ML20006A500
| ML20006A500 | |
| Person / Time | |
|---|---|
| Site: | Fort Saint Vrain  |
| Issue date: | 01/19/1990 |
| From: | Crawford A PUBLIC SERVICE CO. OF COLORADO |
| To: | Weiss S NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM), Office of Nuclear Reactor Regulation |
| References | |
| P-90020, NUDOCS 9001290037 | |
| Download: ML20006A500 (8) | |
Text
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Public Service' ECOL..
P.O. Box 840
. January 19,-1990 Denver co 80201 0840 Fort St. Vrain Unit No. 1 A. Clegg Crawford P-90020 vic p,..io.ni Nuclear operations U. S. Nuclear Regulatory Commission ATTN: Document Control Desk Washington, D.C.
20555 Attention: Mr. Seymour H. Weiss, Director Non-Power Reactor, Decommissioning and Environmental Project Directorate Docket No.-50-267
SUBJECT:
PCRV Concrete Core Drills
REFERENCE:
(1) PSC letter,- Crawford to Weiss, dated June 30, 1989 (P-89228)
(2) NRC letter, Erickson _to Crawford, dated October 4 1989 (G-89338)
(3) PSC letter, Crawford to Weiss, dated ' December 1,
1989 (P-89459)
Dear Mr. Weiss:
The purpose of this letter is to notify the NRC of PSC's intent to bore as many as ten concrete core drills into the PCRV barrel.section c
'at various elevations. Analysis of the concrete cores removed from the PCRV will provide data on the activation of the concrete and bonded reinforcing steel (rebar) inside the PCRV, which is necessary for decommissioning planning.
This notification is being made in accordance witn the NRC's guidance to-PSC received during the October 25, 1989 meeting on decommissioning.
The NRC requested notification of any activity being undertaken by PSC, prior to NRC approval of the Fort St. Vrain Proposed Decommissioning Plan,.
that could be considered a decommissioning activity.
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P-90020
- Page 2 January 19, 1990 In-Reference 2,
the NRC requested that PSC provide a preliminary estimate of occupational exposure associated with major decommissioning tasks and the basis for the estimate, in order to demonstrate that major technical actions involved in the Fort St.
Vrain deconnissioning will be carried out safely in accordance with 10CFR50.75(f)(2).
Reference 3 provided exposure estimates that were based on a conceptual plan prepared to evaluate early dismantlement.
PCRV concrete and rebar activation levels used as the basis for estimating occupational exposure during PCRV dismantlement were calculated using a computer code.
Reference 3 states that PSC lans to include further refinements of estimated radiation exposure p(derived from specific radiological survey information) in the Proposed Decommissioning Plan.
The NRC also requested in Reference 2 that PSC provide estimates of the costs to remove PCRV concrete and dispose of it, and provide the estimated depth of removed concrete along with the basis for selecting these values.
PSC has sent out Requests for Proposals (RFPs) to several vendors requesting their plans and cost estimates for various phases of the Fort St. Vrain deconsnissioning process, including partial. dismantlement of the PCRV. Vendor plans and valid cost estimates will depend in large part on the actual depth and degree of activation present in the PCRV.
To these ends, PSC. is planning to perform PCRV core drillings in order to make an accurate determination of actual concrete and rebar activation levels in the vicinity of the core. This will either confirm or be used to modify the previous analysis which was based solely on computer models. These core drillings will consist of up to ten 3-in. diameter holes (concrete cores will be 3-in.
in diameter; holes will be slightly larger but will not exceed 3.5-in. in diameter) drilled to a depth of 8 ft. 6 in, into the PCRV concrete from various locations on the PCRV sidewalls towards the PCRV centerline.
PCRV concrete is 9 ft. thick in this barrel section.
There are to be as many as eight drillings on one sidewall face and as many as two on one other face at varying elevations. The 8 ft. 6 in. depth of the holes has been selected to prevent contact with the PCRV liner (and those portions of the PCRV cooling tubes which are directly. attached to the liner) while also maximizing the amount of sample concrete and rebar available for activation analysis.
-Precautions have been taken in the selection of drilling locations to I
ensure that PCRV structural integrity, PCRV liner integrity, the PCRV l
prestressing tendons, and the PCRV Liner Cooling System are not adversely impacted.
The attached Safety Assessment summarizes the engineering analyses which shows the effects of the PCRV core drills on the PCRV liner, the structural integrity of the PCRV, the PCRV liner cooling system, radiological /ALARA concerns and helium permeability.
It is PSC's position that FSV is permanently shutdown and that there are no known circumstances or events foreseen that would require a return to power oper6 tion. Additionally, FSV is partially defueled and several plant systems have been taken out of service. Segment 10 fuel has been sold.
However, if FSV were to be returned to power
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' 'P-90020 Page 3 January 19, 1990 operations, this activity would not prevent the PCRV from being pressurized' up to its Reference Pressure of 845 psig.. Based on the engineering analyses contained in the core drilling Change Notice (CN), PSC considers this activity to be safe.
The schedule for this activity calls for the core drillings to begin on February 5, 1990 with completion by the end of February, 1990.
Due to various planning impacts, it is essential that we maintain this schedule.
In this respect, any comments regarding this activity or its schedule should be directed to M. H. Holmes at (303) 480-6960 prior to February 5,1990.
Very truly yours, a$-
Q A. Clegg Crawford Vice President i
Nuclear Operations l
Attachment ACC/TLH:tmk cc: Regional Administrator, Region IV ATfN: Mr. T. F. Westerman, Chief Projects Section B Mr. Robert Farrell Senior Resident Inspector Fort St. Vrain Mr. Robert F1. Quillen. Director Radiation Control Division Colorado Department of Health
n Attachment to P-90020 Page 1 January 19, 1990 Safety Assessment of PCRV Core Drills 1.0 Introduction t
PSC plans to bore core drills into the Prestressed Concrete Reactor Vessel (PCRV) barrel section.
This is necessary to accurately identify activation levels of concrete and rebar, and the radioisotopes producing these activation levels, for decommissioning planning.
Up.to ten core drills will be bored, as many as eight on the southwest face and as many as two on the northwest face.
The attached figure shows the general locations of the proposed core drills. The eight core drills on the southwest face extend in a vertical line from jt st below the core support floor to the top of the PCRV barrel section, near the PCRV top head. These core drills will be offset from each other approximately 4 to 6 ft. Analysis of concrete and rebar samples from these core drills will result in determination of activation levels at varying elevations of the PCRV adjacent to the core. The two core drills on the northwest face are near the core midsection.
They will be offset from each other by approximately-6 ft.,
and will help identify any asymetry in activation profiles due to asymmetrical neutron flux geometry around the core.
All core drills will be 3 in. in diameter (could be as much as 3.5 in. in diameter due to the possible thickness of the drill bit) and extend no more than 8 ft. 6 in. into the PCRV.
Since the PCRV 1/4 in.) thick at the concrete is 8 ft. 11 and 1/4 in. (+
or locations of the core drills, the end of each hole will be approximately 5 in, from the PCRV liner.
The concrete and rebar activation analysis will help PSC to decide whether to dismantle radioactive portions of the PCRV early in the FSV-decommissioning process, or enter a SAFSTOR mode and permit the radioisotopes in the PCRV to decay for approximately 40 years before PCRV dismantlement.
Accurate activation data is necessary for planning PCRV dismantlement, since radiation protection measures and disposal of PCRV sections depend on activation levels.
2.0- Evaluation 2.1 PCRV Liner The core drillings are planned to attain a minimum depth of 8 ft. 5 in, and a maximum depth of 8 ft. 6 in. A mechanical drill stop will be installed on the drill in order to provide positive assurance that the drilling cannot proceed further than 8 ft. 6 in. into the PCRV, hence preventing damage to the PCRV liner. Therefore, the PCRV liner, which serves as the reactor coolant pressure boundary, will not be effected by these core drills.
2.2 Structural Integrity of the PCRV The PCRV prestressing tendons are designed and loaded to assure the reactor vessel is in a state of net compression, even up to PCRV 1,
y Attachment to P-90020 1
Page 2 January 19, 1990 j
ReferencePressure(845psig). This objective can be met even with a number of tendons unloaded, as described in FSAR Section E.14.2.
The locations of the tendons were determined from original design drawings. The selected core drilling sites are such that the possibility of encountering any tendons is minimized.
- Should, however, a tendon be encountered, drilling will be stopped immediately and that hole will be abandoned.
Periodic inspections of the drilled cores will be made to determine if a tendon is
. encountered.
If evidence indicates that a tendon was cut, permission from PSC plant management will be obtained prior to proceeding with further core drillings, i
Localized cutting of rebar will occur in some of the core drills, and
'is desirable for purposes of the activation analysis confirmation.
However, the remaining rebar and concrete would not exceed allowable stresses if the PCRV were to be pressurized to Reference Pressure (845 psig). Due to the core drill locations, these core drills will not hit more than one percent of the total longitudinal rebar nor more than two percent of the total circumferential rebar in the-barrel section of the PCRV.
Since the core drills are widely separated (from approximately 4 ft. to 6 ft. apart), the effects of cutting the above amounts of rebar on PCRV structural integrity is considered insignificant.
Based on the above summarization of the engineering analyses, it is concluded that the structural integrity of the reactor vessel will not be significantly degraded. The capability to operate the PCRV at pressures up to its Reference Pressure will be retained.
2.3 PCRV Liner Cooling System The core drill locations have also been selected to minimize the probability of impacting any of the PCRV liner cooling tubes.
The liner cooling supply and return subheaders are located outside of the PCRV.
Individual tubes travel from the supply subheaders through the PCRV sidewall on one face of the PCRV, attach to the liner and circle the liner barrel once, then travel back through the PCRV sidewall to the return subheader on the same face of the PCRV where the supply tube is located. PCRV Liner Cooling System tubes are separated by 3-.75 in, vertically along the liner barrel in this area, with adjacent tubes supplied from alternate loops (FSAR Section 5.1).
For the PCRV faces of interest (the southwest face and the northwest face), no Loop 2 cooling tubes are routed through the concrete.
The possibility of encountering loop 2 cooling tubes has been eliminated since Loop 2 cooling tubes are part of both proposed fire protection cooldown trains and provide the necessary liner cooling required for defueling.
Therefore, it is only possible, though unlikely, that the core drills could impact Loop 1 cooling tubes.
in any set of four tubes (pecifications require that no more than one The FSAR and Technical S on the PCRV sidewalls) and no two adjacent PCRV liner cooling tubes be out of service during power operations l
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Attachment to P-90020 Page 3 January 19, 1990 (Technical Specification LC0 4.2.14 FSAR Sect.
5.9.2.4).
The CN design analysis indicates that no possibility of severing more than one in any set of four tubes exists, due to the location of the drilling sites.
If a liner cooling tube were cut, the damaged tube would be plugged, and the system would remain operable.
Based on the above, it is concluded that precautions have been taken to minimize the probability of a core drill impacting any liner cooling tubes. However, if loop 1 cooling tubes are cut by the core drills they will be plugged and would not prevent FSV from operating at elevated power levels.
2.4 Radiological /ALARA Concerns The radiation levels (from shine through the core drill holes in the PCRV) to be encountered during the core drilling evolution are not expected to create a high radiation area. The actual core drilling samples have been calculated to read a maximum of 6 millirem /hr on contact.
Levels of less than 50 millirem /hr are predicted to be experienced as shine from the 3-in, diameter holes.
Grout will be placed into the holes to completely fill the holes, hence reducing radiation outside the PCRV to an insignificant level.
It ~ is not possible to accurately predict the contamination levels that would be created by the drilling process, such as those resulting from the generation of dust and water by the drill and by the grinding of any activated metal that may be encountered.
All such potential contaminants will be controlled by the Health Physics department (HP) via standard operating procedures and established methodologies. HP will be monitoring the drilling process throughout its course and provide shielding and adequate containment or employ other means to ensure personnel occupational exposure is kept ALARA as well as within 10 CFR 20 guidelines and limits.
2.5 Helium Permeability Extensive analyses (FSAR D.2.1.2) have shown that the PCRV liner will not fail..even during a permanent loss of forced circulation (LOFC) accident from elevated power levels.
For the DBA-1 accident analysis, PSC made a conservative and non-mechanistic estimate' of PCRV leakage by assuming liner failure.
In this scenario, only the concrete permeability would control helium leakage.
Assuming an internal PCRV pressure of 5 psig, a leak rate of 0.2% per day was calculated. Subsequently, this leak rate, which was used for fission i
product escape from the PCRV, has been found to be overly i
conservative by a factor of 200.
The corrected PCRV concrete L
permeation leak rate at 5 psig is calculated to be 0.001% per day.
However, for conservatism, the leak rate of 0.2% per day is utilized in the DBA-1 accident analysis.
If FSV were to be operated at power l
in the future, based on the 200 fold factor of conservatism in this l
number, the possible effects of a reduction in permeability resulting from the replacement of PCRV concrete with grout is considered I
minimal and without negative impact on any accident analyses. Under the current defueling conditions, in which a permanent LOFC will not
Attachment'to P-90020 Page 4 January 19, 1990 result in any fuel failure, the concrete remaining is adequate to serve as the secondary containment boundary.
3.0 Conclusions I
i Based on the engineering analyses contained within the core drilling Change Notice (CN), as sumarized above, it is concluded that the core drilling is safe and does not require any changes to the Technical Specifications.
It-is further concluded, based on the CN engineering analyses, that the permanent alterations resulting from the core drilling should not prevent the PCRV from being pressurized l
up to ~its Reference Pressure of 845 psig in the future, though PSC has no intention of ever operating FSV ogain.
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