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PostPosted: Sep 29, 2011 6:48 am 
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In the USA, nuclear pressure boundary materials must be made from a limited selection of alloys. Because this code is tailored to light water reactors, they are all low temperature, below 371 degrees Celcius to be specific. MSRs operate at much higher temperatures. Getting new materials such as Hastelloy N certified would be great but will take a lot of time and money. The ASME website has a discussion on the problems with this with the PBMR, that required high pressure high temperature materials.

http://files.asme.org/Divisions/NED/16798.pdf

This had an interesting solution to this problem that avoided the need for new codes on the pressure boundary.

Quote:
Keep Pressure Boundary at temperature below 371 °C (700 F) (ASME Section III Subsection NB/NC)


Since we have a low pressure reactor we could try to internally insulate the vessel with low thermal conductivity graphite or carbon. Then cool the vessel externally to a temperature lower than 371 Celcius. Added benefit is higher vessel alloy strength and probably cheaper vessel alloys.

This will work well with the piping also.

For the heat exchanger the same could also be done: put it in a seperate vessel that is cooled and argue that the internal heat exchanger is not a pressure boundary (which is of course true) so that it can be made of Hastelloy N (which is already certified for non-nuclear components). The same approach is taken for valves.

Perhaps we can use a very low melting fluoride as buffer salt pool to put the vessel and HX in.

This seems like a promising idea to me. Any thoughts?


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PostPosted: Sep 29, 2011 8:08 am 
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Cyril R wrote:
Any thoughts?

Yes.
Sounds familiar to me -- I proposed that reactor PHT components be merely considered as "flow guides", not containment boundaries requiring code materials.
The hot cell wall should be the primary containment wall, and it can certainly be kept cool, below 371 °C (700 F).

On the other hand, one must wonder how the primary containment of TRISO-type fuel passes NRC review, given that none of the material its made of appears in ASME code :lol:


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PostPosted: Sep 29, 2011 8:46 am 
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Cyril R wrote:
MSRs operate at much higher temperatures. Getting new materials such as Hastelloy N certified would be great but will take a lot of time and money.


Dr. White at Haynes (maker of Hastelloy-N) thought it would take about $2M to code-qualify Hastelloy-N. Not that much in the grand scheme of things.


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PostPosted: Sep 29, 2011 7:10 pm 
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jaro wrote:
Cyril R wrote:
Any thoughts?

Yes.
Sounds familiar to me -- I proposed that reactor PHT components be merely considered as "flow guides", not containment boundaries requiring code materials.
The hot cell wall should be the primary containment wall, and it can certainly be kept cool, below 371 °C (700 F).

On the other hand, one must wonder how the primary containment of TRISO-type fuel passes NRC review, given that none of the material its made of appears in ASME code :lol:

I really like that approach, no matter what we do leaks are a possibility and must be catered for, so the careful choice as to what's a flow guide and what's containment is inspired. We also know that we have high temperature metal alloys that will perform at these temperatures, but we have a gap between that knowledge and what's certified.

On the plus side of the equation, as I understand it high temperature materials certifications are an issue for many Gen IV designs and that there are a number of ongoing materials programmes associated with Gen IV, but I don't know how well that work will transfer to a fluoride salts.


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PostPosted: Sep 29, 2011 8:39 pm 
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In a 1.5 or 2 fluid design I think it makes a lot of sense for the first wall to be considered a flow guide. That should help considerably with the lifetime due to neutron damage as I think that would change things from when does the damage weaken the pressure boundary to when does the damage cause flaking.

For the piping the industry study for MSBR found that putting a "blanket" salt around the fuel salt was necessary to help with thermal shock under accident conditions.

For the HX I don't understand the advantage. We have the fuel salt at maximum pressure at the top end of the HX and the coolant salt is at minimum pressure there so it seems to me we have a pressure boundary. Further, this is also a fission product boundary as we presume the fission products (except tritium) are not migrating through to the secondary salt. Normally one looks for three barriers for fission products. The wall between the fuel salt and coolant for the primary HX is the first of these. Perhaps in a system with a third HX we could not count the primary.


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PostPosted: Sep 30, 2011 4:19 am 
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Lars wrote:
In a 1.5 or 2 fluid design I think it makes a lot of sense for the first wall to be considered a flow guide. That should help considerably with the lifetime due to neutron damage as I think that would change things from when does the damage weaken the pressure boundary to when does the damage cause flaking.

For the piping the industry study for MSBR found that putting a "blanket" salt around the fuel salt was necessary to help with thermal shock under accident conditions.

For the HX I don't understand the advantage. We have the fuel salt at maximum pressure at the top end of the HX and the coolant salt is at minimum pressure there so it seems to me we have a pressure boundary. Further, this is also a fission product boundary as we presume the fission products (except tritium) are not migrating through to the secondary salt. Normally one looks for three barriers for fission products. The wall between the fuel salt and coolant for the primary HX is the first of these. Perhaps in a system with a third HX we could not count the primary.


We can of course use the binary nitrate salt storage loop. This grabs tritium.

If we put everything in a buffer salt pool we get that thermal management thing fixed and can operate the actual dry hot cell portion at lower temperature and have passive hot cell wall cooling (so that it is a passive heat sink).

It would be awesome if we could do what Jaro suggests: the hot cell is the pressure boundary. Everything in it can then be from non-nuclear non-pressure boundary materials (which is almost any material). Will the regulators buy it? I think they will if they consider the passive safety features and lack of stored energy in the 'pressure' boundary.

Lars is correct that the primary HX is a trouble spot in this philosophy. The primary HX will be inside the hot cell. This means we need an isolation valve in the secondary salt, inside the hot cell, and argue that the isolation valve is part of the hot cell wall (the 'pressure vessel'). But the isolation valve is normally open and at higher temperatures than the materials in ASME section III nuclear primary boundary part. The valve can be cooled however at low heat loss since it is normally open.

BWRs have only two barriers: the cladding and the condenser tubes. We'll have the hot cell wall (our 'vessel') and another hot cell wall (our containment). We also have a barrier in the fluoride salt itself: it is low volatility and has excellent fission product sequestration capabilities. More importantly the MSR will be fully passively safe. Walk away safe. BWRs are not walk-away safe so it does not matter how many barriers extra barriers you have, the walk away safe plant will always be safer. If Fukushima had more barriers it would not have helped much. If Fukushima had passive decay heat cooling, as in the latest BWR designs, no core damage would have happened.

Does anyone feel as optimistic as Kirk with regards to nuclear primary boundary code qualification of Hastelloy N?


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PostPosted: Sep 30, 2011 4:01 pm 
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Cyril R wrote:
Will the regulators buy it?

They buy it for hot cells where chemical processing of irradiated nuclear fuel is performed.

Seems to me that the obvious difference -- the safety of the fission chain reactions (criticality, etc) -- depends on the physics of the reactor core design, not on containment:
Again, its a case of "divide & conquer".

Its even possible to design containment ("hot cell") for nuclear explosions (i.e. the PACER project) -- these are of course extremely large.
But the wall liner material is just plain steel -- not some magic material that can withstand the millions of degrees in the nuke blast.


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PostPosted: Sep 30, 2011 4:55 pm 
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Jaro, do you have any ideas about the primary heat exchanger? If the hot cell is your nuclear 'pressure' boundary then the heat exchanger becomes the problem since the plates or tubes that seperate the primary from the secondary side must operate at high temperature by definition, unlike the hot cell walls. Is it enough to have an isolation valve on the secondary side?


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PostPosted: Sep 30, 2011 5:45 pm 
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Cyril R wrote:
Is it enough to have an isolation valve on the secondary side?

Typically, nuclear piping design codes require that two isolation valves in tandem ("double isolation valve") be used on containment penetrations. No big deal.


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PostPosted: Oct 01, 2011 3:08 am 
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jaro wrote:
Cyril R wrote:
Is it enough to have an isolation valve on the secondary side?

Typically, nuclear piping design codes require that two isolation valves in tandem ("double isolation valve") be used on containment penetrations. No big deal.


That's the inside hot cell valve + another just outside it concept, right?

So we are allowed to have these valves normally open when operating - even though it would be part of the nuclear pressure boundary? If there are fission products on the other side of the heat exchanger, and your heat exchanger is considered not part of the nuclear pressure boundary, then the isolation valves are both the 'cladding' and the 'pressure vessel' and they would be normally open!


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PostPosted: Oct 01, 2011 8:31 am 
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Cyril R wrote:
If there are fission products on the other side of the heat exchanger, and your heat exchanger is considered not part of the nuclear pressure boundary, then the isolation valves are both the 'cladding' and the 'pressure vessel' and they would be normally open!

The point is that under normal operating condition - no leak in the HX - there is no radioactive material passing through the pipe.
When the HX starts leaking to the secondary side, the containment valves are closed, and not allowed to open until the HX is repaired or replaced.
Moreover, the shutoff point is based on a certain contamination level in the secondary stream -- a very small leak can be tolerated, but not a significant one (the set point likely being based on radiation levels tolerable on unshielded equipment & piping outside containment).

Every nuke plant has to be able to get stuff in-and-out of containment.
This goes for both radioactive and nonradioactive material -- the former typically being various forms of "waste", everything from spent fuel rods to "spent resins" used for periodic cleaning/decontamination of various parts of piping & equipment inside containment.
In the case of HWRs, it also includes the D2O moderator and PHT fluid -- which may either be piped to an on-site de-tritiation facility, or bottled up and trucked to a plant that has one (which may be hundreds of kilometers away).
In all cases, piping that penetrates containment must have that double isolation valve feature, regardless of whether it normally has radioactive stuff in it, or has the potential to have some.


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PostPosted: Oct 01, 2011 8:42 am 
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Thanks Jaro. What I'm trying to find out is what materials we can use for these valves. If the HX is analogous to the cladding, then the isolation valves are analogous to BWR pressure vessel isolation valves. Must they be made from the same code section as nuclear pressure boundaries? Because then the temp is too high and the valves must be cooled.


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PostPosted: Oct 01, 2011 9:15 am 
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Cyril R wrote:
What I'm trying to find out is what materials we can use for these valves. If the HX is analogous to the cladding, then the isolation valves are analogous to BWR pressure vessel isolation valves. Must they be made from the same code section as nuclear pressure boundaries? Because then the temp is too high and the valves must be cooled.

OK. Good point.
But cooling is in fact a workable option in this case, since its such small equipment -- the valves will have a thick internal liner made of graphite or SiC (optimising thermal insulation), and the valve body could be equipped with cooling fins or other passive heat dissipation devices, to keep the temp. below 370C.
Again, not that big a deal.

Perhaps there might also be a possibility to use a more exotic material for the entire valve -- which can't be used in large pieces of equipment like reactor pressure vessels.
But the code issue remains, unless an all-SiC or all-graphite valve can be built, and we claim that these materials are already accepted for primary containment in TRISO fuel particles, where the temperature is easily in excess of 1000 C.

In this case, I would probably try to go with the cooled valve option, as being the simpler one to engineer & build, as well as to get approved by regulators.


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PostPosted: Oct 01, 2011 9:58 am 
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Cyril R wrote:
That's the inside hot cell valve + another just outside it concept, right?

No.
Both of the valves are on the outside -- the little piece of pipe upstream of them is considered an extension of the containment, therefore subject to the same code as the valves.
The length of the projecting pipe is kept to a minimum -- subject to accessibility to the isolation valves for monitoring/inspection/maintenance/replacement.

Considering the cooling option discussed above, one would need to do the same thing with this short piece of projecting pipe (but beyond the isolation valves, the pipe is non-nuclear).

From an engineering design point of view, the easiest thing to do would probably be to increase the metal pipe diameter by a size or two, at least a meter inward from the containment wall, so that a nice thick layer of internal thermal insulation liner can be accommodated -- running through the containment wall, the two isolation valves, and a bit past that. This should facilitate passive cooling of both valves and pipe.
Furthermore, the cooled section of pipe with containment isolation valves could be joined at either end using flanged connections with thick thermal insulation gaskets, to cut heat conduction through the outer metal wall.

The hot cell (containment) wall through which the pipe passes (and is physically joined to, around the circumference) is also cooled well below 370 C, so the pipe joint area will assist in heat dissipation.

Next problem ?


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PostPosted: Oct 01, 2011 10:10 am 
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Thanks Jaro, that's what I was hoping for. Looks like we can really get started already without waiting for new codes!


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