A spent fuel electrorefiner must be continuously maintained at 500 C using electric heaters. If electric power is lost to the heaters for more than 5 hours, cadmium metal in the electrorefiner may solidify. Reheating the solidified cadmium may result in rupture of the electrorefiner vessel due to cadmium thermal expansion. The electrorefiner is supplied with commercial normal power. Loss of commercial power activates a start switch to a primary emergency diesel generator. If the diesel generator starts, an automatic transfer switch is activated resulting in power being supplied to the heaters.

A separate backup emergency diesel generator is also available. The backup diesel generator also has a start switch that receives the signal to start following loss of normal power. Rather than an automatic transfer switch, the backup generator must be manually connected to the heaters. A procedure exists directing facility technicians how to manually connect the backup diesel generator to the heaters, but the procedure is never practiced. An experienced risk analyst estimates an 80% chance of failure to connect the backup diesel generator to the electrorefiner within 5 hours.

A portable non-emergency (common) diesel generator is also available to supply power to the electrorefiner heaters. The portable generator must be started by an operator and manually connected to the electrorefiner. A procedure exists directing the plant services technicians how to manually start the non-emergency diesel generator and the procedure is practiced once per year. An experienced risk analyst estimates a 30% chance of failure to start the non-emergency diesel generator within 5 hours. A procedure exists directing facility technicians how to manually connect the non-emergency diesel generator to the electrorefiner, but the procedure is never practiced.

Assume that diesel generators, start switches, and the transfer switch cannot be repaired if they fail. Also assume that wiring failure can be neglected and start switches and transfer switches operate instantaneously if they do not fail. Also assume that no attempt is made to switch to a new diesel if a diesel fails while running. Assume that once a DG starts, it will continue to run as long as needed.

The system block diagram is shown below.

1. Draw a fault tree for the electrorefiner heater power with the top event being electrorefiner heaters not powered for more than 5 hours.
2. Write the Boolean algebra expression representing the fault tree

0 ATS N. Power ER Man. Con. Man Conn DG 1 DG 2 Port DG Auto SW Auto SW Man sw Batt. Batt Batt