Pebble Bed Reactor: WHO CARES?
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2019-06-06 10:05 by Karl Denninger
in Energy , 164 references Ignore this thread
Pebble Bed Reactor: WHO CARES? *
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More than 50 U.S. companies are developing advanced reactor designs that will bring enhanced safety, efficiency and economics to the nuclear energy industry.

X-energy, located just outside the nation’s capital in Greenbelt, Maryland, is working on a pebble bed, high-temperature gas-cooled reactor that the company says can’t meltdown.

X-energy is developing its Xe-100 reactor and specialized uranium-based pebble fuel that could be available in the market as early as the late 2020s.

Who gives a shit?

Seriously, I mean it.

This design does have advantages -- don't get me wrong.  It's also not new.  The premise is that you construct fuel "pebbles" (about the size of a cueball, so more like "fuel rocks" rather than pebbles) that contain the fuel inside an allegedly "impervious" sphere.  The pebbles, being spherical, allow gas (Helium in this case) to pass between them, which takes the reaction heat away, and you use that to produce electricity through a traditional heat exchanger mechanism.  The moderator is graphite and in the reactor vessel; the fuel is cycled through from top to bottom, which means it is continually refueled in operation, with each fuel unit running for about three years.

Traditional water-cooled reactors use zirconium for the fuel rods.  Zirconium is "transparent" to neutrons; that is, it neither interrupts their passage nor does it get "activated" (absorbing them and becoming a radioactive isotope.)  This is good; you want what looks like a window to the sun for neutrons, because they have to get into the fuel in order to cause fission.

But zirconium has some problems.  Chief among them is thermal tolerance.  This is not a problem provided the reactor remains flooded with water, since water has a critical point of ~3200psi and ~705F.  Therefore you must keep the pressure below that and the temperature below it too, since water is also the moderator.  Above 705F it's steam no matter the pressure.  For this reason water-cooled reactors tend to run around ~1,000psi in normal operation for a BWR and ~2,200psi for a PWR.  BWRs are simpler in that as water boils it loses its moderation; this is a negative feedback on the power level and makes designing control systems, and their inherent safety, easier.

However in the event of loss of circulation (the ability to dump heat) or coolant (e.g. pipe break, etc) you have a severe problem because zirconium melts at ~3,300 F -- and once it does, you're screwed.  Silicon carbide, which is what the pellets in a pebble-bed reactor have their outer shell made of, doesn't melt until nearly 5,000F.  That's a huge safety factor.

But, there's a rub.  The "safety analysis" has run tests that postulate that in an accident the temperatures should not exceed 1,800C.  I note that this is below the melting point of zirconium, yet as we know in Fukushima and elsewhere, that temperature is indeed exceeded in bad situations.

There are also general issues with graphite moderators; they're manageable however, albeit at some cost.

So how safe is this thing?  Well, good question.  But in the end, it doesn't matter.

No fission design is safe end to end, which is all that matters, until and unless you have a closed fuel cycle.  The problem is that the burn-up in a TRISO fuel reactor -- that is, a pebble bed, while much better than a BWR or PWR (20% .vs. ~10%, roughly) still sucks in that 80% of what you put in there comes out and has to be reprocessed somewhere or discarded as high-level waste.

There is no reprocessing in the United States today, and hasn't been since Jimmy Carter shut it down.  Therefore any plant design that does not inherently separate and reprocess its own fuel as an inherent part of its operation is manifestly unsafe and unsuitable for deployment until and unless there is a viable reprocessing cycle available in the United States.

There is only one way to safely deal with most transuranics, which remain dangerous for tens or even hundreds of thousands of years.  You have to put them back into a reactor and burn them up.

Short-lived isotopes that reach a stable, non-radioactive element with half-lives in the range of single-digit years or less we can deal with.  After 10 half-lives basic mathematical theory tells us that the substance is no longer dangerous no matter how high-level of radiation it emitted originally.  But that's not something you can fudge; anything with half-lives in the tens, hundreds or thousands of years has to be returned to a reactor and reduced in this fashion until it reaches either a stable isotope or one with a half-life of less than 10 years.


Now there will always be a small amount of waste that isn't amenable to this, but if it's small enough in volume it never has to leave the plant until the plant is decommissioned.  What we cannot accept is a no-reprocessing paradigm, which is what we have now, where fuel comes out of these units full of hundred or thousand-year or more half-life highly-radioactive elements for which we have no rational disposal mechanism.  Without reprocessing we cannot put those elements back into a reactor and burn them up and we have nowhere we can safety put them either.

Nuclear power safety is not solely about meltdown safety, although pebble bed designs look promising in that regard.  In addition these designs have other challenges, one of them being that they use Helium as a coolant -- and Helium is a non-renewable gas that is in short supply and in addition it's a very small molecule so it leaks like crazy.  Helium, incidentally, is used as a coolant in these units for a number of reasons -- among them is that it is not easily activated (that is, it doesn't capture more neutrons easily) and when it does it decays extraordinarily quickly, so it doesn't form dangerous reaction products.  This means that if it's released (e.g. due to a pipe break) it won't hurt anyone as any activated isotopes will decay before it can get out of the building.  It also has a pretty good specific heat ratio; that is, it carries heat well as gases go (much better than air, for example), so it's a good choice for that reason as well.  Being inert it has no reactive issues with the various materials inside the reactor either, which is a big bonus.  And it has a very low neutron cross-section, so it doesn't interfere with the fission reaction itself.

Finally, due to the use of gas as a coolant and the much higher temperature tolerance of the fuel these units run at materially higher temperatures than a common PWR or BWR, which means they're materially more thermally-efficient.  It also means they can, at least theoretically, be run in places where large-volume water cooling is not available (e.g. inland, and not near oceans, fault lines or huge lakes) with reasonable overall efficiency.  That's a plus.

But on the downside our supply of Helium is basically all from natural gas wells, where it's a trace component of what comes out of the hole.  It's completely non-renewable and non-capturable, in that it is so light it effectively disappears into the upper atmosphere when released.  For this reason consumption of it is a serious long-term problem since our ability to get more of it is inherently tied to natural gas production.

Nonetheless the big problem with all of these types of reactor designs remain -- there is no sane means of dealing with the waste products out of these units.  Of the fission designs currently known and on the board there is only one that is amenable to continual, on-site reprocessing that burns up basically all of the high-level reaction products as part of its normal operation.

That's the LFTR, which uses Thorium as its fuel, is started on Uranium (since Thorium is fertile and not directly fissile) and since the fuel is dissolved in the working fluid it can be reprocessed chemically online in the plant itself, thereby allowing on-site burn-up of most of the high-level reaction products.

Oh, and it is also passively safe since are no fuel pellets or rods that can overheat, crack and release the material inside, and we know that passive safety system works because it was run for several years at Oak Ridge in the 1960s and when the scientists went home for the night they literally just turned the power off to the systems and walked away.

I wrote an article on a viable hydrocarbon replacement strategy here, and also covered it extensively in my book Leverage in Chapter 10.  It's as valid today as it was then; go read it.

The LFTR was abandoned, incidentally, because being Thorium fuel-cycle based it is almost entirely unsuitable for the production of nuclear bombs -- and we wanted dual-use nuclear technology.

Go figure.

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