Breeder Reactors

Uranium contains a fissile isotope 235U, as 0.72% of the naturally mined and chemically refined metal. Current commercial power plant designs, which are remarkably successful and reliable, use only the fissile isotope and some of the incidentally synthesised plutonium, and in the USA they essentially throw away the rest. Indeed, since the fuel rods do not contain all of the fissile isotope from the enrichment process, and fail to sustain the nuclear reaction when half of it is consumed, they only use about 0.3% of the raw, natural uranium.

There are two types of breeder reaction:

  1. A fast neutron breeder reactor uses neutrons which have not been slowed down by a moderator, to turn some of its non-fissile isotope 238U into 239Pu, a fissile isotope of plutonium. This plutonium is a neutron-capture product. The fuel atoms which have not yet split may capture more neutrons, forming americium, curium, and perhaps more. These elements are also called transuranic.
  2. Molten Salt liquid fueled reactors (MSRs): Thorium, uranium, and plutonium fluorides can be dissolved in lithium fluoride, or a eutectic mixture of lithium and beryllium fluorides. One of the huge advantages of liquid fuel in a reactor is that the worst upsetters of the neutron chain reaction are gaseous, and can be collected in a chamber at the surface. 232Th --thorium --can be made into 233U, a different fissile isotope of uranium, in the same way, only it doesn't need fast neutrons. There is a recent improvement upon the design, called the Transatomic Power reactor (TAP) that is calculated to run at the enrichment level at which the fuel for Light Water Reactors (LWRs) is called "spent" and thrown away.
    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 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.

    How the IFR differs from current nuclear reactor designs

    How the ARC 100 Differs from the IFR


    Uranium Enrichment

    is the process which separates natural uranium into depleted uranium and enriched uranium.
    Enriched uranium is uranium with enough of the fissile isotope that it can sustain a nuclear chain reaction. The fissile isotope 235U differs from the non-fissile 238U only by three neutrons in its nucleus, and the mass difference is all that the enrichment operation can use. Two exceedingly tedious methods can work. Both work with uranium hexafluoride, UF6, whose molecules have molecular 'weights' of 349 and 352, depending upon the uranium isotope.
    A centrifuge makes use of this mass difference. At the huge pseudo-gravitational force of the centrifuge, the heavier gas collects on the outside, and the lighter enriched gas in the center.
    OR
    A gas diffusion apparatus can force the gases through porous membranes (brick-like) through which the gases diffuse at speeds proportional to the square root of the molecular weight.
    Gradually, the isotopes become somewhat segregated.
    As I said, the process is tedious. It is far more energy-intensive than mere chemical refining. Before this, the fluorine is combined with, and after is dissociated from, the uranium. Reactor-grade enriched uranium for Pressurised Water Reactors (PWRs) is 3.6% 235U. At uranium.org, there is an example of the enrichment results which starts with 8.05 tonnes natural uranium, producing 1.0 tonne of reactor grade (3.6% U-235) uranium and 7.05 tonnes uranium depleted to 0.3% fissile U-235. In 1999, the USA had stocks of depleted uranium amounting to about 480,000 tonnes.
    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.

    Fission Products

    When a nucleus responds to collision with a neutron by splitting, this is called nuclear fission, and there are usually two fragments of about half the atomic weight of the nucleus that was split, and some more neutrons. These are called fission products. Many of them are so unstable that they decay in under a second to become another atomic nucleus and a small particle such as a helium nucleus or an electron, these being also called alpha and beta radiation. The radioactivity of an unstable isotope is inversely proportional to its half-life. It should be noted that at ten times the half-life, one 1024th of the original is all that is left. You could call it the one-thousandth-life.
    So a freshly removed spent fuel rod is vastly more radioactive than one that is a day old! After a year, the radioactivity of fission products, if they were separated from the long-lived plutonium, is quite small.
    Even in terms of its radioactivity, the statement that plutonium is the most toxic substance known is utter rubbish. It's twenty times less radioactive than radium, and a microgram of radium is nothing compared with a microgram of radon, because radon decays in days, not thousands of years. The fission products and further decay products are correctly called nuclear waste, although those elements that are no longer radioactive after a decade or two might be worth extracting. Neutron capture products, being usable as fuel, should not be classified as waste. They are simply being wasted.

    Meltdown Accidents

    There have been, in the period from 1975 to the present, three meltdown incidents in the entire world. Of these, the Chernobyl incident is irrelevant, corresponding to the dangers of automobile designs that are no longer manufactured. It was of the dangerously obsolete Soviet RBMK design. Each occurred when a reactor or reactors was shut down and had no power available to keep the coolant water circulating. The first was at Three Mile Island, in 1979, and since then none have occurred in the USA. It was caused by misinterpretation of a faulty instrument. The second was at Chernobyl, in a thermally unstable RBMK reactor, followed by serious mismanagement. The loss of three reactors at Fukushima was a result not just of an earthquake. The Tohoku earthquake did not directly damage the reactors but killed tens of thousands of people (without any help from the reactors). It was ALSO the result of the following tsunami, which destroyed the ability of the emergency diesel generators to cool the already properly shut down reactors. The earthquake had also cut the reactors off from the grid, which otherwise would have saved them. One good earthquake-proof SMR (small modular reactor) could have saved the entire complex. A nuclear powered submarine engine, in a vessel floating in a suitable pond, might suffice. But an MSR or IFR would be better.

    Neutron Capture by Xenon, 135Xe

    This problem affects reactors with solid ceramic fuel pellets. Intrinsically it can be eliminated with liquid fuel reactors, and was found to be absent with the metallic fuelled, liquid sodium cooled IFR.

    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 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.
    There is no advantage in using "renewable" energy sources to reduce the rate at which nuclear power is serving base load.

    If the 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 to give the reactor time to wait for most of the xenon to decay away, before restarting. In a liquid fuelled reactor, both the iodine and the xenon are gases, and can be taken out of the fuel at the surface.

    Meltdowns

    Liquid water has a tremendous heat capacity. It is excellent for carrying heat from a furnace or a reactor to somewhere else. (Cold water feels far colder than air at the same temperature.) Unfortunately, its heat capacity drops sharply when it becomes vapor. Normally, in a reactor, water under very high pressure carries heat from the fission process to a second circuit which drives turbines and generators. If such a reactor is shut down, the water must still be circulated, which requires pumping power that may not be available.

    Blowdowns

    Note that if a forest of wind turbines is hit by a storm force wind, stronger than it is designed to use, the standard response is to feather the propeller's blades completely, stop its rotation, and keep its axis pointing into the wind. If its supply of power fails, the motors which turn the immense, heavy nacelle will fail to do so, and with luck, only some of the turbines will be destroyed, and nothing else.