U.S. Army researchers are developing batteries powered by radioisotopes that could last for decades, or longer. The long-lived power sources could lighten the logistics load on the battlefield and energize sensors and communications nodes for extended periods, offering enhanced situational awareness and opening up operational options for warfighters that do not exist today.
Scientists at the U.S. Army Research Laboratory (ARL), Adelphi, Maryland, are developing prototype batteries powered by tritium, a radioisotope produced in nuclear reactors. “The benefits of this different approach are that the replacement of chemical batteries that we have to go through now would be much reduced. Reducing the logistics is something that certainly helps save lives and allows us to focus the energy on the battlefield toward the main goal and not toward resupply,” says Marc Litz, an electronics engineer with the ARL’s Power and Energy Division. The ARL is part of the Army’s Research, Development and Engineering Command. The laboratory engages in high-risk, high-reward basic science.
The long-lived power offers another benefit as well. “Isotope power sources last so much longer that we actually create different kinds of operational needs that didn’t exist before. In particular, battlefield awareness is always an important aspect, knowing what’s happening around us. Having persistent sensing available for better decision making is always important. If we’re able to put sensors out there that last for a long lifetime, this is a big step forward for battlefield awareness and long-term situational awareness,” Litz asserts.
The new batteries have a 13-year half-life. “By definition the half-life is the period of time after which you’ve used half the power of the battery. The other half still remains. So, truthfully, the lifetime of these batteries is much longer,” Litz explains. “If you plan your missions correctly, after the first 13 years, you’ll have half the power, and then after another 13 years, you’ll still have 25 percent of the power still remaining.”
The ARL’s Alternative Energy team is testing the batteries specifically for use with sensors that could be left unattended on the battlefield much longer than sensors powered from conventional chemical batteries. “Let’s say you need to send out a radio signal every day, and you plan the amount of power that you need, and for the first 13 years you’ll be able to do that. For the next 13 years, the sensor will still operate, but you’ll need to send that signal out every other day instead. It is still functional and still providing a capability, and if your operational need can take advantage of that, you’re well-suited.”
Today’s prototypes use tritium, but researchers intend eventually to use other isotopes capable of lasting much longer. “Even if you don’t use the power that comes out of a chemical battery, the chemicals degrade after 10 years, so you no longer have access to that power. Now you have a decaying isotope that can last from 100 years for nickel63 to 432 years for americium,” Litz explains.
An isotope is one of two or more atoms that have the same number of protons but a different number of neutrons. Radioisotopes are radioactive. “The isotopes are merely the source of electrons or beta particles, and these beta particles hit a semiconductor or a crystal, and you have a source of energy,” Litz explains. To convert that energy into an electrical current, wide bandgap semiconductors create electron-hole pairs, which create a trickle charge of electrical current. Electron-hole pairs, he adds, are widely used. “This is common in a semiconductor. When we create these semiconductors, we artificially create them to have electrons on one side and holes or ions on the other side. They’re artificially created that way, and then in the context of usage, you can combine them and create electrical currents.”
Researchers chose tritium in large part because it is more readily available and considered safe. Tritium already is used in a number of common components, including exit signs in public buildings and aircraft, compasses and gun sights. Despite the radioactive nature of isotopes, Litz affirms they are safe in low doses. Researchers compare the radiation level to that of X-rays. The number of rules and regulations involved in shipping and handling radioactive materials made tritium an easy choice. “The reason we started with tritium is not necessarily a scientific reason, but it gets to that whole safety issue or the appearance of safety. Tritium is already a commodity in the Army and in our supply system,” he asserts.
The prototypes developed at the Army lab already use levels safe for soldiers to handle, but Litz predicts safety will continue to improve. “The safety can be increased by our better understanding how to match the isotope with the best energy converter so that we can make the device more efficient; and when we do that, we also reduce the amount of isotopes required, so that goes into the whole safety of the device,” he says.
Within the next year the research team intends to begin field testing the prototypes. “This process of getting the science into the hands of soldiers will first occur from direct requests for small field tests that have come down to us from other agencies. It will first start to happen in smaller locations where we will be able to transport these isotope batteries to other installations—Fort Bragg for instance,” Litz reveals.
While other services have shown interest, Litz makes the case that the Army has the greatest need. “I have to say the Army’s requirement or great demand for unattended ground sensors has been our focus, and they do have the biggest demand for this type of activity. The Army wants to put these sensors in many places around the world where they may only get there once, and they would love to have a source that could last for long periods of time,” he says.
The technology could be available within five years if the demand is strong enough to result in funding. “If it were just a matter of the push from the laboratory, I would say that certainly within five years these could be fielded. But there has to be a pull from the customer as well. And, I would be the first to say that while the customer wants this, time and money are always coupled,” he says.
He adds that the safety issue could raise doubts. “Every time we use the word ‘nuclear’ in a sentence, many people start to ask questions two or three times. We have a clear path for generating the materiel and handling the materiel, but it’s one of those unknowns,” he concedes. “From my point of view, in five years, devices could be available for enhanced battlefield awareness.”
The technology also could offer commercial benefits. “You could have a power source that you could bury inside of a bridge or a building, and now you have a sensor that is constantly available to check the stability of the materials, whether it’s the iron or the concrete. You have a sensor that can last for the infrastructure’s lifetime,” Litz says.
Additionally, it could be used for pacemakers, an area of study that was first begun decades ago. “The idea for isotope power sources has really been around since the 1950s when it was first investigated for the purpose of pacemakers,” Litz says, explaining that the idea was to reduce the number of times surgeons would have to replace the batteries in pacemakers. “Since then, new materials have surfaced, like new semiconductors—in particular, new wide-band gap semiconductors. Because of that, the efficiency of these devices looks like it could be much better than they ever would have imagined back in the 1950s.”
The current prototype is modeled after the military’s BA-5590 battery, which Litz describes as a 2-inch by 4-inch by 3-inch brick. It even uses the same connector as the current chemical battery. “We have now come to a prototype that looks like the standard BA-5590 battery that the soldier is familiar with and is comfortable using. So, we’re matching it to the common tools that the soldier needs,” he says.
Because of the nature of isotopes, however, future prototypes could be made much smaller and lighter. One feature of the isotope power source is that it offers five orders of magnitude greater energy density than chemical batteries because the forces that hold neutrons and photons together in the nucleus are so much stronger than the forces that bind molecules together in their chemical counterparts.
Litz envisions a world in which soldiers will tie together a wide array of energy sources to meet their needs. “As we move on in the future, we’re going to have a combination of solar energy, of isotope power sources, of small little generators, of wind, of water. Wherever they are, they’re going to have to learn to use the energy around them so we don’t have to send as much energy out. You want to be able to not only take advantage of those sources, but be able to combine them in such a way that the soldier can use these to his best advantage,” he states.