Reactivity Worths of Annular Control Rods in a Pressure-Tube-Type Heavy Water Lattice

The control rod effect has been experimentally studied in the Deuterium Critical Assembly (DCA) by using annular absorbers that simulate control rods of the FUGEN reactor, a prototype heavy-water-moderated, boiling-light-water-cooled, pressure-tube-type reactor. The DCA cores for this experiment are of the 1.2%-²³⁵U-enriched UO₂ lattices, and consist of 28-pin fuel clusters arranged in a square array of 22.5-cm lattice pitch. The experiment has been carried out with various control rod patterns and with varying coolant void fraction. Experimental results were analyzed by the “absorption area method,” which was employed in the FUGEN control rod design calculations. The calculated reactivity worth agreed with the experiment within ±10%. The calculations somewhat overestimated the absorber worths in the nonvoided core and underestimated them in the voided core. This tendency was found to be greatly improved by considering the anisotropy effect in the migration area of the cluster lattice. The experimental results were also analyzed by the “logarithmic derivative method.” This method more poorly predicted the worths, but described better the flux shape around the rods.