ML20056A793
| ML20056A793 | |
| Person / Time | |
|---|---|
| Site: | Summer  |
| Issue date: | 08/06/1990 |
| From: | Office of Nuclear Reactor Regulation |
| To: | |
| Shared Package | |
| ML20056A792 | List: |
| References | |
| NUDOCS 9008090203 | |
| Download: ML20056A793 (6) | |
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- CCic UNITED STATES
[(I g NUCLEAR REGULATORY COMMISSION 5 C 8 W ASHINGT ON. D. C. 20b55 SAFETY EVALUATION BY THE OFFICE OF NUCLEAR REACTOR REGULATION RELATED TO NATURAL CIRCULATION C00LDOWN VIRGIL C. SulNER NUCLP.t_ STATION SOUTH CAROLINA ELECTRIC & GAS COMPANY DOCKET NO. 50-395
1.0 INTRODUCTION
Branch Technical Position (BTP) RSB 5-1, " Design Requirements of tne Residual Heat Removal (RHR) System," requires that test programs for pressurized water reactors (PWRs) include tests with supportirg analysis to (1) confirm that adequate mixing of borated water added prior to or during cooldown can be achieved under natural circulation conditions and permit estimation of the times required to achieve such mixing, ed (2) confirm that the cooldown under natural circulation conditions can be achieved within the limits specified in the emergency operating procedures. In 6ddition, the plant is to be designed so that the reactor can be taken from normal operating conditions to cold shutdown using only safety grade systems. A comparison of performance to that of previously tested plants of similar design may be substituted for these tests.
For the purposes of implementing the BTP, plants were divided into three classes. Virgil C. Summer Nuclear Station Unit 1 (Summer), with a construction permit. docketed prior to January 1,1978 and an operating license issued after January 1,1979, is classified as a Class 2 plant for which partial implementation of the BTP is required.
A natural circulation / boron mixing /cooldown test was performed at Diablo Canyon l Unit 1 on March 28-29, 1985. By memorandum dated February 4,1987, the staff determined on the basis of the Diablo Caayon Unit 1 tests and submittals and the Brookhaven National Laboratory (BNL) technical evaluation report (TER),
I that the Diablo Canyon Unit 1 systems meet the intent of BTP RSB 5-1 for a Class 2 plant.
By letter' dated January 26, 1990, the licensee for Summer submitted a l Westinghouse analysis to show the applicability of the Diablo Canyon cooldown i
test results to Summer rather than conduct such a test at its plant. The Westinghouse analysis, WCAP-12464, " Virgil C. Summer Nuclear Station Natural l Circulation Evaluation Program Report," avaluates the natural circulation flow and the boron mixing capability of Summer relative to the requirements of BTP RSB 5-1. In a letter dated March 9,1990 and a follow-up telephone conversation, Summer provided additional information to the staff which clarified the original submittal.
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,. The staff Safety Evaluation for Diablo Canyon Unit 1, with the attached BNL TER entitled " Technical Evaluation Report for Diablo Canyon Natural Circulation, Boron Mixing, and Cooldown Test," identified the plant parameters that may affect application of the test results to other plants. These parameters are the basis for the staff's evaluation and are discussed below.
2.0 EVALUATION Natural Circulation Diablo Canyon Unit 1 is rated at 3338 megawatts thermal (MWt) and has four loops in its reactor coolant system (RCS). Summer is rated at 2775 MWt and has three loops in its RCS. The general configuration of the piping and components in each reactor coolant loop is the same for Summer and Diablo Canyon Unit 1.
Both plants have the same model reactor coolant pumps (Model 93A). Sumer uses a Model 03-1 steam generator while Diablo Canyon Unit I uses a Model 51 steam generator. Significant parameters governing natural circulation are 1 1
hydraulic flow resistance and thermal driving head. To demonstrate similarity in design for natural circulation, these two parameters were compared.
Data from the Westinghouse report showed that the Summe' hydraulic resistance coefficien1s at normal flow conditions were slightly 1Ner than Diablo Canyon's.
For a given driving head the Summer flow per loop would be 8.7% greater than Diablo Canyon's. However, thermal driving head because of a difference in 4 steamgeneratortubelengths,was5-10%higherforDiabloCanyon. The report '
stated that the increased natural circulation thermal driving head for Diablo Canyon and the lower overall piping flow resistance for Summer would result in a natural circulation flow ratio (Summer /Diablo Canyon) in the range of 0.98 to 1.03. Therefore the licensee concluded that the natural circulation loop flow rate for either p,lant would be nearly the same.
RCS Cooldown 1
The capacity of the atmospheric steam dump (ASD) valves control cooling of the RCS at a specified rate, assuming a sufficient supply of auxiliary feedwater and a subcooled RCS. Steam flow through these valves removes the sensible heat and decay heat throughout the cooldown period. At the end of the cooldown period when the steam generator pressure is low, the capacity of the valves is !
mostilmited. The energy to be removed is determined by the water inventory and the amount of structural material in the RCS, and the level of decay heat.
At Summer,'each steam generator is equipped with a safety-grade ASD valve. The licensee has stated in Section 5.5.7.1.1.3 of the Final Safety Analysis Report (FSAR) that in the event of a single failure of an ASD, two steam generators can provide sufficient natural circulation flow to provide adequate core cooling from Hot Standby to 350'F. The residual heat removal system is then available to complete the cooldown to cold shutdown conditions. The licensee also provided the individual ASD capacity (flow at pressure) for Summer and Diablo Canyon. The flow capacity per source pressure of Summer and Diablo
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. Canyon are approximately within 5% of each other and for this reason the staff finds the ASDs similar. The ASDs can be manually cperated by handwheel if power sources to these valves are not available. This was verified during the plant hot functional testing program (see FSAR Section 5.5.7.1.1.2).
Therefore, the staff finds that there is reasonable assurance that the ASD valves have the capacity to cool down the RCS to the RHR initiation temperature; therefore, the ASDs are acceptable.
Bypass Flow and Upper-Head Cooling A potential exists for void formation in the upper-head of the reactor vessel '
during the cooldown/depressurizetion of the RCS under natural circulation conditions if the upper head is relatively isolated from the rest of the PCS and its fluid temperature remains higher than the coolant temperature in the main flow paths of the RCS. Upper-head cooling under natural circulation conditions is influenced by core bypass flow and mixing in the upper head.
Westingheuse plants may be divided into two groups according to the magnitude of the bypass flow. They are designated as T and T plants. For T plants which have a relatively low bypass flogotsuch af0bIablo Canyon 1. Ms results in upper head temperature ranging between the cold-leg and the hot-leg temperatures and raises a possibility of void formation in the upper-head region. For the T plants, such as Sumer, sufficient bypass flow exists to makethetemperatufS)Nftheupperheadfluidessentiallysqualtothecold-leg temperature, it is elso important that the reactor vessel spray nozzle between the downcomer and the upper-head region for Sumer has a significantly larger flow area than that of Diablo Canyon. This circumstance allows better flow comunication and mixing in the upper head during natural circulation. The staff finds that there is reasonable assurance that the potential for void formation is low since Sumer is a Teold plant and has good flow comunication and mixing in the upper head.
The licensee stated that with the enhanced flow mixing capability of the T i plantamaximumRCSnaturalcirculationcooldownrateof50*perhourmay68d empicyed under normal conditions. This is twice as fast as the recomended cooldown rate of 25* per hour for Diablo Canyon. The staff notes that the Sumer Emergency Operating Procedure (EOP) 1.3, Revision 3, restricts the cooldown rate not to exceed 50' per hour to preclude the formation of a void in the reactor vessel head. In thi event of void formation the E0P also includes procedures for void collapse. We find that the licensee has procedures which provide reasonable assurance that void formation in the reactor will be minimized during cooldown.
Boron Mixing The Diablo Canyon boron mixing test evaluation demonstrated adequate boron mixing under natural circulation conditions when highly borated water was injected into the RCS. The mixing effect created as the flow passes through
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~4-the reactor coolant pumps and the steam generator tubes further diffuses the ,
boron. The ability to achieve the proper shutdown margin, however, depends ,
mainly on the injection rate of boron relative to the total inventory of water '
in the RCS, During the test, the required change in concentration of about 300 l ppm was achieved in less than 1 hour1.157407e-5 days <br />2.777778e-4 hours <br />1.653439e-6 weeks <br />3.805e-7 months <br />. 1 The boric acid tanks (BATS) are the normal supply of boron for Sumer. Makeup in excess of that needed for boration can be provided from the refueling water storagetank(RWST). Because these tanks have a baron concentration l
significantly less than that of Diablo Canyon, a larger quantity of borated !
water needs to be added over a longer time to reach the desired concentration change. The licensee has stated that the Summer design has sufficient capability to borate the RCS to cold shutdown conditions while performing a j natural circulation cooldown. The licensee's calculated results show that a boration time of 225 minutes is required for a change in concentration of 300 ppm using only a path via the reactor coolant pump seals. While the Sumer j boration time is greater than Diablo Canyon, t1e staff finds that there is reasonable assurance for adequate boration to provide shutdown margin in a natural circulation cooldown.
Depressurization l
The Diablo Canyon Unit I test demonstrated that the RCS could be depressurized J from cooldown conditions to the RHR initiation pressure under natural l circulation flow conditions using the pressurizer auxiliary spray and/or pressurizer power-operated relief valves (PORVs). l l
RCS pressure control is available at Sumer using the normal pressurizer spray t valves or the pressurizer auxiliary s aray systems. These valves are not safety i grade, and may not be available for tie RCS depressurization under some t conditions. The pressurizer PORVs, however, are available as a backup method l for depressurization. The PORVs are powered from separate vital AC electrical l
power supplies and have two seismic Category 1 air supply accumulators. ;
1 Cooling Water The BNL TER estimated a 360,000-gallon auxiliary feedwater requirement for Diablo Canyon on the basis of a 43-hour cooling time for the upper head. This 1 calculation was based on the followin 1) no heat. loss from the upperheadtothecontainment;and2)gassumptions:
a limited amount of bypass fluid mixes with fluid.in the bottom of the upper head. The staff further concluded that had Diablo Canyon been a T head to containment considOM,the plant, and water cooling with heat transferwould requirement from the haveupper been significantly less than the 360,000 gallons. Diablo Canyon also has a thermal l rating about 20 percent greater than Summer and therefore requires more cooling water to remove decay heat. The staff considered the 360,000 gallon estimate for Diablo Canyor, to be bounding since Sumer is a Tcold plant with less decay heat to be removed.
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l A Seismic Category I condensate storage tank (CST) with a technical ,
specification (TS) capacity of 172,700 gallons is the primary source of i
auxiliary feedwater at Summer. This tank is normally maintained with 400,000 gallons. A 500,000 gallon capacity demineralized water tank is another source of auxiliary feedwater. Filling the CST from the demineralized water system is covered in procedure E0P-1.3, " Natural Circulation Cooldown". The service water system also provides a safety-related backup source for the CST ,
emergency feedwater supply. Since the capacity of the water sources far l exceed the cooling requirement at Summer we conclude that there is reasonable
' assurance that sufficient cooling water Inventory exists to meet the proposed plant cooldown method.
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3.0 CONCLUSION
l The staff assessed the capability of Summer to meet the requirements of RSB BTP '
5-1. The staff has identified and evaluated the plant parameters that may affect l application of the Diablo Canyon natural circulation test results to Summer. -
On the basis of the licensee's submittals, and the staff's evaluation as discussed '
above, the staff concludes that the licensee has demonstrated that the Diablo that Canyon natural circulation tests are applicable to Summer; and therefore, the Summer design meets BTP RSB 5-1 and satisfies License Condition 4 of t he
- Summer license.
Dated: August 6, 1990 1
l Principal Contributor: D. Katze '
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?! J. Hayes 14-B-20
" ! OGC(Forinform.Only) 15-B-18 ;
E. Jordan MNBB-3302 ACRS(10) P-315 D. Katze i Summer File '
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