Fission products from uranium dioxide fuel

Use of a steam generator to separate the primary loop from the secondary loop largely confines the radioactive materials to a single building during normal power operation and eliminates the extensive turbine maintenance problems that would result from radio-actively contaminated steam. Radioactivity sources are the activation products from the small amount of corrosion that is present in the primary loop over the 12-18-month reactor cycle, as well as from the occasional (<1 in 10,000) fuel rod that develops a crack and releases a small portion of its volatile fission products. Uranium dioxide fuel is very resistant to erosion by the coolant, so the rod does not dump its entire fission product inventory into the RCS. 

Fuel Recycle Requirements. We asstime that the final product returned for fuel fabrication and recycle is a mixed uranium-plutonium dioxide material, partially decontaminated from fission products. The questions of fissile material enrichment, radiation levels, and required handling facilities are not addressed. 

Preliminary investigation has shown that most fission products are not soluable in alkali metal nitrate melts and that they are not dissolved by addition of 100% nitric acid vapor. If these characteristics are verified by further experiments, a fission product separation is easily envisioned. One could react the fuel with the molten nitrate, dissolve the uranate with the addition of 100% nitric acid, and separate the uranium from the remaining solids, which should consist of both plutonium dioxide and fission products. 

Spent fuel rods from nuclear power stations are a major source of nuclear waste. Nuclear fuel is composed of uranium dioxide, UO2. After some years of use, when 1—4% of the uranium has undergone fission, the performance of the fuel rods falls, and these are then replaced. The spent fuel rods consist of uranium dioxide together with fission products. 

The basic nuclear reactor fuel materials used today are the elements uranium and thorium. Uranium has played the major role for reasons of both availability and usability. It can be used in the form of pure metal, as a constituent of an alloy, or as an oxide, carbide, or other suitable compound. Although metallic uranium was used as a fuel in early reactors, its poor mechanical properties and great susceptibility to radiation damage excludes its use for commercial power reactors today. The source material for uranium is uranium ore, which after mining is concentrated in a "mill" and shipped as an impure form of the oxide UjO (yellow cake). The material is then shipped to a materials plant where it is converted to uranium dioxide (UO2), a ceramic, which is the most common fuel material used in commercial power reactors. The UO2 is formed into pellets and clad with zircaloy (water-cooled reactors) or stainless steel (fast sodium-cooled reactors) to form fuel elements. The cladding protects the fuel from attack by the coolant, prevents the escape of fission products, and provides geometrical integrity. 

As to accident 9, here, of course, is the crux of the matter so far as population hazard is concerned. One is left to make up one s own mind as to what parameters to assume—as is the case with what may be thought to be corresponding though different circumstances for gas-cooled reactors (see section II,B). At the present state of the art, this is perhaps inevitable. So far as fission product release from fuel is concerned, we clearly have to consider fuel at temperatures up to the melting point of uranium dioxide, 2800 C some representative values are given in Section III. 

The fuel used in most nuclear reactors consists of uranium dioxide (UO2), a ceramic material. This is in the form of small pellets (less than an inch in diameter) contained within metal tubes, usually of an alloy of zirconium (as in the RBMK) or of stainless steel. These tubes are collected into bundles (fuel elements) and, in the RBMK, are inserted into the pressure tubes to form the core. The vast majority of the radioactive material produced by the fission process is held by the ceramic material itself and what little does escape from the ceramic matrix is retained by the metal tube surrounding the pellets. Fission products can only be released from the fuel elements if these overheat. This is a three-stage process ... 

Fissionable uranium is present in the HTR-Module in spherical fuel elements. Each fuel element has a diameter of 6 cm and contains approximately 11 600 coated particles within the inner graphite matrix. Each of these coated particles consists of a fuel kernel (uranium dioxide, UO2) with a diameter of about 0.5 mm, which is coated with layers of pyrocarbon (PyC) and of silicon carbide (SiC). These layers enclose the fuel kernel, thus preventing a fission product release from the fuel element to the highest degree. One fuel element contains a total of 7 g of uranium (equivalent to a moderation ratio of approx. 69(X)) with a 235 U enrichment of 7.8%. 

Nuclear fission energy of the type currently in use has the potential to provide enough energy for the operation of civilization, but it presents much the same supply lifetime problem as fossil fuels. The waste products present a severe environmental problem. The problem is very different from that presented by fossil fuels but possibly more dangerous. Despite much criticism of the use of fission nuclear power, its use may be preferred to fossil fuels because of the lack of other peaceful use for uranium and the fact that the waste products can be confined. Remember, fossil fuels wastes are not confined. They are dispersed through the ecosphere as acid rain and carbon dioxide. 

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