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 Post subject: Re: Meltdown Risk?
PostPosted: Dec 13, 2013 5:45 am 
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fab wrote:
Thanks Cyril, I guess Charles F. was maybe too optimistic, unless its concept has really a great quantity of BDBA salt. Moving heat into the atmosphere is the most convenient way to remove decay heat I suppose. The properties of the ground are very complicated and depending of the location, composition, water content, season, etc ...


I did some quick modelling for you to find out what the peak temperatures of the cavity become when there is only ground cooling (but with cold air above a thick layer of ground). By simply taking a "black box" approach, with a block heat source buried in the ground with only ground around it, we can see how useful ground conduction is. The goal being not to precisely model a specific system but just to get the thermal properties and heat source right.

It is far worse than I expected.

With very optimistic assumptions, such as ground conductivity of 3 wmk, only 1 kWt/m3 decay heat source, and high boundary heat loss rates (not realistic - theres just more low conductivity rock outside the model boundary), the peak temperature outside the heat sourcerose to over 5000 degrees Celsius. Yes that is not a typo, 5000 C. At that point I stopped the model, temp was still rising but it is clear everything was already molten so no longer realistic (model can't cope with stuff melting).

Of course this is just a simple thing, but it goes to show ground conduction is nearly completely useless as a long term heat sink, it is at best a small bonus heat sink in the short to medium term if another passive cooling system is also available.

Attachment:
AHTR ground conduction test.png
AHTR ground conduction test.png [ 318.01 KiB | Viewed 1505 times ]


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 Post subject: Re: Meltdown Risk?
PostPosted: Dec 13, 2013 6:06 am 
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Next is to up the ante with a higher thermal conductivity value for the ground of 5 wmk and a ground emissivity of 0.9

The temperature now gets to around 4000 Celsius in the ground near the cavity wall.

Still a complete meltdown.

This does not bode well for the AHTR idea to conduct heat to the ground upon loss of DRACS.


Attachments:
AHTR ground conduction test high conductivity.png
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 Post subject: Re: Meltdown Risk?
PostPosted: Dec 13, 2013 7:13 am 
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Now to compare to air cooling. If we place the same heat source, out in the open air. The peak temperature at the side of the heat source is now below 300C. This shows how effective air cooling is compared to ground conduction.

Attachment:
AHTR air convection test.png
AHTR air convection test.png [ 814.45 KiB | Viewed 1500 times ]


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 Post subject: Re: Meltdown Risk?
PostPosted: Dec 13, 2013 12:53 pm 
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Thanks for the modelling.

So the yellow box is an homogeneous heat source and you assume a thermal power of 1kW/m3 and groud conductivity of k=3W/(m.K) and k= 5W/(m.K)

Two questions :

What is the volume of your heat souce ?
You assume no material between the heat source and the ground ? (perfect thermal transfer between the heat source and the ground)

Even with all those very optimistic assumptions you have more than 4000 °C outside the heat source ...

So the ground is effectively a damned good insulator.


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 Post subject: Re: Meltdown Risk?
PostPosted: Dec 13, 2013 6:24 pm 
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Yes it assumes perfect thermal transfer between the heat source and the sink. Also the cube has a homogeneous conductivity of 250 wmk. (also optimistic since the surface won't have that, only the internal molten salt flow in decent natural circulation will get that).

The heat source is a 12x12x12m cube. So 1728 m3 (and that many kWt). 1.7 MWt. For comparison, a 2400 MWt AHTR still makes over 4 MWt of decay heat after one month post shutdown.

Ground is indeed, a very good insulator, once it has heated up to near equilibrium conditions. Perhaps this should not surprise us - deep beneath our feet, after all, being red hot mantle, much of this heat being billions of years old and still stuck inside the planet.

If we consider a 12 meter deep column making 12x1 = 12 kWt, 12000 W/m2 beneath your feet. Then compare this to a typical geothermal heat flux of 0.1 W/m2. We get a factor of a hundred thousand times the natural geothermal heat flux. That may help to explain these temperatures.


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 Post subject: Re: Meltdown Risk?
PostPosted: Dec 13, 2013 8:09 pm 
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Ok so it's impossible to use conduction in the ground.

I wanted to ask some questions about your pool type MSR design. I don't want to bore you so if you are tired of questions, say it.

You said that you couple the 2 containments thermally by using flexible thermal shunts, so I guess that these are bands of flexible high conductive metal (like some kinds of spring).

Even if they are flexible can we imagine that they are broken ?

If they fail you still have :

  • convection in the inert gas between the two containment
  • thermal radiation between the two containment

Are these thermal transfers sufficient to prevent the failure of your pool vessel ? (I know that if something destroy the thermal shunts it will also probably break the vessel and the containments but let's imagine that everything else is still intact to discuss the efficiency of the thermal transfers).

In that case I guess you will ironically have to break the containments yourself (open valves) for allowing air ingress and air cooling of the buffer salt, but the non volatil radionuclides are still contained in the fuel salt ( or the buffer salt if the reactor vessel fails ).


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 Post subject: Re: Meltdown Risk?
PostPosted: Dec 13, 2013 11:59 pm 
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Is the color of the medium indicative of the temperature? If so, why is the temperature showing uniform across the entire volume? With a poor conductor, wouldn't the color gradient be uniform throughout the medium?

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 Post subject: Re: Meltdown Risk?
PostPosted: Dec 14, 2013 5:51 am 
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KitemanSA wrote:
Is the color of the medium indicative of the temperature? If so, why is the temperature showing uniform across the entire volume? With a poor conductor, wouldn't the color gradient be uniform throughout the medium?


It is, except for heat sources that are always yellow. But the settings for the scale are the standard ones, so everything hotter than 100C or so will be white. The actual gradient just continues on and is steeper closer to the heat source. I wasn't interested in the gradient though, only peak temperatures.


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 Post subject: Re: Meltdown Risk?
PostPosted: Dec 14, 2013 5:58 am 
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fab wrote:
Ok so it's impossible to use conduction in the ground.

I wanted to ask some questions about your pool type MSR design. I don't want to bore you so if you are tired of questions, say it.

You said that you couple the 2 containments thermally by using flexible thermal shunts, so I guess that these are bands of flexible high conductive metal (like some kinds of spring).

Even if they are flexible can we imagine that they are broken ?

If they fail you still have :

  • convection in the inert gas between the two containment
  • thermal radiation between the two containment

Are these thermal transfers sufficient to prevent the failure of your pool vessel ? (I know that if something destroy the thermal shunts it will also probably break the vessel and the containments but let's imagine that everything else is still intact to discuss the efficiency of the thermal transfers).

In that case I guess you will ironically have to break the containments yourself (open valves) for allowing air ingress and air cooling of the buffer salt, but the non volatil radionuclides are still contained in the fuel salt ( or the buffer salt if the reactor vessel fails ).


It is sufficient to prevent failure. My approach is to keep peak temperatures as low as possible.

In reality a total shunt failure wouldn't be plausible. Some might break off creating temperature differentials across the containments. This would not be a problem as the containments are free standing.

Later on though I tended to prefer just using a single containment as the peak vessel temperature is lower than that of a double containment even with all shunts intact, so ironically it is safer to have fewer barriers.


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 Post subject: Re: Meltdown Risk?
PostPosted: Dec 14, 2013 9:24 am 
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Ok thanks. I really like your concept, with the buffer salt which offer liquid containment (cannot be broken) and high thermal inertia; and the fact that the convection loops are "open" ( I mean in large spaces and not in pipes). It's seems extremely difficult to stop the passive cooling. Even if the building collapses (bombing) the debris fall at the bottom of the buffer salt (if your pool is big enough) and you still have passive cooling with air convection in contact of the buffer salt.

You should try to patent it (but I guess you need a precise and well established design for that).

Quote:
so ironically it is safer to have fewer barriers


But for regulatory reasons you need 3 barriers. Maybe you can have a containment vessel close to your reactor vessel (or use double pipes if you have a loop design). The problem is that complicate the thermal transfer between you reactor vessel and your pool of buffer salt.

Or you use the vessel of your pool of buffer salt like a second barrier by closing it (sodium reactor style). Problem : the transfer between the buffer salt and the containment's gas becomes less efficient, (and if some debris fall onto the pool vessel and increase the thermal resistance, ... I prefer the previous solution).

But you've already thought about all of that.


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 Post subject: Re: Meltdown Risk?
PostPosted: Dec 14, 2013 11:01 am 
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I'm not sure if the regulators demand 3 barriers. BWRs have two barriers, the fuel cladding and condenser tubing. In the US, the regulators demand a pressure capable containment (at least 3 atmospheres of gage pressure). That is the only troublesome regulation that I can think of (we don't need 3 atmospheres of pressure ever, we would like a slightly negative gage pressure, filtered confinement, not a containment).


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 Post subject: Re: Meltdown Risk?
PostPosted: Dec 14, 2013 1:15 pm 
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Well, I have seen so many times these "3 barriers" that I thought this was a regulatory requirement.

For BWRs I believed that several isolation valves can isolate the turbine building from the reactor building and serve as a " third barrier " (at least two valves on each line).

For a PWR in case of failure of fuel clading and rupture of the steam generator tubes, we can also have radioactive products going in the turbine building. So I guess that there are also isolation valves in the secondary loop which serve as a "third barrier". But I don't know.

In any case I think that the public will prefer a concept with at least the same number of barriers than actual PWRs. I think that your concept with a double steel containment plus the shield building will reassure the public.


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 Post subject: Re: Meltdown Risk?
PostPosted: Dec 14, 2013 2:36 pm 
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But isolation valves are normally open so they can fail to close. During the most serious operation state, full power normal operation, the isolation valves are open and not a containment. All you have is a flimsy cladding and a similarly flimsy condenser tube. Plus the coolant itself is radioactive so for the coolant itself there is only one barrier, a flimsy condenser tube.

For the buffer salt, we can pretend that it is another barrier. It shields radiation, is nonvolatile, and sequesters any activity coming out of the primary loop in case of a primary loop failure.


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 Post subject: Re: Meltdown Risk?
PostPosted: Dec 14, 2013 3:26 pm 
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Quote:
But isolation valves are normally open so they can fail to close.


Well, in that case I guess the containment is not impervious and we have some radioactive releases. I suppose we have to check in the regulatory autority instructions in order to see what are their confinement's requirements if we want to know.

Quote:
For the buffer salt, we can pretend that it is another barrier.


Yes but volatil products like noble gases and iode can escape, right ?


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 Post subject: Re: Meltdown Risk?
PostPosted: Dec 14, 2013 3:39 pm 
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Not sure about iodine, it will dissolve in buffer salt. There's a lot of buffer salt to keep volatility of dissolved components down, plus the passive cooling will prevent high temperatures in the buffer salt that could release the volatiles. Iodine that does get out, tends to settle on the inside of the containment.

I must stress also, that one of the main advantages of the buffer salt suspension, is that it protects the primary loop from being damaged by earthquakes or other events, plus from overheating by passive cooling. That takes away all of the drivers that lead to a large failures of the primary loop.


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