ML20149H596

Requests NRC Approval of Proposed Remedial Actions to Be Used If Instruments Adversely Affected During Future Defueling Operations.Fee Paid

ML20149H596
Person / Time
Site:	Crane
Issue date:	02/10/1988
From:	Standerfer F GENERAL PUBLIC UTILITIES CORP.
To:	NRC OFFICE OF ADMINISTRATION & RESOURCES MANAGEMENT (ARM)
References
4410-88-L-0009, 4410-88-L-9, NUDOCS 8802220014
Download: ML20149H596 (5)

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GPU Nuclear Corporation gJ Nuclear

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e Micd etown Penns>%ania 17057 0191 717 944 7621 TELEX 84 2386 Writer's Direct D,al Number:

(717) 948-8461 February 10, 1988 4410-88-L-0009/0208P US Nuclear Regulatory Commission Attn: Docu~ent Control Desk washington, DC 20555

Dear Sirs:

Three Mile Island Nuclear Station, Unit 2 (THI-2)

Operating License No. OPR-73 Docket No. 50-320 Nuclear Instrumentation The purpose of this letter is to request N9C approval of the proposed remedial actions that GPU Nuclear '='ould i@lement in the event that the THI-2 Nuclear Instrunents are adversely af fected during future defueling operations.

GPU Nuclear plans to use an Automatic Cutting Equipment System ( ACES) to cut and remove portions of the Lower Core Support Assembly (LCSA) to provide access to the lower head for defueling. The ACES utilizes a plasma arc torch for cutting operations. This system is discussed in the Lower Core Support Assembly (LCSA) Safety Evaluation Report (SER) submitted via GPU Nuclear letter 4410-88-L-0003, dated January 18, 1988.

A scenario has been postulated in which ele 0trical noise generated by the plasma are torch could adversely af feet the Technical Specification (Tech. Spec.) required Nuclear Instruments (NIs) either temorarily or permanently. However, due to the nature of the concern f1.e., electrical noise problems are dif ficult to analyze theoretically), GPU Nuclear cannot determine conclusively the form or duration of potential disruptions. Therefore, GPU Nuclear will take appropriate action during plasma arc torch operation to comply with the restrictions noted in NRC letter NRC/TMI-87-064, dated August 20, 1987, wtiich approved plasma are cutting of upper end fittings.

In addition, GPU Nuclear is proposing to provide an alternative monitoring system in the event the NIS do not retum to "normal" af ter termination of torch operations. These actions will permit the continuation of core alterations (i.e., defueling) without an exterded disruption, I

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Document Control Desk Fcbruary 10, 1988 4410-88-L-0009

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Technical Specification (Tech. Spec.) Requirements Tech. Spec. Limiting Conditions for Operation 3.3.1.1 requires two (2) source range neutron monitoring instrumentation channels to be operable.

The original plant design source range instrumentation, which is currently in use, consists of two (2) redundant count rate channels that utilize high sensitivity BF3 proportional counters as sensors.

In the i

j event that both of these source range neutron monitoring instrumentation l

1 channels become inoperable, the action statement for Tech. Spec. 3.3.1.1 l

j requires suspension of all activities involving core alterations. The t

j action statement also requires a Special Report to be submitted within 30 l

days of the occurrence. This report must outline the cause of the l

malfunction, the plans for monitoring the condition of the core, and the 7

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plans for resumption of activities involving core alterations.

In anticipation of possible future NI inoperability during plasma are torch l

operations, GPU Nuclear is submitting this Special Report in order to i

i permit the continuation of core alterations (i.e., defueling) without an extended disruption.

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t II. Design Function of the Neutron Monitoring Instrumentation i

Tech. Spec. Bases 3/4.3.1. "Neutron Monitoring Instrumentation," states:

i "Tne neutron monitoring instrumentation, which was included in the normal l

Reactor Protection System Instrumentation, provides information regarcing j

the shutdown status of the core and it will be used to monitor changes in i

neutron generation." Additionally, TMI-2 FSAR Section 7.8.1, "Nuclear j

Instrumentation," states, "Tne nuclear instrumentation is designed to i

provide neutron flux information over the full range of reactor l

cperations."

t The alternate monitoring system, described in Section V of this letter, will have a monitoring capability similar to that of the present source i

j range neutron monitoring instrumentation.

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III. ACES System Description i

i The ACES utilizes three (3) Thermal Dynamics PAK 45 plasma are cutting units. These units are wired in parallel and, as a system, are capable of generating 1,200 amps at approximately 200 volts DC. The cutting i

current is variable between 150 and 1,200 ams with current feedback I

control.

Each unit is fed by a separate 480 VAC, 3 phase, 60 HZ,150 amp j

source, j

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i There are two (2) phases of electrical operation (i.e., arc starting and i

j are cutting). The arc starter generates a 15 HZ saw tooth waveform which j

triggers a Silicon Controlled Rectifier (SCR) firing circuit to generate 1

j a 7,000 volt (peak), 2 microsecond pulse between the electrode and the torch tip. This initiates the pilot arc. The pilot are is extended

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beyond the torch tip chamber when a secondary power supply activates 1

inducing a 360 volt, 8 millisecond pulse across the ionized gas colunn

j within the torch tip. As this voltage decays, the pilot arc goes out and j

the process repeats at the generation of the next saw tooth.

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Document Control Desk February 10, 1988 i

4410-88-L-0009 The arc starter ionizes the primary gas chamber. When the "start" t

4 command is initiated, the main cutting arc is transferred from electrode j

to work piece. During the initial cut, the cutting current is increased to the current command setting and then maintained within 10% of this l

value throughout the cut.

When main arc transfer occurs, the pilot arc l

relay drops out and the arc starter is deactivated.

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There are slight variations in the cutting amperage due to changes in

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material thickness and changes in cutting speeds. These fluctuations are

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minor (i.e., less than 10%) relative to the current conrnand setting. The i

torch for ACES is maintained at the proper standoff distance via a motor i

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driven control unit with position feedback. The cutting voltage will

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fluctuate slightly as the standoff control unit seeks the proper

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4 distance. These fluctuations in voltage are minor relative to the system i

voltage (i.e., less than 10%).

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If main are transfer is lost at any time during the programmed cut, the are starter will automatically reactivate momentarily to reionize the.

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primary gas. The system will then automatically attempt main arc l

transfer.

IV.

Interference Pnenomena j

As discussed above, the ACES will maintain a relatively constant DC i

voltage and current during the cutting process.

It is unlikely that f

sufficient noise will be generated during this phase to interfere with l

the NIs (i.e., create spikes). However, due to the inherent instability l

of the plasma stream, local noise will exist at the torch tip. Although j

J there is natural shielding provided by the water and the heavy case of t

the instrument wells, this noise could affect the NIs.

During the arc starting phase, the arc starter generates a fluctuating l

field which could have an ef fect on the NIs. The are starter circuit is j

activated during the "find height" mode (for positioning the torch prior 3

to cutting), during the initial phase of cutting, and when the are f

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transfer is momentarily lost during a cut. Theoretically, these brief i

operations could create momentary spikes on the NIs.

l GPU Nuclear contacted the vendor to discuss the possibility of damaging l

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the NIS during operation of the plasma arc torch. The vendor does not believe that the BF3 proportional counter will be damaged. However, the vendor did state that other components (e.g., preamplifier) are 4

likely to be affected causing erratic indication on the instrumentation.

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V.

Remedial Actions

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j As discussed previously, momentary spikes may occur on the NIS durirg plasma are torch operations. Therefore, the NIs will be monitored during J

cutting operations.

If disruptions occur, the performance of core alterations other than plasma are torch operations will be prohibited as t

i Document Control Desk Fcbruary 10, 1988 4410-88-L-0009 i

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t specified in the referenced NRC letter. Operators will be advised that interference spikes may occur during torch operations. The NIs will be i

deemed inoperable, per the Tech. Specs., if the instruments do not return to "normal" after termination of torch operations, j

As a remedial action, GPU Nuclear will be prepared to provide an alterriative monitoring system for the inoperable enannel(s). The l

3 alternate monitoring system will consist of a corrrnercially available, i

fission chamber-based, neutron counting instrumentation loop capable of f

operating in the expected count rate range.

Since locating the altemate

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detector at the present NI cetector location is not feasible, the base t

count rate may djffer.

However, the capability to detect count rate changes of interest will be maintained. The instrumentation loop

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includes the following components.

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Fission Chamber I

Coax Cable i

Preamplifier Power Supply

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High Voltage Power Supply j

i Lin/ Log Ratemeter i

Strip Chart Recorder i

Linear Amplifier and Single Channel Analyzer 1

Counter and Timer The location of the alternate NIS in the Reactor Building (RB) will be

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procedurally controlled based on an engineering analysis. The Lin/ Log Ratemeter and Strip Chart Recorder will be remotely monitored from inside 1

the Control Room and/or Command Center.

If it becomes necessary to

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install an alternate monitoring system, surveillance procedures applicable to the system will be irrplemented and performed at the frequencies required in Table 4.3-1 of the Recovery Operations Plan.

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VI. CONCLUSION In anticipation of potential future NI inoperability as a result of

• l plasna are torch cutting of the LCSA, this letter is being submitted as a Special Report per Tech. Spec. 3.3.1.1 in order to pemit the 1,

continuation of core alterations without an extended disruption.

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1 During cutting operations, momentary spikes may occur on the NIs.

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Therefore, the NIS will be monitored during cutting operations. Core l

alterations, other than plasma are torch operations, will be prohibited l

1 if interference disrupts the NIs.

In the event the NIs do not return to I

"nomal" af ter temination of torch operations, GPU Nuclear will be i

i prepared to install an alternative core monitoring system. GPU Nuclear i

believes that these actions will pemit the safe continuation of core I

alterations.

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o Document Control Desk February 10, 1988 4410-88-L-0009 Per the requirements of 10 CFR 170, an application fee of $150.00 is enclosed.

Sincerely, g

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F. R. Standerfer Director, THI-2 RDW/e-f Enclosed: GPU Nuclear Corp. Check No. 009800 cc: Regional Administrator, Region 1 - W. T. Russell Director, TMI-2 Cleanup Project Directorate - Dr. W. D. Travers l

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