itself. Instead, it is designed to bring two different parts of a test together very precisely. It consists of a table with hydraulic presses and vernier adjustment that can handle having a sizeable test apparatus secured to it, and a central pillar to place the other part of the critical assembly being tested. COMET is even older than Flattop, having been used for criticality testing for the Little Boy gun-type atomic bomb. Since then, it was used at Los Alamos’ Pajarito test site until it was moved for that site’s decommissioning, along with the rest of the critical assemblies, to the National Criticality Experiments Research Center at the National Nuclear Security Site in Nevada.
The biggest difference between KRUSTY and Kilopower lies in the reflector of the reactor: rather than having a control cylinder that moves in a fixed reactor core, KRUSTY’s core, along with everything else but the reflector, is placed in a vacuum chamber, which is then mounted on the upper platen of Comet, and a reflector made out of discs of beryllium oxide (BeO) of various thicknesses will be raised around the core using Comet’s lift systems. By using different reflector thicknesses, how much reactivity is available can be changed, and the data collected can be used to finalize the reflector design for Kilopower.
Building KRUSTY: Prototyping and Non-Nuclear Testing
Compromises for the Budget
One of the impressive things about Kilopower is that so far they have managed to keep costs incredibly low. This is wonderful in that more work can be done for much less money, and it’s far more likely to fly when it’s able to be cheaply tested. As we’ve seen though, this means that compromising between the ideal, prototypic testing regime and the one that can be afforded. A great example of this is the Fission Surface Power program, where not only did they not manage to do a single nuclear test, but even the heat rejection system was only half-tested in a vacuum chamber.
This doesn’t mean that we learned nothing from that program, or that there wasn’t extensive testing completed on individual components. For instance, that program looked not only at Rankine power conversion systems, but carbon fiber radiators and heat pipe thermal management as well. Virtually every component in this reactor is similar: it’s been tested before, just not in this specific application. KRUSTY is that system-level series of tests that ensures the flight reactor will function as advertised
Building off this knowledge base and available, off-the-shelf technology, the team at GRC built a mockup of KRUSTY, using stainless steel (SS) instead of uranium for the core (these tests were detailed in the DU thermal testing report). They then did a number of thermal tests using electric resistance heaters to mimic the nuclear reaction, using the models of reactor behavior that the DOE (LANL and Y12, mainly) have shown to be the most likely behavior.
At the same time, a decision was reached that would save significant money for the development effort, but would lead to a change in the power conversion system for KRUSTY. Rather than purchase the full set of eight 125 W Stirling convertors that Kilopower would have, instead the engineers at GRC decided to reuse two 70 W Stirling convertors that had been built and tested for the ASRG, and replaced the other six Stirlings with thermal simulators that they designed for the purpose. This meant that the test wouldn’t be using a prototypic PCS, but this is less of a concern than for other components, especially the core and heat pipes. The PCS can be further refined and tested without the headaches and difficulties of dealing with a critical assembly.
Initial testing focused on the individual components that needed to be verified. This proceeded on two fronts: materials testing for unavailable or inconclusive data from past research, and subcomponent testing. The materials questions included fuel creep, thermal expansion coefficient of the fuel, and diffusion between the fuel and the heat pipes. The subcomponent tests focused on the heat pipes and their connection to the reactor core, ensuring that there would be enough heat transport available for the power conversion system.
The success of these tests led to a full scale thermal prototype test using the stainless steel core. This took place at NASA Glenn in the second half of 2015. These tests took place in a vacuum chamber at GRC, using electrical heaters to heat the core to a design temperature of 800 C while conducting a variety of tests. Of special note were the tests focusing on the heat pipe thermal transport capabilities. In one test, the cold end of the heat pipe was connected to an evaporator, which then discharged into the vacuum chamber that the test was taking place in. This test demonstrated that the Haynes 230 heat pipe was able to transfer 4 kWt over 1 m of distance, showing that the system was able to handle the thermal load required to keep the reactor cool.
With this latest set of successful tests complete, a new dummy core was made, this time out of depleted uranium (DU), to test for any chemical reactions, and for mechanical and thermal interfaces with the heat pipes and other components. This also marked the end to individual component testing for the components that would be used for the KRUSTY test, as they would be integrated with the modified power conversion system for testing with the new core in the last round of non-nuclear thermal testing.
Regulations Rear Their Head Again… But We Can Work With That!
DU is a more difficult substance to work with. This particular core simulator was produced at Y12, contained 8% molybdenum by weight, and the only difference between it and the KRUSTY core is the percentage of 235U (<15%). This is important to characterize thermal effects arising from the density and structural properties of the fuel, and also isolates any chemical, mechanical, or tooling issues with the manufacture of the HEU core to be used by KRUSTY later this year. This was a major step forward for the program, dotting the i’s and crossing the t’s before the nuclear test. This also allowed Y12 to make the molds and other tooling required for the HEU core, and verify that there wouldn’t be any issues in the manufacture.
This also forced a design decision on the power conversion team. Two ways had been discussed to mount the Stirling pistons: the first, and the final as well, is what’s called a dual convertor design. Here, the heat pipes are arranged in pairs radially, with their hot ends toward the center. By managing the stroke of the pistons, their actions cancel each other out from an overall inertia point of view. The alternative was to mount the pistons vertically, and run them in parallel. This requires an active structure to counterbalance the inertial force from the pistons, adding complexity. When it was time to finalize the design, the simpler dual convertor design won. Another compromise from the prototypic flight configuration was the heat rejection system: cold nitrogen gas would be used to remove heat from the cold end of the Stirling convertors and simulators. This allowed for a smaller test apparatus, and also allowed the 75 W Stirlings to simulate a system 50% more powerful.
Due to its mild radioactivity, DU does require special handling as a nuclear material. While this is often “only” a major headache, in this case the Kilopower team looked upon it as an opportunity to get everyone together for a dress rehearsal before KRUSTY.
One of the reasons that NASA is looking to get away from using HEU is due to the security required for its storage and handling. The required personnel and organizational resources don’t come cheap, so the less time the fuel actually has to sit in the reactor on the ground, the better. Fortunately, since all nuclear spacecraft have the reactor as far away from the rest of the spacecraft as possible, and since the reactor core is a single piece, fueling can be held off until much later in the process of vehicle assembly and integration.
Neutronics is a tricky business. When first assembling a new nuclear reactor, there are many unknowns, and accidental criticality lurks behind seemingly innocuous mistakes. Because of this, it would be nice to run through an assembly process WITHOUT having to worry about having a nuclear reaction occur.
So this is what everyone did. Personnel from NASA’s Glenn Research Center and Marshall Spaceflight Center (MSFC), Los Alamos (LANL), and the Device Assembly Facility (DAF) at the National Nuclear Security Site gathered for the first dress rehearsal for fueling the reactor. This way, any hitches found could be dealt with on this far simpler test, and everyone was able to run through their roles in preparation for the big day. No major problems were discovered, the core was installed, and KRUSTY was ready for its last round of non-nuclear testing.
One other note about the preparations for nuclear testing: using the DAF sets limitations on instrumentation and other conditions, for instance on coolant for the cold end of the Stirling engines. In order to make sure that there were no issues here, all the connections to the test stand at GRC were identical to the ones that would be used for KRUSTY at the DAF.
Final integration allowed for assessment of the DU core’s interface with the rest of the reactor, especially the mechanical and thermal connection with the heat pipes. This is one of those critical areas that can be well estimated and modeled, but unless it’s experimentally verified with flight-like components there will be unanswerable questions. Another is the possibility of chemical reactions. With no major problems discovered, testing moved on to preparing for the nuclear test.
A final benefit gained from this test rehearsal is the ability to better estimate fueling requirements for a flight reactor. It was determined that the reactor could be fueled, instrumented, insulated, and canned in 12 hours, and final assembly of the radial reflector and control rod requires another 8 hours. This was conservatively estimated at four working days. This is good to know, since one of the biggest costs associated with HEU is the security required for transport and the highly enriched “Special Nuclear Material” fuel, so the shorter the time that the fuel has to be integrated to the spacecraft, the less time you have to pay for those expensive nuclear security personnel and procedures.
The Final Non-Nuclear Tests
The last round of non-nuclear testing occurred in 2016 at GRC. In these tests, the heaters’ control software was programmed with the projected behavior of KRUSTY during a number of reactor and PCS states. The test profile that was programmed in was meant to mimic as closely as possible the thermal environment the reactor would experience at various points in the testing process.
This does not mean that the reactor system will experience exactly these conditions. This is a model of predicted thermal behavior based on nuclear modeling of components that have only been thermally tested using non-nuclear methods. Further nuclear testing before the full-power test would refine the model, and the thermal test profile is designed to account for any unknown thermal effects during the actual test.
So what did the test look like? As close to the testing that will be done at DAF as possible, so we’ll look at that test timetable and mention any variations from it as they come up.
The first test at the DAF will be a thermal break-in test, where the HEU core is electrically heated. This is a final verification that all of the components are functioning correctly, heating rates can be easily controlled, and thermal interfaces are functioning properly (especially at the hot and cold ends of the heat pipes) before the reflector is raised around the core for the cold (or zero-power) fission test. This is also the test that was duplicated by GRC with the DU dummy core.
After a ~3 hour warm-up period, the sodium in the heat pipes started to boil, and the hot end of the Stirling engines began to warm. Once they reached 650 C, the two Stirlings and six thermal simulators were turned on, dropping their hot end temperatures. The system was then left to reach a steady state thermal equilibrium, which took about 2-3 hours.
Before the test was concluded, though, transient testing was conducted. This was to verify behavior of the reactor in case something failed, such as a piston jamming, reducing the amount of heat that the Stirling convertor would remove. To simulate this, the convertors were stalled, and the simulators were turned up to full power, to verify that a partial loss of power conversion could be accounted for with the rest of the convertors. Then, the convertors were shut down as well, to see the thermal response of the core to a complete loss of cooling. After this, the heater was turned off, the simulators were turned back on to full power, and shutdown occured.
This is identical to the thermal break-in test that will occur in Nevada. From there, the testing at the National Nuclear Security Site’s Device Assembly Facility differs from the tests that can be done at GRC, in much more stringent conditions and with a lot more cost as well, so every lesson that can be squeezed out of this testbed saved the program money and headaches further down the road.
As Dr. Mason mentions in a conference paper on the electrically heated DU core test article, this test was a major milestone.
“Testing of the Kilopower technology demonstration with the DU core, using the test sequence and configuration for the final testing with the HEU core, has reduced the risk of any unexpected issues in fueling, assembly, or test operations at NNSS… The system as it stands is capable of delivering 120 W electric from two ASC convertors with a maximum thermal power draw of roughly 3000 W, which is sufficient to verify neutronics models at the nominal Kilopower operating condition. There were no issues encountered during DU testing that caused unexpected operational issues which would need to be addressed prior to HEU testing.”
As with any well-constructed test, problems were isolated, which led to potential changes in the design. In this case, Dr. Briggs of NASA Marshall points out (in the DU testing final report) one such issue that was isolated, as well as a potential fix:
“Testing of the DU core shows that at nominal operating conditions there is a 200º C temperature drop between the core and the Stirling convertor hot end. The majority of this temperature drop takes place through the conduction plate and throught the ASC heat acceptor, both of which can be eliminated in future design iterations if money comes available for customized convertors and interfaces.”
However, he also notes:
“There were no issues encountered during DU testing that caused unexpected operational issues which would need to be addressed prior to HEU testing.”
Additional advances continue to be made, and there will be more minor changes between KRUSTY and a flight reactor, but these changes are likely to be minor.
Nuclear Testing: Now Things Get Serious
As seen above, nuclear materials are things that are far from taken lightly by NASA. There are arguments to be made both for and against the overabundance of caution that government agencies are required to take (these days), but that’s another topic that we aren’t going to touch today. Instead, what we’re going to look at is what goes into the nuclear testing side of reactor development.
As mentioned in the last post, the critical assembly machines that used to be at the
Pajarito Test Site (in structures known as Kivas) were either decommissioned or moved to the then-new National Critical Experiments Research Center (NCERC) at the National Nuclear Security Site (NNSS) in Nevada. While we were looking at one of the critical assemblies last time, this time we’re looking at what could be called a “critical assembler,” where it integrates critical assemblies without having any nuclear components itself. There are two of these at the NCERC, Comet (which dates back to Little Boy testing), and Planet (which was built to relieve scheduling pressure on Comet, and ended up being a slightly smaller version of the same thing), although there were others in the past that have since been decommissioned.
Why is this important? One of the odd, and difficult, things about nuclear reactions is that they are sensitive to things that we don’t often think of as important. A classic example comes from the dawn of the nuclear age, when Enrico Fermi was still at the University of Cagliari in Italy. Being Italy, marble is a common choice for lab benches, as was wood. When he and his team were participating in early neutronics research, they discovered by accident that the material of the tabletop they were using made a significant difference in the outcome of the experiments they were performing, because the wood tabletop would slow the neutrons more than the granite tabletop would.
This is one example of hundreds that can be offered of the odd effects of neutronics and the condition known as critical geometry, where a self-sustaining fission reaction can occur. All reactors require some form of cooling, and depending on the fissile material, moderators, and relative positions of these components, a reactor can go critical when you aren’t prepared for it to do so. This is called accidental criticality, and is a spectre that hangs in the room of any operation involving fissile material. Accidental criticality reports are made on a regular basis by Los Alamos National Labs, and the lessons from these accidents are then integrated into new handling procedures for nuclear material.
A quick aside on units here, criticality is measured in dollars and cents. One dollar of reactivity is exactly enough to barely sustain a fission chain reaction, and one cent is 1/100 of a dollar. This is also sometimes referred to as keff. When these terms are used, they refer to the reactivity being “inserted” or “removed” from a reactor. In this case, as the reflector is raised around the core, it reflects more neutrons back into the reactor, and therefore inserts reactivity, but removing control rods, rotating control drums, or removing fission poisons all insert reactivity into a reactor as well. There are also a number of ways to remove reactivity, either by inserting control rods, changing moderator configuration, allowing fission product buildup, or in this case lowering the reflector so it’s not reflecting as many neutrons back in the core.
The key to preventing accidental criticality is care in the design of the test, and in the movement of everything in the test area. Sometimes, a known reactor is used (such as Flattop or one of the Godiva reactors), and sometimes a machine is used that is designed to avoid accidental criticality (such as Comet and Planet, but other designs have been used over the years as well). This allows for a level of certainty and precision that takes into account the unique challenges of initial nuclear assembly. To build a critical assembly for the first time takes an incredible amount of forethought, modeling, planning, and preparation.
That being said, I am not a nuclear engineer, merely an enthusiast with a penchant for research and a respect for the limitations of physics and engineering, so explaining the conditions that lead to accidental criticality are not so much off the deep end of my skill set as off the continental shelf. Treat me as the classic “guy on the internet,” I try and provide as many original sources as I can and always love finding more, and as ever if there’s information I’m missing please let me know in the comments!
Comet (and Planet) have been designed to take care of those concerns to a large extent. They are simple machines, with a fixed platen and a movable one. The lower, movable platen on Comet has rough control via a set of hydraulic lifts, and fine control using a screw drive connected to an electric stepping motor. In this case, all of KRUSTY but the core will be placed in a vacuum chamber mounted on the upper platen on Comet. The core will hang down below this assembly, and the lower platen will be used to raise a reflector around the core.
A number of things remained untested (although modeled) after the DU tests, all of the significant ones being on the nuclear side. The first is how much reactivity will be needed for the core to operate In order to account for this, the reflector for KRUSTY is modular in design, containing a number of annular discs (think thick washers) of varying thicknesses that could be selected to tweak how many neutrons are reflected back into the core, and therefore how much reactivity is added. According to Monte Carlo (MCNP) modeling, a reactivity of $1.70 was needed for nominal operating temperatures to occur, but there’s a possible margin of error of up to $1.50. This was an issue, because Comet at the DAF was only authorized to handle $0.80 in excess reactivity, but the site permit changes and modifications were authorized for testing to proceed (a facility basis safety change was approved by the DOE, and there are some indications that even higher reactivity insertion limits may be allowed with the construction of new facilities at the NNSS).
Before the nuclear testing occurs, the reactor will undergo a thermal break-in test, mimicking the electrically heated DU test precisely. This is to ensure that all of the thermal interfaces and thermodynamic properties observed at GRC with the DU core are the same with the HEU one, and that the integration of the new core is done properly. After that, testing can move on to neutronics requirement refinements.
In order to test the needed reflector geometry a number of cold criticality (also known as zero power) tests were conducted over the summer (cold criticals in June or July, warm criticals in September) to refine the reflector geometry. By using different thicknesses of BeO, different amounts of criticality can be inserted into the test article, and this information will then inform the final design of the reflector. By adding components in a step-wise fashion, the reactivity requirements can be pinned down.
After the cold criticality testing data has been fed back into the models to verify the predicted behavior and make any adjustments necessary, low temperature testing can begin. This is important to refine the models of reactor behavior before full-power testing can occur, and again is done in a step-by-step manner, from $0.15, to $0.30, $0.60, and finally up to $0.80 excess reactivity (added in $0.02 intervals). This maximum number was chosen because it ensures that the reaction is sustained by the delayed neutrons and occurs at a lower temperature than what would be found at full power.
With the successful completion of the cold-critical tests, KRUSTY’s operating conditions will be well-enough characterized that any potential issues or unknowns discovered during the cold critical tests can be addressed before the full-power, high temperature testing.
Full Power Test
This is the thing that nuclear engineers have been looking forward to since the 1970’s: the first fission-powered full system test of a space reactor since the 1970s, with SNAP-10a. The wait has been long, but the wait couldn’t have been ended with better test results!
On March 20th, KRUSTY’s core was lowered into the neutron reflector on the Comet test stand once again, beginning the 28 hour full power test. The series of reactor dynamics and simulated equipment failure tests conducted was the same as the electrical heating profile used with the depleted uranium core at NASA’s Glenn Research Center [insert lab name], and the test results show that the modeling that the earlier (non-nuclear) test profile very closely matched the results that are being released today.
First, let’s look at the results of the electrical test and fission test side by side, and look at the individual parts of the test:
KRUSTY achieved full fission and Stirling power in the first hour, and the reactor temperature increased to about 850 C. Since the test profile was designed for 800 C (the slightly higher temperatures weren’t a significant issue, but it’s best to be as accurate as possible), the reactivity of the core was adjusted after about 6 hours to meet the target temperature over the course of the next hour.
Eight hours in, they started playing with things: First, the power drawn from the Stirlings was reduced to 60%, resulting in a small (less than 25 C) fluctuation in core temperature and about 750 W fission power reduction in the core. After an hour, the Stirlings were returned to full power, and then an hour later the Stirling simulators were cranked up to 200% power. This resulted in a large (~1200 W) increase in fission power being produced by the core. An hour later, the reactor was once again returned to nominal full power operating conditions.
Now they started (simulatedly) breaking the heat removal and power conversion systems: First, they simulated a single Stirling failure, resulting in a dip in fission power (and, if I’m reading the graph right, a slight increase in the heat pipe temperature, which I suspect is the blue line on the graph – but the temperature points aren’t labelled, so I can’t be sure). After another hour, they proceeded to remove another Stirling from operation, with similar results. In both cases, the reactor temperature only slightly varied from its’ nominal 800 C temperature.
Another hour of nominal operation, and the Stirlings were once again cranked up to 200% power, with effectively identical results to the first time this was done about 3 hours before. After another return to nominal operation, a series of tests to simulate control rod adjustment were done, including what looks to be (simulated) almost full removal of the control rod 18 hours into the test (this would actually be full insertion of the core into the reflector), resulting in a huge (2500 W) spike in power in the reactor core. Once again, the reactor temperature remained well within the acceptable bounds of the test, despite the rather severe adjustments being made to the amount of reactivity in the core.
With another return to normal operation, they killed most of the heat removal, resulting in a 1500 W drop in fission power – a wonderful demonstration of the strong negative thermal reactivity coefficient that makes Kilopower such an appealing design from a reactor physics point of view. Two and a half hours later the heat removal was eliminated as much as was possible. This resulted in a further, but smaller, drop in fission power being produced. An hour later, two of the Stirlings were restarted, and after the power transients dampened down, the last six were restarted as well, with corresponding increases in fission power.
27 hours after the beginning of the test, all heat removal was once again killed for the core, returning the fission power to the ~1500 W that were produced in the earlier simulation of this situation. An hour later, the reactor was scrammed (all reactivity removed), and the reactor was left to cool down.
Based on the test profile that was designed by the NNSS, KRUSTY was then set aside for the shorter-lived (and therefore more dangerous) fission products to decay.
This highly successful test shows that KRUSTY performed exactly as expected, and that Kilopower is ready for the next step in its’ development: the construction of a flight article for the first new astronuclear reactor design in the US for close to 50 years. Considering all the design and testing work for this system has cost less than $20 million dollars, this is nothing short of an epic achievement on the part of Drs. Patrick McClure, David Poston, and the rest of the LANL space nuclear reactor design team, as well as Marc Gibson at NASA’s Glenn Research Center, and everyone else involved in the program.
Further Development of Kilopower
KRUSTY is a major milestone for US in-space reactor development, but Kilopower has a lot more to offer than just the small 1 kilowatt (electric, kWe) reactor that KRUSTY proved the design of.
The first thing the Kilopower program offers is more power. As a system architecture, Kilopower has four different sizes of reactor, ranging from 1 to 40 kWe, for everything from small, electrically propelled deep space probes to in situ resource utilization and power supply for manned missions, both on planetary and orbital missions.
By moving or adding additional heat pipes, upgrading the power conversion system to match, and increasing the reactor core size, much more power can be drawn out of the potential core configurations of this flexible design architecture. Of course, changing the pattern of heat removal affects the thermal gradients (hot and cold spots) of the fuel element. In this case, the entire core is one single fuel element (known as a monolithic core), an unusual arrangement for a nuclear reactor, so the behavior of this type of reactor isn’t as well studied and understood as the more common type of reactor with many separate fuel elements.
However, Patrick McClure, the head of the Kilopower program at Los Alamos National Laboratory, is confident that any additional testing that is needed to verify the thermodynamic behavior of these larger and more complex designs can be done through electrical heating, similar to what was done at NASA Glenn with the depleted uranium dummy core for the electrical heating test (see the previous KRUSTY post for details on that test), without further fission-powered testing. This means that further development of the larger reactors can be done at only a modest increase in program cost.
Another thermal concern that is common in reactors is known as edge heating, where the edges of the reactor core (or individual fuel elements) are hotter than the center. This is often (including for Kilopower) due to the moderated neutrons being reflected back into the reactor core.
Depending on what materials the fuel elements and core structure are made out of, this can become a limiting factor for heat rejection (and therefore power extraction) in a nuclear reactor. In the case of KRUSTY and the smallest Kilopower reactor, the heat pipes are placed along the edge of the core, where the problem is the worst, but all other designs have the heat pipes internal to the reactor core. Fortunately, Kilopower’s uranium-molybdenum alloy fuel element (U7Mo) has both high thermal conductivity and high thermal limitations, so this isn’t a major concern in this group of reactor cores; however, changes in the fuel element type (for instance, using oxide fuel as is proposed for the Megapower derivative), or the addition of a thermally limited neutron moderator, can make this a much larger issue.