I. PURPOSE AND SCOPE

Pressure switches 1-C71-PS-N003A, B, C & D and 2-C72-PS-N003A, B, C & D are used to indicate whether turbine first stage pressure is greater than 30% of its rated value (~153.5 psig). These switches are used in the reactor protection system to bypass the reactor trips associated with turbine stop valve closure (less than 90 percent open) and turbine control valve fast closure (less than 566 psig decreasing of EHC control oil pressure at valve) when reactor power is approximately less than 30 percent. In order to activate these trips, pressure switches A or C and B or D must sense a pressure increase above the turbine first stage pressure setpoint.

A problem with theses switches is that they are not secured to their mounting brackets and are supported only by the system tubing. The pressure switches are not Q-list items and their present mounting does not create an immediate nuclear safety concern. However, these switches could be susceptible to spurious signals due to physical contact, potentially resulting in a half or full scram. These switches also could be disconnected from the system tubing, which could potentially create an anticipated transient without scram (ATWS) condition. This project involves design, fabrication, and installation of mounting brackets for these pressure switches. Additionally, this project will include verifying the installation of snubbers for the sensing lines of these pressure switches. Follow-up I&C testing will be required as well as drawing updates and verifications.

Because the pressure switches are part of the reactor protection system and are not mounted properly, there is the potential for spurious signals or system tubing failures to impact this system. Depending on the plant conditions, these spurious signals or system tubing failures could result in a scram or a half scram condition or an ATWS situation. In order to cause a scram, at least two pressure switches would need to produce a spurious signal when both reactor power is less than 30 percent and either the turbine stop valve or control valve is closed. In order to cause an ATWS condition, at least two system tubes would need to become disconnected from their pressure switches when reactor power is greater than 30 percent, and then a turbine stop valve closure or control valve fast closure would need to occur. In this case, a turbine control valve fast closure and turbine stop valve trip bypass alarm would be received, requiring reactor power to be reduced below 30 percent.

The success criterion for this project is that the pressure switches will be restored to a more reliable condition, thereby reducing the probability of spurious pressure switch signals that could possibly cause a scram or an ATWS condition.

II. EVALUATION

Schedule Index: 4 - Because of the potential for these switches to cause a scram or a reduction in power, this project has a small positive impact on unit availability (0.2 x 12). Additionally, installing the mounting brackets will increase the reliability of the pressure switches and the operators confidence in their operation, thereby producing a small plant enhancement (0.2 x 8). Although this project will slightly enhance the overall reliability of the reactor protection system, this enhancement will have no appreciable impact on nuclear safety since plant trips are low contributors to the overall plant core damage frequency and other reactor trips, such as the high reactor pressure trip, would provide protection in the event of a turbine stop valve closure or control valve fast closure while the associated trip was bypassed (0.0 x 32).

Economic Aspects: Once this project is complete, no continuing effort will be necessary. The potential for inadvertent scrams and lost generating revenue will be reduced.

Related Standards: The applicable Work Management Policies for this project are material condition and operating parameters.

Other Considerations: Work on installing new mounting brackets would have to be performed during a plant outage because these pressure switches are part of the reactor protection system.

III. CONCLUSION

The installation of new mounting brackets for pressure switches 1-C71-PS-N003A, B, C & D and 2-C72-PS-N003A, B, C & D will be performed as scheduled. Upgrading of the pressure switch mounting brackets will improve the reliability of the reactor protection system and potentially avoid scrams or required reductions in power which could reduce the availability of the BSEP.