Uranium dioxide fission product release from

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. 

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%. 

Tanke, R. H. J. Mass and gamma spectrometric measurements of fission products released from overheated, fresh irradiated, uranium dioxide. J. Nucl. Materials 188, 262-272... 

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 ... 

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. 

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