Thomas wrote:
I'd like to add a point that may go without saying for you, but may not be obvious to the others. The state-of-the-art processing for spent fuel elements is to melt them into a glass. Lays usually connote the word "fuel" with something liquid. Hence they get worried when somebody dumps spent nuclear "fuel" into the sea. What we are talking about is a solid that does not dissolve in water. So even in a leaky container, it will just keep sitting wherever it sits.
I tried to find the percentage of radioactive waste that is actually liquid, but didn't succeed. Do you have a figure, George?
"Radioactive waste" is a term that covers a wide spectrum of stuff, ranging from slightly contaminated tools and work clothing for which the cost of cleaning exceeds its value or replacement cost, to high concentrations of of radioactive heavy elements with half lives in the 10,000 to 20,000 year range found in spent fuel - and everything in between.
The spent fuel waste that goes into the Yucca Mountain repository constitutes no more than a few percent of the total by volume. What goes into the repository includes the (usually zirconium) matrix into which the fuel is blended and the cladding that encloses it. All of this is enclosed in a stainless steel container, so that, by the time it gets to the repository the stored material is no more than about 20% by volume, high level waste.
Generally the economics of disposal strongly favor concentrating the waste by separating the innocuous components from the radioactive ones. This usually means separating the solid component from any waste generated in liquid form. The principal liquid wastes generated are cleaning solvents and, in the case of fuel reprocessing plants, the nitric acid solution into which the spent fuel is dissolved and from which plutonium and other useful nuclides are precipitated. This latter is one of the principal legacy problems at the Hanford Washington site where it was simply stored in large buried tanks and with little control of just what went in them.
Discharged reactor coolant (water) is, I suppose, an important source. However the principal activated component (N-17) has a half life of seconds, so by the time the water is cooled down the only radioactive components are dissolved corrosion products, generally with half lives of a year or so. (In a well-maintained plant these should involve extremeny small concentrations.) These are easily removed in ion exchangers and carbon filters, yielding VERY pure water that meets public source standards.
The great majority of solid radioactive waste generated by nuclear plants is generated by exposure of structural metals to the primary radiation from the reactors, including the corrosion products that coat the interiors of fluid systems. These generally have half lives of about two years, so they are not a long-term hazard. (The worst of them is cobalt-60 which has a 5.1 year half life. Improved valve seat design has eliminated that source in newer plants)
The big emotional issues that surround radiation arise from the facts tha (1) you can't see or feel it, and (2) it is easily and cheaply detected in extremely small quantities. The best medical science can do is to detect a tumor containing millions of cells and billions of atoms, while with an instrument costing just a few hundred dollars one can detect the presence of a few thousand radioactive nuclides. The earth is deterctably radioactive and many emotional radioactive waste public issues involve material less radioactive than the soil in the regions where these issues arise. (We had this problem at Rocky Flats in Colorado)
I agree about oralloy's likely point. I think he was also referring to fast neutron reactors. However, to my lnowledge, there are no operational powerplants of this type anywhere. It is an interesting research possibility and one that promises much cheaper access to fissionable fuel. However there are some serious control issues in the physics of plant design. Most of the world's nuclear plants are water cooled and moderated using thermal neutrons to initiate fission of U-235. I believe the latest designs are gas-cooled variants with improved fuel cladding, permitting much higher operating temperatures and higher Carnot efficiencies.
It is very odd to me that throughout Europe and North America the most avid fear mongers of global warming are also strong opponents of nuclear power. This irrational combination of ideas is an indicator of the emotional and superstitious motivation behid their activism.
