"To get the kind of breeding gains that enable a 7-year doubling time, you need a mighty high breeding ratio, and to get that, you need a mighty fast spectrum and super-rapid reprocessing.
Since one of the tenets of IFR/PRISM is no separated plutonium, please help me understand how you're going to accomplish this? Please also explain how you're going to keep the reactor controllable in this hard spectrum, since resonance absorption (Doppler effect) is really your only self-control mechanism, and you're above all the resonances in energy in this hard spectrum.
Softening the spectrum to make the reactor controllable has been what every LMFBR around the world has had to do to make the reactor even mildly controllable, and this kills off the breeding ratio really fast."
Response by Dr. Yoon Chang:
The metal fuel used in the IFR, due to its high density, results in a most hardened spectrum and the best neutron economy (more excess neutrons that can be used for breeding). Some of these excess neutrons leak out of the active reactor core but captured in the external blankets to convert depleted uranium into plutonium. The harder the neutron spectrum, the higher the breeding ratio. Parenthetically, the neutron economy (excess neutrons) is dictated by fissile isotope and spectrum, and Th/U-233 cycle has a worse neutron economy than U/Pu cycle in fast spectrum, whereas the opposite is true in thermal reactors. Even then, achieving a breeding ratio of unity in thermal
spectrum is a great engineering challenge.
Super-rapid reprocessing is not necessary to achieve the 7-year doubling time. A two-year ex-core inventory is already accounted for in the doubling time calculation. We have two years to reprocess and refabricate.
In the IFR pyroprocessing, all actinides including Pu, Np, Am, Cm, etc. as well as some U and rare earth fission products (trace amounts) are recovered in a single product stream and electrorefining is incapable of separating out Pu from the rest of actinides. The blanket actinides are rich in Pu and less minor actinides. However, actinides from the blanket will be mixed with those from the driver
fuel in the electrorefiner or in the injection casting fabrication furnace. Hence, separated plutonium cannot be produced and the entire reprocessing and refabrication are carried out in the same hot cell, with no accessibility.
The excellent neutron economy also implies that the excess reactivity requirement to overcome the reactivity deficit by fuel burnup is minimal. Hence the reactivity control by control rods (with neutron poisons) is also minimal and the accidental reactivity insertion events can be dealt with simple design features. Doppler reactivity feedback is smaller by about 20-30% compared to oxide fueled fast reactors. But that is still more than adequate to deal with prompt reactivity requirements. What is most important is the overall temperature and power reactivity coefficient. When the coolant temperature rises or the power increses for whatever causes, the IFR responds with a negative reactivity feedback due to coolant density or structure expansion, which tends to reduce the power and hence the temperature.
Even in worst case accident events (loss-of-flow and/or loss-of-heat-sink without scram like TMI-2 or Chernobyl initiators), the initial coolant temperature rise will cause thermal expansion of fuel assemblies which increases neutron leakages, and hence the power is brought down all by itself without operator actions or safety systems. Ironically in these events, as the inherent feedbacks try to bring down the power, the Doppler feedback actually contributes positive reactivity. (Recall that Doppler was necessary to protect against inceasing power. When power is coming down, it tries to raise the power.) This feature is unique only with the IFR. The metal fuel operates at low temperature because of a high thermal conductivity (a factor of 10 higher than oxide), so the stored reactivity, (Doppler coefficient) x (temperature difference), is too small to override the negative feedback due to coolant temperature rise. In other words, it's the temperature difference rather than Doppler coeffient itself that enables this unique inherent safety. Therefore, in IFR the Doppler feedback is adequate to deal with overpower transients, and at the same time it enables inherent safety features in the other extreme accident conditions.