Even though VBWR shutdowns were required for location and removal of five failed fuel assemblies (HPD Program), the increase in fuel exposure was good. Fuel exposures wili pass the values at which cold worked stainless steel cladding was failing under the HPD Program. Failure of the 0.005-inch cold worked stainless steel clad fuel rods in assembly 8L was traced to strain cycling fatigue. A study of tapered fuel rods indicates a potential advantage for us of a variable water/fuel ratio along the flow channel. Natural circulation tests in the hydraulic stability loop were conducted over a range of conditions from stable, to oscillatory with exponential decay, to self-sustaining oscillation of constant amplitude, to unstable oscillations with divergent amplitude. The response to impulses in power input shows the effect of the time delay for transporting steam voids up through the riser. The data permit calculation of oscillation frequency, damping coefficient, time lags, and show the magnitude and character of pressure and velocity changes. The data, which have an experimental scatter of plus or minus 10% maximum, show that burnout heat fiux: decreases with increasing flow up to 2 x 10/sup 8/ lb/hr-ft/sup 2/; has a maximum for hydraulic diameter between 0.25 …

Quarterly progress report discussing activities related to the Vallecitos Boiling Water Reactor (VBWR) and related facilities.