ML20059G351
| ML20059G351 | |
| Person / Time | |
|---|---|
| Site: | North Anna  |
| Issue date: | 08/27/1990 |
| From: | Office of Nuclear Reactor Regulation |
| To: | |
| Shared Package | |
| ML20059G339 | List: |
| References | |
| NUDOCS 9009120264 | |
| Download: ML20059G351 (3) | |
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,e NUCLE AR REGULATORY COMMISSION C.
UNITED STATES s
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q SAFETYEVALUATIONUYTHEOFFICEOFNUCLEAe.R{ACTORREGULATION j
i RELATED TO AMENDMENT NOS.138 AND 121 TO FACILITY OPERATING LICENSE NOS. NPF-4 AND NPF-7 VIRGINIAELECTRICANDPOWERCOMr'.9
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4 OLD DOMINION ELECTRIC COOPERATIVE NORTH ANNA POWER STATION. UNITS NO.1 AND NO. 2 2
DOCKET NOS. 50-338 AND 50-339 MTRODUCTION By letter dat&d April 30, 1990, the Virginia Electric and Power Company (the i
licensee) proposed Technical Specification (TS) changes for the Individual Rod Position Indication System for the North Anna Power Station, Units 1 and 2 (NA-1&2). The proposed amendments would Individus1 Rod Position Indication (IRPI) permit a shift in emphasis from-the I
l Indication (DPI) system (the step counters)ystem to the Demand PositionThe proposed T s
l revise the requirements governing the operability _of the IRPI system, providing a detailed rod position information during the shutdown modes (Modes 3-5) and during startup and shutdown operations.
WSCbSSION i
The licensee has experienced operational difficuM 9s with-the IRPJ system due L
to system non-linearities and both transient a7d w *v-state temperature
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i sensitivities. The proposed TS changes from the Ikr. iystem to the DPI system would t etermine positive rod group position (nformation during' shutdown, and i
pertinent operational modes such as reactor startup. The IRPI system is still expected to provide information and confirmation of rod motion and direction of rod motion, i
The Westinghouse-designed plants employ the IRPI system. The control rod drive mechanism is used to withdraw and insert the control rods and provide sufficient holding power for stationary support. The complete drive mechanism is a magnetic jack device consisting of the latch assembly, the drive shaft assembly, the pressure vessel and rod drive shaft housing and the c>erating coil stack.
The latch assemblies consist of a stationary gripper latc) and a moveable gripper latch.. The latches engage the nround control rod drive shaft when their respective operating coils are energized.
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The calibration and accuracy of individual rod position is dependent on a set of linear variable transformers formed from primary and secondary coils..The rod drive mechanism extends up into a tui,a and serves as a variable core for the transformer. When a constant AC source is applied to the primary side, movement of the control rod changes the primary-to-secondary coupling of the transformer and produces a secondary voltage, where it is assumed that this 9009120264 900827 l
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. i secondary voltage is a linear f unction of the rod position.
In actuality this is not the case. This deviation is normally absorbed by the 112 step allowance for radius alignment.
A second and more serious problem is that the system tr, highl temperature j
sensitive, in addition, there are transient (non-equil1Lrium tein responses associated with not only Reactivity Control System RCS)perature temperature changes, but also with rod motion.
The calibration on sever 61 of the Westing-l house plants is usually carried out at hot operating temperaturer at the begin-ning of the cycle.
As the reactor is cooled down to enter the hot, intermediate j
and cold shutdown modes, the calibration curve becomes inaccurate and may be off by as much as 60 steps, or over one-quarter of the height of the total
- Core, The transient response problem is referred to as " overshoot." This nomenclature stems from the fact that when a rod is withdrawn or insert"1. the IRPI channel l
indicator will show a greater withdrawal / insertion than actual for a period of time. This error could be as high as 25 steps, and the " soak time" for the IRPI system to reach equilibrium could range between 20 and 45 minutes.
EVAL JION The licensee has proposed changes to the NA-1&2 TS which will emphasize the DPI syste.n during the shutdown modes and during startup and shutdown operations.
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The two proposed changes are addressed below.
l During shutdown modes, the IRPI requirements have beer, replaced by the operations l
requirsu nts for a step DPI.
It is worth noting that the IRPI system was not origins.ll," intended to be used in the shutdown modes.
However, some plants have TS r9 quiring IRPI indication during the shutdown modes as added confidence to shutdan margin calculations.
For the shutdown mode, TS require a minimum of 1.77 percent shutdown reactivity margin,1.0 percent of which must always be inserted in the core (i.e., K 0.99).
Procedures do exist to determine the required shutdown boron conceba=tions allowing for calculations and measurement uncertainties.
In addition, industry in general has demonstrated that the DPI is highly reliable. Consequently, this reliability, in combination with the TS requirement that K 0.99, suggests that the IRPI system is not needed to guarantee suberiti b =nd/or shutdown margin in these codes.
a The NRC staff has approved this change on other plants; and finds this change to be acceptable for NA-l&2.
The second proposed change is the incorporation of the so-called 1-hour " soak time." That is, for one in every 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br /> and with power below 20%, the DPI syctem will be the prime indicator of rod position and the IRPI s/ stem's role is to display general rod motion information.
The licensee analyzed the impact of statistically misaligned rods on the core power distribution thron hout core life l
to demonstrate that the condition 11 DNB limits are met for power 'evels up to and including hot full power. The 1 hour1.157407e-5 days <br />2.777778e-4 hours <br />1.653439e-6 weeks <br />3.805e-7 months <br /> " thermal soak" will apply only at power levels less than 50 percent.
In addition, the.1 hour1.157407e-5 days <br />2.777778e-4 hours <br />1.653439e-6 weeks <br />3.805e-7 months <br /> in 24 hcars ensures that the IRPI system is still used a minimum of 96 percent of the tihe even at low power.
The NRC staff find this TS change acceptable.
Therefore, based on all of the above, we find the proposed changes to the NA-1&2 T5 to be Occeptable.
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! i ENVIRONMENTAL CONSIDERATION These amendments i-volve a change to a 9quirment with respect to installation or use of a facility component located within the restricted F.ee as defined in 10 CFR Part 20 and changes surveillance requirements. We have S termined that the amendments involve no significant increase in the amounts and no significant l
change in the types, of any effluents that may be released offsite, and that 7
I there is no significant increase in individual or cumulative occupational l
radiation exposure. The Commission has previously issued a proposed finding l
that these amendments involve no significant hazards consideration and there has heen no public comment on such finding. Accordingly, these amendments meet the eligibility criteria for categorical exclusion set forth in 10 CFR 51.22(c)(9).
Pursuantto10CFR51.22(b),noenvironmentalimpactstatementorenvironmental assessment need be prepared in connection with the issuance of these amendments.
l CONCLUSION Wehaveconcluded,basedontheconsiderationsdiscussedabove,that(i)there
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is reasonable assurance that the health and safety cf the public will not be endangered by operation in the proposed manner and (2) such activities will g
be conducted in compliance with the Commission s regulations, and the issuance of the amendments will not be inimical to the connon defense and security or to the health and safety of the public.
Date:
August 27. 1990 l
l Principal Contributor:
A. C. Attard T
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