Eyes inside: radiation-tolerant CCTV

?CCTV, closed-circuit TV, video cameras can be used in nuclear power plants to inspect critical equipment, to observe outage activities, and also for workplace familiarization for training purposes.

Generally there are no regulatory requirements regarding the tolerance of CCTV cameras used within a nuclear power plant, however cameras installed in the containment typically have to be seismically qualified (to avoid the potential damage that falling cameras could cause to equipment during an earthquake). It is up to the customer to specify the camera requirements; they would usually select a combination of cameras suited to different applications; some with high, medium and low radiation tolerance. That decision depends on factors such as maximum (lifetime) total dose, maximum (dynamic) dose rate, service interval and cost of service (replacement parts and labour). This article focuses on technology options for high radiation environments.

?Uses
PWR containment
BWR reactor building, drywell, and wetwell
HP/RP oversight of critical outage evolutions
Reprocessing and hot cell activities
Cask loading activities
Early detection of critical equipment malfunction to mitigate reactor trips

There are two principal types of radiation tolerant CCTV technology: the older-style CRT (cathode ray tube) cameras or digital technology such as CCD (charge-coupled device) and CMOS (complementary metal-oxide semiconductor) devices that are made using a semiconductor target instead of a vacuum tube. In general, tube-type cameras are more radiation-tolerant and require less maintenance than CCD technology. However, tube cameras may have higher maintenance costs and images could suffer burn-in sensitivity, lag, and over exposure.

Tube-type cameras include the R981 supplied by Mirion Technologies and the Diakont D70. Both of these designs, housed in a stainless steel casing, have a radiation tolerance of 1 MGy or 108 Rad and are able to operate in ambient temperatures of up to 55°C in air.

The D70 (resolution 600+ HTV lines), which has been supplied to about three dozen plants, mainly in Russia and Eastern Europe, is also available with active cooling (fans that dissipate heat away from the unit), for operation in temperatures up to 100°C, as would be needed under the reactor core. The black-and-white D70 uses a 2/3" E-Chalnicon tube (Chalnicon is one brand of tube) and can work at up to 3x105 Rad/hour and at a minimum illumination of 16 lux. The camera is equipped with an x20 optical zoom and has a field of view ranging from 2.25° to 45°. It is protected against complete and continuous submersion in water (IP68) up to a depth of 40 m.

Mirion Technologies' offering, the R981, can operate at a slightly lower maximum dose rate of up to 1kGy/hour (105 Rad/hour) with a Chalnicon tube, or at up to 30kGy/hour (3x106 Rad/hour) with a Vidicon tube. It has a 6x optical zoom, with ±180° pan and tilt capability and a maximum travelling speed of 6° per second. The IP66 version of the 13 kg R981 is "dust tight" and protected against powerful jets of water. The camera horizontal resolution is 550 HTV lines.
The camera's main control system could be some kilometres away from the cameras. It is common for the cameras to be connected to main equipment racks locally via suitable cabling, which routes through the containment wall using special electrical penetrations. Camera control protocols tend to be compatible with industry standards. Both Mirion R981 and the Diakont D70 products are offered as part of a turnkey solution with control systems.

Rolls-Royce has supplied two radiation hardened black and white video camera systems to nuclear power plants. Both cameras have a radiation tolerance of 1x106 Rad/hr, and total integrated dose of 1x107 Rad. They are also both based on cathode-ray tube technology. Both image sensor tubes operate at a maximum ambient temperature of 120°C and have an operational life of approximately two years. For both, Rolls-Royce carried out design and engineering, mated camera head to a non-browning zoom lens and installed the completed assembly, including a pan & tilt assembly. The first system monitors processes in high-rad areas. The system consists of the camera head which is installed in place, and transmits feed to a separate processing unit located outside of the radiation area. The second system is crane-mounted, and monitors fuel movement in a high-radiation area.

Swedish company Ahlberg Electronics also supplies various cameras for use in containment. Its N82Z high-resolution (720 TV lines in S-Video mode) CCD camera has been on the market for around five years. The IP65-rated N82Z is equipped with x10 optical zoom and x10 digital zoom (x100 total), and has a viewing angle of 5° - 50°. The N82Z has an overall radiation tolerance of 106 Rad. It operates at a maximum does rate of 105 Rad/hour, a maximum temperature of 60°C and at 10 lux minimum illumination. The stainless steel housed camera, when combined with an optional H250DC unit with full 360° pan and tilt capability, weighs around 20 kg.

The lifetime of camera module for the N82Z or closely-related N129Z, which comes with active cooling, depends on the reactor design and dose rate. Modules typically need to be replaced every three to five years, according to Peter Legarth, sales manager at Ahlberg Electronics. However, as the electronics and optics are integrated into the camera module no adjustments are required during module replacement, which takes around 15 minutes.

Legarth says that Ahlberg is selling the N82 to new Chinese reactors, including AP1000s, and to Eastern Europe (Czech Republic, Slovakia) for upgrades. Both Ahlberg products are compatible with Pelco or Grundig (P850, P848) communication protocols.

Finally, Korean firm Nucron Co Ltd has also developed a radiation-tolerant 3CMOS colour camera, the KR17090ZWC (105 Rad/hour; 107 Rad overall, 6x optical zoom) in collaboration with Korea Hydro & Nuclear Power.