ML20203G481

Safety Evaluation Approving Commencement of Bulk Defueling Activities Limited to Core Region

ML20203G481
Person / Time
Site:	Three Mile Island
Issue date:	07/24/1986
From:	Office of Nuclear Reactor Regulation
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ML20203G477	List: ML20203G471 ML20203G481
References
NUDOCS 8608010178
Download: ML20203G481 (8)

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SAFETY EVALUATION FOR THI-2 CORE REGION BULK DEFUELIflG ACTIVITIES IflTRODUCTI0fl By letter dated hay 15, 1986, GPU Nuclear Corporation (GPUNC) requested flRC approval of Revision 10 to the Defueling Safety Evaluation Report (SER)

(Reference 1). Additional information regarding the use of the core boring equipment was provided by GPUNC on July 23, 1986 (Reference 5). This evaluation addresses those defueling activities proposed in References 1 and 5 that are restricted to the core region and the area down to the lower grid forging. The remaining activities proposed in Reference 1, such as core support assembly and icwer head defueling, will be addressed in a subsequent j

I;RC safety evaluation.

SAFETY ISSUES The safety issues associated with the proposed core region bulk defueling activities are similar to those analyzed for early defueling activicies, and the potentiel consequences of bulk defueling activities are bounded by previously evaluated activities. This safety evaluation references previous NRC approvals as appropriate anti .inalyzes the unique aspects of core region defueling, as proposed in References 1 and 5.

CRITICALITY The potential f$r a recriticality event during defueling activities is effectively minimized by maintaining a high boron concentration in the Reactor Caolant System (RCS). In the NRC Safety Evaluation for Early Defueling (Reference 2), the staff referenced an earlier conclusion that, at an RCS boron concentration of 4350 ppm, the damaged core will remain subcritical with a shutdown margin of at least one percent for any postulated fuel configuration. Additianal margin exists since GPU will administratively maintain the actual RCS boron concentration at 4950 ppm. Experience in defueling efforts to date has also demonstrated the effectiveness of this approach to maintaining subcriticality.

In Reference 2, the staff also concluded that sufficient margin existed to maintain subcriticality in case cf the inadvertent introduction of foreign materials into the RCS. The tools and equipment to be used for bulk defueling have been analyzed to ensure that a one percent shutdown margin will exist for all credible events. We therefore conclude that appropriate means are being enployed to assure adequate margins exist to mininize the potential for

recriticality of the remaining fuel in the Till-2 reactor vessel.

' Canister handling and storage procedurcs will not differ significantly from those currently in use; therefore, the conclusions of Reference 2 also apply to cure region bulk defueling activities, i.e., the potential for a P

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criticality event in the handling and storage of defueling canisters is acceptably low.

BOR0ft DILUTI0f!

In Reference 2. th" staff concluded that GPU had implemented acceptable controls to minimize the potential for a boron dilution event and to effectively mitigate the consequences of such an event during early defueling.

These controls, which will remain in effect during bulk defueling, include the use of multiple barriers to isolate patential dilution sources and the boration of the hydraulic fluid used in defueling equipment. Additional potential sources of boron dilutien during core region defueling that were not evaluated for early defueling activities are the hydraulic systems for the core bore equipment and the ultrahigh pressure decontaminatinn water. In the NRC safety evaluation for Core Stratification Sanple Acquisition (Reference 3), the staff concludec: that the two unborated hyiraulic fluids used with the core bore equipmnt did not present a credible source for a dilution event resulting in advertent criticality of the core. This conclusion was based on the following: 1) the sources were of small volume (1.4 and 27 gallons),

compared to the large volume of borated RCS watar above the core region (20,000 gallons); 2) the fluids would tend to mix well with the borated RCS water; and 3) the fluids would be introduced near the surface of the RCS Wdter, aWay from the core. This conclusion also applies to the use of the core bore equipment for defueling purposes.

In Reference 1, GPU describes the physical and administrative controls that will be in place to prevent a boron dilution event resulting from improper alignment of the ultrahigh pressure pump. This pump is used with a borated water supply when in-vessel abrasiva/ water jet cutting is performed. Uhen used for reactor building decontamination, the pump is supplied with unborated (or under borated) water. Consequently, GPU has adepted procedures to prevent the inadvertent introduction of this water into the RCS. The two separate supply hoses are permanently coupled to their respective nozzles, with a comon mating fixture attached to the pump. The set of hoses used for decontaninstion will be permanently tagged with a warning to personnel not to use those hoses for in-vessel operations. Procedures will require verification of proper hose a!ignment prior to use of the ultrahigh pressure pump.

Following the use of this pump in the decontamination mode, approximately 5 gallons of unborated water sould remain in the system and could be injected into the RCS when the pump is used in the in-vessel cutting mooe.

Administrative controls will require that the discharge from the in-vessel cutting nozzle be directed upward, away from the core and other system suction lines for a minimum of five minutes following the use of the pump in the decontamination mode. Additionally, the RCS boron concentration will be verified to be at or above 4950 ppa prior to use of the pump in this mann'er.

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! We conclude that, with these precautions, the potential for boron dilution due l to the introduction of unborated water used with the ultrahigh pressure pump is acceptably low. Therefore, we further conclude that the potential for a l

boron dilution event resulting in core recriticality during core region l defueling activities is acceptably low.

RELEASE OF RADI0 ACTIVITY Although the proposed core region defueling activities will involve the displacement of core debris to a greater extent than early defueling activities, it is not expected that significant increases in reactor building radiation levels or in off-site releases will result, based on previous i defueling experience. The systems equipment and procedures used to minimize l the potential for, and consequences of, a release of radiation during early l defueling activities will continue to be used during bulk defueling. All gaseous release pathways to the environment will be monitored and filtered, I

and all reactor building exhaust points can be isolated as needed. flonitoring for Kr-85 and alpha-emitting particulates will be conducted. The off-gas systm uill be operated, as necessary, to filter particulates and disperse gases that may collect under the defueling work platform (DWP). All equipment and tools will be flushed upon removal from the reactor vessel to limit the spread of contamination. A water cleanup system will be operated to maintain the RCS at an acceptable activity level. The analyses approved in Reference 2 for potential releases of radiation during nonaal and accident conditions are also bounding for any release scenario associated with bulk defueling. We therefore conclude that potential releases of radioactivity within the reactor building or to the environment as a result of bulk defueling activities will be naintained at acceptable levels in cenpliance with applicable regulatory limits.

i PYROPil0RICITY l

l Culk defueling activities will include considerable sizing and cutting operations, resulting in the creation of smaller particles of core debris, including zirconium compounds. Despite the generation of more finely divided particles, the potential for a pyrophoric reaction remains very low. During pilot ignition tests conducted on debris samples and previous defueling operations, no pyrophoric characteristics were observed. Although smaller particles are expccted to be created, the increase in the voluoe of particles '

in the range of concern, below 50 microns, is not expected to be great. Bulk defueling activities will be conducted underwater; as discussed in Reference 2 and 3, this condition effectively prevents pyrophoric events even if finely divided zirconium conpcunds are present. In Reference 2, the staff also concluded that the potential for a pyrophoric event in a filled, dewatered, i defueling canister was acceptably low. Canister loading, handling and storage I will be conducted in a similar manner during bulk defueling, therefore, our previous conclusion is applicable. In Reference 3, the staff concluded that the use of the core bore drill would not present an unacceptable potential for l

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a pyrophoric event, provided the drill bit is continuously flushed to rapidly remove the heat generated by drilling. The drill unit is designed to shutdown upon loss of flush water; therefore, removal of the frictional heat generated l l by drilling is assured. He conclude that the potential for a pyrophoric event I is acceptably low for the proposed core region defueling activitie . I OCCUPATIONAL EXPOSURE I In Reference 2, the staff approved the licensee's program for maintaining .

radiation exposures to workers as low as reasonably achievable (ALARA). This '

l comprehensive program will continue to be implenented through defueling activities. This program limits occupational exposure through decontamination l of work areas, design of defueling equipment, training of defueling workers, j and development of appropridte operating procedures. Due to earlier decontamination octivities in the reactor building, measured dose rates in work areas are currently at or below the licensee's target levels for early defueling.

1 Bulk defucling tools are designed to be compatible with existing tooling (e.g. l l

hydraulic systems, long-handled tool extensions) which permits defueling '

activities at a safe distance from the core debris. Radiation shielding for  !

defueling workers is provided by the water in the reactor vessel and by the i lead shield plates on the DUP. The design of the defueling canisters and  ;

handling equipment provides additional shielding when the canisters are -

t ransferred in air between the vessel and spent fuel pool "A".

The level of activity in the RCS is controlled to limit its contribution to area dose rates. The Defueling Test Assembly (DTA), a full-scale mock-up of i the reactor vessel and DNP is used to train defueling workers in the use of i l new tools and procedures, thereby reducing time spent in radiation areas and j limiting exposure. As defueling continues, existing procedures will be l revised or new procedures developed as needed to maintain exposures ALARA. l l Defueling experience to date has demonstrated the effectiveness of the l licensee's ALARA program in minimizing occupational exposure. In Reference 1, the licensee reported that 213 person-rems of radiation exposure had been incurred from defueling hetivities thrcugh 11 arch 31,1985. The licensee currently estimates that an additional 1200 person-rems will be incurred through the completion of defueling operations. Although measured dose rates I

are generally leur than predicted, the licensee's revised exposure estimate

' of approximately 1400 person-rem for defueling activities is higher than earlier estimates due to an increased estimate of job hours required for completion of defueling. This increase is based on a more accurate assessment of the remaining activities and the time required for performing those activities. As discussed in Supplement 1 to NUREG-0683, the Programmatic Environmental Impact Statenant (PEIS) for TMI-2 cleanup, the staff estimated that reactor disassembly and defueling activities would result in a total exposure of 2600 to 15,000 person-rem, over half of which would occur from

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q defueling activities alone. Therefore, the licensee's revised estimate, based on the most recent defueling experience, still falls at the lower end of the range of the staff's estiraate. Based on our earlier conclusion in Reference 2 and on the effectiveness of the licensee's ALARA program as evidenced by defueling experience to date, we conclude that the licensee has established an acceptable program for maintaining worker exposures ALARA during bulk defueling activities.

REACTOR VESSEL INTEGRITY .

The use of the core boring equipment for core region defueling creates a potential for imparting loads to the incore instrumentation nozzles that could result in weld failure ar.d an unisolable leak of RCS water. In previous approvals for the use of this equipment in core sampling applications, the staff imposed appropriate limitations to minimize the potential for failure of the incore nozzle welds.

Reference 1, ti.e licensee states that a solid face drill bit will be used with the core boring equipment for defueling. Such use is clarified in Reference 5, which details the technique and restrictions to be applied. The general protocol for core boring was previously addressed in Refercnce 4. Reference 5 restricts core drilling operations to non-instrumented fuel assemblies to a depth above the lower grid support structure. He .aclude that core bore defueling activities can be safely conducted in ' care region, subject to the following restrictions. 1) The maximum allredie drii. depth will be limited to the top of the lowe .; .d suppoi< n ture. Such depth will be sufficient for core region def uering and will pu/ide asturance that significant loads will not be imparted to the incore nozzles and welds. 2)

The drilling locations will be procedurally controlled to prevent the direct dppliCation of the drill to existing incore instrument stings, unless that string and supporting guide tube are verified to have been cut. Such location control will rely on the theodolite system as described in Reference 5, unless an alternative method is approved by the NRC.

These precautions reduce the potential for unisolable RCS leakage due to failure of an incore weld resulting from the proposed core bore defueling activities. In the unlikely event of such leakage, the licensee has provided equipment and developed procedures to quickly identify the leak and establish sufficient makeup or recirculation of barated water to the RCS to maintain subcriticality of the core.

OTHER SAFETY ISSUES i Other safety issuis, specifically heavy load handling, decay heat removal,  !

hydrogen control,'and fire protection have been addressed in earlier NRC safety evaluations. The proposed core region defueling activities do not '

present an increase in the likelihood or consequences of potential accidents beyond the bounds of those analyzed in References 2 ard 3 for these issues.

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f CONCLUSIONS Our review of the licensee's Defueling Safety Evaluation Report - Revision 10 (Reference 1) and the licensee's supplemental information (Reference 5) has been limited to those activities to be conducted in the core region, i.e.,

above the lower grid support structure. The remaining activities addressed in Reference 1, including debris removal from the CSA and lower vessel head regions are currently under staff review and will be the subject of a separate safety evaluation.

In our review of the proposed core region defueling activities, we have evaluated the safety issues of criticality, baron dilution, release of radioactivity, pyrophoricity, occupational exposure, and reactor vessel integrity. We have determined that the safety considerations of heavy load handling, decay heat removal, hydrogen control, and fire protection have been adequately addressed in previous NRC safety evaluations, and that our earlier conclusions are applicable to the proposed activities. Based on our review, we find that; 1) acceptable precautions are in place to assure that a sufficient margin will be maintained to prevent recriticality due to fuel reconfiguration or boron dilution; 2) potential releases of radioactivity to the reactor building and to the environment during normal or postulated accident conditions will not pose a significant risk to the work force or public; 3) radiation exposure to workers will be maintained ALARA; 4) there is little potential for a pyrophoric event; and 5) the restrictions imposed on the use of the core bore equipment for defueling in the core region provide adequate assurance that the likelihood of unisolable RCS leakage resulting from an incore instrument nozzle weld failure will be mininized, and that adequate borated RCS makeup sources are available if needea. We also find that,1) the proposed activities fall within the scope of those analyzed in the PEIS, and 2) these activities do not constitute an unreviewed safety question per 10 CFR 50.59. Therefore, we conclude that the proposed core region defueling activities, subject to the limitations stated herein, can be safely conducted with mininal risk to the health and safety of the onsite work force and off-site public.

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REFERENCES

1. GPU Defueling Safety Evaluation Report, Revision 10,11ay 15,1936.

2. NRC Safety Evaluation for Early Defueling, November 12, 1935.

3. flRC Safety Evaluation for Core Stratification Sample Acquisition, llay 5, 1986.

4. GPU Safety Evaluation Report for Core Stratification Sample Acquisition, Revision 3, December 31, 1985.

5. Letter dated July 23, 1986 from F. R. Standerfer, GPUti to W. D. Travers, NRC, "Use of Core Stratification Sanple Acquisition Tool For Defueling."

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- - - ~- ~

TMI-2 SETVTCETIST Dr. Thomas Murlsy Willis Bixby, Sita Managsr  ;

Rtgional Administratcr U.S. Department of Energy U.S. Nuclear Regulatory Connission P.O. Box 88 Middletown, PA 17057-0311 631 Park Avenue King of Prussia, PA 19406  :

David J. McGoff John F. Wolfe Esq., Chairman Office of LWR Safety and Administrative Judge Technology Atomic Safety and Licensing Board Panel NE-23 U.S. Nuclear Regulatory Commission U.S. Department of Energy Washington, D.C. 20555 Washington, D.C. 20545 Dr. Oscar H. Paris William Lochstet Administrative Judge 104 Davey Laboratory Atomic Safety and Licensing Board Panel Pennsylvania State University U.S. Nuclear Regulatory Comission University Park, PA 16802 Washington, D.C. 20555 Frank Lynch, Editorial Dr. Frederick H. Shon The Patriot Administrative Judge 812 Market Street Atomic Safety and Licensing Board Panel Harrisburg, PA 17105 U.S. Nuclear Regulatory Commission Washington, D.C. 20555 Robert B. Borsum Babcock & Wilcox Dr. Judith H. Johnsrud Nuclear Power Division Environmental Coalition on Nuclear Power Suite 220 433 Orlando Avenue 7910 Woodmont Avenue State College, PA 16801 Bethesda, MD 20814 Ernest L. Blake, Jr., Esq. Michael Churchhill, Esq.

Shaw, Pittman, Potts, and Trowbridge PILCOP 1800 M. Street, NW 1315 Walnut Street, Suite 1632 Washington, D.C. 20036 Philadelphia, PA 19107 Atomic Safety and Licensing Board Panel Marvin I. Lewis U.S. Nuclear Regulatory Connission 7801 Roosevelt Blvd. #62 Washington, D.C. 20555 Philadelphia, PA 19152 Secretary Jane Lee U.S. Nuclear Regulatory Cosmission 183 Valley Road Washington, D.C. 20555 Etters, PA 17319 Frederick S. Rice, Chainnan Walter W. Cohen, Consumer Dauphin County Board of Comissioners Advocate Dauphin County Courthouse Department of Justice Front and Market Streets Strawberry Square, 14th Floor Harrisburg, PA 17101 Harrisburg, PA 17127 Thomas M. Gerusky, Director Mr. Edwin Kintner Bureau of Radiation Protection Executive Vice President Department of Environmental Resources General Public Utilities P.O. Box 2063 Nuclear Corporation Harrisburg, PA 17120 100 Interpace Parkway Parsippany, NJ 07054 Ad Crable Lancaster New Era 8 West King Street Lancaster, PA 17601