Coolant mixing and subchannel velocities in an LMFBR fuel assembly

Abstract

Experimental mixing studies were conducted using water with a 91-pin wire-wrapped fuel assembly to establish a data base for calibrating thermal-hydraulic codes and to supplement and clarify existing mixing data. An electrolytic tracer was employed in conjunction with an isokinetic sampling technique which permitted mixed mean subchannel concentrations and flowrates to be measured without incurring errors resulting from local subchannel gradients. Emphasis was placed upon the behavior of peripheral subchannels. A complete set of mixing data was obtained which can be used to calibrate thermal-hydraulic codes. Isokinetic velocity measurements revealed that the pitch-average corner, edge, and central subchannel velocities were nearly equal to the bundle average velocity. It was shown that the average peripheral swirl velocity was approximately 1.2 times greater than predicted assuming the fluid follows the wire-wraps. Also the magnitude of the average swirl flow decreases with increasing bundle size. Parametric studies revealed that neither the velocity nor concentration profiles were sensitive to Reynolds number in the range Re = 9000–24000.