IE Circular No. 78-11, Recirculation M-G Set Overspeed Stops
CR78011 UNITED STATES NUCLEAR REGULATORY COMMISSION WASHINGTON, D. C. 20555 June 7, 1978 MEMORANDUM FOR: B. H. Grier, Director, Region I J. P. O'Reilly, Director, Region II J. G. Keppler, Director, Region III K. V. Seyfrit, Director, Region IV R. H. Engelken, Director, Region V FROM: Norman C. Moseley, Director, Division of Reactor Operations Inspection, Office of Inspection and Enforcement SUBJECT: IE CIRCULAR 78-11, RECIRCULATION M-G SET OVERSPEED STOPS The subject circular is transmitted for issuance no later than the close of business on June 15, 1978. The circular should be issued to all holders of BWR Operating Licenses or Construction Permits. Also enclosed is a draft copy of the transmittal letter. Norman C. Moseley, Director Division of Reactor Operations Inspection Office of Inspection and Enforcement Enclosures: 1. Draft Transmittal Letter 2. IE Circular No. 78-11 CONTACT: C. J. DeBevec, IE 49-28180 . (Transmittal letter for IE Circular 78-11, to each holder of a BWR Operating License or Construction Permit.) Addressee: The enclosed IE Circular No. 78-11, is forwarded to you for information. No written response is required. Should you have any questions related to your understanding of this matter, please contact this office. Sincerely, Signature (Regional Director) Enclosures: 1. IE Circular No. 78-11 2. List of IE Circulars Issued for 1978 . UNITED STATES NUCLEAR REGULATORY COMMISSION OFFICE OF INSPECTION AND ENFORCEMENT WASHINGTON, D. C. 20555 June 15, 1978 IE Circular No. 78-11 RECIRCULATION M-G SET OVERSPEED STOPS In August 1977, personnel at the Cooper Nuclear Station reported finding the recirculation M-G set overspeed mechanical and electrical stops at 112 percent and 120 percent of rated core flow. The licensee had believed the set point to be 107 percent for these stops, although a search of Station preoperational and startup test records could not establish the actual initial set point. Subsequent to this finding, the licensee set the mechanical stop at 102 percent of rated core flow. The technical specifications require that the Minimum Critical Power Ratio (MCPR) be maintained greater than 1.22 for 8 X 8 fuel. The technical specifications further require MCPR to be above a minimum of 1.22 times Kf whenever operating at less than rated flow. The correction factor Kf is always greater than or equal to 1.0 and thus increases the MCPR limit when at less than rated flow. Technically, this factor increases the required MCPR thermal margin in the event a recirculation M-G set speed control failure would occur and allow the recirculation pump to overspeed. The lower the initial core flow the more severe is the flow transient, thus Kf increases as actual core flow is decreased. The Kf factor is read from a curve included in the technical specifications. A family of Kf curves is given in the figure in the technical specifications. The proper curve to use is the curve corresponding to the set point of the manual flow control overspeed stop (the M-G set mechanical stop). A lower set point of the overspeed stop results in a small Kf correction factor for a given core flow, as the flow transient on an M-G set speed control failure is less severe. Thus, it can be seen that by using a lower curve (e.g., 107, percent curve) than that required by the actual overspeed stop set point (believed to have been set at 112 percent), a nonconservative Kf correction factor was being used. The M-G set overspeed stops were set during the preoperational test program by a General Electric Company startup engineer. The stops were checked by a GE preoperational test procedure. This procedure did not include a requirement to specifically record the stop setting. It 1 of 2 . IE Circular No. 78-11 June 15, 1978 included only a requirement to verify that the stop setting had been made. It appears likely that these same procedures, and perhaps the same engineer, were involved in the preoperational/startup test programs at several other sites. All holders of operating licenses for Boiling Water Reactor plants employing recirculation M-G sets should be aware of the potential for improperly or undocumented settings on recirculation M-G set speed stops. It is recommended that the following be considered in your review: 1. Facility records should be searched to determine if the actual existing set point on the recirculation M-G set overspeed mechanical stops is positively established. 2. If the setting of the mechanical and electrical overspeed stops is in doubt, then steps should be initiated to determine the setting or additional conservatism should be included in the determination of Kf. 3. The actual Kf being used for the process computer calculations in the determination of MCPR limits should be verified to be appropriate to (or more conservative than) the actual setting of the recirculation M-G set mechanical overspeed steps. This circular is also issued to holders of BWR Construction Permits for information or use as appropriate to assure that steps are taken during the preoperational test program to confirm that the M-G overspeed stops are set properly and that the proper Kf will be used with the process computer. No written response to this circular is required. If you require additional information regarding this matter, contact the Director of the appropriate NRC Regional Office. 2 of 2
Page Last Reviewed/Updated Tuesday, March 09, 2021
Page Last Reviewed/Updated Tuesday, March 09, 2021