ML19340A335
| ML19340A335 | |
| Person / Time | |
|---|---|
| Site: | Crystal River  |
| Issue date: | 09/29/1976 |
| From: | Office of Nuclear Reactor Regulation |
| To: | |
| Shared Package | |
| ML19340A334 | List: |
| References | |
| NUDOCS 8003160022 | |
| Download: ML19340A335 (5) | |
Text
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STRUCTURSL' ENGINEERING BRANCH
. C0tHENTS AND REQUEST FOR ADDITIONAL INFORMATION ON CRYSTAL RIVER UNIT NO. 3 REACTOR BUILDING DOME DELAMINATION INTERIM REPORT AND SUPPLEMENT NO.1 I.
GENERAL C0tt4ENTS In the report the applicant discussed all possible factors which could have caused the delamination of the dome. No single or overriding mechanism has been positively identified as the cause of the delamination.
However, the following facts are significant.
1.
The indication of a tension failure along the delaminated surface.
2.'
The complete fracture of the coarse aggregate on the de-laminated surface.
3.
Large variations in the strength values obtained from the direct tensile tests of the concrete.
4.
The presence of cracks of various sizes and extents in the concrete below the delamination as indicated by core borings.
On the basis of these facts, the sequence of events that led to delamination could be surmised:
From the evidence indicated above, one could conclude that (1) the characteristics of the dome concrete are such that it is crack-prone, and localized cracks may have existed even before the prestressing force was applied, and (2) the coarse aggregates are fragile thus, instead of acting as crack arresters, they became the path of cracks.
With the existence of precracks and the presence of fragile coarse aggregates, the radial tension accumulated from all sources
.was so large that it overcame the very limited tensile strength 4
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It has been found by various investigators that cracking of con-crete under compmssion is slight for leads below 30 to 50 percent of the ultimate.
This is basically the recson why the allowable concrete compressive stress is limited to 45% of the ul timate.. The cracks, if any, which initially may have developed in the dome concrete as a result of prestressing are unstable.
They increase in length and width until either they eventually stabilize or' ultimate failure occurs.
The slow crack growth in concrete under sustained loading is most likely associated with cmep.
The postulation of the delamination mechanism and the understanding of concrete crack initiation and propagation are essential for the establishment of the dcme repair procedure and its evaluation.
The following repair procedure is being pursued by the applicant:
1.
Holes will be core drilled into the lower concrete; 2.
Top delaminated concrete will be removed; 3.
Final inspection of 24" structure will be perfomed; 4.
Lower level cracks will be grouted with epoxy;
- 5. -Radial anchors will be set and the holes grouted;
- 6. _New reinforcement and concrete will be added;
-7.
.18 tendons will be re-tensioned; 8.
Structural Integrity Test will be performed.
On the. basis of the: postulation of the delamination mechanism
. and understanding of concrete crack initiation and propagation as discussed above,~ the staff has reviewed and evaluated the repair -
procedure.
However, before the staff can finalize its evaluation,
'the applicant should respond to the staff's concerns as indicated
. below:
LII.- 00ME REPAIR 11 AnLanalysis. of the mpaired dome should be made for the. following m.
e 3-conditions:
(a) Before the hardening of the cap concrete.
(b) _ After the hardening of the cap concrete, including all the loading conditions as described in the FSAR.
Indicate the stresses and strains in the mainly-reinforced concrete cap portion and in the prestressed concrete lower porti on.
2.
Provide a description of the final design of the radial anchors and indicate how the combined action of the cap concrete and the lower dome concrete is ensured.
3.
It was _ indicated that two layers of reinforcing steel will be provided in the cap.
For the meridional reinforcing steel, if only one layer can be spliced to the existing meridional steel' near the ring girder, indicate how the other layer can effectively carry the load if it is not spliced to the existing steel, noting that under internal pressure, dome concrete may crack in tension.
4.
Since the repaired dome becomes a unique structural element off the containment structure, indicate any special considera-tions to meet the requirements of Regulatory Guide 1.18 in executing the structural integrity test of the centainment.
5.
The original dome design concrete strength, f', is based on 5000 psi; now a concrete strength of 6000 psi is used for evaluating the. repaired dome.
The basis for using 6000 psi is that the actual strength of the existing structure possesses that strength.
It is a ~well-known fact that concrete strength increases with age beyond 28 days and stabilizes after a certain time.
Generally, designers of concrete-struc-tures do-not take such increases into consideration mainly to offset'" ignorance factors"in areas of design and construction.
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J Provide a justification that such additional margins of safety are not required in the case of a concrete containment, noting that there is a reduction in' dome concrete area due to the presence of cricks, sheathing ducts and other possible voids, and if such reduction of concrete area is disregarded in the stress computation, the computed membrane compressive stress may be less than the actual.
6.
The cracks in the dome concrete as discussed in the general comments, have reached stability.
The structural integrity test (SIT) will affect such stability.
Provide an evaluation of SIT on die lower level cracks of concrete which may not
.be grouted with epoxy.
Provide the data on the effectiveness of epoxy grout in controlling concrete cracks.
III.
CAUSES OF DELAMINATION 1.
On Page C-3 in. Appendix C under the subsection on' "Of rect Tensile Test hesults" the applicant indicates that the range of direct tensile tests on 6 core sampler was 230 psi to 505 psi with an average value of 420 psi.
In view of these low results, the a11cwable membrane tensile stresses indicated in table 2-2 appear high.
Discuss the caust.
of these low tensile ultimate stresses, the r6ason for the wide scattering of the test results and the possibility that the delamination phenomer, was caused by the poor quality of the aggregate, and the propagation of local cracks along the whole surface of the dome as surmised in the general comments above.
2.
The applicant presented in Fig. 3-22 the plane strain finite element model used to evaluate some stiess concentrations at the tendon ducts.
a.
Present'a detailed descriptien o' 'oundary conditions (especially at the duct) and initial conditions intro-
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-duced in the computer analysis -for all cases of stress-concentration.
b.
Justify the use of plant strain to analyze what is essen-I tially. a three-dimensional problem.
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