AECL is moving toward licensing of the ACR-700 and/or ACR-1000–safer, enhanced versions of the CANDU that has operated in Canada for years.
From “Energy Policy in the Shadow of Chernobyl” (April 26):
-Pressurized Heavy-Water Reactors (PHWRs) use heavy water (the heavy hydrogen in the H2O is twice as heavy as ordinary hydrogen) in place of light water. Heavy water is a better moderator than light water, which allows a PHWR to use natural uranium and produce more fuel than it consumes under almost all circumstances. PHWRs usually have subtle design differences from PWRs due to the high cost of heavy water and to allow exploitation of the better moderator. The most common PHWR uses heavy water as the moderator and coolant but in separate pressurized systems; this type is known as a CANDU (some more modern ones, using light water coolant and heavy water moderator, are known as Advanced CANDU Reactors or ACRs). CANDUs place their fuel rods in heavy-water-filled high-pressure tubes, which are embedded in a tank of heavy water. Heavy water is pumped through the tubes, which then boil light water to generate steam to turn a turbine. In LWRs, the coolant and moderator are the same thing: the water. In the CANDU, the coolant and moderator are separate, which can cause a problem: if the coolant absorbs neutrons (which moderators also all do to some degree), removing it will allow more neutrons to get through. Instead of slowing down the reaction, boiling the coolant can then speed it up. The design of each pressure-tube reactor is different, though, which is why the CANDU’s positive void coefficient is small enough for the other passive safety effects and active safety systems to compensate, but the other major design that uses separate water coolant and moderator–the Russian RBMK–can go out of control, and the ACR’s void coefficient is actually negative. A meltdown in CANDU reactors would probably involve only one pressure tube, and would immediately stop the reactor. CANDUs can also run directly on nuclear waste from LWRs and refuel online. They do not use burnable poisons but rather distribute old and new fuel with the online refueling system.
Please support licensing–and put pressure on the NRC to keep the review going.
Filed under Activism, Physics, Politics and Regulation, Safety
Posted on May 31, 2006 by Stewart Peterson |
