ML20216J192
| ML20216J192 | |
| Person / Time | |
|---|---|
| Site: | Fort Saint Vrain  |
| Issue date: | 06/17/1987 |
| From: | Office of Nuclear Reactor Regulation |
| To: | |
| Shared Package | |
| ML20216J189 | List: |
| References | |
| TAC-59936, NUDOCS 8707020235 | |
| Download: ML20216J192 (4) | |
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UNITED STATES 8
NUCLEAR REGULATORY COMMISSION o
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WASHINGTON, D. C. 20555 4.0 ENCLOSURE 1 l
SAFETY EVALUATION BY THE OFFICE OF NUCLEAR REACTOR REGULATION RELATING 10 M0ISTURE INGRESS (TAC NO. 59936)
PUBLIC SERVICE COMPANY OF COLORADO FORT ST. VRAIN NUCLEAR GENERATING STATION DOCKET NO. 50-267 J
- 1. 0 INTRODUCTION Los Alamos Nation'aI Laboratory (LANL) was contracted by.the NRC to evaluate the effects produced by moisture ingresses into the prestressed concrete reactor vessel (PCRV) which have occurred at the Fort St. Vrain (FSV) plant over thy'ppjt,few years.
LANL subcontracted part of this work to Advanced Scienes and Technology Associates, Inc. (ASTA).
ASTA issued a 3
report summarizing the results of their work which the staff reviewed and issued in a letter dated June 16,1987.
The staff also reviewed the report prepared by LANL and the final draft of that report is enclosed.
Thissreport summarizes the work performed by LANL under FIN No. A-7290 on moisture ingress.
O LANL was contrac6 d'by NRC to perform the following tasks:
1.
Identify all the moisture ingress occurrences in the FSV plant since its startup in 1974.
2.
Identify all components and systems that could be affected by the ingressed moisture and by helium contaminants.
3.
Outline the effects of these moisture ingresses on the components and systemsg s
i, 4.
Eyaluate the' current magnitude of these effects and predict the degree by which they will affect the future behavior of the plant in terms of material degradation and structural integrity.
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The first three of the above tasks were performed under subcontract by ASTA and reported in their report.
The remaining task was the responsi-bility of LANL.
The LANL final report includes a summary of the entire effort including the work performed by ASTA and identifies the conclusions and recommendations resulting from the study.
8707020235 B70617 ADOCK0500g7 DR
o 5 g 2.0 EVALUATION The following individual tasks were discussed in this report:
2.1 Identification of Moisture Ingress Occurrences Forty-five moisture ingres,s occurrences were identified between July 19, 1975 and July 24, 1985.
A total of 9,155 gallons of water has leaked into the prestressed concrete reactor vessel (PCRV).
The two major sources of water inleakages were the helium circulator's auxiliary system, and the economizer / evaporator /superheater section of the steam generators.
Moisture inleakage through the helium circulators was oy far the most important source as it was responsible for more than 99 percent of the moisture that entered the PCRV.
Moisture entered the primary system through the following three paths:
(1) along the shaft of a helium circulator, (2) through the buffer helium lines to a circuiator, and (3) through the helium pressurization lines serving the PCRV penetrations in which the circulators are located.
In addition to these main sources of moisutre inleakage, ASTA identified several potential sources of moisture.
However, with the exception of the purification cooling system, none of them was considered a potential major source of water ingress.
The actuai amounts of ingressed moisture were specifically known for some incidents.
However, for others, where no reliable data existed, estimates were made.
ASTA identified two main causes of the moisture ingress incidents:
(1) improper operation, especially during main-tenance of the circulator water seals and subsequent surveillance testing, and (2) malfunction of electrical systems.
ASTA also provided recommendations for corrective actions for some of these incidents.
2.2 Identification of Components and Systems ASTA identified several components and systems inside the PCRV which
- ould become exposed to the ingressed moisture. These systems were generally located on or within the PCRV bottom head since the most likely place for ingressed moisture to finally settle was in the Kaowcol insulation in contact with tne bottee liner. The following are the components likely to be exposed to the ingressed moisture
1.
PCRV bottom head liner; 2.
Core support columns; 3.
Kaowool insulation; 4.
Insulation mounting studs, seal sheets and cover sheets; 5.
Helium c'"ctlators; 1
J
e gi; 6.
Steam generators; 7.
Helium circulator, steam generator and bottom access penetrations, up to the primary closure; and I
8.
Helium purification system.
Those components located away from the bottom of the PCRV head liner are unlikely to be exposed to the ingressed moisture even though there are some mechanisms by which moisture could be transferred to them.
LANL identified the following moisture transfer mechanisms within PCRV:
(1) liquid droplet entrainment, (2) moisture ingress from sources other than the circulator shaft, and (3) evaporation /
condensation in the PCRV top head penetration from a helium / water vapor inixture.
A second list of the components likely to be affected by ingressed moisture was prepared by the contractor to Public Service Company of Colorado, the Fort St. Vrain licensee, and docunented in a report dated March 25, 1985.
The licensee's list identified several components in addition to those on the ASTA listing, fwo of them of significant importance are the instrument and sensor lines, and the control rods and control rod drive (CRD) mechanism assembly. This latter component experienced actual damage by chloride induced corrosion caused by ingressed moisture.
2.3 Outline of Effects of Moisture Ingresses on Components and Systems In their evaluation report, ASTA provided a brief description of the effects of ingressed moisture on the individual components identified in their list. The description considered the postulated effect of inleaked water as well as some effects of the impurities which may be introduced by this water.
The particularly imp 9rtant corrosion effects of chlorides were considered.
The AMA evaluations did not, however, include actual predictions of compe,nent failures.
- Further, it was not possible from them to determine if those accidents which could be caused by corrosion damage would exceed the design basis accidents analyzed in the Final Safety Analysis Review (FSAR) for the plant.
A more complete analysis provided by the licensee in their March 25, 1985 report considered the effects of corrosion on the components identified in the report.
The results of this analysis indicated that it was very unlikely that chloride induced corrosion could produce significant enough degradation of plant components to result in their failures.
Even if such a failure were to occur, the corresponding accidents would not be more severe than those already considered in the FSAR.
2.4 Quantitative Evaluation of Moisture Ingress Effects Quantitative evalation of moisture ingress effects was the responsi-bility of LANL.
It consisted of developing a method by which the megnitude of the effects induced by moisture ingress could be evaluated.
t
.,. Of particular importance was the ability to predict the effects produced by chloride induced corrosion.
LANL determined that in order to properly address this concern, they would need to develop a very complex computer code which would include models of the different corrosion mechanisms respcnsible for components degradation.
LANL further determined that development of such a code would require l
spending a considerable amount of effort extending far beyond the original scope of the work.
Even if such a code were developed, it was uncertain that it would make accurate predictions because environ-
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mental characteristics in the PCRV during moisture ingress are not well known.
LANL did, however, develop a very rough prediction of the corrosion effects on the PCRV liner and concluded that the liner would not fail during the life of the plant.
A similar conclusion was reached by the licensee in their March 25, 1985 report after making predictions of corrosion effects on several other components.
This information indicates that there will likely be no component failures due to moisture ingresses, even though a detailed and accurate prediction is not possible without expending extensive resources.
3.0 CONCLUSION
S AND RECOMMENDATIONS Based on the staff review of the final LANL report on maisture ingress, we have the following conclusions and recommendations:
1.
The evaluation of moisture ingress into the PCRV at Fort St. Vrain performed by LANL and its subcontractor ASTA provided the necessary additional information to supplement the findings from the initial study sponsored by the licensee and reported in their submittal dated March 25, 1985.
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l 2.
ASTA identified the major moisture ingress occurrences, and listed those components and systems which were likely to be affected by moisture ingresses.
This information forms the most important
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contribution to the study.
The study did not develop a means for quantitatively predicting the effects of ingressed moisture and accompanying impurities (especially chlorides), because the development of exact methods would be very difficult, f
3.
We fully concur with the LANL recommendation that periodic inspections and preventive maintenance be performed on pertinent components. We also agree that operational performance should be continuously upgraded and the viability of the power distribution systems be improved since these two factors caused most of the moisture ingress occurrences. We also recommend that any moisture ingress occurring in the future be carefully evaluated so that knowledge about the nature of these ingresses can be continuously improved.
Principal Contributor:
K. Parczewski Dcte: June 18, 1987 L-
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