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PostPosted: Jun 24, 2009 5:34 pm 
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Longtime nuclear naysayer Steve Thomas has a piece on the BAS website claiming that the PBMR is "dead" in light of decreasing South African commitment to the design:

Bulletin of Atomic Scientists: The demise of the pebble bed modular reactor

In particular, Thomas cites this 2008 report from the Juelich Center in Germany about their experience with the AVR:

Does anyone with more background with the PBMR care to comment? I've never studied the PBMR like I have either various Soviet reactors or the ORNL molten-salt proposals.


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PostPosted: Jun 25, 2009 4:08 pm 
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Robert Hargraves is the PBMR guy around here. The South Africans recently signed a coopweration agreement with the chinese. Developiing a reactor might be a little expensive for a small country like South Africa, but the rewards might be good. The problems with the AVR appear to be related to abbrasion on the fuel. This problem is heat related, and might limit opperations temerature.


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PostPosted: Jun 25, 2009 4:46 pm 
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I'm no expert. I asked Andy Kadak to comment on the Juelich paper. The BAS article seemed realistic. Here's a subsequent statement from PBMR.
http://www.reuters.com/article/rbssConsumerGoodsAndRetailNews/idUSLN72588120090623


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PostPosted: Jun 25, 2009 5:00 pm 
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When I was at the ANS conference, several people mentioned to me that the graphite balls in the helium do not form the protective and lubricating layer that they form in air, and that binding up of the balls was a real problem. This was presented to me as an argument for the Per-Peterson version of the pebble-bed, with fluoride salts lubricating the balls and conducting the fission heat.


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PostPosted: Jun 25, 2009 6:07 pm 
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Kirk Sorensen wrote:
When I was at the ANS conference, several people mentioned to me that the graphite balls in the helium do not form the protective and lubricating layer that they form in air.....

Did anyone mention what they've done to investigate this? I would guess the oxidised coat on SiC is silica-like, but any decent research university will have a surface science department that could find out. Once formed, is the coating stable - to heat and to neutrons - in the absence of oxygen? Has anyone tried what happens with CO2? Apart from the irradiation stability test, these are all things that could go into an MSc project.

Luke


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PostPosted: Jun 25, 2009 8:30 pm 
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Kirk Sorensen wrote:
When I was at the ANS conference, several people mentioned to me that the graphite balls in the helium do not form the protective and lubricating layer that they form in air, and that binding up of the balls was a real problem.


Has anyone explored secondary coatings as a solution to this problem?


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PostPosted: Jun 26, 2009 9:32 am 
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You find a short description of this indeed crucial lubrication item on p.11 in:

http://www.nuclear-engineering-journal. ... 221619-119

It was a 'congenital defect' of the Pebble Bed Reactor to assume that the well known lubrication properties of graphite are an intrinsic property of this material. This was originally suspected by Bragg, the nobel price winner, who evaluated the graphite structure in 1928: His opinion was found until 1960 in many textbooks, although it was discovered already during WWII, that a certain amount of moisture is required for good lubrication. All experiments on graphite pebble beds were for cost reasons made in air (except of the reactors itself).

SiC coatings do not help, that was examined. MoS2 coatings can probably be ruled out e.g. for activation reasons.

In the THTR300 where friction between in-core rods and graphite led to destruction of many fuel pebbles, they tried to use NH3 injection: This does not work on graphite but forms a lubricating nitride layer on metals. Unfortunately this led to major corrosion on metals.


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PostPosted: Jun 26, 2009 10:40 am 
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rainier wrote:
In the THTR300 where friction between in-core rods and graphite led to destruction of many fuel pebbles.....

Driving rods through the pebble bed was the cause of the destruction of many fuel pebbles: Bad idea to begin with.

AVR had no such in-core rods, and worked fine for many years.

PBMR Project was well aware of the THTR problems, and steered clear of in-core rods too.


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PostPosted: Jun 26, 2009 11:46 am 
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I agree, AVR had no in-core rods. But it had also huge problems with inhomogeneous pebble flow.

More important: If you look into the first July issue 2002 of Nucleonics Week, you will find there the statement from AVR company that AVR is the most heavily beta-contaminated nuclear installation worldwide (Strontium-90!!). We talk about contaminations which are power weighted for Sr-90 more than 7 orders of magnitude larger than in LWRs. For Cs-137m this difference is only (?) 5 to 6 orders of magnitude.

Also for that AVR was by no means successfull but probably the most unsafe reactor ever operated in Western Europe.


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PostPosted: Jun 26, 2009 11:49 am 
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rainier wrote:
It was a 'congenital defect' of the Pebble Bed Reactor to assume that the well known lubrication properties of graphite are an intrinsic property of this material.


Very interesting Rainier! Welcome!

I would definitely recommend downloading and reading Rainier's paper.


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PostPosted: Jun 26, 2009 12:42 pm 
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rainier wrote:
AVR was by no means successfull but probably the most unsafe reactor ever operated in Western Europe.

Are you sure that's not a slight exaggeration ?

Quote:
Pebble bed scientists defend safety of nuclear power station at global talks
Cape Times, 6 October 2008
Melanie Gosling

SCIENTISTS working on South Africa’s pebble bed modular reactor (PBMR) have defended the safety of their nuclear power plant at an international conference in Washington, saying critics had ignored scientific advances in this field.

They were responding to the findings of a German scientist, Rainer Moormann of the Juelich Research Centre (FZJ), who maintains that all pebble bed nuclear reactors should be equipped with a leak-tight safety barrier, known as a secondary containment. All modern nuclear reactors have a containment to prevent release of radioactivity in the event of an accident.

Moormann raised safety problems concerning pebble beds after re-examining the pebble bed prototype reactor, called the AVR, shut down in Germany in 1988.

Tom Ferreira, spokesperson for the PBMR Company, said PBMR scientists had told the Washington conference that the design of the PBMR demonstration model to be built at Koeberg had a “confinement” safety structure, which was not inferior to the containment structure.

PBMR scientists had established that this method would reduce the risk of radiation leaks to the public, and so was safer. A confinement structure, which allowed for the filtered release of the coolant, but which removed the radioactivity, was the best solution for a gas-cooled reactor like the PBMR. A gas-tight containment was appropriate for water-cooled reactors, Ferreira said.

Moormann had found huge radioactive contamination in the pebble bed prototype, which he said came from inadmissibly high core temperatures. He said in the last 20 years there had been over 10 different explanations for these high core temperatures, but not one was convincing. This had created problems in dismantling the old power station.

The contamination could not be blamed on the use of inferior fuel in the prototype pebble bed, as a similar fuel had been used in the Peach Bottom reactor in Pennsylvania, in the US, where there had been no remarkable contamination.

PBMR scientists argued that the Peach Bottom had used only one fuel type, whereas the pebble bed prototype had used over 20. The early fuel was still contributing to contamination when it was shut down, they said. The PBMR would use fuel based on the “latest high quality” used in Germany, and PBMR scientists were confident the contamination would be “several orders of magnitude lower” than the prototype.

Moormann told the Cape Times that it was unlikely that his findings would be accepted or rejected by the “high temperature reactor community” now or in the near future, as there had been almost no research on high temperature reactors worldwide for the last 20 years, so the number of experts with long-term experience was very small.



Quote:
NUCLEONICS WEEK OCTOBER 2, 2008
Containment requirements debated for pebble bed design
A German research scientist says he remains convinced that pebble bed reactors should have a “gas-tight” containment that meets the same standard as required for LWRs, but agreed with those challenging his view that new PBRs also need to have a way to filter exhaust gas.

Rainer Moormann, a scientist at the Juelich Research Center, or FZJ, in Germany sparked debate with his assertion that containments are necessary for PBRs. Based on his research of the 46-MW thermal (15 MWe) AVR, a demonstration pebble bed reactor that operated in Germany from 1967 to 1988, Moormann said he believes a containment should be built for all future high-temperature reactors (NW, 25 Sept., 1). “The containment should be explosion proven or inertized in order to prevent from potential dust or burnable gas explosions in accidents,” he wrote in a paper entitled “A safety re-evaluation of the AVR pebble bed reactor operation and its consequences for future HTR concepts.”

Moorman drew criticism for some of the key conclusions in his paper, which he presented September 30 at the 4th International Topical Meeting on High Temperature Reactor Technology in Washington, DC. One conference participant called it a “leap of logic” to require a containment, arguing that there needs to be a correlation between the fission products released and the source term in accident conditions. For HTRs, he said, the “driving force” for fission product releases is a “non-condensable coolant.”

Andrew Kadak, a professor at the Massachusetts Institute of Technology’s department of nuclear engineering, also questioned the need for containment. In an August 19 letter to Moormann, provided to Platts last week, Kadak said it was important to consider the differences between fuel used at the AVR and fuel that would be used at future PBRs. He stressed that the AVR used fuel of “varying quality and design,” which would not be the case at new pebble bed reactors. Also, allowing for venting before the core is heated up would provide additional safety benefits. “This early venting followed by low pressure isolation, as you know, avoids the problem of containment bypass or failure sequences when higher radionuclide concentrations may be present at high pressure,” Kadak wrote.

Moormann said that the old type of fuel used at AVR has been blamed for contamination problems at the plant. But he said that was an “unsatisfying explanation” because a similar type of fuel was used at the now-closed Peach Bottom-1 high-temperature gas reactor in the US, and there were not any notable strontium releases there. A more likely explanation of the contamination was the “unacceptably” high temperatures in the AVR core, he said.

The accumulated fission product releases from the fuel elements are an indicator of the high hot gas temperatures, he said. “Assuming that maximum temperatures measured in 1986-87 are not higher than in 1974-76 it becomes clear that the enhanced fission product release is correlated to overheating: hot gas temperatures of 850 degrees C led to release rates by 2 - 4 orders of magnitude smaller,” he wrote.

In his paper, Moormann said that the newer Triso fuel coating “strongly reduces the uranium contamination of graphite and thus the release rates of iodine and noble gases in normal operation.” But he said at this week’s conference that the “breakthrough of cesium diffusion in modern [silicon carbide] coatings is faster than in old [high-temperature isotropic pyrocarbon] coatings.”

Kadak said he agreed with Moorman that there would be a dust problem from the graphite coating on the pebbles, but he disagreed about the extent of its accumulation because of dust cleanup systems and fuel that is less “failure-prone,” he said.

Kadak credited Moormann with raising some issues that should be explored, including conducting more research on the AVR core temperatures and peak temperatures in the pebbles. He said the Pebble Bed Modular Reactor design has a central column in the reactor that “can be instrumented to provide for more direct measurements adjacent to the fuel region.”

Moormann said he is unaware of any way to install an instrument capable of measuring the temperature of individual pebbles. If this is perfected, he said, that would be a significant improvement from the past pebble bed reactor designs.

-Jenny Weil, Washington


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PostPosted: Jun 26, 2009 1:01 pm 
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Thanks for those articles Jaro--here I was about to send Dr. Kadak an email with Rainier's paper, but it sounds like he's already read it! I would recommend others read it too--I think it is very interesting.


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PostPosted: Jun 26, 2009 1:40 pm 
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Very interesting stuff. I only follow PBMRs from the sidelines but the picture certainly isn't as rosy as many portray. Beyond the friction problems I was surprised to hear it mentioned that there was proof that some pebbles were reaching temperatures over 1400 C (by inserted metal test strips found melted). That sort of problem sounds far more serious to me. I never did like the idea of "we don't need a containment structure because nothing can go wrong!".

David L.


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PostPosted: Jun 26, 2009 1:49 pm 
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David wrote:
"we don't need a containment structure because nothing can go wrong!".

There *is* a containment structure.

In fact there are millions: a multi-layered one around each UO2 kernel.

Just because its small, doesn't mean its not there !


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PostPosted: Jun 26, 2009 3:50 pm 
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rainier wrote:
SiC coatings do not help, that was examined. MoS2 coatings can probably be ruled out e.g. for activation reasons.


That hardly sounds exhaustive one would think that there might be a plasma applied coating that might meet the neutronic requirements and provide for better mechanical performance of balls.


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