Robert Steinhaus, a 57 year old male from Castro Valley asks on August 30, 2008,Can a fast breeder reactor like the famous Candu series of Canadian designed fast reactors be modified such that the waste ultimately produced by these reactors can not be used to make weapons? Can these fast reactors be modified to only "burn" plutonium and long lived high level waste products like the minor actinides through fission and leave nothing but short lifetime, high activity waste products that would be useless for making a nuclear weapon?
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First, a correction: The CANDU reactor is not a "fast breeder" reactor but a "thermal" reactor, which means that it must slow down (or "moderate") the neutrons that are born in the fission chain reaction before they can be used to cause further fissions.
Secondly, a clarification: The spent fuel from a CANDU reactor is already not very attractive a material for making weapons. It contains very little uranium and plutonium, and neither of these leftover fissile materials is enriched enough to be very useful in a weapon - particularly to a country trying its hand at weapons construction for the first time. That said, the risk is not zero and this is why both fresh and spent reactor fuel are carefully tracked through international safeguards administered by the U.N.'s International Atomic Energy Agency (IAEA) in Vienna.
Finally, a commendation: Your suggestion of using CANDU reactors to "burn" plutonium and other long-lived actinides is a fine example of seeking technological solutions that benefit humankind, rather than politically expedient solutions that can be catastrophic (such as, say, ordering all technologies with measurable risks to be shut down).
A CANDU reactor can, in fact, be a very efficient "garbage burner", due to both its high availability of neutrons and its unique ability to accept a wide range of fuel types. This capability is attracting the interest of government agencies in the U.S. and elsewhere that are looking for ways to reduce the amount of spent fuel being stored around the world. The best machine for "burning" the waste is, as you allude, a "fast breeder" reactor, but the second best machine currently on the market is probably a CANDU reactor. Furthermore, unlike fast reactors, CANDUs are off-the-shelf and do not require either enriched uranium or separated plutonium to operate.
The good news is that, by recycling the spent fuel from reactors through "garbage burner" reactors (a combination of thermal and fast reactors is most efficient), we can theoretically extract up to 100 times more energy from the same mass of fuel that we currently use "once-through" in conventional light-water and heavy-water reactors.
As a small, relatively easy step in that direction we can put CANDU reactors next to light-water reactors and "burn" their spent fuel almost directly (since spent fuel from light-water reactors is still slightly enriched compared to natural uranium, which CANDUs use), thereby extracting another 50% of the energy it has already produced. This is called the "DUPIC" fuel cycle, for "Direct Use of PWR fuel in CANDU", and it's part of a suite of strategies being examined in the international effort to extract more energy from our uranium resources without increasing proliferation risks.
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