Talk-Polywell.org

Don't know if this has been posted yet. If so, my apologies.


Pebble Bed Advanced High Temperature Reactor Can Enable Lower Costs and Enable Deep Burn and Advanced Fuel Cycle Reactors

The Pebble Bed Advanced High Temperature Reactor (PB-AHTR) is a liquid salt cooled, high temperature reactor design developed at UC Berkeley in collaboration with Oak Ridge National Laboratory and other national labs.

http://nextbigfuture.com/2010/08/pebble ... ature.html


I've read of the pebble bed reactor design years ago. I'm inclined to think that if it has "nuclear" in it's description anywhere, the flighty people will freak out. The biggest impediment to safe nuclear is ignorant people.

There's a guy over at NSF who is a true expert in systems like the Pebble Bed. He posts under the moniker "vanilla" and I can highly recommend all his writings. IIRC, his criticism of the Pebble Bed was that it is a dangerous design that will eventually melt down. It was slated for use in the Timberwind, but that was a very short duration vehicle design that IIRC, was originally intended for missiles that don't need to burn for long at all. They're "throw away".

Vanilla was originally angry at the Pebble Bed concept because it seemed obvious to him it would melt down. Anyone proposing a different use than very short term either knows some things vanilla doesn't, or misunderstands the limits of the design. Either way, NSF is probably the best place to go to get the real scoop. Don't trust my memory!

GIThruster wrote:There's a guy over at NSF who is a true expert in systems like the Pebble Bed. He posts under the moniker "vanilla" and I can highly recommend all his writings. IIRC, his criticism of the Pebble Bed was that it is a dangerous design that will eventually melt down. It was slated for use in the Timberwind, but that was a very short duration vehicle design that IIRC, was originally intended for missiles that don't need to burn for long at all. They're "throw away".

Vanilla was originally angry at the Pebble Bed concept because it seemed obvious to him it would melt down. Anyone proposing a different use than very short term either knows some things vanilla doesn't, or misunderstands the limits of the design. Either way, NSF is probably the best place to go to get the real scoop. Don't trust my memory!

Perhaps I am misunderstanding something. I thought the "Pebbles" were made of nuclear fuel surrounded with a layer of Silicon Carbide, and the spacing is such (because of the thickness of the Silicon Carbide layer) that the max temperature is limited to well below what Silicon Carbide can withstand. If this is true, how is it possible for this thing to melt down? I had always read that the "melt down proof" design was one of it's biggest selling points.

Diogenes wrote:Perhaps I am misunderstanding something. I thought the "Pebbles" were made of nuclear fuel surrounded with a layer of Silicon Carbide, and the spacing is such (because of the thickness of the Silicon Carbide layer) that the max temperature is limited to well below what Silicon Carbide can withstand. If this is true, how is it possible for this thing to melt down?

No, it relies on the fact that U-235 has a larger fission cross section for slow neutrons. The fuel temperature coefficient of reactivity is large and negative because of doppler-broadening.

Diogenes wrote:I had always read that the "melt down proof" design was one of it's biggest selling points.

The german AVR and chinese HTR-10 have tested withdrawing all control rods and stopping all coolant flow to see what the level of damage to the fuel was. Some small fraction of the tiny TRISO particles encased inside the graphite pebbles do fail, the graphite balls remain intact, the reactor doesn't melt/leak/rupture.

I don't much like pebble beds because the waste form is very difficult to reprocess and takes up a lot of space in the final repository.

Yeah, I also thought that Pebble Bed Reactors were safe...

Skipjack wrote:Yeah, I also thought that Pebble Bed Reactors were safe...

Safe from meltdown, but there are other problems:

http://www.neimagazine.com/story.asp?storyCode=2052589

Warthog wrote:

Skipjack wrote:Yeah, I also thought that Pebble Bed Reactors were safe...

Safe from meltdown, but there are other problems:

http://www.neimagazine.com/story.asp?storyCode=2052589

I liked this bit:

Over the period 1988-89 both German PBRs were taken out of operation. Mainly, it was safety concerns that led to the permanent AVR shutdown: lack of sufficient protection against external impact leading to an air ingress with a core fire, and a potential positive void coefficient of reactivity in water ingress events.

One of the disadvantages of living on a planet that is 2/3rds water.

Honestly, I HATE graphite in reactor cores. Even though it has unique engineering properties for reactors.
Chernobly sits heavy on my mind.

I don't quite understand the water problem. If it increases reactivity so what, it boils the water off, then no more water, then reactivity goes back down.

kcdodd wrote:I don't quite understand the water problem. If it increases reactivity so what, it boils the water off, then no more water, then reactivity goes back down.

It can be inconvenient while the water is boiling.

At a conference, Exelon Corp. Chairman and Chief Executive Officer John W. Rowe spoke about the PBMR and why Exelon Corp has not bought any of this type of reactor.

In a nutshell: they are just too small. Exelon Corp wants BIG nuclear; aka economies of scale.

PS: PBMR power density is low.

kcdodd wrote:I don't quite understand the water problem. If it increases reactivity so what, it boils the water off, then no more water, then reactivity goes back down.

Carbon tends to react with water at high temps, yielding hydrogen and carbon monoxide (the "water-gas" reaction). Used to be used to make "town gas" from coal, before the advent of wide availability of natural gas. Also inconvenient from the perspective of fuel integrity. I don't know if the PBMR runs hot enough to trigger it, but I suspect it is.

There is a new variant in PBMR technology that is being developed by Dr. Per Peterson that solves many of the shortcomings of the current high temperature gas cooled version. See:

http://www.google.com/url?sa=t&source=w ... M4PQObIl4Q

Do you have a slighly more direct link that doesn't require a download?

All this is why I kind of prefer the Molten Salt reactor (aka LFTR). Very high temperatures so high thermal efficiencies and effectively "meltdown proof" in that it is already molten and any loss of system power automatically "pulls a cork" and the fuel drains out of the core. Neat!

And these are plausible in a VERY wide range of power outputs, low megs to gigs.

Oh, and an intermediate machine has been designed, the liguid salt cooled PBR.