On 26 March, the SM-1 plant experienced excessive control rod drive seal leakage rate. During a vapor container entry for instrumentation check, the leakage rate from each seal was measured. The maximum rate was found to be 6.3 gallons per hour compard to a normal valus of approximately .7 gph. Two seals were replaced with rebuilt assemblies. This reduced leakage rates to an allowable operating rate. Shim no. 3 seal was found plugged, and back flushing eliminated the plugging. Visual inspection of the disassembled Rod-B seal indicated leakage increase was due to the increased clearance probably caused by overheating due to loss of cooling water at various times in the operating period.