Thermo-acoustic nuclear fuel rods

It’s been more than three years since the Fukushima Daiichi power plant melted down, and despite nearly unlimited interest from the public we still have virtually no idea what’s going on in there. Detailed findings report the conditions immediately around the plant, beneath it in the soil, and above it in the atmosphere — but the core of the nuclear power station is so heavily shielded that its status remains largely unknown to this day. Experimental rad-shielded robots inch a bit further in every month, but their progress is slow. Scientists are desperate enough that they’re trying to make use of passing cosmic rays, which are occasionally powerful enough to pass through the core and ferry out some precious intel. But why are we just figuring this out now?
A new technology developed by academics and the Westinghouse nuclear company could keep this from being a problem in the future. When temperatures or pressures start to fluctuate, their new thermo-acoustic devices emit a corresponding auditory frequency that can be interpreted in real-time — it naturally whistles its status, in other words. Rather than use some complex monitoring rig that would fail in the intense environment of a working nuclear reactor, these thermo-acoustic sensors are based on passive physical forces. Changes in temperature, pressure, or even radiation dosage around the device cause natural shifts in resonant frequency — and thus, the tone of the whistle.

A simple diagram of the thermo-acoustic sensor.

By necessity, the design is as simple as can be. A resonator (long hollow rod) abuts a series of small parallel chambers called the stack. Temperature or pressure differentials across the stack, or changes in the rod’s physical shape due to intense radiation, produce predictable changes in the frequency of resonance inside the resonator — that frequency is our output information. This thermo-acoustic nuclear sensor uses 1100 parallel chambers made of a durable ceramic, and can be made small enough to fit virtually anywhere within the reactor core. Most interestingly, the team suggests that their monitors could be built into nuclear fuel rods themselves, turning fuel containers into sensors that intrinsically report changes without needing any outside power or supervision.
These simple devices would only be able to monitor one attribute of the core in one location, so an array of specialized thermo-acoustic sensors would be placed throughout the core to monitor different variables. Resonators of specifically tailored lengths and designs would produce a multi-voice chorus in a reactor, providing nuanced, real-time information with no need for energy input. If there’s any justice in the world, these scientists will at least try to tailor any “meltdown” frequencies to sound ominous and panicked, or perhaps like the monolith from 2001.

Stored nuclear fuel rods glow an eery, distinctive blue.

Regardless, if Fukushima had sported these devices from the start, we would almost certainly know much more about the state of its core today. The preference in nuclear engineering is now for these sorts of “passive” safety measures which rely on relatively fool-proof principles like thermodynamics. Ideas like “freeze plugs,” which require active cooling of a stopper which otherwise melts and totally drains the reactor due to gravity alone, represent the kind of fool-proof design we demand of nuclear technology these days.
While we of course want to keep watch for any cascading “meltdown” reactions, Fukushima’s nightmare scenario showed just how bad things need to get to foul up a modern reactor; the team sees their sensors as ultimately more useful to fundamental nuclear research. If a good portion of nuclear reactors were providing detailed records to some giant national database, analysts could probably derive useful suggestions for safety or efficiency upgrades.
This sort of functionality has been tantalizingly close for a while, being technically possible but infeasible in the real world due to costs and the high rate of equipment destruction. With simple designs like this, nuclear companies like Westinghouse could finally be able to look inside the fuel rods in an average nuclear plant directly and in real time. That’s the sort of upgrade you install quietly, hoping nobody notices that it wasn’t there all along.
source:  http://www.extremetech.com