# Energy From Thorium Discussion Forum

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 Post subject: Pressure at fuel salt surfacePosted: Dec 28, 2013 9:55 am

Joined: Jul 28, 2008 10:44 pm
Posts: 3063
In a full blanket system we have one blanket thickness of blanket salt above the top of the core. If the blanket is 1 meter thick and the density is 4 grams/cm^2 this implies we have added 4 atm of pressure at the surface to the natural 1 atm of pressure.
But there is also a desire to hold the gas pressure at the surface to a mild vacuum to encourage off-gassing and a general desire to keep any gas pressure in the system low to reduce the motive force to push radioactivity into the biosphere in an accident.

The tradeoff gets worse if we increase the thickness of the fluid above the fuel salt level - such as we might do in a pool type design.

I'm wondering if we need to partition the blanket (and pool) salt into above and below the fuel salt level and have sufficient structure in place to bear the weight of the blanket (pool) salt above the fuel salt level without applying pressure on the salt below. (Rather the opposite problem that Holger was worried about).

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 Post subject: Re: Pressure at fuel salt surfacePosted: Dec 28, 2013 4:06 pm

Joined: Jun 05, 2011 6:59 pm
Posts: 1335
Location: NoOPWA
I think you slipped a decimal point. Or even more. Its more like 0.2 bar, not 4.

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DRJ : Engineer - NAVSEA : (Retired)

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 Post subject: Re: Pressure at fuel salt surfacePosted: Dec 28, 2013 4:42 pm

Joined: Jul 28, 2008 10:44 pm
Posts: 3063
Yep. 100cm = 1m (not 1000cm=1m). So the blanket salt adds 0.4 atm if it is 1 meter thick (which is a pretty thick blanket but not much for pool salt). Makes the problem less severe but still makes it hard to have a partial vacuum in the gas space above the fuel salt. But probably we could reduce the blanket to just 25 cm thick above the fuel salt level. This would allow a gas pressure at 0.1atm which is likely sufficient for the job.

The vessel outside the blanket salt could be strong enough to tolerate the extra pressure from having a significant amount of pool salt above it.

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 Post subject: Re: Pressure at fuel salt surfacePosted: Dec 28, 2013 9:23 pm

Joined: Jun 05, 2011 6:59 pm
Posts: 1335
Location: NoOPWA
Still off by 2,
10m H2O = ~1bar.
FLiBe SG = ~2.
1m FLiBe=~0.2 bar.

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DRJ : Engineer - NAVSEA : (Retired)

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 Post subject: Re: Pressure at fuel salt surfacePosted: Dec 28, 2013 11:30 pm

Joined: Jul 28, 2008 10:44 pm
Posts: 3063
But blanket salt is not pure FLiBe. It is full of thorium so it is around 4 g/cm^3.

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 Post subject: Re: Pressure at fuel salt surfacePosted: Dec 29, 2013 11:47 pm

Joined: Jun 05, 2011 6:59 pm
Posts: 1335
Location: NoOPWA
Lars wrote:
But blanket salt is not pure FLiBe. It is full of thorium so it is around 4 g/cm^3.

That much? What % is that?

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DRJ : Engineer - NAVSEA : (Retired)

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 Post subject: Re: Pressure at fuel salt surfacePosted: Dec 30, 2013 12:55 am

Joined: Jul 28, 2008 10:44 pm
Posts: 3063
Around 20% ThF4 is common. The more thorium the less neutron capture by Pa.

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 Post subject: Re: Pressure at fuel salt surfacePosted: Dec 30, 2013 6:06 am

Joined: Jun 05, 2011 6:59 pm
Posts: 1335
Location: NoOPWA
Ok.

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DRJ : Engineer - NAVSEA : (Retired)

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 Post subject: Re: Pressure at fuel salt surfacePosted: Dec 30, 2013 8:25 pm

Joined: Jul 28, 2008 5:01 am
Posts: 461
Location: Teesside, UK
Lars wrote:
Around 20% ThF4 is common....

And that is 20% mole/mole. By mass, the blanket is >50% thorium, most of the rest fluorine, and only 6-7% Li

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