ML20086F667
| ML20086F667 | |
| Person / Time | |
|---|---|
| Site: | Davis Besse  |
| Issue date: | 11/26/1991 |
| From: | Shelton D CENTERIOR ENERGY |
| To: | NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM) |
| References | |
| 1992, GL-88-17, TAC-M69738, NUDOCS 9112030368 | |
| Download: ML20086F667 (4) | |
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Donsid C. Shohen 300 Madison Avenue MPN W Toledo, OH 436520001 (419)249 2300 I
Docket Number 50-346-License Number NPF-3 Set!al Number 1992 November 26, 1991 United States Nuclear Regulatory Commission Document Control Desk Vashington, D. C.
20555
Subject:
Containment Vessel Structural Canacity for External Pressures Gentlemen:
Duting a review of Davis-Besse Nuclear Power Station, Unit No. 1 (DBNpS) Updated Safety Analysis Report (USAR) Sections 3.6.2 and 3.2.2 by Toledo Edison, questions were raised regarding the containment pressure vessel's capacity to withstand external pressures.
Specifically, the maximum calculated pressure due to a postulated high energy line break outside the containment was greater than the design external differential pressure (0.5 psid) stated in USAR Section 3.8.2.1.4d, On March 78, 1991, the NRC staff was notified by telephone of thin potential condition and of the compensatory measures taken while these values vere being evaluated by Toledo Edison..This letter documents Toledo Edison's evaluation and resolution of this issue.
As a part of itc. evaluation, 1oledo Edison reviewed the evolution of information provided in the USAR.
In 1972, subsequent to the design and construction of the DBNPS containment pressure vessel, high energy line break.s (HELBs) outside containment were identified as an industry 31cens.ing issue.
In 1974, the NRC requested and was provided information on the Davis-Besse containment annulus pressure response due to pipe breaks outside the containment. As part of its response to this request, Toledo Edison provided the calculated !!ELB pressure in the annulus of 1.1 psig and the structural capacity of the containment, 1.93 psid,. based on the theoretical critical buckling pressure for the cylindrical portion of the containment.
The use of 1.93 psid provides
-a safety factor of approximately 1.75.
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-Docket Number 50-346
License Number NPF '
Serial Number-1992 Page 2 This structural capacity value was documented in the Final Safety Analysis Report (FSAR)-Table 3-6aa and subsequently USAR Table 3.6-6.
Based on the.information presented in the FSAR and the USAR, Toledo Edison concludes that the licensing basis for the DBNPS containment
-structural capacity for high energy line breaks outside the containment is 1.93 psid which is based on the theoretical critical external pressure.
Since the initial notification of the NRC on this issue, Toledo Edison has had several discussions with the NRC staff.
During these discussions, the NRC staff requested'that Toledo Edison exagine the issue further by evaluating the safety factors using appropriate knockdown factors provided in the American Society of Mechanical Engineers Boiler and tressure Vessel (ASME B & PV) Code and the effects of asymmetric pressures in the annulus.
The DBNPS containment annulus does not contain any compartments which will result in a significant asymmetric pressure betvaen different regions of the annulus. The asymmetric loading on J a co tainment has been analyzed as described below. The containment an miss was divided into_tventy sub-regions for analytical purposes. The pressures in each region vere calculated for a steam generator blowdown (SGBD) line break inside the annulus and a main feedvater line break (FVLB) in the Mechanical Penetration Room (Room 314). The results indicate that the maximum pressure differential of 0.019 psid occurs between two subregions of the containment annulus for a-FVLB in Room 314. The asymmetric pressure differentials for SGBD breaks are lover than 0.019 psid. These asymmetric pressures are considered to have no significant effect on the containment structural capacity. Additionally, localized loading on the.containn.ent vessel due to the effects of the break vere also evaluated.
Since the FULB resultc in a maximum mass and energy release to the containment annulus, the loading due to this break on the containment vessel in the vicinity of the opening from Rocu 314 to the containment annulus was evaluated. The impact of this loaaing on the containment structural capacity was assessed by combining the local load with the calculated containment annulus pressure. These evaluations confirmed that containment integrity vill be maintained.
Although safety factors are less than those specified in the ASME ~.de,
. sufficient marg.i to containment structural capacity exists.
In a systematic effort to reduce the co"tainment annulus pressure following postulated high energy llae
,ks outside the containment Toledo Edison has considered various p
..t modifications. The
-Mechanical Penetration Rooms, Room 314'and Roam 303 have blevout panels, which are set to lift if the room ptrasure exceeds the set pressure.
Currently these panels are sc* to.lovout at 1 psid as described in USAR Section 3.6.2.7.1.6.
Tb?se penetration rooms are interconnected through the containment annulus openings and through L
other interconnecting compartments. The containment annulus pressure L
response calculations shoved that the peak annulus pressure is l -
sensitive to.the blovout panel neticant.
Although the blevout panels in the Mechanical Penetration Room with Ene break lift almost L
l
Docket Number 50-346
- License Number NPF-3 Serial ilumber 1992 Page 3 instantaneously, the pressure in the compartments connected to this room continues to increase due to-the continued blovdown from the break.
When the pressure in the opposite Mechanical Penetration Room reaches the blovout panel setpoint, its blovout panel vill also litt, providing a relief path and thereby controlling the pressure transient in the annulus.
/
lt is noted that the same blovout panels also serve another protective function.
These panels maintain the negative pressure boundary in the 4
auxiliary building-following a postulated design basis loss of coolant, accident (LOCA).
A premature lifting of a blowout panel following a postulated LOCA could result in an unfiltered release path to the environment. The blowout panels are currently set to lift at 1 psid.
This setpoint-was conservatively selected based on the calculsted pressure response following a postulated LOCA. These calculaticns performed in support of the 1 psid setpoint have used conservative assumptions, similar to the assumptions given in NUhEG-0800, the Standard Reviev Plan (SRP), Section 6.2.3. Consistent with SRP Section 6.2.3, these calculations have assumed no outleakage through the negative pressure boundary throughout the transient although the calculated pressures are significantly positive.
These calculations have further assumed a constant flow of 8000 cfm for the Emergency Ventilation System (EVS) fan.
Based on the fan curve it is determined that the flov through the EVS vill be-significantly higher than that assumed in the calculations used to determine the blowout panel setpoint.
The SRP assumptiot.s allow credit for fan performance charecteristica to be utilized.
The negative pressure boundary pressure response following a postulated LOCA was re-evaluated by taking-credit for additional flow through the EVS based on fan characteristics and zero outleakage. These-calculations show that the peak pressure in the negative pressure boundary vill be less than 0.64 psig assuming that the system flov
~
resistance corresponds to the minimum EVS ilov allowed by DBNpS 3
Technical Specification 4.6.1.5.d.
The calculated negative pressure boundary pressure vill be approximately 0.55 psig for typical flow resistance conditions.
Based on conservatisms in this evaluation Toleco Edison has concluded that if the blovcut panel setpoint is changed to 0.65 psid it vill assure that these panels will not prematurely lift during afpostulated LOCA.
Based on the above evaluation, Toledo Edison has dec ded to revise the i
blovout panel setpoint to 0.65 psid, This setpoint provides additional-margin for containment structural capacity following a HELB outside the containment while at the same time providing adequate protection during a postulated design basis LOCA A blovout panel setpoint of 0.65 psid vill result in a calculated peak pressure of approximately 0.7 psid in the annulus for HELBs outside the containment.
The calculated differential pressure of approximately 0.7 psid is very close to the allowable external design pressure of 0.67 psid-calculateu using equations provided in 198S Edition of the ASME B & PV Code.
l I
o Docket Number 50-346 3.
__ License Number NPF-3 Serial Number 1992 Page'4; This change in blowout panel netpoint assures that the overall' safety factor in containment structural capacity is improved.
In addition, if the positive pressure that normally exists inside the containment vessel _is considered, the: differential pressure across the containment vessel following postulated HELBs outside containment will be voll within the external ptessure value calculated using 1986 ASHE B & PV Code equations.
Toledo Edison plans to revise the blovout panel setpoints during the next operating cycle (Cycle 8) provided the modification does not tequire an outage. In the event an. outage is required, thjs modification is being planned to be performed during-the eighth refueling outage, currently scheduled to commence in Harch 1993.
Toledo Edison believes that this modification provides additional-margin and sctisfactorily resolves this issue.
Should you have any questions or require additional information, please contact Mr. Robert V. Schrauder, Manager - Nuclear Licensing, at (419) 249-2366.
Very t rt
- yours, (Lo v %
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Davis, Regional Administrator, NRC Region III J. B. Ilopkins, NRC Senior Project Manager V. Levis, DB-1 NRC Senior Resident Inspector li'.ility Radiological Safety Board-l
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