ML20076A836

Eddy Current Test Has Been Performed.No Tube Defects Discovered & No Corrective Action Based on Tube Defects Required.Preliminary Analysis of Results of Insp Indicates Denting Rate Has Been Reduced

ML20076A836
Person / Time
Site:	Millstone
Issue date:	04/13/1979
From:	Counsil W NORTHEAST UTILITIES
To:	Reid R Office of Nuclear Reactor Regulation
Shared Package
ML20076A842	List: ML20076A836 ML20076A843
References
TAC-46174, NUDOCS 7904190079
Download: ML20076A836 (3)

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l April 13,1979 Docket No. 50-336 Dirc:ctor of Nuclear Reactor Regulation Attn:

Mr. R. Reid, Chief Operating Reactors Branch #4 U. S. Nuclear Regulatory Commission Washington, D. C.

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Reference:

(1)

W. G. Counsil letter to R. Reid dated February 12, 1979.

Gentlemen:

Millstone Nuclear power Station, Unit No. 2 Steam Generator Inspections In Reference (1), Northeast Nuclear Energy Company (NNEC0) proposed revisions to Technical Specifications concerning steam generator inspections.

It was proposed that the inspection requirements adopted as a result of the condition of the steam generators following Cycle 1 operation, and pertinent only to the March,1979 inspection, be deleted.

This concept was technically justified con-sidering the corrective actions taken, and presumed that the inspections following Cycle 2 operation would demonstrate acceptable steam generator perfomance.

NNEC0 has performed eddy current and visual inspections of both Millstone Unit (D

No. 2 steam generators during the present refueling outage that began March 10, V

1979. Although the data obtained are still being analyzed, preliminary results are currently available, and these preliminary results are provided below tc facilitate disposition of the Reference (1) request.

The eddy current testing program specified in the Millstone Unit No. 2 Technical Specifications (Section 4.4.5.0) has been perfomed.

Results, summarized in Table 1, are based on testing approximately 2700 tubes in Steam Generator 1 and approximately 2200 tubes in Steam Generator 2.

No tube defects were discovered, as indicated in Table 1.

Therefore, no correc-tive action based on tube defects is required.

It is significant to note that this complete absence of defects includes 100% of the peripheral, exposed tubes that were examined. This region would be expected to be most susceptible to

" loose-part" induced damage.

The deformation manifested by constriction of the tube ID to block passage of a 0.540 inch diameter probe at a Tube Support plate (TSPL) elevation indicates active dent-related processes.

The number of such blocked tubes has decreased 790419ooJg-g

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dramatically during the last operating period to three tubes in the current outage.

Likewise, dent progression at TSPL elevations was essentially insig-nificant when test accuracy is considered, zero in Steam Generator 1 and 42 mils in Steam Generator 2.

Statistical plots of dent progression by TSPL are shown in Figures 1 - 8.

" Dent" signals had been observed at tube sheet and egg crate elevations during the January,1978 outage.

Based on the magnitude and frequency of these indi-cations, they represented o relatively minor failure threat, as compared to TSPL indications.

Current dent indication results for these areas are also given in Table 1.

No progression was apparent although the frequency of these occurrences may have increased slightly in localized areas.

Visual inspection of the secondary sides of both steam generators was perfonned both from the top of bundles, and from the lower hand holes.

O The following observations were made:

(1) The gap between the upper support plates and the shroud, established by the January, 1978 rim cutting, appeared essentially unchanged.

i No plate / shroud contact was observed.

(2) The deposits appeared to be unchanged in general and some reductions were noted.

(3) Several " loose parts" were present.

(4) The general condition appeared similar to the condition existing after the repairs performed during the last outage.

The largest " loose part" observea is shown in Figure 9.

It is a piece of TSPL Q

that had apparently broken loose. Tube holes, flow holes, and ruptured ligaments are evident.

The piece's dimensions are irregular, bounded roughly by a 5 cm. x 3 cm. rectangle. The piece was found on the No.10 TSPL outer rim, on the cold leg side of Steam Generator 1.

It is probable that this piece came from the No.

11 TSPL.

Note the retention of a sharp angular geometry.

It was also noted that the nonprotective magnetite at the tube hole surface was essentially intact.

These observations, along with the absence of tube defects in the area of the No.10 TSPL, support the conclusion that postulated impact-type movements that could lead to tube damage are not a serious threat.

Several areas around the existing, upper TSPL rim had cracked tube hole to flow hole ligaments. This situation is believed to have resulted from the synergistic effects of normal operating vibrations and the partially cracked ligament condi-tions existing in January,1978. To minimize the possibility of complete separa-tion during the next cycle, similar pieces that were loosely attached have been manually removed.

. Corrective or preventative actions instituted by NNECO as a result of this in-spection consisted of manually removing the loosely attached, and unattached, TSPL pieces, as discussed above and pluggins a total of five (5) tubes, two (2) in Steam Generator No. I and three (3) in Steam Generator No. 2.

The five plugged tubes included the three blocked tubes, tubes that would not pass the EC probe, one tube whnse hot-leg plug had inadvertently been omitted during the 1978 plugging campaign, and one suspect " weld leaker". These actions are more conservative than the tube plugging criteria established during the previous ou tag e.

In sununary, preliminary analysis of the results of this steam generator inspec-tion indicate that the denting rate has been reduced significantly, if not completely arrested.

Furthermore, no tube defects were detected in an eddy current test sample representing approximately 30% of the total tube population.

It is believed that the corrective actions instituted by NNEC0 during and sub-sequent to the November,1977 outage are responsible for this favorable situation.

O Two (2) actions of particular importance were:

(1) the phasing in of a full flow condensate polishing system using deep-bed demineralizers, and (2) the " rim cut".

The former action maintained feedwater purity, thus avoiding a " denting chemistry",

and the latter minimized potentially harmful secondary effects associated with any possible denting brought on by residual impurities. The gap between the upper support plates and the shroud has been maintained for Cycle 3 operation, and the full flow condensate polishing system will remain in operation for Cycle 3.

Although detailed analyses of the nearly 5000 tubes inspected is in progress, the review conducted to date is sufficient to conclude that the steam generators are suitable for service during Cycle 3.

The Technical Specifications proposed in Reference (1) are in conformance with or in excess of the requirements of Regulatory Guide 1.83, and are appropr' ite for Millstone Unit No. 2.

We trust the above information is sufficient for you to favorably disposition our request of Reference (1).

Very truly yours, NORTHEAST NUCLEAR ENERGY COMPANY 1

,h ['N0L W. G. Counsil Vice President Attachment I

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