TLAA 112MAIN CIRCULATION PUMP FLYWHEEL

The flywheels in reactor coolant pumps(RCP) provide inertia to ensure a prolonged pump rundown providinga slow decrease in coolant flow in order to prevent fuel damage after a loss of power to the RCP motors.

The ageing effect of concern for the RCP flywheel is fatigue crack initiation and growth over itsservice life.A flywheel has sufficient kinetic energy to produce high-energy missiles should it fail. Over-speed of the pump rotor assembly during a LOCA transient increases both the kinetic energy and the potential for failure of the flywheel.The safety consequences of a flywheel failure could be significant because of possible damage to the reactor coolant system, the containment, or other equipment or systems important to safety.

For the RCP flywheel, two types of analyses with time limited assumptions may be encountered:

  1. The first type isbased on the design analysis in whichthe cumulative fatigue usage factoris calculated for a specifiednumber of startup/shutdown cycles of the RCP considered. In this case, the time-dependent parameters include the component operating time or the number of startup/shutdown cycles.If an explicit fatigue analysis was performed at the design stage, the analysis parameter is the cumulative fatigue usage factor. In some cases, however, such an underlying fatigue analysis may be missing.
  2. The second type of time limited analysis is the fatigue crack growth analysis (fracture mechanics analysis of the flywheel as foreseen for instance in US-NRC Regulatory Guide 1.14[1-2]). In this second type of analysis, the time-dependent parameters include number of startup/shut down cycles and other transients causing cyclicstresses on the flywheel. The analysis parameters associated with this time-dependent parameterincludethe initial crack length at which fatigue crack growth starts for the given stress range, and the critical crack length at which sudden fracture failure will occur. The fracture mechanicsanalyses of the RCP flywheel is based on appropriate conservative assumptions for stress level, flaw size, temperature and fracture toughness at the location of interest. The effect of cracks emanating from structural discontinuities as key-ways and bolt holes is to be evaluated. Inspection of the RCP flywheel is a part of the plant specific ISI programme based on CSA N285.4 [3], AERB/NPP/SG/O-2 [4] or applicable Code/standard such as ASME Code Section XI [5]to ensure that possible cracks are below the acceptance limits with requested margin. The flaw size used in fracture mechanics analysis isthe actual flaw size detected by inspection or the largest flaw (e.g. 0.25 inch [2]) which could conceivably be missed by inspection or non-destructive examination.

Analysis Type I:

For the RCP flywheel with restrictions on the component operating time or on the number of startup/shutdown cycles (the first case as defined above), the TLAA is found acceptable under (i), (ii), or (iii) based on the following:

  • Acceptance by (i):

If the projected specified number of RCP startup/shutdown cycles during the intended period of operation does not exceed the number that wasoriginally considered, then the existing analysis is still valid and acceptable for the intended period of operation.

  • Acceptance by (ii):

If the number of RCP startup/shutdown cyclesis projected tobe greater than the originally assumed number of startup/shutdown cycles forthe intended period of operation, the fatigue analysis of the flywheel needs to be updated and reassessed. Arevised baseline of startup/shutdown cycles, determined from a review of past operating history,can provide for amore accurateprojection of thenumber of startup/shutdown cycles for the intended period of operation. If the updated analysis meets the applicable acceptance criterion (e.g. cumulative usage factor below 1), then the TLAA is acceptable for the intended period of operation.

  • Acceptance by (iii):

In this case, flywheel integrity can be managed during the intended period of operation by an adequate in-service inspection frequency including any necessary analyses as described above in (ii) for any selected time period of operation.Refer to AMP101,AMP102, and AMP146 which provide inspection requirements and corrective actions, including revision of operating procedures, adequate in service inspection frequency, and more rigorous analysis to demonstrate that the design code limit will not be exceeded.

Analysis Type II:

For primary RCP flywheel integrity evaluations (second case as defined above), the TLAA is found acceptable under (i), (ii), or (iii) based on the following:

  • Acceptance by (i):

If the specified number of RCP startup/shut down and other transients expected during the intended period of operation does not exceed the number used in the existing fly wheel integrity analysis using the relevant actual or postulated crack size, then the present analysis is still valid and acceptable for the intendedperiod of operation.

  • Acceptance by (ii):

If the number of cycles will exceed the specified limits during the intendedperiod of operation, a new fracture mechanics analysis will be needed in order to ensure flywheel integrity. The new analysis is toconsider the results from latest inservice inspection. A revised baseline of startup/shutdown cycles, determined from a review of past operating history, can provide for a more accurate projection of the number of startup/shutdown cyclesto use for the updated crack growth calculations. It is shown that the above required safety margins towards crack initiation and corresponding critical speeds will be maintained. If the resultant updated analysis meets the applicable acceptance criteria (e.g., the calculated crack size is less than critical crack length), then the TLAA is acceptable for the intendedperiod of operation.

  • Acceptance by (iii):

In this case flywheel integrity can be managed during the intended period of operation by an adequate in service inspection frequency including any necessary crack growth calculations and fracture mechanics analyses as described above in (ii)for the intended period of operation. Refer to AMP101,AMP102, and AMP146 which provide inspection requirements and corrective actions, including revision of operating procedures, adequate in service inspection frequency, and more rigorous analysis to demonstrate that the design code limit will not be exceeded.

If the above TLAAs cannot found to be acceptable, the RCP flywheel is replaced.

References

[1]UNITED STATES NUCLEAR REGULATORY COMMISSION, Regulatory Guide 1.14, USNRC, August 1975.

[2]UNITED STATES NUCLEAR REGULATORY COMMISSION, NUREG-0800, Standard Review Plan, 5.4.1.1 Pump Flywheel integrity, Rev. 2, USNRC, March 2007.

[3]CANADIAN STANDADS ASSOCIATION, Periodic Inspection of CANDU Nuclear Power Plant Components. CSA N285.4, CSA, Toronto, Canada.

[4]ATOMIC ENERGY REGULATORY BOARD, Inservice Inspection of Nuclear Power Plants, AERB/NPP/SG/O-2, AERB, Mumbai, India, March 2004.

[5]AMERICAN SOCIETY of MECHANICAL ENGINEERS, ASME Section XI, Rules for Inservice Inspection of Nuclear Power Plant Components, The ASME Boiler and Pressure Vessel Code, New York, NY.

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