New Japanese WiFi can withstand nuclear radiation levels

Researchers have made a WiFi receiver that’s tough enough to work inside a nuclear reactor. They hope the receiver might be part of a wireless communications system for robotics used to decommission reactors.

Yasuto Narukiyo, a graduate student at the Institute of Science Tokyo, presented the wireless receiver at the IEEE International Solid-State Circuits Conference (ISSCC), in San Francisco in March. The receiver endured a total radiation dose of 500 kilograys, orders of magnitude higher than the doses typically tolerated by electronics in outer space.

After the 2011 nuclear disaster at the Fukushima Daiichi plant, engineers began using robots to help characterise and clean up the site. Most of these require local area network (LAN) cables that can get tangled, says Narukiyo. His team, which includes his advisor Atsushi Shirane and Masaya Miyahara of Japan’s High Energy Accelerator Research Organisation (KEK), is aiming to develop a wireless system for controlling robots in this harsh environment.

Even under less dramatic circumstances, nuclear plants don’t last forever, and they need to be safely dismantled and decontaminated so the sites can be reused, a process called decommissioning. The process is lengthy, and risks exposing people to radiation, which is why engineers hope robots can come to the rescue.

The need for such robots is only growing. According to a 2024 study, of 204 reactors that have been closed, only 11 plants with a capacity over 100 megawatts have been fully decommissioned, and 200 more reactors will reach the end of their lifetimes in the next 20 years.

While electronics for space exploration are typically required to endure radiation doses of 100 to 300 grays over three years, a robot operating in a nuclear reactor needs to endure more than 500 kGy over the course of six months, says Narukiyo - at least 1,000 times the dosage. A robotic arm made by KUKA was able to withstand just 164.55 Gy of damage before failing. For comparison, the lens of the eye absorbs just 60 milligrays during a CT scan of the brain.

To “harden” the 2.4-gigahertz WiFi receiver against intense levels of radiation, Narukiyo and his team changed its mix of components, minimised the total number of transistors, and tinkered with the geometry of the transistors that were left.

The transistors, silicon MOSFETs (metal-oxide semiconductor field-effect transistors), contain an oxide layer that’s particularly vulnerable to radiation damage. Blasts of gamma rays can trap positive charges in the oxide, degrading the device’s performance and causing errors. They also changed the design of the transistors themselves. The device’s gate controls the flow of current through the transistor. The smaller it is, the more its performance will be degraded by a dose of radiation. So they made the gates longer and wider.

Secondly, they considered the differences in how radiation affects PMOS transistors, in which current is carried primarily by positive charges, and NMOS, where electrons flow. PMOS transistors are more vulnerable to radiation damage because positive charge gets trapped in both the oxide and at the interface between the oxide and the rest of the semiconductor. These add up and shift the transistor towards the off state, says Narukiyo.

To compensate, the new receiver design minimises the use of PMOS, replacing these transistors with other elements such as inductors that don’t have an oxide layer. NMOS transistors are more resilient, says Narukiyo, because positive charges trapped in the oxide are to some extent cancelled out by negative charges that get trapped at the interface.

Narukiyo and his team measured the performance of the receiver before exposure to radiation, and again after blasting it with a total dose of 300 kGy and then 500 kGy. Before being irradiated, it showed comparable performance to typical Wi-Fi receivers. After reaching the highest radiation dose, the gain of the receiver had decreased by about 1.5 decibel.

Narukiyo says the receiver is hardened enough, and now he hopes to improve its performance. He’s also working on a transmitter, which would allow for two-way communications. This is more challenging due to the need to produce high levels of current to generate the WiFi signal. He says an earlier version he tried was broken by a 300 kGy dose. The group is exploring using other semiconductors, such as diamond, to toughen the transmitter.

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