HTGR Knowledge Base

Conference Article: Effects of the fuel elements pebble bed structural non-uniformity on the HTGR physical and thermophysical characteristics.

Yevsevev, V.I.; Kiryushin, A.I.; Kuzavkov, N.G.; Mordvintsev, V.M.; Sukharev, Yu.P (Experimenta Machine Building Design Bureau, Gorki (Union of Soviet Socialist Republics))

Abstract

The paper features results for the analysis of fuel elements velocities' field and porosity distribution effects in the pebble bed on the core power and temperature distribution in the VG-400 and VGM reactors designed in the USSR. Both normal operation of the plants and the accident with the pebble bed shrinkage under the seismic impact has been considered. The analysis of power distribution has been made on the basis of experimental data dealt with porosity distribution obtained in the USSR during model experiments. The need to take into account the detailed porosity distribution throughout the pebble bed in the reactor physics analysis has been emphasized. It is shown, that the difference in the reactivity rise at the pebble bed shrinkage calculated using the average volumetric porosity changes from that calculated using the detailed porosity distribution can reach about 10%. Increase of the fuel elements velocities' non-uniformity (i.e. changeover from the uniform field to the profile with the maximum non-uniformity) is followed by the radial power distribution non-uniformity increase from 1.15 to 1.3. Porosity distribution changes substantially affect the power level in the area of disturbances (approximately in proportion with the changes of the fissiled nuclei concentration). In the rest of the core the energy release deformation is negligible. Changes of the pebble bed porosity distribution may substantially affect the maximum values and pattern of the fuel temperature distribution. Thus, porosity reduction in the core periphery from 0.6 to 0.32 results in the maximum fuel temperature rise by approximately 200 C. As far as the other physical characteristics changes are concerned, it has been pointed out, that variation of the velocities profile causes the fuel burn-up changes not higher than by 5%, and porosity variation not more than by 10%.

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key words: Gas Cooled Reactor, Nuclear Technology
Reference:
Uncertainties in physics calculations for gas cooled reactor cores. Proceedings of a specialist's meeting held in Villigen, Switzerland, 9-11 May 1990
International Atomic Energy Agency, Vienna (Austria). International Working Group on Gas-Cooled Reactors
IWGGCR--24, pp:72-77