David Weber, owner and founder of Executive Engineering, is developing a technology, the nuclear accelerated generator (NAG), which has been considered for powering miniature autonomous security devices for an undisclosed application. The main breakthrough represented by the technology is the direct conversion of nuclear energy into low-power electrical energy. This enables the technology to extract useful power from extremely small quantities of obtainable radioactive isotopes, including those in regular production by the U.S. Department of Energy.

In general, NAG technology is intended for use with isotopes that emit beta-minus radiation (electrons), which is crucial for the process by which the technology converts nuclear energy into electricity. Isotopes that are theoretically compatible with the technology include strontium-90 (Sr-90), nickle-63 (Ni-63), and promethium-147 (Pm-147). Among these materials, the latter is the most desirable. One reason for this, Weber stated, is that it decays into a stable element (inert promethium) within 30 years.

While Weber is not at liberty to discuss details about the design of systems using NAG technology, or the technology's intended applications, he stated that it is different from less-efficient technologies based on mere charge separation, a phenomenon inherent in the emission of beta-minus radiation. One simple (and mature) way to harness this naturally occurring charge separation is to directly capture the electrons emitted from a radioactive material.

However, Weber stated that NAG technology uses a different energy-recovery mechanism that is more efficient. Specifically, this mechanism has the potential to extract between 60% to 85% of the available energy from the electrons ejected from a beta-radiation-emitting isotope in a large-scale NAG system.

Furthermore, if 80% efficiency can be retained as NAG systems are scaled down to the level of MEMS structures, Weber calculated that a 0.2 mg sample of Pm-147 would enable 0.25 W MEMS devices (with power falling as a function of the half life of the sample, 2.62 years). In fact, some of the initial experimental research into the technology was performed on small scales. For example, in a functional prototype of the device, Weber generated measurable power from an extremely small sample of Sr-90 that was supplied in a cloud-chamber needle (see figure).

Weber stated he had filed an application for a patent for NAG technology that is being reviewed by the U.S. Patent and Trademark Office. He had also confirmed that it had been a potential candidate for DARPA funding on two different occasions. While both of these proposals have been declined in favor of less-costly bids, NAG technology is being reviewed by other government agencies.

Furthermore, according to Weber, one DARPA program remains in need of a power system that has a volumetric-power-density goal of 35 mW/cm3, and Weber believes NAG technology has the capability to exceed this goal by up to a factor of 10. This provides the opportunity to revise and resubmit previous proposals in response to future rounds of funding issued by DARPA to meet this challenge.

Michael Huff, founder and director of the DARPA-supported MEMS Exchange program, was one of the first technologists introduced to NAG technology. Huff stated that while the data for larger NAG prototypes looks encouraging, it is unknown how the efficiency of this technology will scale with size down to the MEMS (i.e., micro) level.

He also stated that based on the experimental data he reviewed, it is more likely that the high projected efficiencies of NAG technology will be retained as the physical scale of the systems is increased (though he re-emphasized high efficiencies may still be achievable for small-scale systems).

Huff added that if full working versions of the technology can be demonstrated, that success would facilitate the use of NAG systems with certain inventory-constrained or controlled isotopes. Some of those isotopes could provide higher performance levels than what would be possible using readily available isotopes.

If NAG technology is proven and successfully applied to industrial applications currently served by long-life primary batteries, it could become as ubiquitous as the nuclear technology currently deployed in modern smoke detectors. Ironically, according to Weber, this is one application for which the technology would probably not be well suited.

For more information about NAG technology, visit Executive Engineering's web page on the topic, www.exec-eng.com/eenag.