ML19209A654

Responds to IE Bulletin 79-21, Temp Effects at Level Measurements. Outlines Liquid Level Measuring Sys Inside Containment Used to Initiate Safety Actions.Tables & Charts Encl

ML19209A654
Person / Time
Site:	Prairie Island
Issue date:	09/14/1979
From:	Wachter L NORTHERN STATES POWER CO.
To:	James Keppler NRC OFFICE OF INSPECTION & ENFORCEMENT (IE REGION III)
References
NUDOCS 7910050127
Download: ML19209A654 (8)

Text

{{#Wiki_filter:! ] NSR2 NORTHERN 5TATES POWER COMPANY M I N N E A PO LI S. M I N N E S OTA 55409 September 14. 1979 Mr. James G. Keppler Director - Region III Of fice of Inspection and Enforcerrent United States Nuclear Regulatory Commission 799 Roosevelt Road Glen Ellym, Illinois 60137

Dear Mr. Keppler:

PRAIRIE ISLAND NUCLEAR GENERATING PLANT Dockets No. 50-282 and No. 50-306 In response to IE Bulletin 79-21, the following is offered: 1. The following liquid D vel measuring systems inside containment are used to initiata safety actions or are used to provide post-accident monitoring information: a. Steam Generator Narrow Range Water Level (Three Channels per Steam Genera tor) The narrow range channels employ an open reference leg with condensate pot located at the elevation of the upper tap. A Foxboro Model E13DH-SAMI transmitter mear. ares differential pressure and provides initial level signals. These channels provide the following safety action inputs for either steam generator: - Turbine trip and feedwater isolation on high-high steam generator water level (2/3 logic). - Reactor trip on Icw steam generator water level in coincidence with steam flow - feed flow mismatch (1 of 2 level plus 1 of 2 flow logic). - Reactor trip on low-low steam generator water level (2 of 3 logic). - Auxiliary feedwater pump initiation on low-low steam generator water level (2 of 3 logic). - Post-accident monitoring function. 1109 336 SEP O F {} } 010050 /1;]

NORTHERN STATES POWER COMPANY Mr. James G. Keppler September 14, 1979 Page 2 b. Stear Generator Wide Range Water Level The wide range channels also employ an open reference leg with c.ndensate pot located at upper tap levet. A Foxboro Model 13DH-SAH1 transtitter measures differential pressure and provides initial level signals. The only safety function of these channels is to provide post-accident monitoring. c. Pressurizer Water Level (Three channels) The pressurizer level channels employ a sealed reference leg with an open condensate pot located at the level of the upper tap. A Barton Model 386/351 sealed reference leg DPU and transmitter provides initial level signals. These channels provide a high level reactor trip signal (2/3 logic) and may be used for post-accident monitoring. 2. Evaluation of the effect of post-accident conditions on indicated water level (See Section 1 for Safety Functions) a. Reference Leg Heatup High energy line breaks inside containment can result in heatup of level measurement reference legs. Increased reference leg water column temperature will result in a decrease of the water column density with a consequent apparent increase in the indicated water level (i.e., apparent level exceeding actual level). Results of our analysis are shown in Table 1 (for Steam Generator Narrow and Wide Range) and 2 (for Pressurizer). b. Reference Leg Boiling Boiling cannot occur in a sealed reference leg (Pressurizer); and, according to Westinghouse, recent containment analyses indicate such boiling would not occur in Steam Generator reference legs, c. Coolant Density Changes A bias in indicated water level may also be introduced by changes in Pressurizer or Steam Generator pressure due to changes in the density of saturated water and steam within those vessels. While prediction of the effects of rapid depressurization requires complex calculations for each specific case, the bias which would exist at low power under quiescent conditions was calculated and is shown in Figures 1 (for Steam Generator Narrow and Wide Range) and 2 (for Pressurizer). 1i09 337

NORTHERN STATES POWER COMPANY Mr. James G. Keppler September 14, 1979 Page 3 3. Safety Function Setpoints a. Steam Generator Narrow Range Water Level Trip Setpoints The only high-energy line rupture within the containment for which the steam generator water level provides the primary trip function is a feedline rupture. For such a case, the low or low-low water level trip must be actuated when the pressure difference between the narrow range level taps corresponds to a zero-level value. Thus the trip setpoints must be at or above the value that would be indicated at zero true level. Because large steam generator pressure changes are not expected before trip, only the reference leg heatup effects need be considered and not the effects of system pressure changes. The accident analysis at Prairie Island was based on a low-low trip value of 0% narrow range level. Plant Technical Specifications require a 5% trip setpoint in order to take into account channel and process measurement accuracy. Because SI is initiated at 4 psig (corresponding containment temperature is less than 1600F), primary protection (Rx trip) is provided by high containment pressure signals at temperatures above 160 F. Therefore, temperature bias is considered in oroer to trip reactor before containment high-pressure setpoint is reached. Temperature bias at 1600F containment temperature is 2% (See Table 1). Because channel accuracy is approximately 1% and temperature bias is 2%, our present low-low level trip setpoint of 13% is quite conservative. The same analysis holds true for low-level trip -- our present setroints are 25%; Tech Specs require 16% or greater. Therefore, no changes to setpoints are planned as a result of IE Bulletin 79-21. b. Pressurizer Water Level Trip Setpoint No credit is taken for this reactor trip function following a high energy line rupture inside containment. Thus the trip setpoint need not be revised to include environmental errors. 4. A review of emergency procedures referring to steam generator and pressurizer level indications was made. The only revisions planned are to statements requiring operators to mair*ain visible (and in some cases specific) levels in steam generators or pressurizers. Because of possible confusion generated by constant revisions to emergency procedures, we will revise such procedures after a commonly agreeable Post-Accident Monitoring i109 338

NORTHERN STATES POWER COMPANY Mr. James G. Keppler September 14, 1979 Page 4 position is established between NSP, Westinghouse PWR Owners Group, and NRC Stendards personnel (Reg Guide 1.97 Working Group). It is expected that these discussions will better itemize required post-accident monitoring equipment and will require other revisions to emergency procedures. Operator training concerning reference leg heating effects will be incorporated. Yours very truly, L. J. achter Vice President Power Production and System Operation cc: Mr. G. Charnoff NRC Office of Inspection and Enforcement Washington, D.C. Enclosure t i109 339

TABLE 1 Correction tc indicated steam generator water level for Reference Leg Heatup effects due to post-accident containment temperature. Correction to S/G Level Containment Temperature (OF) (% of Sran) 1060 0 1600 2.0 200 3.9 2500 6.6 280 8.5 300 9.8 1109 340

TABLE 2 Correction to indicated Pressurizer Water Level for Reference Leg Heatup effects due to post-accident containment temperature. Correction to Prst Level Containment Temperature (OF) (% of Span) 1060 0 160 3.0 2000 5.8 250 9.9 280 12.7 3000 14.7 Note: It is expected that bias due to temperature will be less than shown above due to the inability of the scaled reference leg to absorb large volume changes. 1109 341

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