Fission chambers for ex-core use

Photonis manufactures a broad range of fission chambers for out-of-core use in severe environmental conditions. They are used in thermal neutron detection detectors provide indication for up to 11 decades of neutron flux for wide-range reactor monitoring. They operate in extremely high gamma environments and at temperatures up to 600°C.

Read more
Detection of thermal neutrons in wide flux ranges
Very high safety of use
Customization according to the user’s needs

Fission chambers for ex-core use

Contact sales or request a quote

Contact us

Technical information

Ex-core fission chambers without integral Cable
Basic typeExternal diameter (mm)Length (mm)Operating RangeMax.
   Pulse modeCurrent mode 
CFUM1125.422710 - 107107 - 1011250
CFUL01483371 - 106104 - 1010250
CFUK09603950.3 - 3x105105 - 1010250


Ex-core fission chambers with integral Mineral Insulated Cable
Basic typeExternal diameter (mm)Length (mm)Operating Range

Max. temp (°C)

   Pulse modeCurrent mode 
CFUM1825.426310 - 107107 - 1011250
CFUL0848384.51 - 106104 - 1010250
CFUC06484121 – 105104 - 1010600
CFUK08603620.3 – 3x10^510^5 - 10^10250
CFUP0876.53891 - 106104 - 1010250
CFUG08804190.2 - 2x105105 - 7x1010250




Fission chambers are critical components for reactor neutron flux monitoring and can be used to detect thermal neutrons in high flux, to monitor the reactor fuel burn-up, and to control power in the source, intermediate, and power ranges. Gas filled neutron detectors are designed to operate in three modes.

Gas filled neutron detectors are designed to operate in three modes:

  • Pulse mode analyzes and counts individual detector pulses. 
  • Fluctuation, or Campbell mode, is used to analyze the fluctuation of the direct current from the detector.
  • Current mode is used to measure the mean of the direct current from the detector.

LOCA rated versions of fission chamber standard models are available on demand. These fission chambers are typically designed with a maximum operating temperature of 250°C (although some versions can operate up to 600°C) and are equipped with an integrated mineral insulated cable and a HN connector. 

Their outstanding gamma rejection capabilities allow them to be used in waste monitoring applications such as spent fuel monitoring. 

Due to the extreme operating conditions of fission chambers, Photonis also recommends the use of mineral insulated cable extensions designed specifically for the detector.

Our fission chambers stand resilient in severe conditions, operating with precision in environments where others falter. Designed for thermal neutron detection, these detectors provide invaluable indications across a remarkable 11 decades of neutron flux in gamma fields of 104 Gy/hr and temperatures as high as 600°C.

Why choose Photonis fission chambers?

  • Unmatched Precision: Accurate detection across a wide range for comprehensive reactor monitoring.
  • Resilient Performance: Operates seamlessly at temperatures up to 600°C, ensuring reliability even in advanced reactors.
  • Nuclear Quality: Our detectors are built to perform safety-related functions in the operating nuclear fleet. We manufacture to stringent quality standards, operating according to ISO9001, ISO19443, RCCE, NQA-1, and HAF604.

Essential to reactor neutron flux monitoring, our fission chambers play a pivotal role in detecting thermal neutrons, monitoring fuel burn-up, and controlling power across source, intermediate, and power ranges.

Custom coaxial cable extensions & connectors

By full control of the following processes, Photonis Nuclear Instrumentation can supply custom fission chambers for ex-core use with the highest quality and reliability levels:

  • Adaptation of industrialized fission chambers to customer specific requirements.
  • Development of new fission chambers with our experienced R&D team.
  • Long term collaboration with the French Alternative Energies and Atomic Energy Commission / CEA (theoretical approach, modeling, qualification tests in research reactors).
  • Complete manufacturing process on site.
  • Support from all the Photonis Group activities, experience and knowledge.
  • Certifications & Quality Assurance programs adapted to the customer environment.
  • Technical support, applying expertise.
  • Export control, as an Authorized Economic Operator, Photonis acts in accordance with the EU international regulations.
  • Fissile material management & product storage

Examples of custom versions from basic types

  • Neutron sensitivity tailored to individual requirement.
  • Guard-ring (2 outputs) or coaxial (1 output) structure.
  • Stainless Steel or Nickel alloy for high temperature (>250°C).
  • Adapted mechanical interface with the equipment (Customized external dimensions, springs, threads, hooks).
  • LOCA (Losse Of Coolant Accident) and post-LOCA protection.
  • Ruggedized structure for harsh environment (radiation, temperature, humidity, mechanical stress).
  • Integral cable or integral connector.
  • Coaxial or triaxial mineral insulated cable.
  • Mineral insulated connector type (Coaxial HN, BNC, BNC HT, SMA or triaxial solutions). Reinforced types are available for high pressure, high temperature conditions.
Fission chambers for ex-core use

Main features

  • Structure configuration: Coaxial or guard-ring.

  • Structure material: Aluminum for max. F32 operational temperature. 

  • Integral cable: Coaxial Mineral insulated cable for pulse and current transmission, high immunity. Integral cables are recommended for the signal and for the high voltage when operating in severe -environments.

  • Connector: HN Mineral insulated connector for pulse and current transmission. 

  • Insulators: Photonis only uses high-grade, radiation resistant alumina insulators from qualified suppliers to ensure the measurement of extremely low currents.


The French Alternative Energies and Atomic Energy Commission (CEA) selected Photonis as its technological partner for a cooperative development of neutron and gamma detectors when the French nuclear power program took shape in the sixties.


Our references
  • PWR – wide range neutron flux monitoring – ex-core fission chambers CFUG – Switzerland.
  • VVER – wide range neutron flux monitoring – ex-core fission chambers CFUL – Hungary, Slovakia, Ukraine.
  • Fuel reprocessing plant – AREVA La Hague/ORANO – numerous fission chambers – France, Japan.
  • FBR – wide range neutron flux monitoring – high temperature 600 °C fission chambers CFUC – France.

PHOTONIS Wide Range Fission Chamber