ML12093A098

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Slides Provided by Monticello Licensee for 4/3/12 Meeting Regarding Containment Accident Pressure Credit for EPU Operation
ML12093A098
Person / Time
Site: Monticello Xcel Energy icon.png
Issue date: 04/02/2012
From: Fields J
Northern States Power Co, Xcel Energy
To:
Office of Nuclear Reactor Regulation
Tam P
References
TAC MD9990
Download: ML12093A098 (24)
v • d • e


Text

Status of Containment Accident Pressure (CAP)

Analysis Activities and Path Forward Discussion Monticello Nuclear Generating Plant April 3, 2012 1

Purpose Statement

  • Computational Fluid Dynamics (CFD) analysis is considered by the NRC staff as the cornerstone for the resolution of SECY 11-0014.
  • After nearly one year, industry efforts have been unsuccessful in developing a CFD model that reflects the Monticello pump design (double suction vertical pump).
  • The purpose of this meeting is to propose an alternative regulatory path to demonstrate adequate core and containment cooling is achieved without reliance on CFD analysis.

2

Agenda

  • Background

- SECY 11-0014 Overview

- BWROG/Sulzer work

  • Update Status of BWROG/Sulzer work
  • Proposed Path Forward 3

Background

  • SECY 11-0014 addressed a fundamental question that the ACRS has regarding CAP. Is defense in depth compromised because a plant must have an intact containment (1 fission product barrier) to achieve cooling (1 fission product barrier) of the reactor core?
  • In the NRC Commissioners vote on SECY 11-0014 the philosophy that was endorsed stated:

The Commission, defines defense in depth as "an element of the NRC's safety philosophy that employs successive compensatory measures to prevent accidents or mitigate damage if a malfunction, accident, or naturally caused event occurs at a nuclear facility". This definition does not state that the compensatory measures must be independent.

  • In addition, the SECY vote confirmed that reliance on CAP is not a safety issue. However, it directed that the NRC would use the improved guidance that resulted from ACRS recommendations to include margin and uncertainty determinations in CAP calculations.
  • By selection of Option 1 of SECY 11-0014, the NRC Commission endorsed use of SECY 11-0014, Enclosure 1 which provides technical guidance on the use of CAP in reactor safety analyses.

4

Background

SECY 11-0014, Enclosure 1, Guidance on Use of CAP, addresses the following:

- 6.6.1 NPSHreff - For DBA, include uncertainty in the value of NPSHr3% based on vendor testing and installed operation, including the effects of motor slip, suction piping configuration, air content, and wear ring leakage. For non-DBAs, NPSHr3% without uncertainties may be used.

- 6.6.2 Maximum Pump Flow Rate for the NPSHa Analysis -

maximum flow rate chosen for the NPSHa analysis should be greater than or equal to the flow rate assumed in the safety analyses that demonstrate adequate core and containment cooling.

- 6.6.3 Conservative Containment Accident Pressure for Calculating NPSHa - a 95/95 lower tolerance limit should be used to calculate the containment accident pressure used to determine the NPSHa.

- 6.6.4 Assurance that Containment Integrity is not Compromised

- demonstrate conservatively that loss of containment integrity from containment venting, circuit issues associated with an Appendix R fire, or other causes cannot occur or that they would occur only after use of containment accident pressure is no longer needed.

5

Background

- 6.6.5 Operator Actions - Operator action to control containment accident pressure is acceptable. The NRC staff should approve any operator actions, and the appropriate plant procedures (e.g.,

emergency, abnormal) should include them.

- 6.6.6 NPSHa less than NPSHr or NPSHreff - Operation in this mode is acceptable if appropriate tests are done to demonstrate that the pump will continue to perform its safety functions.

- 6.6.7 Assurance of no Pre-existing leak - licensees proposing to use containment accident pressure in determining NPSH margin should do the following:

1. Determine the minimum containment leakage rate sufficient to lose the containment accident pressure needed for adequate NPSH margin.
2. Propose a method to determine whether the actual containment leakage rate exceeds the leakage rate determined in (1) above.
3. Propose a limit on the time interval that the plant operates when the actual containment leakage rate exceeds the leakage rate determined in (1) above.

6

Background

- 6.6.8 Maximum Erosion Zone - The zone of maximum erosion rate should be considered to lie between NPSH margin ratios of 1.2 to 1.6. Realistic calculations should be used to determine the time within this band of NPSH ratio values.

- 6.6.9 Estimate of NPSH Margin - A realistic calculation of NPSHa should be performed to compare with the NPSHa determined from the Monte Carlo 95/95 calculation.

- 6.6.10 Assurance of Pump Operability for Total Time Required -

The necessary mission time for a pump using containment accident pressure should include not only the duration of the accident when the NPSH margin may be limited, but any additional time needed for operation of the pump after recovery from the accident when the pump is needed to maintain the reactor or containment, or both, in a stable, cool condition but at a much greater NPSH margin. This additional time is usually taken as 30 days.

  • NSPM has been working, independently and in conjunction with the BWROG, to address the issues in SECY 11-0014, Enclosure 1 on use of CAP since the NRC Commission approval of Option 1 of SECY 11-0014 (restarting the EPU reviews) in March 2011.

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Background

  • BWROG/Sulzer Work - Sulzer was contracted to perform CFD analyses in order to characterize the magnitude of several NPSHr uncertainties as compared to NPSHr vendor curves. The Sulzer study is divided into six tasks.

- Task 1A - Determine pump baseline NPSH curve (benchmark)

- Task 1B - Effect of Temperature

- Task 1C - Effect of Inlet Geometry

- Task 1D - Effect of Dissolved Gas

- Task 1E - Effect of Mechanical Wear Ring Clearance

- Task 1F - Combined effects of uncertainties

  • The CFD (Task 1) is applied to:

- The CVDS (3600 RPM double-suction) pumps used at all BWR sites except Cooper (based on Monticello pump).

- The CVIC (1800 RPM single-suction) pumps used at many BWR sites (based on Browns Ferry and Peach Bottom pumps).

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Background

  • The BWROG/Sulzer project scope also includes the following tasks:

- Task 2 - NPSHr changes due to pump speed changes

- Task 3 - effects of pump operation with NPSHa < NPSHr

- Task 4 - effects of extended pump operation (30 days) in the maximum erosion rate zone

- Task 5 - effect of non-condensable gases on pump mechanical seal performance

- Task 6 - NPSHr uncertainties related to pump test instrumentation 9

Background

SECY Section BWROG/Sulzer Scope 6.6.1 Tasks 1, 2, 5 and 6 address pump uncertainty and reliability issues.

6.6.6 Partially addressed by Tasks 3 and 5.

6.6.8 Task 4 addresses maximum erosion zone.

6.6.10 Task 3 addresses full mission time analysis assuming NPSHa < NPSHr. Task 4 includes full mission time (30 days) analysis with effects of operation in the maximum erosion zone.

  • Remaining sections of SECY 11-0014 are being addressed independent of the BWROG/Sulzer work.

10

Update Status of BWROG/Sulzer work

  • Sulzer performed 4 different steady-state CFD simulations (models) using ANSYS CFX in an attempt to produce a NPSHr3%

curve (Task 1A) for the CVDS pump. Each simulation could not reproduce the factory test results. The simulations included:

- A full CFD model of the suction piping, pump, and volute/discharge.

- A split-plane CFD model (half-pump model) to eliminate pump asymmetries from the full model.

- A split-plane CFD model (half-pump model) with a vaneless diffuser to remove unsteady regions within volute.

- A split-plane CFD model (half-pump model) with a vaneless diffuser to remove unsteady regions within volute and a stage grid interface to better handle the rotating and stationary elements of the pump.

  • Sulzer also performed 1 transient CFD simulation case using ANSYS CFX for the CVDS pump, and demonstrated the ability to better predict the NPSHr3% curve*. However, only 1 data point has been analyzed and the simulation could not reproduce the factory test results.

11

  • All data presented in this presentation is considered preliminary and is unverified.

Update Status of BWROG/Sulzer work

  • Recently Sulzer produced a transient CFD case using a different software (STAR-CCM+) for the CVDS pump. Only 1 data point has been analyzed and the simulation could not reproduce the factory test results.

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Update Status of BWROG/Sulzer work

  • Progress on BWROG/Sulzer Tasks 2 - 6:

- Task 2 is complete. The conclusion reached is that the ANSI 1.6 equation for speed-related NPSHr changes is valid for speed increases limited to +/-3% from the nominal.

- Task 3 is in review with the BWROG. The draft report shows, based on previous pump test experience, that a short period of operation with NPSHa < NPSHr does not adversely affect the pump mission time.

- Task 4 is in final review with the BWROG. The draft report indicates significant cavitation erosion margin exists (~ 6,200 days = 17 years) even when operating in maximum erosion zone. A minimum service life of 30 days operation is assumed.

- Task 5 has been reviewed by the BWROG and comments are being resolved by Sulzer. Independent methods confirm that mechanical seal performance is not impacted by non-condensable gas.

- Task 6 is complete. The report shows that the NPSH uncertainties introduced by pump test instrumentation are small (< 1 NPSH at 4000 gpm).

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Update Status of BWROG/Sulzer work

  • The conclusion NSPM has reached based on the work completed over the past year is:

- CFD modeling of the Monticello CVDS pump has not been successful to date. The failure of the steady-state model solution has been attributed to asymmetry in the fluid flow velocity and pressure in the pump impeller eye.

- Evaluation of CFD modeling options for the Monticello CVDS pump continues using a transient solution; however, success is not assured.

- CFD CVDS models may not be able to predict NPSHr uncertainties with the precision necessary to meet NRC objectives.

- The results of non-CFD task reports indicate small changes in NPSHr for the parameters evaluated or negligible effects on pump performance and reliability.

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Proposed Path Forward

  • NSPM will apply the 21% uncertainty (used for Non-CAP plants) to the CVDS pumps for determining adequate margin for pump operation.
  • Sulzer will perform a similarity analysis of the MNGP Core Spray pumps (also CVDS pumps) in relation to the MNGP RHR pumps (applicability of Tasks 2 - 6) and will demonstrate that these pumps also have adequate margin for pump operation.
  • Using this approach NSPM will submit the following analyses to the NRC for approval:

- the ECCS pump margin analyses using 21% uncertainty

- the BWROG non-CFD related analyses to support pump NPSH uncertainty margins

- the Core Spray similarity analysis

- An analysis that demonstrates that negative NPSHr margins for a short time do not challenge ECCS performance or pump reliability

  • In parallel NSPM will continue to stay engaged with, monitor and support the BWROG work regarding NPSH uncertainties for ECCS pumps.

15

Proposed Path Forward

  • Use of 21% uncertainty for NPSHr results in negative margin for only 5 minutes of the 30 day DBA LOCA mission time.
  • BWROG Task 3 will evaluate that NPSHa < NPSHr3% is adequate to demonstrate pump performance and reliability for the 30 day DBA LOCA mission time.
  • Current standard vendor testing to determine NSPH curves results in operation below NPSHr3% curve for periods longer than 5 minutes and does not result in pump damage.
  • Core is reflooded to 2/3 core height within the first 4 minutes of a DBA LOCA, prior to low NPSH margin. After core PCT excursion is mitigated, only 2700 gpm of Core Spray (CS) flow is required to cool core. Analysis will be provided to demonstrate CS pumps will supply this flow with substantial margin.
  • Experts in hydraulic analysis are being utilized to review and provide direction on approaches being considered.

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Proposed Path Forward Example Use of 21% Uncertainty (4000 gpm) 21% Uncertainty 4.62 Total uncertainty applied to MNGP RHR pump to be assumed at 4000 gpm based on NPSHr.

Task 1B, Not Used Positive effect and will not be used, Budris Technical Report on Task Temperature #4 Findings (ML093510164) suggests benefit of 3% or 0.67.

Task 1C, Piping 2.2 Budris recommends 0% - 10% uncertainty in Technical Report on Geometry Task #4 Findings (ML093510164).

Task 1D, Dissolved 1.1 Budris recommends 0% - 5% uncertainty in Technical Report on Task Gas #4 Findings (ML093510164).

Task 1E, Wear Ring Not Used Not significant as long as the pump meets ASME Section XI In-service Clearance Testing Limits, not listed in Budris recommended uncertainties.

Task 2, Speed + 0.88 Task 2 report applied to MNGP RHR pump based on speed assumed in MNGP TS 3.8.1 Surveillance Requirement (2% fast). Appropriate to treat as random uncertainty (square root sum of the squares - SRSS) with other issues.

Task 6, Vendor Test + 0.98 Task 6 report applied to MNGP RHR pump. Appropriate to treat as Instrument random uncertainty (SRSS) with other issues.

Total uncertainty 4.62 This is 21% using SRSS 17

Proposed Path Forward

SECY Section How Complying 6.6.1 Develop 21% uncertainty analysis - includes Task 2, Task 5 NPSHreff and Task 6 from BWROG work to justify low uncertainties and robustness of pumps. Does not include wear ring leakage uncertainty based on successful ASME (IST) pump testing.

6.6.2 Flow rate assumed in the EPU safety analyses demonstrates Pump Flow Rate adequate core and containment cooling. No change in EPU analysis for NPSHa is anticipated.

6.6.3 95/95 Monte Carlo Analysis - MNGP results shown in NEDC -

Conservative 33347P, Containment Overpressure Credit for NPSH or use of Containment Press license basis deterministic analysis.

6.6.4 Demonstrate containment is not compromised in DBA LOCA.

Containment For Appendix R events - will demonstrate that any containment Integrity leakage will not result in loss of core cooling capability.

6.6.5 No new operator actions are required. For beyond-design-basis Operator Actions containment failure operator actions can be added (e.g. raise torus water level) to enhance EOP response if inadequate NPSH is detected.

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Proposed Path Forward

SECY Section How Complying 6.6.6 No testing required - Analysis to demonstrate ECCS pumps will run for full mission time included in 6.6.10, Task 3 and Task 5 NPSHa < NPSHr from BWROG work supports operation when NPSHa < NPSHr.

6.6.7 Will demonstrate: (1) sufficient margin in results from 6.6.4, (2) minimum containment leakage that will provide sufficient CAP, No Pre-existing Leak (3) surveillance test that can detect leakage <La using nominal instrument indications and existing TS limits, and (4) enhanced online monitoring of containment leakage.

6.6.8 Task 4 from BWROG work determines effects of operation in the maximum erosion zone.

Max Erosion Zone 6.6.9 Use 95/95 Monte Carlo analysis to compare with NPSHa -

NRC has said that realistic analysis may be eliminated.

NPSH Margin 6.6.10 Task 3 and Task 4 from BWROG work and CS pump similarity analysis provide basis for pump performance for full 30 day Mission Time mission time.

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Proposed Path Forward Basis for Proposed Approach

  • Adequate core cooling is demonstrated by meeting SECY 11-0014, Enclosure 1 criteria as described.
  • Use of CAP results in a very small increase in CDF as defined by RG 1.174*
  • Use of CAP does not challenge the accepted definition of defense-in-depth.
  • Online monitoring for containment integrity.
  • Enhance the procedural response to mitigate inadequate NPSH under beyond-design-basis containment failure conditions.
  • Provide training to increase Operator awareness and sensitivity to NPSH concerns, that includes pump NPSH monitoring and containment integrity monitoring.
  • Emergency Operations Procedures provide alternate methods to cool the core.

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4 min 6 min 10 min 200 300 21

22 Proposed Path Forward

  • Current Proposed Schedule:

Date Deliverables May 2012 21% Uncertainty analysis complete June 2012 BWROG Task Reports complete July 2012 Revised ECCS analysis complete August 2012 CS Pump similarity analysis complete September 2012 Provide CAP Submittal to NRC March 2013 NRC Draft Safety Evaluation April 2013 ACRS EPU subcommittee review May 2013 ACRS full committee review June 2013 NRC issues EPU License Amendment 23

Discussion 24

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