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PostPosted: Jan 11, 2012 7:18 pm 
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I had an idea, which may or may not be viable, and please feel free to shoot it down if it's not.

One of the big issues with fast spectrum reactors is how to cool them. Water is a neutron moderator, so it won't work if you want to have a hard neutron spectrum. Sodium is one common solution, but sodium is highly reactive, making refueling and responding to leaks complex. Sodium can be corrosive with certain materials, it can solidify if the reactor becomes too cool. Adding potassium reduces the melting point of sodium but also has neutronic disadvantages.

Mercury works well, but is very very toxic and produces extremely toxic vapors. Even a small leak could be a catastrophe.

Lead works okay, but tends to solidify since it has a high melting point. Adding bismuth lowers the melting point but increases neutron absorption, is corrosive and incompatible with certain materials and produces huge amounts of polonium-210, which can present a problem for maintenance and refueling.

Various gasses can be used, helium being the most common, but they don't provide as effective a level of cooling as liquid and may increase the risk of a leak.

Suppose instead of having the coolant flowing around the fuel, the fuel were to be cooled exclusively by conduction, within a solid core structure.

The core would be a solid block of a material of high thermal conductivity. Copper would be the obvious choice, but I don't know if it would have an acceptable neutron cross section. The copper block would have holes drilled into it where the fuel rods would be inserted. The diameter of the holes being slightly larger than the fuel rods in order to allow for thermal expansion. Some material that is soft and could melt, such as lead, could surround the holes drilled through, thus insuring that the rods fit snugly and came into contact with the copper block.

Around the outside it would have fins or be made into the shape of a star or a gear or something, thus increasing the surface area for thermal transfer. The entire thing could then be immersed in water, in a vessel similar to that of a conventional PWR or BWR, but since the water foes not come between the fuel elements, it does not moderate the neutrons and therefore the reactor would have the advantages of a fast reactor for burning transuric elements or to be used as a breeder.


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PostPosted: Jan 11, 2012 8:55 pm 
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You need to do some heat transfer calculations. I'm pretty sure this won't be able to move enough heat.
For starters just estimate the distance from the core to the HX, use 2.4 GWth, and calculate the CX of a solid piece of copper to move that much heat while losing less than say 50C.


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PostPosted: Jan 11, 2012 10:59 pm 
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Los Alamos and NASA MSFC personnel have been working on a solid-core, fast-spectrum reactor that removes heat from the core by heat pipes for years now.


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PostPosted: Jan 12, 2012 4:55 am 
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It's just transferring the problem from the copper to the water. The water needs to be pressurized to make steam for electricity. So you have a pressure vessel that could leak, overpressurize etc. All the problems of the water cooled reactor, but much lower power density and a bigger pressure vessel (or a huge amount of pressure tubes).

All that copper is going to steal a lot of neutrons even in fast spectrum, and produce a lot of activated copper waste. Copper has poor w/m/k compared to moving fluids. Moving fluids typically have 5000 to 50000 w/m/k. Copper can't get past 500.

Fluid coolants with high boiling points are definately the way forward for future nuclear powerplants. I've suggested a solid fuel rods reactor, pool type, NaF-BeF2 cooled, using thorium-plutonium metal fuel and SiC triplex cladding. That would be a quite fast spectrum. Similar to the HEER liquid salt cooled reactor, except that it doesn't use moderating hydride fuel, but thorium metal fuel.


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PostPosted: Jan 13, 2012 12:37 am 
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I would prefer to have thorium double both as fertile fuel and conductive solid. It could also be shaped as 6 fins in a starfish shape for better heat transfer. 20% LEU or reactor grade Pu as PuO2 embedded in uranium/thorium metal could be fissile feed as rods fitting into holes in thorium metal. Thorium could have a SiC cladding vapor deposited on it.
The coolant could be a Mg(65)Al(35) eutectic, safer than sodium due to higher ignition temperature, less neutron hungry and a good conductor. It could have been water for a reduced moderation reactor but that would require a high pressure reactor vessel.
The coolant can circulate through a low pressure tank outside the core with only pipes for steam/gas outside the core requiring a high pressure build.


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PostPosted: Jan 16, 2012 4:01 pm 
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Cyril R wrote:
It's just transferring the problem from the copper to the water. The water needs to be pressurized to make steam for electricity. So you have a pressure vessel that could leak, overpressurize etc. All the problems of the water cooled reactor, but much lower power density and a bigger pressure vessel (or a huge amount of pressure tubes).


I was unaware that PWR's were so problematic. Actually, they seem to work just fine.

Of course, this kind of concept will need to be physically larger, thus a larger pressure vessel and this will mean more steel and that increases capital cost a bit. A few million dollars more of steel is not a huge thing in a multibillion dollar plant that has an 80 year lifetime.

I'm looking at this primarily from a regulatory standpoint. I've come to believe that the only way there's any chance of getting a reactor built in the United States, at least, is to make it as close as possible to existing, conventional reactors. If the reactor is basically a scaled up PWR vessel, but otherwise operates as a PWR except the core is solid and does not contain water, then it might have a shot at getting built. But if you're talking about something that is any more exotic, I don't think it stands a chance.

I think that it's less important to think "what is the best technical way to do this" and start thinking "What actually has a chance of getting NRC approval."

Lets consider the history here: The AP-1000 is one of the most conservative and well reviewed reactor designs ever conceived. It has taken more than a decade to get final approval and it has still faced threats of suspension of approval. The AP-1000 is itself really just a scaled up version of the AP-600, which was approved in 1999 after having been in development since the early 1980's. At this point, the approval process for new reactor technology, even when as conservative as the AP-1000, is pushing on thirty years.

I do not believe that the NRC as it currently exists will EVER approve a gas-cooled power reactor, a liquid metal cooled reactor, a molten salt reactor or any other reactor that is not an extremely conventional light water reactor with as few innovations as possible. In fact, as far as I am aware there is no wholly new reactor design that has ever been approved by the NRC and actually constructed. All the reactors in operation in the US were basically grandfathering their major components from the AEC, which actually would approve new designs.

The only way a reactor with an alternative cooling method has ever been built in the US is if it is built on the grounds of a government facility and declared to be a test or experimental reactor. This skirts most of the NRC regulations. It's not a viable way of making power reactors, however.

There's never even been a heavy water reactor built for commercial power generation in the US, despite numerous attempts.

You can look at some of the more innovative and G4 reactor designs pending: pebble bed reactors, sodium cooled reactors etc. They're in perpetual limbo. None have come even remotely close to regulatory approval. There is no light at the end of the tunnel.

It does not matter how technically elegant something is. The NRC is the gate keeper.


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PostPosted: Jan 16, 2012 5:03 pm 
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Heat conduction through copper or steel is good, but not infinite. If you do some basic heat transfer calculations you'll find that you need to have coolant between the fuel rods, plates or pebbles. Look at how much coolable surface area you need per megawatt. This can't be a monolithic core. You need high surface area, this means not having a big chunk of metal as your core.

It wouldn't offer any safety advantage over the existing PWR either. If you don't remove decay heat it will melt even if it's made of steel or copper. Since it has no moderator it would be dangerous in any core damage event (recriticality). Which is very likely without coolant between your fuel.

Being conservative requires that you use UO2 fuel. This has inherent poor heat transfer. It will run near melting temperatures even in normal operation with lots of coolant around it. With metal to transfer heat it will run much hotter.

Steel and copper are pretty horrible in neutronics as well. These materials will also degrade in the middle of a high fast neutron flux.

Come to think of it, it's a pretty terrible concept, no offense.

PWRs do work fine just the way they are. Getting the NRC out of the loop is an important and necessary step towards a nuclear transition for the USA. Without it any nuclear transition is doomed, including PWRs and [insert favorite nuclear technology x]. The NRC is not a gatekeeper, it's nuclear power's jailor.


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PostPosted: Jan 16, 2012 9:14 pm 
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Cyril R wrote:
Heat conduction through copper or steel is good, but not infinite. If you do some basic heat transfer calculations you'll find that you need to have coolant between the fuel rods, plates or pebbles. Look at how much coolable surface area you need per megawatt. This can't be a monolithic core. You need high surface area, this means not having a big chunk of metal as your core.


Did some "back of napkin" type calculations - very rough ones.

It seems to me like it could potentially work if you the right geometry - like one with lots of fins and heat pipes to increase the thermal transfer and if the water around it moved reasonably quickly.


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PostPosted: Jan 17, 2012 1:27 am 
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drbuzz0 wrote:
I'm looking at this primarily from a regulatory standpoint. I've come to believe that the only way there's any chance of getting a reactor built in the United States, at least, is to make it as close as possible to existing, conventional reactors. If the reactor is basically a scaled up PWR vessel, but otherwise operates as a PWR except the core is solid and does not contain water, then it might have a shot at getting built. But if you're talking about something that is any more exotic, I don't think it stands a chance.

The maximum construction of reactors is now in Asia. US regulation ideosyncracies are not applicable in these cases. Heat transfer is the crux issue technically.


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PostPosted: Jan 17, 2012 9:01 am 
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Joined: Apr 04, 2011 4:11 pm
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drbuzz0 wrote:
Cyril R wrote:
It's just transferring the problem from the copper to the water. The water needs to be pressurized to make steam for electricity. So you have a pressure vessel that could leak, overpressurize etc. All the problems of the water cooled reactor, but much lower power density and a bigger pressure vessel (or a huge amount of pressure tubes).


I was unaware that PWR's were so problematic. Actually, they seem to work just fine.

Of course, this kind of concept will need to be physically larger, thus a larger pressure vessel and this will mean more steel and that increases capital cost a bit. A few million dollars more of steel is not a huge thing in a multibillion dollar plant that has an 80 year lifetime.

I'm looking at this primarily from a regulatory standpoint. I've come to believe that the only way there's any chance of getting a reactor built in the United States, at least, is to make it as close as possible to existing, conventional reactors. If the reactor is basically a scaled up PWR vessel, but otherwise operates as a PWR except the core is solid and does not contain water, then it might have a shot at getting built. But if you're talking about something that is any more exotic, I don't think it stands a chance.

I think that it's less important to think "what is the best technical way to do this" and start thinking "What actually has a chance of getting NRC approval."

Lets consider the history here: The AP-1000 is one of the most conservative and well reviewed reactor designs ever conceived. It has taken more than a decade to get final approval and it has still faced threats of suspension of approval. The AP-1000 is itself really just a scaled up version of the AP-600, which was approved in 1999 after having been in development since the early 1980's. At this point, the approval process for new reactor technology, even when as conservative as the AP-1000, is pushing on thirty years.

I do not believe that the NRC as it currently exists will EVER approve a gas-cooled power reactor, a liquid metal cooled reactor, a molten salt reactor or any other reactor that is not an extremely conventional light water reactor with as few innovations as possible. In fact, as far as I am aware there is no wholly new reactor design that has ever been approved by the NRC and actually constructed. All the reactors in operation in the US were basically grandfathering their major components from the AEC, which actually would approve new designs.

The only way a reactor with an alternative cooling method has ever been built in the US is if it is built on the grounds of a government facility and declared to be a test or experimental reactor. This skirts most of the NRC regulations. It's not a viable way of making power reactors, however.

There's never even been a heavy water reactor built for commercial power generation in the US, despite numerous attempts.

You can look at some of the more innovative and G4 reactor designs pending: pebble bed reactors, sodium cooled reactors etc. They're in perpetual limbo. None have come even remotely close to regulatory approval. There is no light at the end of the tunnel.

It does not matter how technically elegant something is. The NRC is the gate keeper.


Good thing the military does not have to kiss the ring.


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