My van is using 1/2-1 Gal antifreeze a week. No leaks, radiator pressure test OK, oil clean, spark plugs clean. Driveway clean after overnight parking. Someone suggested that it may be leaking out the waterpump shaft, while driving only. Is this possible? what other causes??

 

If it were the Water Pump Shaft, that would show up in your driveway at some point. You need to have your Radiator Headroom (space above the coolant) "sniffed" with a Fuel Gas Analyzer. If it test positive, means a Head Gasket problem.