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Conference Article: Core heat up accident simulation with HTR fuel elements

Nabielek, H.; Schenk, W.; Verfondern, K. (Kernforschungsanlage Juelich G.m.b.H. (Germany, F.R.))

Abstract

The coated particle consists of a 500 mum dia. oxidic kernel surrounded by coating layers of pyrocarbon and silicon carbide. About 20,000 of those particles are imbedded in a spherical fuel element with a diameter of 60 mm. Fuel manufacture and the first phase of irradiation tests of low-enriched UO2 fuels have been finished in 1984. They demonstrated low contamination levels and a negligible failed particle fraction during normal operations. The main effort is now concentrating on testing coated particle fuels under accident conditions. Therefore, irradiated fuel balls are heated in hot cell furnaces to temperatures in the range of 1400-2500 deg. C. The measurement of Kr 85 release is indicative of the failure of particle coating. In addition, the measurement of long-lived metallic fission products yields information on diffusive release from intact particles. First results from temperature ramp and from isothermal annealing tests are being presented. In parallel, work on fuel performance modeling has enabled the prediction of fission product behaviour in a given reactor design. Models to describe the time and temperature dependent gas release in the case of core heatup have been incorporated into the PANAMA-code, which contains the principal mechanisms for coating failure at high temperatures. The diffusive release of metallic fission products during irradiation and annealing has been modeled in the code FRESCO with effective diffusion coefficients as one of the main material parameters. Future plans for the demonstration of HTR fuel performance contain the continuation of accident simulation tests and associated postirradiation work, the improvement of conservative performance models, and the performance of proof tests which are specific to reactor designs.

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key words: cesium 134; cesium 137; coated fuel particles; computer calculations; fission product release; htgr type reactors; krypton 85; laboratory equipment; loss of flow; performance testing; post-irradiation examination; silver 110; strontium 90; very high temperature; accidents; beta decay radioisotopes; beta-minus decay radioisotopes; cesium isotopes; days living radioisotopes; equipment; even-even nuclei; even-odd nuclei; fuel particles; gas cooled reactors; graphite moderated reactors; hours living radioisotopes; intermediate mass nuclei; internal conversion radioisoto; isomeric transition isotopes; isotopes; krypton isotopes; nuclei; odd-even nuclei; odd-odd nuclei; radioisotopes; reactor accidents; reactors; seconds living radioisotopes; silver isotopes; strontium isotopes; testing
Reference:
Specialists' meeting on safety and accident analysis for gas-cooled reactors. Oak Ridge, TN (USA). 13-15 May 1985
International Atomic Energy Agency, Vienna (Austria)
IAEA-TECDOC--358, pp:213-225