Editor's Note: This is part of IEEE Spectrum's ongoing coverage of Japan's earthquake and nuclear emergency.
Tucked behind shielding, most of the electronics in a working nuclear reactor are no more exposed to radiation than the humans that operate them. Problems like the loss of coolant in Japan’s damaged Fukushima reactors can change that, boosting radiation to levels that can threaten control systems and robots that might be sent in for repairs.
How do you protect or “harden” electronics to prevent radiation damage? And are the electronics at the Fukushima Dai-1 nuclear power plant tough enough? IEEE Spectrum Associate Editor Rachel Courtland asked Dan Fleetwood, an expert in radiation-resistant devices at Vanderbilt University in Nashville, Tennessee, to take us through the basics.
How does radiation pose a problem for electronics?
Radiation can ionize atoms and disrupt a semiconductor's crystal structure. For electronics that are very close to a reactor, neutrons will create physical damage to the semiconductor crystal. But most chips will fail first because of leakage that’s associated with the charging of insulators. In something like a metal-oxide-semiconductor device, for example, gamma rays and X-ray radiation will knock electrons off atoms in an insulator to create electron-hole pairs. The resulting trapped positive charges will shift the operating characteristics. Devices are designed to turn on and off at a well-defined point of operation, and if that operating voltage shifts, this can create difficulties.
How do you protect or “harden” electronics against radiation in a nuclear reactor?
It involves all aspects of design, process, and testing. At a power plant there are usually design criteria to keep the most basic operating and control electronics relatively simple and relatively robust so that you can have an event like a loss of coolant and maintain control of the plant.
There are special processing techniques that are used to make the insulators more resistant to the consequences of having electrons knocked out. There are ways to process insulators, for example, so that there are fewer defects, which reduces the number of sites where positive charges can be trapped, and there are ways to dope the regions between transistors to make devices more resistant to the effects of radiation.
Nuclear power plants may also have the most critical electronics shielded in enclosures made of lead or some other very dense material that can help protect them from radiation.
How good can radiation hardening get?
The annual, whole-body limit for radiation workers is usually set in the range of 20-50 millisieverts, or 2-5 rads. Most commercial electronics can survive radiation levels in silicon of at least 500 to 1000 rads. Some commercial devices can survive levels higher than that but you’re just never sure when it’s going to lose functionality unless detailed testing has been done in advance. The most radiation-hardened electronics can survive levels of radiation that are hundreds of thousands of times greater than what a human can survive, more than a million rads.
The higher the dose the less likely you will be able to find a commercial integrated circuit to handle it. Radiation-hardened electronics are typically anywhere from two to four generations behind commercial electronics in terms of their performance. It takes extra time to do the additional engineering.
Can a reactor’s electronics survive a meltdown?
Certainly in the 1960s [when the first Fukushima reactors were built], people were very aware of the risks due to radiation and there were choices of electronics that could be made that would increase the resistance to radiation by a lot.
Very basic control circuits can be made to withstand exceedingly high levels of radiation, but they’re very simple in terms of function. They’re not the kinds of electronics that you could use to run the entire plant. They would just be used to maintain the capability of being able at some point to turn cooling systems on or perform critical switching and control functions.
Is it possible that electronics in the damaged Fukushima reactors could be used to boot the cooling system back up?
That’s a question of how bad the damage is. My guess is the electronics are probably not the weak link there. The mechanical systems could be the weak link, just because of the physical damage that appears to have occurred as a result of the explosions that have been reported. It’s also possible that there could be some failures in the electronics that might recover with time. Not all the effects of radiation exposure are permanent.
Rachel Courtland, an unabashed astronomy aficionado, is a former senior associate editor at Spectrum. She now works in the editorial department at Nature. At Spectrum, she wrote about a variety of engineering efforts, including the quest for energy-producing fusion at the National Ignition Facility and the hunt for dark matter using an ultraquiet radio receiver. In 2014, she received a Neal Award for her feature on shrinking transistors and how the semiconductor industry talks about the challenge.