Section 3.7.1 MSSVs

Question 1

What is T.S. 3.7.1 MSSVs?

Answer 1

During modes 1, 2 and 3

Twenty-three MSSVs shall be OPERABLE

• There are 24
  
  • If more than 1 is not operable, no LCO entry required.
• 1 or more of the remaining 23 inoperable - Repair in 4 hours
  
  • Not repaired in 4 hours, Mode 3 in 6, Mode 4 in 30

Question 2

What are the main steam safety lift settings?

Answer 2

985 psig (1000 psia)

1005 psig (1020 psia)

1025 psig (1-40 psia)

+-3%

Question 3

What is the primary purpose of the MSSVs?

Answer 3

To provide overpressure protection for the secondary system.

Question 4

What and how do the MSSVs protect the PCS?

Answer 4

• They provide protection against overpressurizing the Primary Coolant Pressure Boundary (PCPB).
• They provide a heat sink for the removal of energy from the Primary Coolant System (PCS) if the preferred heat sink, provided by the condenser and Circulating Water System, is not available.

Question 5

What is the design basis of the MSSVs?

Answer 5

To limit secondary system pressure to ≤ 110% of design pressure when passing 100% of design steam flow.

Question 6

The events that challenge the MSSV relieving capacity are what?

Answer 6

Those characterized as decreased heat removal events.

The full power loss of external load event is the most limiting.

The reduced heat transfer causes an increase in PCS temperature, and the resulting PCS fluid expansion causes an increase in pressure.

PCS pressure increases to ≤ 2614.9 psia, this peak pressure is < 110% of the design pressure, or 2750 psia for the primary system, with the pressurizer safety valves providing relief capacity.

The secondary system pressure increases to 1040.8 psia, this pressure is < 110% of the design pressure, or 1100 psia for the secondary system, with the MSSVs providing relief capability.

Question 7

What periodic testing is required of the MSSVs?

Answer 7

• Visual examination;
• Seat tightness determination;
• Setpoint pressure determination (lift setting); and
• Compliance with owner’s seat tightness criteria.

The ANSI/ASME Standard requires that all valves be tested every 5 years, and a minimum of 20% of the valves tested every 24 months.

Question 8

How is drift accounted for in the MSSVs?

Answer 8

Table 3.7.1-1 allows a ± 3% setpoint tolerance for OPERABILITY; however, the valves are reset to ± 1% during the Surveillance to allow for drift.