The Evolution of the Pebble Bed Reactor

Nuclear Street ^ | Nov 27, 2009 | Duncan Williams

[Hostage]

One of the more promising nuclear reactor technologies known today is the pebble bed nuclear reactor.

Offering many advantages over conventional reactors, the pebble bed reactor gets its name from the type of the nuclear fuel it consumes. Just like conventional reactors, the pebble bed reactor can fission uranium, thorium and/or plutonium as its nuclear fuel. But the similarities end there. Instead of forming the fuel into plates or pellets as in conventional reactors, the pebble bed reactor fuel is manufactured into spheres, known as “pebbles,” slightly smaller than a tennis ball. Simply stacking the pebbles together in a pebble bed in the reactor can form a critical geometry that initiates the nuclear fission process. Instead of being cooled by water as in a conventional reactor, helium gas is used to cool the spherical fuel elements.

The most widely used spherical fuel is made up of thousands of coated particles known as tristructural-isotropic (TRISO) particles. As shown in the diagram, the center of the particle is typically uranium dioxide, known as the fuel kernel, and is .5 mm in diameter. The fuel kernel is coated with a layer of porous carbon which serves to capture any fission product particles emitted from the fuel kernel. Three additional layers of carbon are then applied to each particle: an inner layer of pyrolitic carbon; a mid-layer of silicon carbide; and an outer layer of pyrolitic carbon. These layers provide the structural support necessary to endure irradiation during the fission process. Thousands of these TRISO particles are then embedded in a graphite matrix and formed into a sphere.

One of the major advantages of the pebble bed reactor is that it is extremely unlikely that it will ever suffer a catastrophic meltdown at high temperatures.

[rlmorel]

I don’t believe that is the case with a pebble bed reactor. I believe they use a sub-critical process to produce heat, which is used to produce steam.

I could be mistaken on this, but I believe they use the heat generated by nuclear decay inside spheres rather than a full blown fission reaction inside each sphere.

[rlmorel]

Easy, folks.

I thought Mr. Warthog was right here...I do not think there are many operational pebble bed reactors out there. My initial impression was there were only a small handful operating, and to be honest, I thought there were only two or three...

[rlmorel]

I agree with your assessment on PWR, and your recognition of the quality and training of the USN nuclear teams.

PWR is a giant Rube Goldberg device...I think they are serviceable, but we should be able to build a better mousetrap by now.

By the way, thanks for your service...:)

[Hostage]

It matters not ‘how many’ rather ‘how much’.

http://www.iaea.org/inisnkm/nkm/aws/htgr/fulltext/gcr_review_02.pdf

Once operating experience has revealed issues and validated solutions, the tech is made ready for expansion. That’s where Pebble Reactors are now as we speak, and that is why the USA needs to look forward and not back to forever costly PWR designs.

[rlmorel]

You and I are in agreement here.

[Hostage]

Your diagram is one configuration but another basic configuration exists whereby the gas stream is used to turn a turbine directly without the need for heat exchangers.

“From the reactor the helium goes directly to gas turbines that convert the thermal energy in the gas into electricity.”

http://thefraserdomain.typepad.com/energy/2005/10/abouat_pebble_b.html

[Vineyard]

Several additions/corrections, and my own comments.

First - Thorium can’t be used as fuel directly - but as fertile material that is easy to breed (convert to fissile fuel). A “Thorium Reactor” is a reactor that starts with fissile U235 or Plutonium 239 as the material to “start” the reactor - and the Thorium 238 that is part of the fuel mix is readily converted to Uranium 233 ...and extends the time the reactor can operate before refueling....or it can be converted, and the fuel reprocessed to remove the U233 to make new fuel.

Second - The US had a High Temp. Gas Cooled Pebble Bed reactor at Ft. St. Vrain. (And it used microspheres with slightly enriched Uranium and Thorium added.) The reactor operated from around 1977 to 1992 was never economically viable, but lots of lessons were learned.

I agree that the commercial viability of normal PWRs is there - if we don’t let the environmentalists and legal people subject the building of a plant to thousands of challenges and changes, order modifications, etc. There are many plants operating very efficiently today in the US, and they would be shut down if they weren’t making money for the power companies. As to new construction, there would be zero demand if the PWRs were not viable - but there are many countries looking to build nuclear power plants because it can be economically viable. (But by the time the US gets its act together - we are going to be athe the bottom of the priority list in ordering reactor components - and it will make it difficult to restart our own nuclear industry.)

The Inherently Safe Reactor exists in many forms/designs. Many different plant designs can be built that have significant benefits. Liquid Sodium reactors have been built, and demonstrate inherent safety with respect to many considerations. Even the basic PWR plant concept has designs that are relatively safe. The question is - what constitutes “safety” - and who judges ...and to what standards??

Nuclear power could provide safe, clean power, lots of jobs - and the cost MIGHT be fairly low if you keep the Anti-Nuke people from mucking it up. But until Congress passes laws that limit challenges (or allows a “looser pays” for frivolous challenges - we are unlikely to see any new plants get started. Why would a utility select nuclear power if they have all the risks and minimal benefits?

I too, have lots of nuclear engineering experience - Nuclear-trained (also at NY prototype) Submarine Officer ....I also worked at the sodium-cooled Fast Flux Test Facility (FFTF - and that plant proved that it had “inherently safe” design that made liquid metal a potentially good choice for reactor design), and I worked in commercial nuclear power for many years. (And now also have a son undergoing Navy NucPower Training at Charleston.)

Mike

[dr_who]

and the cost MIGHT be fairly low if you keep the Anti-Nuke people from mucking it up

Exactly. You wonder how much of the cost/cost overruns are due to "administrative overhead".

[dr_who]

Even the basic PWR plant concept has designs that are relatively safe. The question is - what constitutes “safety” - and who judges ...and to what standards??

Hopefully not lawyers, judges, politicians, the sort of engineers that designed the space shuttle SRB's, and the sort of scientists that have been advising the UN.

[Wonder Warthog]

"I don’t believe that is the case with a pebble bed reactor. I believe they use a sub-critical process to produce heat, which is used to produce steam. I could be mistaken on this, but I believe they use the heat generated by nuclear decay inside spheres rather than a full blown fission reaction inside each sphere."

Wrong. PLEASE look it up. "Going critical" means precisely what I have said. The "heat generated by nuclear decay" ONLY applies to conditions when the reactor has been "scrammed"(i.e.the fission reaction stopped). IF the the decay heat is not removed, you can have a meltdown, which is what the secondary safety systems in PWRs and BWRs are for.

[Arthur Wildfire! March]

I’m glad you know something about this. I’m no scientist, but it would concern me to live near such a reactor. A helium leak, and the gas rises up, out of the plant. Helium is not as commonly available as water. Is this thing actually more resistant to earthquakes and terror attacks?

Personally, I think we should freeze all nuke plant construction until our borders are ten times safer.

[Wonder Warthog]

"Read some more. It is is now clear you don’t do your homework especially when you make a superficial search and then right off the bat conclude: “The first pebble bed reactor went online in South Africa in 2002.” The above is a false statement."

According to most articles, the specific type of pebble bed reactor under discussion originated in South Africa. There may have been other subtypes earlier. But the fact remains that compared to light water reactors, their track record is miniscule, and not an unblemished record of successes.

[Wonder Warthog]

"No I don’t think hundreds of operating PWR plants refute me, the cost is high, very high and utility rates from nuclear plants bear that out. Coal and natural gas plant are much more economical. If conventional nuclear power plant operations could substantially cut consumer utility bills, you might have an argument. But you don’t and you never will because the design is not *inherently* safe as are the Pebble Bed designs."

So, if they're so expensive, why haven't they been shut down and mothballed. But they haven't been, now have they. I think that supplies sufficient refutation to make my point.

[Hostage]

There were 21 years of successful operatoin of a Pebble Reactor in Germany. That’s plenty of time to evaluate viability.

The South Korans. Chinese, South Africans and others now planning major Pebble Reactor sites with substantial money investments are not gambling.

The long and short of all this is that PWRs are costly and always will be. PWRs are proven but they are not the best tech for nuclear. PWRs are also not unblemished in their success rate so your attempted point is not a point. The development of PWR tech was riddled with problems and those problems led to a costly evolution that still persists to this day.

[Hostage]

Many have been shut down. Those still operating have been considered for shutdown.

In the electric generation business today, a power plant becomes a fixture that is very hard to remove no matter what fuel configuration it uses. Even trying to change or upgrade a power plant is met with a lot of resistance as the impact to utility rates are always a sensitive issue.

An operating nuclear power generation site that is operating under an existing regulatory tariff is very difficult to shutdown permanently because the outcome is usually to replace the site with a more costly alternative or to take power from a more distant power site at a higher rate.

People today will not even allow their grossly inefficient and polluting coal plants to be replaced or upgraded because the replacement costs will drive up rates. Coal is cheap, people can still breathe and function and they need their homes warm in the winter, so don’t change anything.

Samething with nuclear power, if a region shuts it down for say a clean natural gas alternative, it’s going to cost a lot to replace and the rates are going to go up, so most communities will say don’t change a thing, don’t make the rates worse.

[Hostage]

What appeals to me is that all the helium could leak out and the reactor fuel in the Pebbles would get and stay hot but not so hot as to melt surrounding materials. And then of course control rods could shut down the nuclear heat generating reaction.

With conventional pressurized water reactors (PWRs), if the water coolant leaks out and the fuel is exposed to air, then it melts, even if the control rods are blocking the reaction, it still melts because the radioactive decay creates heat that is high enough to melt the fuel and surrounding material.

With the Pebble fuel, the higher the temperature, the less the added heat, so there is a limit to the amount of heat generated and that includes decay heat.

A terrorist bomb of a nuclear reactor would create a radioactuve mess no matter what is the configuration of the design of the reactor. So heightened advanced security is always a necessity.

[Hostage]

From my light reading on Thorium, it was deemed uneconomical in operation even though it is 400 times more prevalent than uranium. So my interest in it waned as I believe economics trumps technology in most business plans.

My reading of the Ft. St. Vrain HTGC Pebble Reactor shows it was not the nuclear issue that made the plant uneconomical but rather non-nuclear plant issues and furthermore these issues were all corrected and the plant was operating well when an economic downturn in the region caused it to be shutdown. Even fossil fuel plants have been shutdown in such circumstances. If a region has no employment, people move away, why pay plant personnel to supply power when it it is cheaper (for the plant owners) to take it from a distant site?

The history shows Pebble Reactor tech is viable and is ready for primetime.

http://en.wikipedia.org/wiki/Fort_St._Vrain_Generating_Station

As for not allowing the ‘anti-nuke’ crowd to ‘muck it up’, that such an action would allow for nuclear siting etc. I don’t believe this is material to the debate. Such people are indeed irritating and annoying but the bottomline is what drives the debate.

Power plant siting and construction is enormously expensive no matter what the fuel configuration. Regions and their communities will approve siting if it makes economic sense period. The only way to get a region to go along with a new nuclear tech is to make the risk-reward ratio worth it and that means loan guarantees, insurance and maybe subsidies.

So now that we are all looking at nuclear seriously again, we should be ‘smart’ and look at the next generation tech that will be more safe, more efficient and above all economically competitive.

[rlmorel]

You are correct. I misspoke. It looks like the walls of the sphere provide the medium to absorb neutrons to keep the reaction in check.

I think the safety factor inherent in this design is that it can withstand the loss of coolant without suffering a catastrophe.

[Hostage]

There are three general levels of criticality: subcritical, critical and supercritical.

When the reactor is starting up it is taken to a critical level meaning the neutron chain reaction is sustaining itself, the fission is releasing neutrons to cause other fissions at an equilibrium level. Think of it as the material ‘glow’ is taken to a brightness level and then stays at that level by itself, then the reactor is critical.

When the reactor is subcritical, the fission rate is not enough to provide a level of neutrons to sustain the ‘glow’, so it begins to fade in intensity.

But....and this is key....once the nuclear material and its packaging have been radiated by the fission process, meaning the material is radioactive, then there are radioactive decays of particles from those materials and the decay generates heat. This decay heat will always exist even if the reactor is shutdown.

In PWRs, the decay heat is so intense that it can melt fuel and surrounding materials unless there is coolant to keep the radioactive material cool.

But with Pebble Reactors, even if all the helium coolant escapes, the heat generated by the radioactive decay is limited, there is a cap on it, even if control rods are not inserted. There are two reasons this occurs by design: the decay heat is absorbed by a clever carbon packaging and the other is using a physics principle called doppler broadening of neutron absorption energies. In this latter, U238 will begin to absorb many more neutrons at higher temperatures than it will at lower temps, thus the neutron flux dies out at higher temps and is rekindled at lower temps, the net result is a power level that is naturally limiting; the ‘glow’ can only get so bright and the good news is the temp at this ‘cap’ is far below any temp that would melt the reactor.

Long story short: Pebble reactor suffering a Loss of Coolant Accident (LOCA) and a Control Rod System Failure will get hot but not so hot to create damage.

But in a PWR suffering a LOCA and Control Rod Jam, the result can be catastrophic damage and even if there is no Rod Jam, the decay heat can melt things and create a nasty radioactive mess.

[rlmorel]

Excellent explanation. Thanks for taking the time to do so.

Funny thing, I was not in the Nuclear Navy (I was a jet mechanic) but when I got out in 1979, they tried to recruit me into it as an incentive to keep me in and offered an astronomical (to me) bonus to do so. I forget how much it was, but I recall it was a lot, but I got out anyway.

Got my degree in Chemistry and ended up in Nuclear Medicine for a long time. They don't spend much time in Nuclear Medicine school on reactor technology and the like, but I understand decay and radiation physics well enough to follow the discussion...:)

Anyway, the spheres are designed in such a way to keep the fission reaction going on its own inside each one without any external manipulation...a self regulating reaction, except that they must all, by necessity, begin to fade?

So, like a blind squirrel that finds a nut, I reasoned that these spheres must be “Subcritical”, in that they slowly begin to wind down over time and must be monitored and removed when they reach a point they are no longer generating heat correctly? That is, they are not self propagating?