ML100350182

Revised Request for Additional Information 24 Response from Licensee

ML100350182
Person / Time
Site:	Diablo Canyon
Issue date:	02/03/2010
From:	Grozan T Pacific Gas & Electric Co
To:	Wang A Plant Licensing Branch IV
Wang, A B, NRR/DORL/LPLIV, 415-1445
Shared Package
ML100350170	List: ML100350175 ML100350182
References
TAC MC4682, TAC MC4683
Download: ML100350182 (2)

Text

NRC RAI 24:

One additional question has arisen in the staffs review of other licensees submittals for GL 2004-02, and the staff requests that the Diablo Canyon licensee address it.

Specifically:

The potential for deaeration of the coolant as it passes through the debris bed should be considered. Please provide an evaluation of the potential for deaeration of the fluid as it passes through the debris bed and strainer and whether any entrained gasses could reach the pump suction. If detrained gasses can reach the pump suction, please evaluate whether pump performance could be affected as described in Appendix A of Regulatory Guide 1.82, Revision 3.

PG&E Response:

Deaeration is not expected to be a concern for the DCPP sump. Evaluation of the dissolved air content of the sump water can be evaluated using the solubility curves for nitrogen. From 167 to 212 degrees F at 400 psia, the dissolved gas fraction of nitrogen is 0.28 ml/g. At the same temperature and 100 psia, the value is 0.08 ml/g. Since the solubility relationship is effectively linear to 0 psia, the effective degassing of sump fluid is (0.28-0.08)/(400-100) = 0.00067 ml/gram of water per psi of pressure drop.

(Industrial And Engineering Chemistry, Volume 44)

The maximum screen pressure drop is 33 inches or 1.2 psid which provides the most conservative deaeration. Thus, assuming that the sump water is saturated at the elevated pressure and temperature conditions present inside containment at the start of recirculation, the total void fraction of the water flowing through the sump downstream of the sump screen would be 0.00067 ml/gram/psi X 1.2 psid = 0.0008, or 0.08 percent void fraction. The deaeration would manifest itself as microbubbles which do not have much buoyancy and would remain entrained in the flow and not accumulate in the sump structure.

As the void migrates down the RHR suction pipe, the hydrostatic pressure from the sump to the pump inlet (a 30 foot drop) will compress the entrained bubbles and further reduce the void fraction. NUREG/CR-2792, An Assessment of Residual Heat Removal and Containment Spray Pump Performance Under Air and Debris Ingesting Conditions, states that For a wide range of operating flow rates RHR and CS pumps should handle volumetric air quantities up to 2% with negligible degradation in performance. The void fraction of 0.08 percent established above is much less than the void fraction threshold of concern discussed in NRC Generic Letter (GL) 2008-01, Managing Gas Accumulation in Emergency Core Cooling, Decay Heat Removal, and Containment Spray Systems.

NUREG/CR-2792 also recommends adjusting the NPSH requirement for the presence of air: The relationship recommended is NPSHR-air/water = NPSHR-water X (1 + 0.5 X Air Fraction). With a maximum void fraction of 0.08 percent, the increase factor for

NPSHR is 1 + 0.5 X 0.08 = 1.04. The 33-inch maximum head loss for the screen is when both RHR pumps running together at a total flowrate of 7,769 gpm. Under these conditions, each RHR pump is running at approximately 3,900 gpm. The NPSHR for these pumps at 3,900 gpm is 16 feet. With an adjustment factor of 1.04, the NPSHR under the deaeration conditions would be16 feet X 1.04 or 16.6 feet which is much less than the 24 feet of NPSHR assumed in the NPSH margin evaluation. Since the NPSHR used in the NPSH margin evaluation is very conservative, the impact due to deaeration as the as the sump fluid passes through the debris bed is considered negligible Any actions taken to address GL 2008-01 will be more than sufficient to address the concerns raised in this RAI. Therefore, deaeration across the sump screen does not present a challenge to the operation of the emergency core cooling system pumps.