Thulium Power! The Rare Earth That’s Perfect for Deep Sea Missions

How thulium-based nuclear power could run the next generation of underwater robots

Thulium Power! The Rare Earth That’s Perfect for Deep Sea Missions Thulium Power! The Rare Earth That’s Perfect for Deep Sea Missions

Uncategorized October 1, 2013 0

Thulium metal and deep-sea coral. Wikimedia Commons photo Thulium Power! The Rare Earth That’s Perfect for Deep Sea Missions How thulium-based nuclear power could... Thulium Power! The Rare Earth That’s Perfect for Deep Sea Missions
Thulium metal and deep-sea coral. Wikimedia Commons photo

Thulium Power! The Rare Earth That’s Perfect for Deep Sea Missions

How thulium-based nuclear power could run the next generation of underwater robots

Thulium? It sounds like something straight out of Comic-Con: “Then lo, Thor brought forth his nightstick of purest thulium, power-metal of Thule, and did beat the daylights out of Loki, etc.”

If you’ve never heard of the soft, silvery metal it shouldn’t surprise you; thulium (Tm), atomic number 69 on the periodic table, is the rarest of the rare earths, a group of related elements that include cerium, europium, neodymium and other mouth-filling names. Rare earths are added to alloys and glasses to create critical qualities in magnets, sensors, lasers and other devices.

Thulium’s rarity limits its uses, but its oddity — a metal that fluoresces — make it useful in lamps (the first halide lamp was thulium-based) and in dentistry, where low radiation exposures are required. However, four scientists at the Lawrence Livermore National Laboratory once proposed an ocean future for the rare metal, by demonstrating the feasibility of thulium-based nuclear power sources for unmanned underwater vehicles.

Batteries can only take you so far at sea, as literally tons of silver-zinc batteries are needed to drive manned submersibles and big unmanned underwater vehicles. Fuel cells hold great promise, but will be relegated to auxiliary power roles for some time.

Stored electrical energy carries its own problems — the Navy lost a $400 million experimental sub when its lithium-ion battery system caught fire. Thus the big push for glider and waverider technologies that harvest energy from buoyancy and waves. But nuclear power is needed for subs that swim deep and stay down long.

During the Cold War, the United States powered undersea eavesdropping devices with radioisotope thermo-electric generators. Originally developed for spacecraft, these compact “nuclear batteries” fueled by plutonium-238 can produce power for astoundingly long times. The ones aboard the Voyager spacecraft have been running since disco was cool. But there’s precious little Pl-238 to go around these days, and plutonium’s obnoxious qualities encourage a search for alternatives.

Thulium isotope power (TIP) design from “Thulium Heat Source For High-Endurance and High-Energy Density Power Systems”

Undersea power

Unlike earlier power-module concepts designed to propel full-size warships, the proposed thulium powerplants were scaled for the smaller sizes and lesser demands of unmanned vehicles. One “hot” thulium isotope is especially attractive. Tm-170 has a half-life of four months and decays into stable ytterbium, which can be then be disposed of as Class-B radioactive waste, like some radiopharmaceuticals widely used in medical scans.

Working off research conducted at the Savannah River Laboratory, the Livermore team selected thulia — one of the most stable oxides known to humanity — for the fuel elements. The pale-green powder would be pressed and sintered into wafer-thin disks, then bathed in neutrons in a reactor core for over a month. After nuclear alchemy transformed Tm-169 atoms into Tm-170 ones, the disks would be assembled into stacks interleaved with graphite, pierced by heat pipes and sealed inside a helium-filled casing with a regulator and heat exchanger.

The entire assembly, including the nuclear heat source, shielding and power generation system, would be remarkably powerful and compact: the larger unit studied would produce 12 kilowatts for four months in a package less than four feet long and weighing less than a ton. Sales manager Pedram Pebdani of Deep Sea Power & Light was impressed with the specifications. Pebdani tells War is Boring:

“Twelve kilowatts for four months is just inside the envelope of what’s currently possible. You can achieve that kind power level and endurance today using current battery technologies and lots of batteries — I mean lots of batteries. Low-power motors and electronics make a big difference, and so do things like solar buoys which can recharge the batteries from the surface. But you’d have to really work for it and it would cost a lot of money.”

Demand for undersea power is increasing dramatically with the growth of seafloor energy production, submarine cables and military activity. Siemens now offers industrial-scale power-grids for seafloor oil and gas fields three miles below the ocean’s surface, while the Navy has identified undersea power as a critical strategic requirement. Projects like DARPA’s Hydra would seem to be excellent candidates for thulium generators.

The greatest obstacle to thulium power likely won’t be its scarcity (as rare as gold or silver), or even its geopolitical angle (thulium is mined commercially only from China’s rich bastnasite deposits, thus grouping it with other strategic rare earths). As pressures on the world’s ocean grow ever greater and nuclear stakeholders continue to perform poorly, the global public’s willingness to tolerate anything atomic in the sea may erode completely.

That would be a shame, if it meant passing up a small gift from the gods.