ML18039A473
| ML18039A473 | |
| Person / Time | |
|---|---|
| Site: | Browns Ferry  |
| Issue date: | 08/11/1998 |
| From: | NRC (Affiliation Not Assigned) |
| To: | |
| Shared Package | |
| ML18039A472 | List: |
| References | |
| TAC-M83348, TAC-M83349, TAC-M83350, NUDOCS 9808180132 | |
| Download: ML18039A473 (24) | |
Text
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1.0 INTRODUCTION
By letter dated July 31, 1992, the Tennessee Valley Authority (TVAor the licensee) described the corrective actions which will be implemented to resolve certain control room emergency ventilation system (CREVS) deficiencies at Browns Ferry Nuclear Plant (BFN) Unit Nos. 1, 2 and 3. The U.S. Nuclear Regulatory Commission (NRC or the staff) also performed an onsite review during the week of December 8, 1992. TVAand the NRC staff met in Rockville, MD, on February 3, 1993, to discuss the CREVS issue.
By letter dated August 10, 1994, TVAprovided additional information in response to NRC staff requests.
By letter dated October 1, 1997, as supplemented October 14, 1997, March 16 and 20, April 1 and 28, May 1,20, and 22, June 12, 17, and 26, and July 17 and 24, 1998, TVAsubmitted a license amendment request to increase the licensed rated thermal power from 3293 MWtto 3458 MWt, an increase of 5 percent.
Since the core inventory of radionuclides (i.e., those significant in consequence assessments) is largely proportional to rated power, there could be an increase in accident radiological consequences, including increased dose to control room operators.
Because of this, the staff has considered information provided by TVAin these submittals as it relates to control room habitability corrective actions. As part of its review, the staff performed confirmatory calculations to evaluate the suitability of TVA's analyses.
The parameters used by the staff are listed in Tables 1 and 2.
2.0 BACKGROUND
Title 10 of the Code of Federal Regulations, Part 50 (10 CFR 50), Appendix A, General Design Criterion (GDC) 19 - Control Room, requires that adequate radiation protection b provided to permit access and occupancy of the control room under accident conditions. Accordingly, the CREYS is designed to automatically start on receipt of a control room isolation signal and pressurize the main control bay habitability zone with filtered outdoor air during accident conditions that could result in radioactive releases.
This filtered air maintains the control room at a positive pressure so that all leakage should be outleakage.
Charcoal absorbers and high efficiency particulate absolute (HEPA) filters in the CREVS assure removal of radioactive iodine and particulate matter.
0 The Control Bay ventilation towers, located on the north wall of the reactor building, provide the outside air for the Control Building supply duct work. Ventilation fans, which are located in the ventilation towers, pressurize the supply duct work that traverses the main control bay habitability zone. These fans operate during the accident recovery period (30 days) and supply necessary cooling for essential equipment.
The existing CREVS units take suction from these positively pressurized ducts.
0
. During the Unit 2 Cycle 5 outage, TVAidentified a specific condition that could potentially impact the ability of the CREVS to provide an environment suitable for personnel occupancy.
The Control Building air supply ducts are not designed or fabricated to be leaktight. Unfiltered outside air could leak from the seams/joints of the supply air ducts that traverse the control bay habitability zone.
This duct leakage could result in make-up air bypassing the CREVS and introducing potentially contaminated and unfiltered outside air into the control bay habitability zone.
During the Unit 2 operating cycle (Cycle 6), TVAtemporarily modified the CREVS operability requirements and implemented certain compensatory measures.
The compensatory actions included operation of all three trains of the Standby Gas Treatment System (SGTS) following an accident to maximize the negative pressure inside secondary containment, and monitoring of plant radiological conditions to provide an early indication that the control room habitability zone may become degraded.
Upon determination that there was a possibility that the iodine uptake dose to the thyroid could exceed 10 rem, potassium iodide (Kl)tablets would be distributed to control room and Technical Support Center personnel.
The licensee indicated that these corrective actions represent a change in BFN's previously approved approach to meeting GDC-19 requirements and, therefore, requested staff review of the proposed corrective actions.
These actions include, but are not limited to, modification of control bay ventilation towers to reduce the concentration of eNuents into the control bay habitability zone, increasing the leak tightness of the control bay habitability zone, establishing procedures and performance of periodic testing of control bay habitability zone, and increasing the CREVS capacity.
3.0 DISCUSSION The staffs review involves the following three issues:
atmospheric dispersion factor; loss -of-coolant-accident (LOCA) release pathways; and applicability of findings on control room habitability to BFN Unit 1. These aspects of the staff review are discussed below.
3.1 Atmospheric Dispersion Factors In performing the reviews on the control room habitability corrective actions, the staff evaluated the atmospheric dispersion factors (X/Q) used in control room assessments.
The Browns Ferry facility has a tall plant stack that towers above the control room ventilation intakes.
Since the control room is relatively close to the base of the stack, releases via the stack tend to leapfrog the control room ventilation intakes, reducing the amount of radioactivity intake during.
accidents.
In 1989, TVAidentified deficiencies in ventilation system configurations that could result in as much as 3700 cfm of unfiltered inleakage to the control room. White mitigative actions were taken (e.g., higher flowfiltered pressurization intake, new intake locations), the design basis inleakage remains at about 3700 cfm. TVAhas been able to demonstrate control room doses within the GDC-19 criteria, largely due to the favorable X/Q values.
In this review, the staff closely considered, the determination of the X/Q values used in TVA's analyses to ensure the acceptability of the large unfiltered infiltration. The staff performed confirmatory calculations using the NRC PAVANand ARCON96 codes for selected release points. The staff identified a non-conservative assumption regarding the distance between the stack and the
limiting control room intake.,TVA provided acceptable values and updated their control room dose analysis results in a letter dated May 1, 1998. This review indicated that TVA's X/Q values were comparable to those developed by the staff, and in many cases, were slightly more restrictive. The staff concluded that TVA's X/Q values were acceptable.
3.2 I
P h
TVAevaluated the offsite doses associated with a release of radioactivity from the containment atmosphere via leakage through the drywell into the secondary containment, where it is collected and filtered by the standby gas treatment system (SGTS) and released via the plant stack. These assumptions are typical of the LOCA analyses performed at the time that BFN was'original licensed.
There are two other release pathways which are applicable to BWRs such as BFN. The first is leakage from systems that recirculate suppression pool water outside the drywell following a design basis LOCA. The second is the design leakage through the main steam isolation valves (MSIVs). Both of these release pathways are potentially significant in that they represent drywell bypasses.
TVAstated that the MSIVleakage component was modeled as part of the overall primary containment leak rate. The staff notes that TVA modeled all primary containment leakage as being collected and processed by SGTS.
However, the MSIVleakage bypasses the secondary containment and is released at ground level. TVA's model underestimates the doses associated with MSIVleakage.
Since the difference in magnitude between stack and ground level X/Qs at BFN is on the order of two to three decades more restrictive, the MSIVleakage release could be limitingfor the control room, especially given the large unfiltered infiltration rate.
In a letter dated June 12, 1998, TVAindicated that the MSIVand emergency core cooling system (ECCS) leakage from outside containment are not considered within the BFN licensing basis.
Based on its review of the BFN docketed information, the staff has determined that consideration of MSIVand ECCS leakage is within the BFN licensing basis as a result of TVA's commitments related to NUREG-0737 Item III.D.34 and the related confirming orders as described below.
Pursuant to f50.34, an applicant for a construction permit must include the principle design criteria for a proposed facility. Part 50, Appendix AGeneral Design Criteria, establishes minimum requirements for the principle design criteria for structures, systems, and components important to safety.
GDC-19, Control Room, establishes a requirement that:
"...Adequate radiation protection be provided to permit access and occupancy of the control room under accident conditions without personnel receiving radiation exposures in excess of 5 rem whole body, or its equivalent to any part of the body, for the duration of the accident...."
0 In 1990, the NRC published NUREG-0737, "Clarification of TMIAction Plan Requirements."
item III.D.3.4 of NUREG-0737, Control Room Habitability Requirements, provided the following:
"...The design-basis-accident (DBA) radiation source term should be for the loss-of-coolant-accident (LOCA) containment leakage and engineered safety features (ESF) leakage contribution outside containment as described in Appendices A
and B of Standard Review Plan Chapter 15.6.5.
In addition, boiling-water reactor (BWR) facility evaluations should add any leakage from main steam isolation valves (MSIV)(i.e., valve-stem leakage, valve seat leakage, main steam isolation valve leakage control system release) to the containment leakage and ESF leakage following a LOCA...."
By letter dated October 31, 1980, the NRC notified licensees of its expectation that the requirements in NUREG-0737 would be met and requested that licensees furnish confirmation that the implementation dates would be met. This letter also noted:
"...A licensee or applicant seeking relief from any element of our criteriashould submit a request for relief, along with supporting justification, in response to this letter..."
By letter dated December 23, 1980, TVAprovided the requested confirmation, with the statement that "...TVAintends to be fully responsive to these requirements..."
On March 17, 1981 and July 27, 1981, TVAprovided additional information on Item III.D.3.4. None of these letters requested a deviation from the requirements of III.D.3.4 with regard to sources of radiation exposure.
The NRC issued a confirmatory order to TVAon July 10, 1981.
Section III of that order provided:
"...The licensee's submittals dated December 23, 1980, March 17, April 9 and June 1, and July 2, 1981, the references stated therein, which are incorporated by reference, committed to complete each of the actions specified in the Attachment...."
A search of the TVAdocket identified some requests for deviation from requirements of,various sections of NUREG-0737. With the exception of some chlorine toxic gas issues, none of these deviations were related to Item III.D.3.4. By letter dated August 30, 1982, the NRC issued a safety evaluation that concluded that the Browns Ferry units acceptably met the requirements in NUREG-0737 Item III.D.34. Based on the information provided by TVAin the March 17, 1981, and July 27, 1982, letters, this safety evaluation concluded that the design meets the criteria identified in Item III.D;3.4. Since TVAdid not request a deviation, and the staff did not explicitly accept a deviation in its safety evaluation, the staff is of the opinion that the requirements of Item III.D.3.4, including MSIVand ECCS leakage considerations, are applicable, and are a part of the licensing basis for BFN.
In support of a temporary technical specification (TS) change to allow plant operation with CREVS technically inoperable, in 1989 TVAperformed analyses of control room dose during DBAs, including an analysis of the LOCA dose due to leakage from MSIVs and by letter dated February 14, 1989, TVAproposed a compensatory measure to have Kl available for control room personnel.
Further, on September 10, 1992, in a subsequent request for a TS change to relax conditions (including the use of KI as a compensatory measure), the licensee addre. sed MSIVleakage consideration by referring to its July 31, 1992, CREVS corrective action letter as the basis for concluding that doses were acceptable.
By letter dated April 9, 1993, the NRC approved the proposed license amendment.
The control room habitability corrective action
I
issue was not within the scope of this license amendment.
Accordingly, the staff has determined that consideration of MSIVand ECCS leakage is within the BFN licensing basis TVAcalculation ND-Q2031-9200075, Control Room Doses, was revised (Revision 1) in 1992 to incorporate 3717 cfm unfiltered inleakage, 3000 cfm CREVS flow, X/Q values associated with new CREVs intakes, and to address MSIVleakage.
The MSIVleakage dose was determined using the General Electric, BWR Owner's Group (BWROG) method documented in NEDC-31858P, BWROG Report for Increasing MSIV Leakage Rate Limits and Elimination of Leakage Control Systems, dated October 1991. The results from this analysis are documented in the current BFN Final Safety Analysis Report. The BWROG method has been submitted for staff review as a generic topical report. While the NRC has considered this report on a case-by-case basis for a limited number of facilities, the report has not been accepted on a generic basis.
The BWROG method is predicated on the ability of the licensee to demonstrate seismic suitability of main steam lines and the main condenser.
In response to the staffs request for additional information relating to TVA's power uprate request (which is currently under staff review), by letter dated June 12, 1998, TVAidentified that certain prerequisites for the methodology used in the earlier analyses may not have been met. TVAhas now proposed a license condition that will make Kl available in the control room as a compensatory measure until such time as the TVAdemonstrates that LOCA doses, including MSIVand ECCS leakage, are within the GDC-19 criterion.
The staff has performed preliminary calculations and has determined that the inclusion of these MSIV and ECCS release pathways would result in acceptable doses.
Nonetheless, the staff requested TVAto evaluate the offsite and control room doses due to MSIVand ECCS leakage and to docket the results.
As discussed above, by letter dated June 12, 1998, TVAproposed a license condition to evaluate these pathways and to submit the results of these additional analyses by March 31, 1999. The staff has concluded that the licensee's corrective actions are acceptable on the basis that: (1) MSIVand ECCS leakage are not explicitly part of the original BFN licensing basis; (2) the staffs analyses of the doses associated with MSIVand ECCS leakage show acceptable results; and (3) the licensee has proposed a license condition for TVA to perform necessary analysis and design modifications to demonstrate compliance with GDC-19.
3.3 li ili f F'ndi TVA's analysis of control room doses contains disclaimers limiting applicability to postulated accidents occurring at Units 2 and 3. Similarly, the staff did not.consider accidents occurring at Unit 1. Therefore, the staff finding on the acceptability of TVA's control room corrective action plan, although docketed for all three units, is limited to Units 2 and 3. Further, as part of the staffs issuance of license amendment No. 234 for conversion to Improved Standard TS, the staff included a license condition that "...p]he licensee shall review the Technical Specification (TS) changes made by License Amendment No. 234 and any subsequent TS changes, verify that the required analyses and modifications needed to support the changes are complete, and submit them for NRC review and approval prior to entering the mode for which the TS applies."
4.0 I
EN IT As part of its application for license amendment for power uprate and implementation of corrective action for CREVS issues, TVAhas made the following commitments:
fVAwillperform an analysis of the de'sign'basis loss of coolant accident to confirm compliance with General Design Criteria-19 and offsite limits considering main steam isolation valve leakage and emergency core cooling system leakage.
The results of this analysis willbe submitted to the NRC for review and approval by March 31, 1999.
Following NRC approval any required modifications willbe implemented during the refueling outages scheduled for Spring 2000 for Unit 3 and Spring 2001 for Unit 2. TVAwill maintain the ability to monitor radiological conditions during emergencies and administer potassium-iodine (KI)to control room operators to maintain doses within GDC-19 guidelines.
This ability will be maintained until the required modifications, ifany, are complete.
The licensee's commitment to have thyroid prophylaxis available to control room operators is a compensatory measure deemed necessary while TVAresolves issues associated with its MSIV and ECCS analyses.
'5.0 Based on the information above, and upon the license conditions proposed by the licensee, the staff finds reasonable assurance that the consequences of postulated DBA accidents will remain within 10 CFR Part 50, Appendix A, GDC-19 acceptance criteria and Part.100 limits and the BFN control room willprovide adequate radiological protection such that control personnel willnot receive radiation exposures in excess of 5 rem whole body, or its equivalent to any part of the body, from postulated accidents occurririg at Units 2 or 3. Therefore, the staff finds the TVA's proposed corrective action acceptable.
a iPr I
de Reactor power (3458 x 1.02), MWt Core peaking factor Number of fuel rods in core Iodine species distribution Elemental Organic Particulate Main condenser volume, ft'GTS Flow, cfm Stack, Elevated Damper bypass, ground level SGTS drawdown time, sec (assume release is ground level during drawdown)
SGTS Filter Efficiency, all species, Dose conversion factors Breathing rate, offsite, m'Is 0-8 hours 8-24 hours
>24 hours Breathing rate, control room,m'ls Control room unfiltered infiltration, cfm Control room filtered pressurization, cfm Control room volume, ft'ontrol room intake filterefficiency, all species, %
Control room occupancy factor 0-24 hrs 1A days 4-30 days C
r Dr Fraction of core Inventory in gap Iodine Noble gases Failed rods 3527 1.5 47,368 0.91 0.04 0.05 125,000 21,995 5
75 90 FGR11/FGR12 3.47EA 1.75EA 2.32'.47EA 3717 3000 210,000 90 1.0 0.6 0.4 0.1 0.1 850
0
Failed rod gap release fraction to vessel Iodine 4
Noble Gases Failed rods that reach melt in n
n rl k
Release rate from main condenser,
%/day Release duration, hrs X/Q values t
R I
fr rv' Release rate from main condenser, cfm Release duration, hrs X/Q values Melted fuel release fraction to vessel Iodine
', Noble gases Fraction of activity released to vessel that enters main condenser Iodine Noble gases Main condenser plateout fraction Iodine Noble gases I
1.0 1.0 0.0077 0.5 1.0 0.1 1.0 0.9 0.0 1.0 24 Table 2, Turbine Building 1850 Table 2, Top of Stack Reactor coolant volume, ft'ample line release, ft'Ibm/hr)
Flash fraction Roof ventilator release prior to isolation, cfm h
Roof ventilator isolation time, sec X/Q values 26,500 220 (10651) 0.36 95,000 Table 2, Reactor Building 8 Top of Stack 0
C a
n inmn k
Core release fraction to CNMT Iodine Noble gases 0.25 1.0
Primary CNMTvolume, ft' Drywell Supression pool air space CNMT leakrate, %/day Secondary containment volume (50% of free volume)
SGTS ground level leakage (base of stack), cfm Volume at base of stack (50% offree volume),
ft'/Q 285,200 159,000 126,200 2.0 1,932,000 10 34,560 Table 2, Top 8 Base of Stack Activitysame as CNMT leakage case Flow rate, cfm CAD operation, days post accident CAD operation duration, hours No mixing in RB X/Q 139 10, 20, 29 24 Table 2, Top 8 Base of Stack Activitysame as CNMT leakage case MSIV leak rate (T/S allowed corrected for temperature/pressure),
ft'/hr Release from main condenser,
%/day 32.5 0.625 Plateout fraction X/Q values 0.9 Table 2, Turbine Building Core release fraction to CNMTsump iodine Noble gases Suppression pool liquid volume, ft'stimated leakage, gpm iodine Flash Fraction 0.5 0.0 128,700 5
0.1 Release mixes in secondary containment and released via SGTS X/Q Table 2, Top & Base of Stack
'These parameters are from NRC staff analysis.
As providedin license condition, /icensee is re-performing analyses to incorporate these release pathways in the design basis.
0 P
10 dl n Fuel rods damaged Decay period, hrs Fraction of core in gap I-131 Kr-85 Other iodines Other noble gases Pool decontamination factor Roof ventilator release prior to isolation, cfm Roof ventilator isolation time, sec Mixing volume, ft3 X/Q 125 24 0.12 0.3 0.1 0.1 100 95,000
~ 15 4900 Table 2, Top of Stack 8 Refuel Floor Bypass a
I a
Reactor coolant activity, uCi/gm dose equivalent I-131 Normal Spike Noble gas release, uCi/sec Mass release, Ibm Steam Liquid Release flash fraction (pressure=1020 psia),
Release duration, sec X/Q values 3.2 26 100,000 25,000 160,000 0.38 10.5 Table 2, Turbine Building
Table 2 Time Period Control Room Unit 1 Unit 3 Site Boundary EAB LPZ 0-0.5 hrs 0.5-2 hrs 2-8 hrs 8-24 hrs 1A days 4-30 days 3.40E-5 5.90E-15 4.29E-15 3.65E-15 2.58E-15 1.57E-15 3.02E-5 9.64E-7 1;89E-7 8.37E-8 1A3E-8 1
~ 13E-9 2.40E-5 9.70E-7 1.30E-5 8.00E-7 8.00E-7 4.00E-7 2.00E-7 6.50E-8 ref: Conbol Room Data fiom TVAItrto NRC dtd May 1, 1998; FAEII.PZfiom FSAR Table 14.6-8 f
I 0-2 hrs 2-8 hrs 8-24 hrs 1A days 4-30 days 3.70E-3 2.38E-3 1.91E-3 1.19E-3 5.97EA 1.2E-3 7.91'.42EA 4.09E-4 2.14EA 1.22EA 5.65E-5 5.65E-5 2.24E-5 7.S4E-6 1.71E-6 ref: Control Room Data fiom TVAItrto NRC dtd May 1, 1998; EAEKPZ fiom FSAR Table 14.6-8 in il 3.48'.92EA 2.50EA 1.97E-4 1AOEA 0-2 hrs 3.48EA 2-8 hrs 2.94EA 8-24 hrs 2.53EA 1A days 2.01EA 4-30 days 1A4EQ reft Control Room Data fiom TVAItrto NRC dtd August 10, EAMPZfiom TVAItrto NRC dtd April 12, 1996 2.70'.32EP 6.02E-5 4.07E-5 1.73E-5 5.10E4 1994-need to be divided by two pnor to use to relbct duel intekes; Refuel Floor Damper Bypass 0-15 secs 1.46'l A6EA ref: L~ttolRoom&RSLPZ ftom FSAR Table 14.6-8 1.22'.65E-5
ENCLOSURE 2