Pebble Bed Reactor - a fuel nightmare?

Andrew Mason
Science Advisor
Homework Helper
7,549
318

Main Question or Discussion Point

I am not sure why the Pebble Bed modular reactor (PBMR) is being touted as the great solution to problems with nuclear power. It uses a once through, virtually non-reprocessible fuel. It produces a high volume of waste consisting not only of the fuel but the casing/moderator which has about 50 times the volume of the fuel and 20 times its mass. It is essentially a uranium hogging once-through thermal reactor, but with a bigger waste problem. The graphite moderator in each pebble could also be a fire hazard. If the graphite moderator in the pebbles should catch fire, there could be a catastrophic release of radiation.

Its chief advantages seem to be the high efficiency due to its ability to use the helium coolant that is in the core to drive turbines directly. Supposedly, there will be no radioactivity in the helium coolant. I am not so sure about that.

It seems to me that the PBMR is a vastly overrated reactor.

AM
 

Answers and Replies

745
20
Well, I was talking specifically about propulsion applications for it.
 
Astronuc
Staff Emeritus
Science Advisor
18,543
1,685
Andrew Mason said:
It produces a high volume of waste consisting not only of the fuel but the casing/moderator which has about 50 times the volume of the fuel and 20 times its mass.
I don't think those numbers are correct. Here is an example of a TRISO fuel particle - http://en.wikipedia.org/wiki/Nuclear_fuel#TRISO_fuel

http://www.romawa.nl/nereus/fuel.html

The pyrolytic carbon and SiC coatings have some volume similar (maybe 3-4x) to the fuel kernel, and the mass IIRC is slightly less.

However, clearly there is still a waste issue.

Reprocessing is difficult, but not impossible.
 
Last edited by a moderator:
Andrew Mason
Science Advisor
Homework Helper
7,549
318
I don't think those numbers are correct. Here is an example of a TRISO fuel particle - http://en.wikipedia.org/wiki/Nuclear_fuel#TRISO_fuel

http://www.romawa.nl/nereus/fuel.html

The pyrolytic carbon and SiC coatings have some volume similar (maybe 3-4x) to the fuel kernel, and the mass IIRC is slightly less.
According to http://en.wikipedia.org/wiki/Pebble_bed_reactor#Containment" there is 9 g. of U in a 210 g. 60 mm diameter ball (volume:113 cm^3) .

The density of U is 18.7 g/cm^3 so the U would occupy .5 cm^2. So this fuel pebble occupies over 200 times the volume of the U. I was being kind at saying 50 times.

However, clearly there is still a waste issue.

Reprocessing is difficult, but not impossible.
Not only the waste in terms of handling and storing it, but in terms of making efficient use of uranium. Although U is a plentifiul element, economic deposits are not abundant. There is a serious world shortage of U at the moment, as the 700% increase on price since 2003 indicates.

AM
 
Last edited by a moderator:
Astronuc
Staff Emeritus
Science Advisor
18,543
1,685
The Wikipedia article - Pebble_bed_reactor#Containment - may be incorrect. I checked the Idaho site and a report on the calculation of Dancoff factors for a typical pebble has the typical diameter of a TRISO particle at 25 mm. I know several people involved with the fuel development, so I can get better data. Petti et al reported about some of the fuel development work at the ANS meeting last June in Reno, NV. I'll be seeing these guys later this year.
 
Andrew Mason
Science Advisor
Homework Helper
7,549
318
The Wikipedia article - Pebble_bed_reactor#Containment - may be incorrect. I checked the Idaho site and a report on the calculation of Dancoff factors for a typical pebble has the typical diameter of a TRISO particle at 25 mm. I know several people involved with the fuel development, so I can get better data. Petti et al reported about some of the fuel development work at the ANS meeting last June in Reno, NV. I'll be seeing these guys later this year.
There are a number of sites that provide the same information as the Wikipedia article:

These papers describe in detail the fuel pebbles:
http://www.pbmr.com/download/FuelSystem.pdf [Broken]
https://odin.jrc.nl/htr-tn/HTR-2004/B15.pdf [Broken]

This was on the World Nuclear.org site:
http://www.world-nuclear.org/sym/1999/kemm.htm

See also this abstract:
http://linkinghub.elsevier.com/retrieve/pii/S0029549303000062 [Broken]

The U fuel is in the form of UO2 which has a density of about 10 g/cm3. It appears from the above that the UO2 consists of tiny .5 mm diameter grains each of which are coated with various layers of different carbon material and then mixed in a resin to form a ball and then coated with a final ceramic carbon layer. 9 grams of UO2 in a 210 gram pebble with a 60 mm diameter.

AM
 
Last edited by a moderator:
24
0
I am not sure why the Pebble Bed modular reactor (PBMR) is being touted as the great solution to problems with nuclear power. It uses a once through, virtually non-reprocessible fuel.
The fuel is not "once through", if that is meant in the literal sense. Each "pebble" will be analyzed when it is ejected from the core and diverted either for re-use or to a waste stream.

Supposedly, there will be no radioactivity in the helium coolant. I am not so sure about that.
Helium has a relatively low activation cross section so if it is maintained pure then it will not activate from neutron irradiation.

One of the main PBMR advantages is its inherent safety feature of a negative temperature co-efficient of reactivity and the fact the fuel design is less attractive for potential proliferation activities.
 
Andrew Mason
Science Advisor
Homework Helper
7,549
318
The fuel is not "once through", if that is meant in the literal sense. Each "pebble" will be analyzed when it is ejected from the core and diverted either for re-use or to a waste stream.
"Once-through" refers to the lack of reprocessing. Once the U235 is down to levels that will not sustain a reaction, the pebble, and the uranium in it, is treated as waste.

Helium has a relatively low activation cross section so if it is maintained pure then it will not activate from neutron irradiation.
I am having difficulty imagining how you can run the gas repeatedly through a turbine and not contaminate it with matter that will capture neutrons. But I take your point.

One of the main PBMR advantages is its inherent safety feature of a negative temperature co-efficient of reactivity and the fact the fuel design is less attractive for potential proliferation activities.
I think that can be achieved with much better fuel usage using other designs.

AM
 
Astronuc
Staff Emeritus
Science Advisor
18,543
1,685
The alpha particle (nuclear of He-4) is extremely stable. If it absorbed a neutron to become He-5, He-5 decays to He-4 (alpha) and a neutron. Basically the He-4 does not become activated.

However, if cracks develop in the carbon or SiC coatings, then fission products, e.g. noble gases and volatile radionuclides like I-131, I-135, can escape and deposit on the fuel or in the primary system.
 
24
0
Yes, the coolant itself will not become active itself, certainly much less so than other coolant materials such as carbon dioxide, water, sodium, etc As well as any escaped fission products, there will always be impurities present in any reactor cooling system, such as activated trace metals from metal components and grease, lubricants, etc used to construct and maintain the containment. These are virtually impossible to eliminate entirely, even with good scrubbing systems, although they can be reduced by appropriate choice of materials. It is difficult to avoid steel though, & the associated activated Co-60. Not perhaps a contender for long term waste consideration but certainly for radiation protection considerations.

"Once-through" refers to the lack of reprocessing.
This could be an American term? I have not heard it. I will conduct a discrete investigation amongst my colleagues.

PBMR-type design technology was dropped in the UK & Germany. It would be interesting to know if this was purely from an economic standpoint or whether there were issues with it.
 
125
2
All reactor design was dropped in the UK when the future for electricity generation was seen as gas-fuelled. The South African PBMR does however draw on German expertise but it's seen as a Generation III technology. Six different reactor technologies have been chosen for the Generation IV programme.

http://www.world-nuclear.org/ is a good source of information, albeit not unbiased.
 
Morbius
Science Advisor
Dearly Missed
1,125
6
All reactor design was dropped in the UK when the future for electricity generation was seen as gas-fuelled.
rdt2,

I'm sorry to hear that the UK dropped their work on nuclear reactors in favor of gas.

While gas may be the "cleanest" of the fossil fuels, it probably has the shortest
longevity of any fossil fuel, as well as continuing to be a green house gas [CO2]
emitter; as are all fossil fuels.

Dr. Gregory Greenman
Physicist
 
125
2
I'm sorry to hear that the UK dropped their work on nuclear reactors in favor of gas.
Oh, while the public posture is still anti-nuclear, the technological illiterates in Westminster seem to have realised that they have no option but nuclear power if they want to:

a) provide the capacity for the whole country to make a cup of tea when the six-o'clock news starts

b) guarantee power in the face of the increasingly-aggressive Gasprom, who supply the gas from Russia.

So, we've signed up for the Generation IV programme ( http://www.gen-4.org ). The problem is to re-establish expertise, particularly in fast reactor technology, where the UK once led the world. Partly to that end, there's a new nuclear research facility at Manchester Uni ( http://www.dalton.manchester.ac.uk/ ). But it might have been easier if British Nuclear Fuels, who owned Westinghouse Electric, hadn't sold it to Toshiba.

However, while the UK government may have woken up, our local parliament here in Scotland is still hiding its head in the sand - in spite of the fact that two of our major nuclear plants (Hunterston and Torness) are due for decommissioning soon. Reality will fall on them when the power-cuts start.
 
Last edited by a moderator:
Astronuc
Staff Emeritus
Science Advisor
18,543
1,685
Nuclear Fuel Performance Milestone Achieved [with TRISO carbide fuel]

ScienceDaily (Mar. 13, 2008) — Researchers at the U.S. Department of Energy's Idaho National Laboratory, in partnership with three other science and engineering powerhouses, reached a major domestic milestone relating to nuclear fuel performance on March 8.

David Petti, Sc.D., and technical director for the INL research, says the team used reverse engineering methods to help turn the fuel test failures from the early 1990s into successes in 2008. "We wanted to close this loop for the high-temperature gas reactor fuels community," he said. "We wanted to put more science into the tests and take the process and demonstrate its success."

The research is key in supporting reactor licensing and operation for high-temperature reactors such as the Next Generation Nuclear Plant and similar reactors envisioned for subsequent commercial energy production.

"Hats off to the R&D fuels team on this major milestone," said Greg Gibbs, Next Generation Nuclear Plant Project director. "This is a major accomplishment in demonstrating TRISO fuel safety. This brings us one step closer to licensing a commercially-capable, high-temperature gas reactor that will be essentially emission free, help curb the rising cost of energy and help to achieve energy security for our country."
. . . .

Research details

The research to improve the performance of coated-particle nuclear fuel met an important milestone by reaching a burnup of 9 percent without any fuel failure. . . .
That's close to 90 GWd/tHM, which is a significant burnup level.

There will probably be some papers and discussion at the upcoming ANS Summer Meeting in Anaheim, CA this June. I'm planning on being there.
 
mgb_phys
Science Advisor
Homework Helper
7,744
12
While gas may be the "cleanest" of the fossil fuels, it probably has the shortest
longevity of any fossil fuel,
When the electricity market was privatised it was done in such a way as to make time to market the most important factor. Gas fired are the quickest to build - although many of them have had to be rebuilt only 5years later.

We don't have to worry about gas supplies, although the north sea has run out we can get gas from Russia and Iran so supplies are perfectly secure.
 
745
20
Nuclear Fuel Performance Milestone Achieved [with TRISO carbide fuel]

That's close to 90 GWd/tHM, which is a significant burnup level.

There will probably be some papers and discussion at the upcoming ANS Summer Meeting in Anaheim, CA this June. I'm planning on being there.
So that's for TRISO fuel.
But what's the highest burnup achieved for any type of fuel?

http://en.wikipedia.org/wiki/Burnup

Is the previous record 60GWd/tHM?
In which case, that would make this a 50% improvement. Wow.
And they said they expect to achieve 14% burnup by yearend? Wow, so more than a doubling of nuclear fuel efficiency.
 
Last edited:
2
0
I don't think those numbers are correct. Here is an example of a TRISO fuel particle - http://en.wikipedia.org/wiki/Nuclear_fuel#TRISO_fuel

http://www.romawa.nl/nereus/fuel.html

The pyrolytic carbon and SiC coatings have some volume similar (maybe 3-4x) to the fuel kernel, and the mass IIRC is slightly less.

However, clearly there is still a waste issue.

Reprocessing is difficult, but not impossible.
Can this technology (massive amounts of very low grade spent fuel) place us closer to the old Jimmy Carter dream for long term storage of spent fuel, ie., by diluting in ceramic blocks at "natural" (3%) concentrations in locations like the Marianas trench?
 
Last edited by a moderator:

Related Threads for: Pebble Bed Reactor - a fuel nightmare?

  • Last Post
Replies
3
Views
4K
  • Last Post
Replies
17
Views
1K
  • Last Post
Replies
2
Views
3K
Replies
8
Views
1K
Replies
1
Views
851
Replies
9
Views
4K
  • Last Post
Replies
4
Views
6K
Replies
10
Views
4K
Top