ML20196J210

Proposed Tech Specs Reducing Containment Tendon Design Stresses to Incorporate Addl Design Margin

ML20196J210
Person / Time
Site:	Byron, Braidwood, 05000000
Issue date:	03/02/1988
From:	COMMONWEALTH EDISON CO.
To:
Shared Package
ML20196J199	List: ML20196J197 ML20196J210
References
NUDOCS 8803140184
Download: ML20196J210 (6)

Text

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CONTAINMENT SYSTEMS

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SURVEILLANCE REQUIREMENTS (Continued)

b. Performing tendon detensioning, inspections, and material tests on a previously stressed tendon from each group (dome, vertical, and hoop). A randomly selected tendon from each group shall be completely detensioned in order to identify broken or damaged wires and deter-mining that over the entire length of the removed wire or strand that:

1) The tendon wires or strands are free of corrosion, cracks, and d amag.e ,

2) There are no changes in the presence or physical appearance of the sheathing filler grease, and *

3) A minimum tensile strength of 240,000 psi (guaranteed ultimate strength of the tendon material) for at least three wire or strand samples (one from each end end one at mid-length) cut from each removed wire or strand. Failure of any one of the wire or strand samples to meet the minimum tensile strength test is evidence of abnormal degradation of the containment vessel structure.

c. Performing tendon retensioning of those tendons detensioned for inspection to their observed Tift-off force with a tolerance limit of +6%. During retensioning of these tendons; the changes in load and elongation should be measured simultaneously at a minimum of three approximately equally spaced levels of force between zero and the seating force. If the elongation corresponding to a specific load differs by more than 5% from that recorded during installation, an investigation should be made to ensure that the difference is not related to wire failures or slip of wires in anchorages;

d. Assuring the observed lift-off stresses adjusted to account for elastic losses exceed the average minimum design value given below:

Dome 133t e ksi Vertical 121ibb6 ksi .

Hoop 13340} ksi

e. Verifying the OPERABILITY of the sheathing filler grease by assuring:

1) No voids in excess of 5% of the net duct volume,

2) Minimum grease coverage exists for the different parts of the anchorage system, and .

3) The chemical properties of the filler aaterial are within the tolerance limits as specified by the manufacturer.

BYRON - UNITS 1 & 2 3/a 6-9 G803140184 880002 PDR ADOCK 05000454 P DCD _

CONTAINMENT SYSTEMS SURVEILLANCE REQUIREMENTS (Continued)

b. Performing tendon detensioning, inspections, and material tests on a previously stressed tendon from each group (dome, vertical, and hoop). A randomly selected tendon from each group shall be completely detensioned in order to identify broken or damaged wires and deter-mining that over the entire length of the removed wire or strand that:

1) The tendon wires or strands are free of corrosion, cracks, and damage,

2) There are no changes in the presence or physical appearance of the sheathing filler grease, and

3) A minimum tensile strength of 240,000 psi (guaranteed ultimate strength of the tendon material) for at least three wire or strand samples (one from each end and one at mid-length) cut from each removed wire or strand. Failure of any one of the wire or strand samples to meet the minimum tensile strength test is evidence of abnormal' degradation of the containment vessel structure.

c. Performing tendon retensioning of those tendons detensioned for inspection to their observed lift-off force with a tolerance limit o f +6%. During retensioning of these tendons, the changes in load and elongation should be measured simultaneously at a minimum of three approximately equally spaced levels of force between zero and the seating force. If the elongation corresponding to a specific load differs by more than 5% from that recorded during installation, an investigation should be made to ensure that the difference is not related to wire failures or slip of wires in anchorages;

d. Assuring the observed lift-off stresses adjusted to account for elastic losses exceed the average minimum design value given below:

Dome 13314+ ksi '

Vertical It.r) 444 ksi Hoop / g 440 ksi

e. Verifying the OPERABILITY of the sheathing filler grease by assuring:

1) No voids in excess of 5% of the net duct volume,

2) Minimum grease coverage exists for the different parts of the anchorage system, and

3) The chemical properties of the filler material are within the tolerance limits as specified by the manufacturer.

BRAIDWOOD - UNITS 1 & 2 3/4 6-9

4 ATTACHMENT B DESCRIPTION AND SUP9tARY OF PROPOSED CHANGES The proposed changes involve Technical Specification 4.6.1.6.1.d for the Byron and Braidwood Stations.

Basis for Changes to Tendon Design Stress Values The design tendon stresses for Byron /Braidwood containments specified in the current Technical Specification can be reduced as follows:

Dome from 143 ksi to 133 ksi Vertical from 144 ksi to 127 ksi Hoop from 140 ksi to 133 ksi Original Desian Basis The containment tendon forces balance loads, in particular pressure, that put the containment shell in tension. The tendon force must be sufficient to balance the tension forces to keep the reinforcing steel within the allowable stresses.

Two load ccmbinations and load factors given in Table 3.8-3 of the PSAR which control the design of the tendon forces are:

For hoop and dome tendons: .

Abnormal: 1.5P a =Ta (LE) - D ,

For vertical tendon:

Abnormal / Severe Environmental: 1.25 (Pa + (OBE) + Ta (LE) - D where Pa = the design internal pressure of 50 psig Ta (LE) = liner expansion effect due to temperature difference between liner and concrete OBE = Operating Basis Earthquake D = Dead Load 1

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Basis for Revision The new design stress values were derived by taking into account the following refinements that had not been considered in the original design values.

1. Additional Tendon Forces @ P a Force in the tendons increase corresponding to the outward deflection of the wall when internal accident pressure (P a ) is applied, compatibility dictates that the tendons are extended by the outward deflection due to the pressure. Additional tendon forces of 1.73%,

1.75%, and 3.8% of the forces produced by Pa are accounted for dome, vertical and hoop tendons respectively. They are based on the deflections observed during the Structural Integrity Tests.

2. Reduction in Liner Expansion Force @ Pa The applied internal accident pressure, Pa , also extends the liner.

The liner is at a higher temperature than the concrete and when it tries to expand it puts the concrete in tension. The extension of the liner due to Pa relieves the liner expansion forces assumed in the original design for dome, vertical and hoop tendons, respectively, by 10%, 25%,

and 18%. The required tendon forces in the wall are consequently reduced.

3. Use of Refined Seismic Analysis The containment seismic forces used in the original tendon design were calculated assuming a maximum overturning moment obtained from a

. simplified seismic "stick model" was applied over the containment wall sections. The seismic forces can be reduced by 13% for the section at the basemat-wall junction when the refined DYNAX seismic model which utilizes a 3-dimensional analysis is used. This substantially reduces the force required in the vertical tendons.

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ATTACHMENT C EVALUATION OF SIGNIFICANT HAZARDS CONSIDERATIONS Commonwealth Edison has evaluated this proposed amendment and determined that it involves no significant hazards considerations.

According to 10 CFR 50.92(c), a proposed amendment to an operating license involves no significant hazards considerations if operation of the facility '

in accordance with the proposed amendment would not:

1) Involve a significant increase in the probability or consequences of an accident previously evaluated; or

2) Create the possibility of a new or different kind of accident from any accident previously evaluated; or

3) Involve a significant reduction in a margin of safety.

The proposed amendment of Specification 4.6.1.6.1.d reduces the containment tendon design stresses to incorporate additional design margin not reflected in the values currently in the Technical Specifications. The design stresses provided in the Technical Specifications correspond to the average tendon stress after 40 years of losses. Losses occur due to relaxation in the tendon wires and shrinkage and creep in the concrete.

Most of these losses occurred in the first few years after the tendons were stressed (1980 to 1982 at Byron and Braidwood). Thus the predicted tendon force determined when performing the Technical Specification surveillances can approach the design force. The difference between the calculated tendon force and the design force is small enough that factors such as different methodologies for performing lift-offs at installation and at the in-service inspection and tolerances on the equipment can result in tendon forces at variance with technical specification valves. When these limits are not met, the Technical Specifications require that the containment integrity be restored, an engineering evaluation be performed and a special report be submitted to the Commission or that the station be shutdown.

The new design stress values were derived by taking into account such factors as additional tendon forces based on the deflections observed during the Structural Integrity Tests and reduction in the liner expansion force which consequently reduces the tendon forces in the containment wall.

Also the vertical tendon forces were reduced by the use of a refined DyNAX seismic model which uses a three-dimensional analysis. The proposed reduced design stresses will provide a larger margin between the expected tendon forces and the Technical Specification values.

The proposed amendment does not involve a significant increase Thein the probability or consequences of an accident previously evaluated.

changes do not alter the existing tendon stresses or the containment system. They merely incorporate new design stress values as determined by the Architect / Engineer based on refinements not included in the original design basis. The new design stress values meet the Byron and Braidwood PGAR acceptance criteria for use in monitoring containment prestressing.

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i The change does not add or modify any existing equipment, nor introduce a new mode of plant operation. The containment tendon lift-off stresses will continue to be The verified in accordance containment with system remains Technicalin its as-built Specification 4.6.1.6.1.d.

configuration and the design is in accordance with the Byron and Braidwood PSAR acceptance criteria. As such, the possibility of a new or different kind of accident from any accident previously evaluated is not created.

The proposed amendment does not involve a significant reduction in a margin of safety. The containraent structure and the existing tendon stresses remain unchanged. The revision establishes new tendon design stress values for monitoring the prestress. In addition, this ensures the containment maintains its integrity following an accident that generates an internal design pressure.

The new values were prepared in accordance with The new design the acceptance criteria in the Byron and Braidwood FSAR.

stress values were determined in accordance with accepted methodologies so the margin of safety is not impacted.

Therefore based on the above analysis, commonwealth Edison concludes that the proposed amendment to the Technical Specification does not involve significant hazard considerations. t 4301K l

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