NASA’s New Space Reactor Is Powered by Nuclear Fission
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NASA’s New Space Reactor Is Powered by Nuclear Fission

February 18, 2020

From a technology standpoint, there’s not
too many greater challenges than nuclear power in space. The physics involved in it is quite intense
— it has that luster of something totally different. We start with that highly enriched core, the
splitting of the atoms generates that heat that we need at the right power levels, at
the right temperature levels, and then we transfer that heat up to the power conversion
system. You convert that to electricity at that engine,
reject some of the waste heat, and then use the electricity for whatever you’re trying
to power, whether it’s a coffee maker or a very expensive instrument for science. It sounds pretty simple because it kind of
is simple. It’s just the engineering pieces aren’t always
simple. Nuclear energy and space flight have a deep
history that stretches all the way back to the Cold War. At the time, scientists were looking for new
ways to harness the power of the atom, and nuclear offered something special for deep-space
missions. We don’t have to rely on the sun. When we go out in deep space or whether we’re
in a shadowed crater on the moon or whatever, we’ve got our own energy source. That’s the biggest plus. The secondary piece of that is the amount
of power that energy can provide. Fission power is very, very high on the power
density scale. As a quick refresher, nuclear fission releases
heat energy by splitting atoms, and fusion combines two lighter atoms into a larger one. They both release an enormous amount of energy,
but scientists found fission easier to control for space missions. To tap into nuclear’s potential, NASA launched
a research program called SNAP. This kickstarted the development of radioisotope
thermoelectric generators, which use plutonium-238 and thermocouples to convert heat into electricity. These units have been on a number of famous
missions like Curiosity, Cassini, Pioneer, and even Voyager 1, which is currently cruising
on RTG power 21 billion kilometers away from us. The program also developed SNAP 10A, a fission
reactor that’s considered the U.S.’s first and only known nuclear space reactor. It took off in 1965, failed after 43 days
into the mission, and will be orbiting Earth for another 3,000 years. All subsequent nuclear space programs, like
NERVA, which looked into nuclear powered rockets, were shuttered or just didn’t get off the
ground. Now, given, they were working on bigger power
systems, which was part of the problem, I think. But, they were also chasing materials and
processes and things that weren’t available at the time. Today, nuclear power systems are seeing a
comeback, and that’s thanks to new mission targets. When we start talking about putting men on
the moon in 2024 or on Mars in the 2030’s, you need a lot more power. That’s where fission really kicks in. Kilopower, is one to ten kilowatts of electrical
power. That’s starting with the fuel source. It’s uranium, molybdenum alloy fuel. Highly enriched. We have sodium heat pipes that are attached
to that core. That sodium heat pipe carries that heated
vapor up to the Stirling power conversion system, and then we have to cool the cold
side of the engines to get that temperature difference that we need for that power conversion
and keep the engines from overheating. The other pieces of the reactor that are,
obviously, very important are the neutron reflector. That helps direct those neutrons back into
the core and keep that chain reaction going at the power levels that we need. Before operating this, we have a poison rod
inside the center of that core and then, when we get to our destination we have a mechanism
that would pull that rod out, make sure that those neutrons will not be absorbed anymore
and they can start the actual chain reaction that we need. One of the biggest attributes, I think, to
the success of this program was that we kept everything very small. And, we’re looking to make it as lightweight
as possible. Once the design was approved by NASA, the
system and the people behind it were shipped off to Nevada for testing. KRUSTY was the experiment, which stood for
Kilopower reactor using Stirling technology. That experiment was really a follow-on to
the Duff experiment, a demonstration using flat top fissions. Los Alamos, typically, or always likes to
name their nuclear experiments after Simpsons’ characters. We run through all those mission scenarios,
where we would turn off the cooling to the whole top end of the power conversion system
and see how the reactor reacted. And then, how we could really mess with them
and see is it going to be a stable controlled reactor. The experiment culminated in a 28-hour test,
from reactor startup to shutdown. It operated at 800 degrees Celsius and produced
over 4 kilowatts of power. The highest power nuclear mission we’ve ever
completed was Cassini, that had 870 watts. So, already our lowest is already higher than
any mission that’s ever been done. We did things to that reactor that you shouldn’t
do to a reactor, but it was really neat the way it handles. From concept to test was about three and a
half years. That’s a real quick timetable. To me, that’s the most impressive piece. Me and one of my buddies down at Marshall,
as we were cleaning things up afterwards. You know, the thousands of CAD models that
we had and the information that was generated, we’re like, how did we do all that work? I’m like, I don’t know. Some days, I don’t know. We worked a lot of hours and just had a lot
of passion. The fact that it worked really well was just
a bonus. This was a major step in demonstrating nuclear
power’s feasibility in the new space age, but there are still hurdles ahead. The real challenges are really political and
how we develop all the safety and security pieces to launching a nuclear system. There’s a whole lot of work related to making
sure that there’s no way you accidentally start that reactor up. And then there’s what we call the launch safety
piece, which also has a criticality accident associated with it, so if the launch vehicle
fails, and it falls into the ocean or on a beach or in the general public, we have to
show that it’s not going to turn itself on. Political will might be in favor this time. Congress recently passed a bill earmarking
$100 million for NASA to develop nuclear thermal rocket engines. So a future of fission, for both propulsion
and power, is looking ever more promising. We’ve just finished some studies with JPL
on how we use this for what we call nuclear electric propulsion. So, you couple our reactor with an electric
propulsion system and we can go do deep space missions, carrying much higher payloads, faster
times. We’ve got design concepts that couple four
of those to give us forty kilowatts electricity on one lander that now you can use for your
human habitat. Once we improve the technology and it’s available
as a power source, you’ll start to see some really neat propulsions on what they can do
with that extra power.

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