ML19224B391

Health Physics Procedure 1754,Revision 1:operation & Calibr of Eberline PNC-4 Neutron Counter

ML19224B391
Person / Time
Site:	Crane
Issue date:	02/16/1977
From:	Metropolitan Edison Co
To:	Mullinix W NRC/IE
References
1754, TM-0132, TM-132, NUDOCS 7906150044
Download: ML19224B391 (10)

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TMI DOCUMENTS W~

DOCUMENT NO:

h OF DOCUMENT PROVIDED BY COPY MAEE ON EDIJONCOMPAfPI.

l METROP3LITAN

'Vilda R. Mullinix, NRC e

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Revision 1

02/16/77 THREE MILE ISLAND NUCLEAR STATION STATI0ti HEALTH PHYSICS PROCEDURE 1754 OPERATI0tl At4D CALIBRATI0ti 0F EBERLIt4E PtiC-4 tiEUTR0t1 COUt4TER Table of Effective Pages PS Date Revision Pace Date R evision Paae Date Revision 4

1.0 02/16/77 1

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50.0 75.0 Unit 1 Staff Recommends Approval Unit 2 Staff Recommends Approval Approval N

Date 2/84/7'?

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Cognizant Dept. Head Cognizant D ept. Head i

Unit 1 FORC Re )mmer'ds A roval Unit 2 PORC F ommedds

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1754 Revision 1

02/16/77 TfiREE MILE ISLAND NUCLEAR STATION STATION fiEALTH PilYSICS PROCEDURE 0peration and Calibration of Eberline PNC-4 Neutron Counter 1.0 PURPOSE 4

The purpose of this calibration is to explain in detail the operation and calibration of the PNC-4 Neturon Counter.

2.0 DISCUSSION 2.1 The PNC-4 neutron counter detector tube is sensitive to slow neutrons directly.

To detect fast neutrons the detector must be inserted in the moderator.

Here the 0.03 inch cadmium lining captures the slow neutrons and passes the fast neutrons; the fast neutrons are then moderated by the paraffin so that their energy can be absorbed in the detector.

2.2 When the 8 atom inside the tube captures a neutron, 'it releases an 10 I

Alpha particle. The energy released ionizes the gas in the tube. The negative ions are accelerated to the anode wire because of the pos-ltive p

high voltage applied. As they approach the wire, they have sufficient energy to ionize more atoms causing ion amplification.

The amou,nt of amplification is controlled by the high voltage. The resulting r

current pulse is coupled to the instrument amplifier and ultimately displayed on the meter as counts per minute.

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3.0 REFERENCES

3.1 Eberline PNC-4 Technical Manual i

3.2 10 CFR 20 4.0 EQUIPMENT 4.1 PNC-4 Eberline Neutron Counter 4.2 AmBe Source (4.59 Ci) 4.3 Poly Bricks 20'c 091 -

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1.0

1754 Revision 1 4.4 Eberline MP-1 Mini Pulser 137 4.5 Cs Source 5.0 OPERATING INSTRUCTIONS 5.1 External controls and connectors (Figure 1754-1).

4 5.1.1 Three position switch turns instrument "0FF" "0N" and "BATT CHECK".

Spring returns switch from "BATT CHECK"to "0N" posi tion. l 5.1.2 High voltage dials control magnitude of high voltage applied to the detector. Clockwise rotation increases the voltage.

Radial marks are provided around control for reference.

5.1.3 Phone connector is provided for connection of the headset, speaker or external scaler.

5.1.4 Connector is provided for connection of detector.

5.2 Internal controls 5.2.1 Four calibration controls R2, R4, R3, and R5 are used to adjust the meter reading to agree with input frequency (Figure 3.1 in PNC-4 Tech Manual. Ref. 3.1).

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5.2.2 A twenty turn sensitivity control adjusts the input sensit vity.

5.3 Using the Instrument.

5.3.1 Turn the switch to the "BATT CHECK" position.

The left pointer 7

(Black) should read in the green area on the meter dial.

Release the switch and it will return to the "0N" position.

If the s

pointer reads below the green area of the meter dial, replace the batteries as described in Section IV. A.2 of the PNC-4 Tech Manual (Ref. 3.1).

5.3.2 The Lin-Log scale is read by obse ving the pointer that is on

- scale.

The black po' iter is read up to 5,000 CPM, above whkh i

2.0

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2/8/74 1754 Revision 0 5.3.2 (con't) it disappears from view.

The red pointer then is on scale and is read up to 500,000 CPM. The fluctuation of the pointer is normal and is caused by the random nature of radioactive y

emissions.

Watch the pointer long enough to obtain the average i

readiag.

5.3.3 The detector, when out of the moderator, detects slow neutrons.

When it is in the moderator it detects fast neutrons.

5.3.4 For the converstion from CPM to mrem /hr fast and slow neutrons refer to Figures 1754-2 and 1754-3 attached to this procedure.

(These figures are also attached to the sides of the PNC-4 instru-ment).

NOTE:

Conversion from CPM to mrem /hr is valid only if neutron energy spectrum is thermal (slow) or 5 meV (fast).

Con-siderable error is possible for a broad energy spectrum or i

in intermediate energy regions.. A possible procedure to I

correctly interrupt the PNR-4 response is to compare PNR-4 readings to PNC-4 readings to develop conversion factors in each room, or area, where the neutron energy spectrum can differ.

5.4 Calibration

5.4.1 To completely set-up and calibrate the PNC-4, three steps are required plus an additional verification check with a neutron source.

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5.4.2 Input Sensitivity:

Remove the PNC-4 from its case and connect the MP-1 output to the spring connector (junction of R8 and.C2).

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"175'4 2/8/74 Revision 0 5.4.2.1 Set MP-1 CPM at 40K (40 x IK) counts per minute and amplitude on the 3 millivolt (mV) range.

5.4.2.2 Slowly turn the Amplitude Fine Control up until the PNC-4 meter responds and setties at a reading (reading should be 40K GPM but this is not critical at this point).

5 4.2.3 Adjust the Amplitude Fine Control down until the PNC-4 reads 1/2 of that reading above.

The MP-1 meter should read 2.0 + 0.5 m Volts.

If outside of these limits the sensitivity must be adjusted.

5.4.2.3.1 Adjust MP-1 Amplitude Fine Control for 2.0 m Volts and adjust the serisitivity (located on the AMP-DISCR-BINARY card; see Figure 2-2 in Tech Manual) for a PNC-4 reading of approxi-mately 1/2 of that obtained in 5.4.2.2 above.

5.4.3 Meter C;1ibrhtion; Connections same as 5.4.2 above.

Set MP-1 amplitude at 3 millivolts.

NOTE:

Sections 1 and 2 of this driver combine to deflect meter one, and sections 3 and 4 to deflect meter two. The calibration controls adjust this combination for proper meter reading vs input count rate.

Each pair of sections and meter is completely inderaH:,t of the other.

The first control (R2 and R4 respectively) adjust the slope of the meter current vs frequency line for both sections.

The second control (R3 and R5 respectively) adjusts the point where the lower frequency section saturates.

This saturation forms a stable base upon which higher frequency section operates.

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'1754 2/8/74 Revision 0 5.4.3.1 Turn R3 and R5 (located on PNC-4 chassis; see Figures 2-1, 2-2 in Tech Manual) maximum clockwise.

5.4.3.2 Set MP-1 at 400 CPM (40x10) and adjust R2 for a PNC-4 meter reading of 400 CPM.

j 5.4.3.3 Set MP-1 at 4 K CPM (40x100) and adjust R3 for a PNC-4 meter eading of 4 K CPM.

5.4.3.4 Set MP-1 at 40 K CPM (40x1K) and adjust R4 for a PNC-4 meter

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reading of 40 ) CPM.

5.4.3.5 Set MP-1 at 40 K CPM (40x10K) and adjust R5 for a PNC-4 meter reading of 400 K CPM.

5.4.3.6 Disconnect MP-1 and replace PNC-4 in its case.

5.4.4 High Voltage 5.4.4.1 Expose the detector assembly (either in or'out of the n:oderator) 60 137 to a gamma field (either Co or Cs) of approximately 10 R/hr and adjust the H.V. adjust control until the meter reads approxi-mately 100 CPM.

NOTE:

The high voltage comes down slowly.

If a downward adjustment was made, turn the instrument off for about L

one minute, then back on and verify the setting.

5.4.4.2 Remove the instrument from its case and. measure the voltage on the spring connector with an elect ostatic voltmeter. Adjust the HV ADJUST control for a high voltage 50 Volts less than that measured. Turn the instrument off until the voltage drops significantly, then turn back on and verify the setting.

5.4.5 Neutron Test; Expose the detector (in the moderator) to a known fast neutron field and read the meter.

The reading should agree 202 095 7

5.

1754 Revision 1 02/16/77 5.4.5 (con't) with the field within + 20%.

For the conversion from CPM to mrem /hr for fast (AmBe) neutrons refer to FIGURE 1754-2.

Distance vs dose relationship for AmBe neutron source is 5.4.5.1 as follows:

D=

(Q)

N

.139 (mrem /hr)

=.011(Q ) mrem 2

2 2

(4n d )

cm sec x..(n/cm sec)

(d )

hr or (Q) d =.011 (D) = cm

where, D = mrem /hr dose rate 7

Q = neutrons /sec emitted by sour:e (1.21 x 10 n/sec) 4n = constant (12.57) i i

.139 - Relation between dose equivalent and flux for 5 Mev

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neutrons (average energy for AmBe source is approxi-y mately 5 Mev).

t d = distance from source center to detector center in cm.

5.4.5.2 Do not attempt to get closer than 50 cm between source center h

and detector center. Closer than this will cause e cessive geometry errors.

5.4.5.3 The space between the source and the aetector should be free air (no obstructions) to prevent scattering or mode"ation of the neutrons.

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