Report from the conversion to use of slightly enriched uranium in DR3 during 1988-90

The research reactor (DR3) at Risoe National Laboratory was converted from the use of highly enriched, to slightly enriched uranium in the fuel elements during 1988-1990. At first the 8 slightly enriched uranium elements were evenly distributed over the core until the holding of highly enriched uranium was gradually used up and substituted with slightly enriched uranium. Graphs and tables illustrate the conversion period, indicating the mean thermal neutron flux, the maximum fast neutron flux and the mean power, total burn-up etc. Neutron flux and reactivity values were predicted and compared to actual results, using the Coarse Control Arms calibration curve until the highly enriched uraniun was used up. Calibration methods are described. Temperature coefficient measurements and calculations are also described in detail. During the conversion period, the neutron flux, the mean power and the reactivity values were strictly monitored and there was at no time any risk of instability or other hazards and no increase in fission products was observed. There was a reduction of the thermal neutron flux density of 15% in the core center with a matching reduction in relation to the thermal and fast neutron flux density, but this was insignificant. (AB)