There are two types of breeder reaction:
Enthusiasts for thorium-fed reactors cite the low production of "transuranic" elements as an advantage, but this is merely catering to the superstitious notion that "transuranic" is un-"natural". U-233 is just as unnatural as plutonium. Radon, with its decay products, which occurs "naturally" is millions of times as deadly as plutonium.
In the long run, therefore, a breeder reactor can extract about 200 times as much energy from a ton of fuel as the non-recycling designs. This is pretty impressive, considering that about 50 tons of uranium suffice to provide as much energy in today's reactors as 1.25 million tons of coal in coal burning plants.
There have been several designs tried for constructing a viable breeder reactor for civilian use.
The most successful was at Argonne National Labs., twenty years ago. It was called the Integral Fast Reactor, IFR for short. It worked, and in the very month that the Chernobyl reactor failed, the project's EBR II (Experimental Breeder Reactor II) had already proven itself immune, by deliberate test, to that kind (and the TMI and Fukushima-Daiichi kind) of failure. It was cancelled in 1994 in response to people who erroneously think themselves environmentalists. There exists now (July 2010) a proprietary plan to build packaged 100 MWe reactors using the proven technology of the EBR II reactor.
Note that if ARC100 can produce 20 years' worth of 100 MW from 8% of 20 tonnes, that's 20 years at 100 MW for 1.6 tonnes of depleted U, or 20 GW-yr for 16 tonnes, so for 480,000 tonnes of that reserve stock, we'd have enough for 600 thousand GW-yrs of energy
Suppose we wanted an additional 60 GW of nuclear power. That's 600 such ARC100 reactors. Initial loading requires 20 tonnes each of uranium, at an enrichment of "less than 20%", actually three zones of 10.1, 12.1, 17.2 percent. The average is in fact just under 13.5%. means we'd need 600 x 20 x 0.135 tonnes of fissile isotope to start up. That's 12,000 tonnes of uranium, containing just under 1,620 tonnes of fissile U-235 or Pu-239. There's probably enough plutonium in the civilian spent fuel dumps to supply all that, and without actually separating it entirely from the U-238.
135Xe captures neutrons and becomes 136Xe which is stable.
Or it decays to 135Cs, which is mildly radioactive, with a half life of 2.3 million years, and is not to be confused with 137Cs, which has a half life of 30 years, and is therefore 76,000 times as radioactive.
In a reactor, 95% of the 135Xe comes from the decay of isotope 135I of iodine, which has a half life of 6.6 hours. This is why it is difficult, or even dangerous, for generation 2 reactors, with solid ceramic fuel pellets, to respond accurately to very large changes in load. Readjustments of the control rods, which absorb neutrons surplus to the power requirement, have to be matched to the variation in the waste product neutron absorption rate.
It is of course rational in any case that since the fuel cost is small compared with the capital and staffing costs, that nuclear power is generally used primarily to cover base load.
It is worthless and perverse to use "renewable" energy sources to reduce the rate at which nuclear power is serving base load. In Ontario, the practice of doing so is actually increasing the GHG emissions, because the wildly erratic behaviour of the wind resource necessitates constant idling or under-used gas turbine reserve for backup. But although Gen IV nuclear power could drive equally responsive steam or gas turbines, there would still be no advantage in running solar powered "renewables".
If a solid, ceramic-fueled reactor is shut down in an emergency, the quantity of 135Xe goes up, until the supply of 135I runs out.
Then the decay of the 135Xe takes over.
To restart the reactor, the control rods are withdrawn enough that the combined rate of capture by the xenon and the control rods is low enough for the reaction to start.
But the first thing that happens is that the rate of capture of neutrons goes down, and the reactor's response will go higher than was desired. It is necessary to insert the control rods at a rate sufficient to balance the rate of conversion of 135Xe to 136Xe by the neutron flux.
Meanwhile, 135I is being produced, and decaying to 135Xe.
The safest approach is instead, to give the reactor time to wait for most of the xenon to decay away, before restarting. The same phenomenon makes it difficult for currently widespread reactors to respond smoothly to large changes in demand. But note that US Naval nuclear power propulsion plants are quite successful in doing so. In a liquid fuelled reactor, both the iodine and the xenon are gases, and can be taken out of the fuel at the surface.