A bug was discovered on the Raspberry Pi 2, a small computer intended to promote computer science education to children. While new to the Raspberry Pi, this is old news to the nuclear power industry . . . .
The bug is this: when exposed to a camera’s flash, the Raspberry Pi 2 crashes. It shuts off. No harm comes to the device, it’s just an unexpected effect. A similar event occurred in the Connecticut Yankee nuclear plant control room in 1997.
A training representative was taking pictures of the fire detection system control panel to produce an upgraded user’s manual. He was using a Canon Powershot 600, one of the first digital cameras produced for consumers. He took a series of pictures, and what happened next is best described by the NRC‘s information notice:
The first flash caused an annunciator inside the panel to sound. The cabinet door on the panel was closed and an examination of the front panel showed no lock-in alarm indications. The cabinet door on the panel was reopened and a second flash photograph was taken within 2 minutes of the first picture. The second flash caused a second alarm with a different tone, indicating that system actuation was imminent.
Within 3 to 5 seconds of the second flash, Halon discharged from the overhead nozzles. The discharge occurred at 9:47 a.m. and lasted for 10 to 12 seconds. It was characterized by a loud roar, fog, and significant air turbulence. The discharge scattered loose papers around the control room and dislodged several ceiling tiles, support frame pieces, and lighting fixture plexiglass covers. One ceiling panel support piece broke the cover glass and bent the case fastener on a relay that provided backup line protection to a 345-kv line.
One falling ceiling tile struck an operator as he exited the control room (he was not injured).
They traced the cause to the camera’s flash. Follow-up testing “had confirmed that the light from the camera flash affected an electronic programmable read-only memory (EPROM) microprocessor located inside the Halon FDS control panel…discussions with the manufacturer indicated that a strong light source could cause an unpredictable perturbation within the EPROM, depending on the light intensity and the angle of incidence to the circuitry through the EPROM’s window.”
THE PHOTOELECTRIC EFFECT
The photoelectric effect is the term that describes when electrons are knocked loose in conductors when they are hit by light. Essentially, certain wavelengths and intensities of light, when shone on conductors, cause a small current to flow. This effect is the heart of how most modern semi-conductors work. The Raspberry Pi 2 engineers describe the problem like this:
Silicon junctions (the types that are responsible for making diodes and transistors and other such electronic miracles function) can be ‘upset’ by this photoelectric effect if it is large enough (i.e. if enough light of the right energy [i.e. colour] is fired at them). This seems to be what is happening to our power supply chip – somewhere in the complex silicon chip circuitry there are some transistors or diodes that malfunction when hit by high energy bursts of light, causing the power supply to ‘drop out’, so the Pi reboots.
Most digital cameras use a xenon flash, which releases a high-energy pulse when discharged. The flash is a tube filled with xenon gas, and when a high-voltage impulse is applied across it a light flash is released.
This broadband light emission is intense enough to activate the photoelectric effect in conductors and semi-conductors, which usually produces undesired effects (as we have seen).
The Raspberry Pi engineers recommend blocking the offending chips with a special kind of putty that will keep the bright flash from hitting it. They stress that other kinds of light have no effect: bright daylight, indoor lighting, and bright LEDs are all safe.
The nuclear power plant engineers came up with a similar solution: block the EPROM chips with tinfoil or black electrical tape (though, being that aluminum is a conductor, it seems non-conductive black electrical tape would be the wiser choice).
But, considering the multitude of uncovered electronics in a control room, the main solution these days has been simpler than that: turn off your camera flash when in the control room. Just make sure you take your pictures using a steady hand because, without the flash, any slight movement tends to produce fuzzy photos.
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