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PostPosted: Jul 24, 2010 3:48 pm 
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The MSRE had a big decommissioning problem with the fuel salt. FLiBe radiogenically produces fluorine gas when solid at low temperature. So, not defuelling and cleaning the salt turned out an expensive mistake (I think decommissioning cost totalled tens of millions but am not sure).


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PostPosted: Jul 24, 2010 5:14 pm 
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Yes, and what was really bad is that some of the uranium volatilized and ended up in an air filter, causing worries about criticality. Sad, because they already removed all the uranium from the salt to switch from U-235 to U-233, so they knew how to do it.

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PostPosted: Jul 24, 2010 5:40 pm 
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pstudier wrote:
Yes, and what was really bad is that some of the uranium volatilized and ended up in an air filter, causing worries about criticality.

....unexpected safety problems can arise when using HEU and even LEU fuel.


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PostPosted: Jul 25, 2010 3:07 am 
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The whole post shut down problem with the MSRE could have been avoided had the AEC been willing to pay for its decommissioning at the time of the shutdown. ORNL told the AEC that there would be problems if the MSRE were not properly decommissioned, but they refused to come up with the money. This decision can be laid at the feet of Milton Shaw. Why did ORNL not finance the shutdown on its own? The answer is simply, it had lost a large amount of funding, and was downsizing, discharging hundreds of employees in the process.


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PostPosted: Jul 25, 2010 12:39 pm 
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Charles Barton wrote:
The whole post shut down problem with the MSRE could have been avoided ...


This may be the Achilles heel of the MSR. They may be more resistant to incompetence while running than a LWR, but with respect to the intermediate term fuel storage, they are less resistant.

The Magnox have a similar problem. The fuel must be reprocessed, or the cladding will corrode.

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PostPosted: Jul 25, 2010 2:11 pm 
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Are you sure? I have no idea what would happen to an LWR if you just shut it down and left the fuel rods in the core with the primary coolant spiked with boric acid.


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PostPosted: Jul 25, 2010 2:45 pm 
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I think that is basically what happened at Three Mile Island after cold shutdown with the pumps turned off. IIRC, it was years before they went into containment and removed the head of the reactor, and found out how much had melted. What ever the actual hazard, I think the image of toxic, fissile and radioactive gases being generated, and the hazards of the roof or basement leaking is scarier than fuel slowly corroding into sludge in the bottom of a pool of water.

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PostPosted: Jul 26, 2010 7:54 pm 
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I don't believe there was any issue with MSRE after a few years either. It was more of a problem after 50 years of near neglect. This is not a problem that should give anyone heartburn. It can easily be handled with some simple procedures.

The ORNL designs called for the entire area where fuel salt was to be hot to avoid dealing with reliability problems of individual pipe heaters. We will eventually run the numbers but I am guessing you could cut all power to a properly designed LFTR and walk away for 50+ years and the fuel salt would still be liquid.


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PostPosted: Jul 26, 2010 8:05 pm 
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pstudier wrote:
I think that is basically what happened at Three Mile Island after cold shutdown with the pumps turned off. IIRC, it was years before they went into containment and removed the head of the reactor, and found out how much had melted. What ever the actual hazard, I think the image of toxic, fissile and radioactive gases being generated, and the hazards of the roof or basement leaking is scarier than fuel slowly corroding into sludge in the bottom of a pool of water.

There was more to it than "fuel slowly corroding into sludge in the bottom of a pool of water" !

That water had to be constantly circulated & cooled to remove the decay heat.

If the cooling had stopped for an extended period, you can be sure that a more "scary" situation would have developed.

Of course there is no reason why cooling would not be reinstated, even if existing system were somehow compromized.
But it the fact remains that an active cooling system was definitely required.

As for storing fission product waste from MSRs, we have touched upon this topic in other threads.....
For safe, passive (air-)cooling, I think most people agree that a big part of the solution lies in diluting the FPs in an inert solid matrix of some sort.
With a sufficiently high dilution ratio, the self-shielding of the material will virtually eliminate serious radiolytic decomp issues.


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PostPosted: Jul 28, 2010 4:27 pm 
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jaro wrote:
There was more to it than "fuel slowly corroding into sludge in the bottom of a pool of water" !

That water had to be constantly circulated & cooled to remove the decay heat.

If the cooling had stopped for an extended period, you can be sure that a more "scary" situation would have developed.


I had no idea. So LWR reactors are vulnerable to neglect also.

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