XPost: alt.christnet.second-coming.real-soon-now, alt.religion.c   
   ristian.last-days   
   From: none@none.com   
      
   On 9/22/2010 5:36 PM, Dr. Yuri Blinov wrote:   
   > Christopher wrote   
   >   
   >> Thorium-232 is the only primordial isotope of thorium and makes up   
   >> effectively all of natural thorium.   
   >   
   > When optimized for breeding, thorium breeder reactors may require on-site   
   > reprocessing to remove protactinium-233 from the breeding blanket so it   
   > can beta decay to uranium-233 instead of neutron capture to uranium-234.   
   > This might allow diversion of fuels to weaponry, so a decision can be made   
   > to simply allow Pa isotopes to remain in the salts, especially since there   
   > are far more effective neutron absorbers in the fission products, such as   
   > Argon, which will be removed continuously as gas at the reactor's plenum.   
   > Chemical extraction of protactinium will extract any isotopes of   
   > protactinium including 231Pa which has a half-life of 31,000 years. Thus,   
   > there's good reason to simply allow Pa to remain within the salt, and so   
   > decay, or be converted by neutron capture.   
   > # The uranium-233 contains trace amounts of uranium-232, which produces a   
   > hard gamma emitter thallium-208 in its decay chain. This gamma radiation   
   > would increase the difficulty of making nuclear weapons.[8] Removal of U-   
   > 232 by isotopic separation would be even more difficult than enrichment of   
   > U-235 in natural uranium. These features may offer some non-proliferation   
   > advantage, over conventional, enriched-uranium reactors.[citation needed]   
   > If the uranium is purified of thorium and other elements, its   
   > radioactivity is initially low and increases with accumulation of thorium-   
   > 228 (halflife 2 years) and further short-lived thorium series decay   
   > products. An easier route to produce nuclear weapons already exists by   
   > enrichment of natural uranium.[citation needed]   
   > # Fluoride salts naturally produce HF when in contact with moisture, which   
   > may lead to release of hydrofluoric acid fumes during reactor shutdowns,   
   > decommissioning, or flooding. However, competent reactor designs would   
   > never allow the salt plumbing to ingest or become exposed to moisture or   
   > other contaminants, whether in operation or shutdown. Similarly, de-   
   > commissioning procedures for all reactors are always fastidious, and the   
   > nature of the MSR's continuous operation makes de-commissioning   
   > infrequent.   Molten salt reactors, nevertheless, present a number of   
   > design challenges. Known issues include:   
   >   
   >      * High neutron fluxes and temperatures in a compact MSR core can   
   > change the shape of a graphite moderator element, causing it to require   
   > refurbishing in as little as four years of operation. Eliminating graphite   
   > from sealed piping was a major incentive to switch to a single-fluid   
   > design.[9] Most MSR designs do not use graphite as a structural material,   
   > and arrange for it to be easy to replace. At least one design used   
   > graphite balls floating in salt, which could be removed and inspected   
   > continuously without shutting down the reactor.[11]   
   >      * The high neutron density in the core rapidly transmutes lithium-6 to   
   > tritium, a radioactive isotope of hydrogen. In an MSR, the tritium forms   
   > hydrogen fluoride (HF). Tritium fluoride is a corrosive, chemically   
   > reactive, radioactive gas. Because of this, if a MSR design uses a lithium   
   > salt, it uses the lithium-7 isotope in order to prevent tritium formation.   
   > The MSRE proved that lithium-6 removal from the fuel salt worked to   
   > prevent tritium formation. Since lithium-7 is at least 14% heavier than   
   > lithium-6, and is the most common isotope of lithium, the lithium-6 is   
   > comparatively easy and inexpensive to extract from naturally occurring   
   > lithium. Vacuum distillation of lithium achieves efficiencies of up to 8%   
   > per stage and only requires heating of raw lithium in a vacuum chamber.   
   >      * Some slow corrosion occurs even in the special nickel alloy,   
   > Hastelloy-N used for the reactor. The corrosion is faster if the reactor   
   > is exposed to hydrogen which forms corrosive HF gas. Exposure to water-   
   > vapor within the piping causes uptake of corrosive amounts of hydrogen, so   
   > practical MSRs operate the salt under a blanket of dry inert gas, usually   
   > helium.[12]   
   >      * When cold, the fuel salts radiogenically produce corrosive,   
   > chemically reactive fluorine gas. Although a very slow process, the salts   
   > should be defueled and wastes removed before extended shutdowns to avoid   
   > (non-radioactive) fluorine gas production. Unfortunately, this was   
   > discovered the unpleasant way, while the MSRE was shut-down over a 20-year   
   > period[13].   
   >   
   > An MSR based on chloride salts (e.g. sodium chloride as the carrier salt)   
   > has many of the same advantages. However, the heavier nuclei of chlorine   
   > are less moderating, which causes the reactor to be a fast reactor.   
   > Theoretically, it wastes even fewer neutrons and breeds more efficiently,   
   > though it may be less safe. It would require isotopically-pure chlorine-   
   > 37, to avoid neutron activation of chlorine-35 into the long-lived   
   > radioactive activation product chlorine-36, Chlorine-36 is not a problem   
   > in itself, but decays into sulfur, which forms volatile sulfur   
   > tetrafluoride. SF4 is a poisonous, corrosive gas that degrades nickel   
   > alloys and forms HF on contact with water, including human mucosa.   
   > [edit] Fuel cycle concerns   
   >   
   >      * There is no need for fuel fabrication. This reduces the MSR's fuel   
   > expenses. It poses a business challenge, because reactor manufacturers   
   > customarily get their long-term profits from fuel fabrication. Since it   
   > uses raw fuel, basically just a mixture of chemicals, current reactor   
   > vendors don't want to develop it. They derive their long-term profits from   
   > sales of fabricated fuel assemblies. A government agency could, however,   
   > type-license a design, which utilities could replicate. An alternative   
   > business model might be to charge for maintenance and reprocessing of the   
   > salt.   
   >      * A safe thorium breeder reactor using slow thermal-energy neutrons   
   > also has a low breeding rate. Each year it can only breed thorium into   
   > about 109% of the uranium-233 fuel it consumes. This means that obtaining   
   > enough uranium-233 for a new reactor can take eight years or more, which   
   > would slow deployment of this type of reactor. Most practical, fast   
   > deployment plans would start the new thorium reactors with plutonium from   
   > existing light-water reactor wastes or decommissioned nuclear weapons.  To   
      
   [continued in next message]   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)