What's at the forefront of reactor research? A must-read digest/discussion.

[PamW]

16. Then you don't know the negatives.

I agree it would be much better than what we have, but MSRs appear to have all the advantages of IFRs with none of the negatives.
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First, even IF the IFR had a pin failure and melting and the fuel dissolved in the liquid sodium; then you have EXACTLY the same cleanup problem that you ultimately have with a molten fuel reactor.

Expansion of the fuel in not the only mechanism. Another mechanism is fuel-assembly bowing. Because the fuel assemblies are "canned", i.e there is an outer "duct", the response to non-uniform temperatures as experienced in an accident will cause the fuel assembly to "bow" - like an archer's bow - convex side toward the high temperature side. In an IFR, the assembly is only restrained at the bottom, so this bowing causes the core to "flower" outward - and the resultant displacement negates the excess reactivity. An MSR lacks this feedback mechanism.

You are WRONG about the MSR having none of the negatives. The MSR has some negatives of its own when it comes to neutronic stability concerns. Because of the IFR's heterogenity between coolant and fuel, it has a very strong neutron resonance Doppler feedback that can terminate runaway reactor transients promptly. The MSR doesn't have that. The ability to promptly terminate a neutron runaway is one of the most important feedback mechanisms in my book. Again, fuel assembly displacement is also another feedback that the MSR lacks.

The MSR still has good transient feedback response as detailed in:

http://www.coal2nuclear.com/MSR%20-%20Stability%20Analysis%20-%20ORNL-TM-1070-ocr.pdf

However, it's just not as good as the IFR transient feedback response.

PamW