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Model 875 High Range Containment Monitor

High Range Containment Monitor

Model 875

The 875 High Range Containment Monitor consists of a detector, readout, and cable (in metallic tubing) connected through a sealed penetration in the containment wall. The detector walls are thick enough to withstand loss of coolant accident (LOCA) pressures, but thin enough to provide full response to 133Xe (80 keV).

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Key Features

  • Unattenuated, in-containment detector location. Responds fully to 133Xe (80 keV) and other fission products with photon energies as low as 60 keV
  • Monitor designed and tested to meet NRC Reg. Guide 1.89 (class 1E qualifications) and NRC Reg. Guides 1.29/1.100 (seismic qualification)
  • Permits continuing in-containment post-LOCA radiation assessment
  • Detector and cable designed and tested to meet IEEE 323 (1974) for operation in containment. Readout meets IEEE 323 (1974)


The system measures a seven decade range from 100 to 107 R/h. Conditions in containment which could produce 108 Rad/h (surface dose) (NRC Regulatory Guide 1.97, paragraph C3a) of mixed radiation are characterized by an effective measurement of less than an exposure rate of 107 R/h (photons above 60 keV).


The system operates with detector/readout spacing up to 500 feet and greater distances are possible. Normally, redundant detector/readout combinations are utilized. To assure the integrity of the detector/cable/readout system, an electronic check source is automatically initiated every 17 minutes. A successful check keeps the green front panel Fail indicator illuminated. An unsuccessful check initiates a relay closure (failsafe), and turns the green Fail indicator off. The channel TEST button on the readout injects a signal to test operation of the alarm lights and relays. If a battery is connected to the readout, the system automatically switches to this supply in the event of failure of the AC input.

Detector (Model 877)

The Model 877 detector is a hermetically sealed, stacked parallel plate, 3-terminal, guarded ionization chamber, operated in the saturated mode. For in-containment use, all external surfaces are 316 stainless steel. A stacked plate construction eliminates the need to pressurize the ion chamber, and the associated loss of sensitivity if the pressurization is lost. A stainless steel mounting bracket is provided. Electrical connections are brought out through two hermetically sealed, welded connectors. Model 878-1-9 Radiation Resistant Cable (in stainless steel tubing) is used between detector and containment penetration.

Readout (Model 876A)

The readout presents a seven decade logarithmic range on a mirrored 4 inch panel meter. Each readot contains its own independent power supply, dual independent radiation alarms, each of which can be set anywhere across the scale, and a failure alarm held off by the electronic check source. The alarm circuits actuate Form C, 5-amp failsafe relay contacts. A popular mode of operation is to set the low radiation alarm set point at minimum scale to initiate the operation of an optional external recorder when the radiation level in the containment exceeds 1 R/h. The radiation alarms are available in either the manual or automatic reset mode. The display can be expanded by a selection switch to show any three consecutive decades across the full scale. Up to two readouts may be mounted in the Model 876-1-55 chassis, which fits a standard 19-inch opening.


Model Name


Model 875 High Range Containment Monitor High-Range Containment Monitor, includes: Detector (877) Readout (876A) Radiation-Resistant Cable (878-1-9)


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