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 Post subject: Idea: the unsealed core
PostPosted: May 05, 2014 2:12 am 
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There are many systems for handling liquids that do not rely on seals for operating without leaks. For example, a roof with overlapping shingles or a gutter with sections inserted one into the other. These systems rely on gravity and the inertia of the liquid to enable it to safely pass over a joint or other connection between separate sections with no seals.

I think this principle can also be applied to a molten salt core.

The unsealed core is designed for one specific flow rate and therefore one specific dynamic and static pressure at all relevant unsealed joints. If the pumps run faster than nominal the input buffer overflows into a channel that bypasses the core. If the pumping rate is too low, the core develops voids and goes non-critical. Varying the heat output can be done by adjusting the flow rate of the secondary salt circuit. If the pumps stop, the core soon empties into the lower sump.

Why?

This type of construction may be more suitable for brittle materials that are difficult to seal while accommodating for dimensional changes resulting from thermal expansion and neutron irradiation. With a suitable design, all parts under high neutron flux may be inserted or removed by simple vertical motions. This makes it relatively easy to replace these parts with a crane if their lifetime is significantly shorter than that of the rest of the circuit. It can also make failure of such parts non-catastrophic for the reactor.

The parts of the circuit not under high neutron irradiation have sealed joints and may be constructed with more conventional metal alloys. The unsealed core would obviously be contained within a fully sealed enclosure. The walls of this containment are protected from neutron irradiation by shielding blocks and lined to protect them from any salt splashes and collect the salt safely on the event of major failure of the unsealed parts. Other than the fuel channels there are no significant voids in the core, only small expansion gaps. This makes it impossible for the fuel to assume unexpected shapes that may be more critical.

So what's the difference between this design and a fully flooded core? Why not just let the salt fill the gaps, too?

The fuel can avoid contact with the moderator, for example. It can be contained in pipes of a more durable material. This reduces the requirements on the moderator to "be there, and be carbon". No problem of fission product penetration. surface erosion or even minor cracks. This could significantly increase the lifetime of the moderator. Another interesting possibility is a solution to the "barrier material problem" on a two-fluid reactor. The blanket can be constructed from a series of wedges such that all paths from the core pass through sufficient thickness of blanket salt before reaching the core wall. With this design there is greater choice of possible barrier materials and it can be more easily replaced, if necessary.

There are obviously many potential problems with this idea and design constraints that may not be compatible with other core design goals. I am wondering whether the potential benefits are interesting enough for further study of the concept.


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PostPosted: May 05, 2014 8:11 am 
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Joined: Nov 30, 2006 9:18 pm
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Location: Montreal
Owen T wrote:
With a suitable design, all parts under high neutron flux may be inserted or removed by simple vertical motions. This makes it relatively easy to replace these parts with a crane if their lifetime is significantly shorter than that of the rest of the circuit. It can also make failure of such parts non-catastrophic for the reactor.

Nicely written – thanks!
I think the unsealed core idea is well suited to the calandria-type HW-MSR.

https://dl.dropboxusercontent.com/u/116 ... SR_sml.jpg

https://dl.dropboxusercontent.com/u/116 ... iping4.jpg

https://dl.dropboxusercontent.com/u/116 ... iping4.jpg

https://dl.dropboxusercontent.com/u/116 ... ctions.jpg


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PostPosted: May 05, 2014 10:16 am 
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Jaro, the salt part of your HWMSR uses the same ideas of the unsealed core. The connection from the top manifold to the vertical pipes can be achieved without the syphons, though. As long as everything is designed for one flow rate the right cross section profile and a bit of overlap can ensure no leaks - and no need for the complex syphon priming sequence.


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PostPosted: May 05, 2014 11:50 am 
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Owen T wrote:
no need for the complex syphon priming sequence.

That's one way of looking at it.

The way I see it, the sequence allows a gradual, safe approach to criticality, one fuel channel at-a-time. The system also allows for very quick shutdown, by simply opening the vacuum line(s) to outside air. Fuel boiling will also kill the siphon circulation & shut down the reactor, without any operator action.

Also, if the little vacuum lines can be easily disconnected, then removal/replacement of individual fuel channels can be a relatively simple affair.


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PostPosted: May 07, 2014 12:26 am 
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A partially molten fuel with light water moderator-coolant tubes would be simple, safe and easy.
Phase change latent heat of fuel will ensure constant temperature in spite of any coolant loss till dilution of fuel and loss of moderator cools off things. The only sealed structure required is water tubes which are similar to boiler tubes. You could have NaF-ZrF4 blocks melting at or just above operating temperature as heat sinks.


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