Wednesday, March 30, 2016

Power Reactors

In the United States, power reactors are entirely of the PWR (pressurized water reactor) or the BWR (boiling water reactor) types. In both cases, water is used as the coolant and the moderator, which provides a very interesting advantage that probably no one has bothered to mention to you: If the coolant is lost, the chain reaction stops. Depending on the length of time the fuel has been producing power, the fuel rods may or may not be thermally and radioactively "hot" from the daughters of the fissioning process. Even in the worst case, the heat generated is no more than 1% or 2% of that during normal operation. This is why the "disaster" at Three Mile Island didn't really happen - except in the minds of the uninformed.

While the Japanese installed the first Advanced Boiling Water Reactor (ABWR) in 1996, none of the new, modular designs have seen the light of day in this country. Not only have we been blinded by the non-threat of low-level radiation, but the cost of building a nuclear plant has escalated by a factor of seventeen, after considering inflation - mostly from construction delays caused by environmentalist lawsuits. (The above-mentioned Japanese ABSR plant took fifty-two months to build - compared with more than eleven years for the most recent plant in the United States.) I would say the new designs are even safer than the old - but how do you get safer than no deaths, no injuries, and no negative effects to the public from several thousand reactor years of operation with thousands of gigawatt-hours of life-enhancing electrical energy having been generated? [Some of the media scream "disaster" when ten gallons of water with 1/80 the radioactivity of salad oil leak out in the process of heating and otherwise providing life-giving energy to an entire city. Why doesn't it make front-page news when some one falls off the roof to his death trying to clean the solar collector - which provides a few puny kilowatts of solar energy for warming the hot water... when the sun is shining?]

Nonetheless, neither the PWR or BWR has much promise for miniaturization and "local" use as - by nature - they operate with high-power densities, which have the potential to cause a messy and expensive loss-of-coolant accident. They also require pumps, back-up pumps, and relatively elaborate controls.

All of these U.S. power reactors use enriched uranium as a fuel, as do reactors in France (where 80% of the electrical power comes from nuclear energy), Japan, England, and most other countries. The enrichment process starts with natural uranium, which is dissolved in acid to produce uranium hexafluoride gas. This ultra-corrosive gas is then pumped thousands of times through membranes where the lighter U235 passes through just a little bit easier than the U238. For power reactors, the U235 is enriched from 0.7% to about 3.5%, which takes not only lots of time but considerable energy. ["Bomb grade" U235 must be enriched to 90% - an extremely difficult process. Thank goodness, or any crackpot might be able to do it.)]