To put to rest the erroneous claim that a reactivity excursion and sodium boiling can't occur in a fast reactor, please see the following results for a transient with EM pump failure from the @NRCgov preapplication safety review for the PRISM reactor, the model for the Natrium.
1. Positive sodium density reactivity increase versus negative reactivity from core expansion. 
2. The resulting rapid, unbounded power increase.
3. Sodium temperature. Note boiling point is 1,156 K.
Fuel temperature increase. Note the fuel melt temperature is around 1,400 K.
In conclusion, sodium boiling and rapid, positive reactivity excursions can occur in metal-fueled fast reactors. One may debate how probable such events are, but to say they are precluded by fundamental physics is utterly wrong.
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