International Introduction to HTGR Technology
Q: How did you get into the nuclear space?

A: Donald Carlson grew up in Omaha, Nebraska and got exposed to the energy industry early
on as his father was an electrical engineer who worked with hydroelectric power. He eventually
chose to pursue Engineering Science at Iowa State University, followed by a Master’s degree in
Nuclear Engineering. During his Master’s program, Donald worked at Fort Calhoun during its
first two cycles. This plant recently shut down, struggling with the ability to deal with floodings as
part of the safety analysis. After completing his program and receiving his degree, Donald took
a six month exchange program at the Nuclear Research Center in Jülich, Germany. He ended
up staying five years in Germany and received his PhD from Aachen under the mentorship of
Rudolf Schulten, the protege of Werner Heisenberg. Donald’s previous experiences had left him
as an subject matter expert on pressurized water reactors, but Three Mile Island - which
happened during his time at Aachen - was an epiphany for Donald. Until this point, he thought
the passive safety features of Schulten’s technology were interesting, but not a selling point.
The pebble-bed HTGR technology offers such a degree of inherent safety that it can be treated
almost like any other energy technology: suitable for rapid deployment in the developing world
and developed without special oversight. Nobody knew at the time that the releases at Three
Mile Island were not so bad, and even the long-lived radionuclide releases from Chernobyl and
Fukushima were not as bad as predicted. Donald left Germany in 1983 to work on exotic
reactors for exotic applications at Los Alamos National Labs. After Los Alamos, he moved on to
be the lead principal investigator for the new production high temperature gas reactor (HTGR),
an interesting, but short, project which used HTGR technology for weapons material production.
Donald then went to work for the Nuclear Regulatory Commission (NRC) in 1991 because of his
expertise in non-light water reactors. After twenty-five years at the NRC, Donald began to see
an organization that was farther from being able to consider and accommodate innovation than
when he first joined. It was difficult to recruit a team of engineers to become experts on HTGR’s,
or other advanced reactor technology, because it was safer long term for people to stay within
the parameters of what the NRC specialized in: light water reactors. Newcomer countries will
have less organizational and cultural inertia to overcome, compared to the NRC. Donald
Carlson felt frustration with the way the Nuclear Regulatory Commission made decisions about
advanced reactor technologies, which was predominantly with a refined focus on light water
reactors.

Mythbusting Misconceptions About Graphite Fires
Q: My claim is there is no physical mechanism to get the radionuclides into human bodies in
sufficient quantities even in the worst case of a light water reactor accident.

A: Donald Carlson advocates for the importance of rigorous, independent peer review and
analysis to confirm claims about mechanisms for getting radionuclides into human bodies.
Putting educated individuals’ careers at stake is part of the inertia that must be challenged in
these situations. One of the silver linings may be that this inertia may be easier to overcome in
emerging countries. A Nuclear Regulatory Commission (NRC) representative gave a closed
seminar voicing frustration about Fukushima and experimental evidence suggesting cesium
does not come out to anything like the codes predict. Some of nuclear’s most revered experts
have misconceptions about the worst reactor accident in history. A widespread misconception is
that graphite burns like coal. Reactor-grade graphite can be made to burn, but it is not easy,
requiring high temperatures and air flow. The Windscale reactor fire in the UK was called a
graphite fire, but in 2006, photos taken of the damaged reactor core showed that most of the
graphite was still intact. Windscale was an air-cooled weapons material production reactor that
used metallic fuel elements. These elements got badly overheated and burnt, taking some
graphite with it, but most of the graphite remained. At extremely high temperatures, a solid
graphite block will air oxidize to carbon monoxide. Research has been done to see exactly how
that happens and how that will be credible - or not - in an extreme accident in a high
temperature gas reactor. Everybody thinks this happened at Chernobyl, but it did not. Chernobyl
went supercritical and exploded, blowing the core to bits. Within a fraction of a second, the core
went from producing ten percent of its nominal rate of power to ten times its nominal rate of
power, vaporizing the fuel and the zirconium. The Chernobyl-type RBMK reactor has twice the
zirconium in it than the comparable light water reactor. It is easy to conclude all of the zirconium
violently, exothermically oxidized in the first second of the accident and that the rapid oxidation
served as an accelerant to make a little bit of the graphite burn. No more than ten percent of the
graphite at Chernobyl burned, even though everybody says there was a raging graphite fire that
dispersed radioactive fission products via a thermal plume all over Europe. There was a thermal
plume - which was very hot, powered by air rushing into the demolished core - but the valid heat
source was not the burning of graphite or zirconium, but simply nuclear decay heat.

Light Water Reactors’ Weak Link
Q: Tell me about your residual concern for the current light water reactor system.

A: With a few minor changes to the currently light water reactor system, Donald Carlson sees a
possibility for a large fission product release that could even dwarf what came out of Chernobyl.
The Nuclear Regulatory Commission’s (NRC) Fukushima Near-Term Task Force (NTTF)
included two key recommendations that Donald felt should have been implemented, but were
not implemented due to industry opposition. The major recommendation that should be
implemented is an accelerated transition from high density pool storage of spent fuel to dry cask
storage spent fuel. There are critical scenarios, such as a major solar flare, that would wipe out
huge portions of the electric grid by burning up transformers, causing a months-long blackout.
There has been a lot of work, sponsored by the NRC at Sandia National Laboratories, to do full-
scale modeling of what happens in a spent fuel pool drain-out accident, leading to a raging
zirconium fire. A drain-down accident is where the pool gets a hole in it and drains down,
exposing the fuel. This would take down one reactor at a time, but if there was a region-wide
grid blackout, all the reactors in that region would be facing the same scenario. Decades worth
of cesium and strontium could potentially come out of spent fuel pools without the other
mechanisms seen in light water reactors, or even Chernobyl, that make that release less than
predicted. This air-fueld zirconium fire is very exothermic and drives the plume of fission
products. While local geologic repositories absent of groundwater are physically possible, there
would be political outrage that must be addressed. The nuclear industry has tried to manage
risk by instead managing outrage. China has shown a certain amount of independence. During
a trip to the Chinese regulator many years ago, Donald was amazed that China was trying to
license one of everything in China and learning from the best in the process. China has that
diversity in the regulator to think about things that aren’t light water reactors. They are very
close to starting up the first modular HTGR at Shidaowan site.

Influential Factors in Nuclear Policy
Q: Why is spent fuel left in the pools for so long instead of moving it to dry storage?

A: Donald Carlson sees spent fuel pools as the biggest risk of light water reactors. Spent fuel is
left in the pools for long periods of time because it is cheaper than dry storage and the regulator
allows it. During his time at the Nuclear Regulatory Commission (NRC), Donald worked for six
years helping license spent fuel casks. In his efforts to bring a more rational approach, he
helped the NRC acknowledge that spent fuel is much less reactive than fresh fuel. The spent
fuel casks that were licensed were far more expensive than they had to be. There is an idea that
the canisters inside the shielding annulus of the spent fuel cask will be taken and placed into a
geologic repository someday. This repository is thought to have limited space, so as many
assemblies as possible need to be able to fit in the repository. Another idea considered for
nuclear waste is a deep borehole disposal (DBD), which would be cheaper than disposal in a
geologic repository. It is somewhat of a mystery to Donald why Germany turned their backs on
nuclear power. Germany was leading the way in nuclear technology and Professor Schulten’s
high temperature gas reactor (HTGR) concepts were emulated in China, hopefully leading to a
successful demonstration of the first modular HTGR. It also started in South Africa, but failed
due to a number of issues. Around the world, people admire the work of Professor Schulten, but
inside Germany during the current times, he is viewed as somewhat of a villain. In ten to fifteen
years, Donald sees the construction of multiple reactors that are acknowledged to be safe
enough and far better than the alternative, both in terms of pollution and public health, but also
reversing climate change. At the turn of the next century, fusion may start to take on that role.