During normal operation, instrument air drier operation lineup changed from A train to B train according to normal operating procedure. Just after changing train, discharge valve of drier 'A' was gone out and lost its leaktightness. Instrument air flowed to reverse direction of this check valve and vented to atmosphere through vent valve of drier 'A'. Decreasing of instrument air pressure made main feedwater control valves closed. Reactor was tripped due to the 'steam generator level low' and 'steam/feedwater mismatch' signals. There was no release of radioactive materials outside the reactor building and no unplanned exposure to employees.
