Critical Experiments for PL-2 and Their Analysis

Abstract

The critical experiments showed that the original specification of 1.0 wt% for the boron concentration could be reduced to 0.25 wt% without reducing the one-rod-out shutdown margin below 2 1/2% reactivity as measured by rod drop experiments. Good agreement was obtained between calculated and measured eigenvalues for the small, clean cores which were analyzed. In cores with no control rods and containing all fuel, 0.0, 0.25, or 1.1 wt% boronstainless steel, and with all 0.25 wt% boron-stainless steel and fuel elements surrounded by aluminum-water displacers, the calculated eigenvalues were 0.7 to 1.0% greater than the measured values. In a core with 0.25 wt% boronstainless steel and with the center control rod fully inserted, the calculated eigenvalue was 1.0% larger than measured. In one case, a core composed entirely of fuel and with the center control rod fully inserted, the calculated eigenvalue was 0.33% DELTA k smaller than that measured. The calculated power distributions compared extremely well with those measured in the fuel assemblies. Shutdown measured by the boron substitution method, where the worth of the boron was measured by calibrating both a single control rod and a bank of rods, was found to give consistent results which in turn agreed to within 0.5% DELTA k with shutdown measured by the rod drop technique. The rod drop measurements with 0.25 wt% boron gave a shutdown of 2.52% with the central control rod withdrawn. Correcting for the small difference between the mockup and the reference core, the shutdown margin for the PL-2 was 3% DELTA k/k. Shutdown measurements obtained by moving a calibrated control rod from all into its critical position and maintaining criticality by the compensating motion of another rod or rods, were found to give widely differing results depending on the combination of rods used. Measurements in a boron-stainless steel zoned core where the boron-stainless steel poison rods were removed from the outer 12 peripheral subassemblies and replaced with fuel elements, show that the center rod remains the strongest, that the center-rod- stuck-out shutdown margin is reduced very little (--0.28% DELTA k) from the core containing boronstainless steel in all subassemblies, and that 1.09% reactivity is gained at beginning of life by removing this boronstainless steel. (auth)