S. Africa Developing Safe "Uranium Pebble" Power

[tsipple]

South Africa is preparing to lead the world in a new form of uranium-based energy production using a technology called the "Pebble Bed Modular Reactor" (PBMR), according to an article in Wired News.

The PBMR has these major advantages:

- Cheap to build.
- Inexpensive to operate.
- Quickly constructed.
- No greenhouse gases.
- No air pollution.
- No water pollution.
- Radiologically safe.
- Cannot be used for weapons proliferation.

PBMR plants are highly efficient in generating electricity. That electricity can be sent directly onto an electrical grid or used through electrolysis to separate hydrogen from water, to store energy for use in a hydrogen-based economy.

Actually, the PBMR is not really all that new. Germany had a demonstrator plant over four decades ago. But, in a perverse accident of history, electric utilities preferred the gigantic (and radiologically risky) steam power plants for generating massive amounts of power.

How does the PBMR work? It's brilliant in its simplicity, actually -- and that makes its operation idiot-proof. The "secret" lies in the fuel itself, which is manufactured into precisely calibrated "pebbles." Here's an illustration:



Each little pebble is essentially its own reactor containment structure. Helium, an inert gas, is circulated within a simple graphite-lined container "tub" that holds the pebbles -- almost a half million of them in a small pressure vessel about 20 m tall and 6 m in diameter. Each pebble has the precise amount of uranium that generates heat -- not too much -- when it finds itself near a lot of its partners (and the graphite reflectors), and the heated helium then drives turbines to generate electricity. Each pebble gives off a certain amount of heat until its uranium decays, and then old pebbles are removed and new ones take their place, without shutting down the whole plant. Pebbles circulate in the tub so that they are consumed evenly.

That's the whole system. It's so safe because of the fuel design. The evacuation zone is 400 m around the plant. (For example, if something crashes into the plant, the pressure vessel ruptures, pebbles scatter a few meters, and the whole thing shuts itself down. The plant only runs when the pebbles meet each other, and then they only throw off so much heat.) A used pebble can be reprocessed or disposed. (That's the lone disadvantage with uranium: unless you reprocess, you've got waste to keep secure. Not a huge volume of it -- and we already have that problem -- but it's not a perfect answer. It's darn good, though, in a world of very bad choices.)

Meanwhile, the Republicans (and Senator Tom Daschle) are trying to pass a monstrosity of an Energy Bill which gives enormous tax breaks to fossil fuel companies and apparently does little or nothing for alternative energy, including the PBMR. (Did you know that coal-fired electric plants pump not only mercury but also radioactive substances into the environment? I just learned that.)

Leave it to the South Africans, though, to take advantage of a bit of Germany ingenuity. I think America ought to look at the PBMR to help achieve energy independence and to help us shift to a hydrogen-based economy (produced increasingly with wind, solar, hydro, and, yes, PBMR, as we improve the cost economies). Fossil fuels are all short term answers. PBMR looks promising for a medium-term, inexpensive solution as wind, solar, and perhaps hydro and geothermal sources mature and get more affordable.

[NNadir]

1. The pebble bed reactor is a wonderful example of modern

nuclear reactors with passive safety features.

This is not to minimize the success of the pressurized water reactors that you criticize. We have thousands of reactor-years of experience with these reactors, and not one nuclear accident leading to a loss of life. Indeed, only one of these reactors has failed during operation, Three Mile Island, and even then the fail safe system (the containment building) prevented injury.

Of course these reactors were built using 1960's and 1970's technology. They should be retired before they fail, and replaced with modern systems using the advances in computers and materials science that so defined the end of the millenium.

Other promising reactors: The Molten Salt reactor, and other types (besides the pebble bed) of high temperature gas cooled reactors (HTGR).

[tsipple]

2. I Agree 100%

You make an excellent point about the safety record of the existing nuclear power plants in the United States and around the world. They do an excellent job, but it looks like these new technologies, like the pebble plant design, take even the human element out of the safety equation.

I've been reading up on the pebble bed plant, and I got one or two things wrong, so here are a some corrections (and elaborations):

1. The fuel is not designed for reprocessing. The spent fuel is not recoverable (in any feasible manner), which is a good thing because it combats proliferation risk. (It's also really easy to count those pebbles.)

2. The pebbles themselves are triple sealed, so waste storage of spent pebbles should be easier. The fuel is encased in ceramics which are stable. Also, if one of the pebbles should shatter, inside there are 15,000 little fuel granules which are also sealed. Transport is also safe for the same reason.

3. I alluded to the coolant problem, so here's some more information about the idiot-proof aspect to this. The vessel is designed to lose all coolant and require no attention for at least 48 hours. At that point you hook up a fire hose from an ordinary fire truck and you're back in business. If you don't even bother to do that you might have some heat damage to the vessel -- and an expensive repair bill -- but the little pebbles stay intact. (There's not enough heat.) Then the whole thing slowly cools off.

They know this because they actually tested this aspect of the design in Germany. Ran the thing at 100%, pulled the coolant, and then observed what happened.

4. This sort of plant is capable of "peaker" production, meaning that, unlike old fashioned nuclear plants, it can actually respond to different demand loads. This is the whole secret to economical electricity production. Electricity prices spike during periods of peak demand. It also has a lower generating capacity than a giant plant and is modular, so the economic profile is much better, because the plant can grow as demand grows. (You're not paying for a bunch of excess capacity until you need it.)

You can fit three modules (vessels) in a total area the size of a football field. Each module produces about 165 MW of power. Three football fields and you have your biggest (10 module) power complex. So it's very land efficient.

5. There are no mechanical parts involved in circulating the pebbles. Each pebble is loaded at the top, while the plant is operating, and the old pebbles are extracted at the bottom, thanks to simple gravity. (Did I mention the idiot-proof part?) Each pebble makes a few trips through the reactor (to "burn" evenly), and then it is retired. The vessel is design capable of operating continuously for six years until its regular maintenance shutdown, and it goes through six or seven cycles over its 40 year design life.

6. South Africa's utility will be the launch customer. The U.S. may have a plant in production by 2007 if all goes according to plan, and it would be with Excelon in Illinois or Pennsylvania most likely. The United Kingdom is heavily involved in the program, and there are other export markets.

South Africa's energy alternatives are pretty slim. They're out of hydro options, and over 90% of their electricity comes from low-grade coal. (Yuck.) They have no good supply of natural gas, and they have to import their oil. Uranium they've got, and there's plenty of it. One huge problem is the growth of economies in the second and third world, and that's rapidly increasing production of greenhouse gases and other pollutants. This program looks like a good answer.

7. Here are some interesting statistics.

If the U.S. shut off all of its old fashioned nuclear reactors today and replaced them with fossil fuel plants, the additional greenhouse gas emissions would be equivalent to 90 million cars -- or almost the entire U.S. fleet.

How much waste does one of these pebble plants generate? Well, a typical coal-fired plant burns about six trainloads of coal per day and spews mercury, radioactive substances, sulphur compounds, and lots of other nasty stuff. One of these new plants generates the same amount of electricity by using one truckload of pebbles per year.