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PostPosted: Feb 04, 2015 3:30 pm 
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Joined: Mar 07, 2007 11:02 am
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Location: Ottawa
Kirk,

I recently viewed a video of your recent technical presentations at UC Berkeley. An excellent overview of the Two Fluid Breeder approach developed by ORNL in the mid 60s before they switched to Single Fluid concepts. Something I thought quite important to bring up however was that someone asked about criticality concerns of the fuel salt once it is drained to a dump tank. You answered that there are no concerns as criticality is impossible as there is no moderator in the dump tank. That is certainly the intuitive answer and 100% true for dump tanks of Single Fluid thermal spectrum designs but I'm afraid it is not true of dump tanks for Two Fluid designs.

In the Two Fluid design as there is no fertile material in the fuel salt it actually makes it highly reactive when it is separated from the thorium containing blanket salt (the two are typically interlaced in the core as shown in your figures to give larger critical core diameters). The critical diameter of pure Two Fluid fuel salt (say of the 1960s ORNL blend) is roughly 1.5 meters without moderator or reflectors (I'd have to double check old calculations of mine from years ago or find ORNL ones to be more precise). This is a smaller critical diameter than the fuel salts proposed by fast spectrum Single Fluid MSRs. Fast spectrum fuels do indeed have at least 10 times the fissile concentration of a Two Fluid fuel salt but they also have hundreds of times more neutron absorbing fertile like Th or U238. In a Two Fluid fuel salt there is only a little bit of U234 and U236 and the rest is fissile U233, U235 and very low cross section carrier salt elements so it is very reactive stuff (typically about 65 to 70% fissile fraction in the uranium of Two Fluid fuel salts).

There are still of course numerous ways to make the dump tanks of a Two Fluid critically safe, from proper geometry to assuring neutron absorbers present but I wanted to be sure this point was clear to you for your Two Fluid design efforts. There will be the same questions on dump tank criticality for Two Fluid designs that the fast spectrum Single Fluid folks are addressing.

David LeBlanc


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PostPosted: Feb 04, 2015 3:44 pm 
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Joined: Nov 30, 2006 3:30 pm
Posts: 3731
Location: Alabama
Thanks David, I will investigate that issue in greater detail.


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