Reactivity effects of a research reactor (HOR) during the transition of a HEU to LEU core

The HOR is a pool-type research reactor at the Interfaculty Reactor Institute (IRI) of the Delft University of Technology and contains MTR-type fuel elements. Since the beginning of 1998, the high-enriched uranium (HEU) fuel elements are replaced step by step by low-enriched uranium (LEU) fuel elements while the core size is simultaneously reduced by replacing fuel elements by beryllium reflector elements. At present about 75% of the HEU fuel elements are replaced by LEU fuel elements. In this paper, an overview is given of the calculational methods used for the transition from HEU to LEU fuel. The three-dimensional Monte-Carlo code KENO-Va and the three-dimensional nodal diffusion code system OSCAR-3 are used. Special attention is given in this paper to the modelling of the control rods with the focus on the reactivity effects. Different calculated effective multiplication factors (k-effective) for a number of cycles are shown and compared with measured results. Then a detailed comparison is made between measured and calculated reactivity values for different cycles. The integral reactivity results (s-curve) are given for the four control rods in two different cycles. Taking into account the accuracy that can be achieved for such small critical cores using Monte-Carlo and nodal diffusion methods, the results are useful. Work to improve the results in this transition phase and the final LEU core is still in progress. (author)