RBMK graphite reactors can run on natural uranium but the Doppler coefficient with natural uranium makes them dangerously unstable as they have not only a positive coolant void coefficient they also have a positive power coefficient as well. This means that as the coolant voids and that positive feed back drives Keff farther past one the increase in fuel temperature effect on the neutron Doppler coefficient is also positive as fuel temp goes up so does the Keff in a never ending feed back loop until the fuel metal melts and attains a noncritical geometry. CANDU heavy water have a slightly positive void coefficient but a negative temperature Doppler coefficient even with natural uranium. The solution for RBMK was to use 1_2% enrichment which lowers the thermal Doppler coefficient to slightly negative but this means that the fuel must be enriched and also means that harvesting weapons grade PU is no longer an option since the burnup needed to justify enrichment would taint the the PU with 240. Put simply the RBMK was the cheapest reactor possible that could use no enrichment not heavy water enrichment while using graphite and light water without a pressure vessel or containment structure and have a burnup low enough to create copious amounts of weapons grade PU
It’s been a while since I read Midnight in Chernobyl, but I think that Chernobyl was running natural metal on the night of the explosion.
But a previous experiment with RBMK had proven that natural metal should never have been a operational state in the first place (it was in the trial), as this problem had reared its head before, just not so violently.
CANDU can do it, and not even sweat... but the heavy water is PRICEY!
I wonder if good fuel is cheaper than heavy water? I don’t keep up with those markets.
“The solution for RBMK was to use 1_2% enrichment which lowers the thermal Doppler coefficient to slightly negative but this means that the fuel must be enriched a”
Backwards. Doppler coefficient was always negative. Fixed absorption units were installed to reduce void coefficients necessitating an increase in enrichment.