Section 3.7.2 MSIVs

Question 1

What are the requirements of T.S. 3.7.2 Main Steam Isolation Valves (MSIVs)?

Answer 1

During Mode 1

AND

During Mode 2 and 3, when MSIVs are not closed and de-activated.

Two MSIVs shall be OPERABLE.

One MSIV inoperable in MODE 1. Restore MSIV to OPERABLE status in 8 hours.

Question 2

If one MSIV in inoperable in mode 1, what happens?

Answer 2

Restore MSIV to OPERABLE status in 8 hours.

If not completed.

Be in Mode 2 in 6 hours.

Question 3

What is the function of the MSIVs?

Answer 3

• To isolate steam flow from the secondary side of the steam generators following a High Energy Line Break (HELB) downstream of the MSIV.
• MSIV closure terminates flow from the unaffected (intact) steam generator for breaks upstream of the other MSIV

Question 4

What closes MSIVs?

Answer 4

Steam Generator Low Pressure

Containment High Pressure

Either switch to close.

Loss of air. There is an acculating tank for each.

Question 5

The design basis of the MSIVs is established by what accident analysis?

Answer 5

Main Steam Line Break in Containment (MSLB).

• Not part of original licensing. Required Probabalistic Risk Analysis. Three accidents analyzed. One for fuel integrity and two for containment integrity.

Pressure and Fuel Integrity Bounding

• The pressure and Fuel Integrity bounding accident is a MSLB at hot zero power.
• At zero power, the steam generator inventory and temperature are at their maximum, maximizing the analyzed mass and energy release to the containment.

Question 6

What is the bounding accident assumed in the PRA for containment temperature?

Answer 6

Temperature Bounding

The temeperature bounding accident is a MSLB immediately following a turbine trip at 100% power. Hot full power - stored energy in PCS is maximized.

Question 7

What is the bounding accident for fuel integrity and containment pressure in the MSIV PRA?

Answer 7

Pressure and Fuel Integrity Bounding

The pressure and Fuel Integrity bounding accident is a MSLB at hot zero power.

At zero power, the steam generator inventory and temperature are at their maximum, maximizing the analyzed mass and energy release to the containment.

With the most reactive rod stuck out, the reactor could go critical again.

The core is ultimately shut down by a combination of doppler feedback,
steam generator dryout, and borated water injection delivered by the
Emergency Core Cooling System.

Question 8

What is the outcome of the MSLB outside containment?

Answer 8

With tube rupture, the MSLB outside containment upstream of the MSIV is limiting for offsite dose, although a break in this short section of main steam header has a very low probability.

Question 9

Summarize the MSLB accident analysis.

Answer 9

MSLB inside containment.

• Steam is discharged into containment from both steam generators until
  closure of the MSIV in the intact steam generator occurs. Steam is
  discharged into containment only from the affected steam generator at this point.

MSLB outside containment and upstream from the MSIVs.

• Not a containment pressurization concern. Closure of the MSIVs limits the blowdown to a single steam generator. Prevents excessive PCS cooldown and positive reactivity addition.

MSLB downstream of MSIVs.

• Isolated by the closure of the MSIVs. Events such as increased steam flow through the turbine or the turbine bypass valve will also terminate on closure of the MSIVs.

SGTR

• Closure of the MSIVs isolates the affected steam generator from the intact steam generator and minimizes radiological releases.

Question 10

How are the MSIVs deactivated?

Answer 10

Air is removed. Supply and vents are unlatched.

Question 11

What makes an MSIV operable?

Answer 11

Closure time of each MSIV is ≤ 5.0 seconds on an actual or simulated actuation signal from each train under no flow conditions.