Nuclear Power Plant A

Type of neutron detector Fission counter Type of signal cable RG-71/U double shielded

Location of pulse preamplifier Near loading-face shield at top of detector wells Location of pulse amplifier In amplifier cabinet at control room Distance between pulse amplifier and preamplifier Approximately 120 ft Distance between neutron detector and preamplifier Approximately 28 ft

Method of grounding Multiple-point grounding system Both the preamplifier and amplifier are grounded to the building ground at the point of their installation The signal-cable shield is grounded at the preamplifier and the amplifier The neutron detector is grounded through the signal-cable shield back at the preamplifier

Operating problems and modifications During the first year of operation, it was noted that the excessive noise was eliminated by adjustment of the discriminator threshold bias control on the pulse amplifier Toward the end of the first year of operation, the noise became so excessive other methods were required to correct the problem. Examination disclosed that a No 8 conductor was installed between the preamplifier and amplifier cabinets to establish building grounds between these units The conductor resistance measured greater than 3 ohms Ground circulation currents caused by an excessive pickup of r-f noise from a-c and d-c machinery throughout the reactor building produced an emf across the grounding conductor The emf modulated the signal to the amplifier cabinet, causing excessive noise in the neutron-monitoring channels This source of noise was eliminated by removing the No 8 conductor and installing two No 2/0 grounding conductors in its place Other shielding problems Noise showed up in the automatic servo-control system during the first year of operation. Examination disclosed that a shield on a multiconductor shielded cable had not been terminated to ground by the construction contractor The problem was corrected by properly terminating the shield to the reactor building ground

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