Monday, April 4, 2016

Five Million Miles on a Pound of Plutonium?

In the early 1950s, when I was an almost-teenager reading all the popular magazines on science and mathematics I could read at the drugstore newsstand, there were articles on "atomic propulsion" for every conceivable vehicle from motorcycles to space ships. (Well, maybe not motorcycles.) A lot of the concepts were just that: concepts. Grandiose ideas sold magazines but would probably not have done much to power your fishing boat.

Only a few years later we learned the horrors of exposure to radiation. I can remember reading - even before age thirteen - not only the heart-rending stories of A-bomb victims, but also about the unfortunate Japanese fishermen who were accidentally exposed to H-bomb test fall-out. I thought they all had died. (Although billed by the press as "Lethally Exposed," except for the one who died from acute radiation sickness, none of the other twenty-two was a cancer fatality as of twenty-five yars after exposure.) [Kumatori, T. Ishihara, T., Hirshima, K., Sugiyama, H., Ishii, S., and Miyoshi, K. Follow-up studies over a twenty-five-year period on the Japanese fishermen exposed to radioactive fallout in 1954. The Medical Basis for Radiation Preparedness, Hubner, K.F., and Fry, A.A., editors, Elsevier, New York, 1980.]

It wasn't long after this that the "atomic power" articles, and much of the interest in nuclear technology, dried up. The Linear No-Threshold hypothesis was soon to send all of those ideas into the black hole of radiation avoidance - which later became radiation hysteria. Did we miss the "nuclear vehicle" boat because of our fears and the rules imposed on the nuclear industry? "What might have been" is truly an impossible question to answer. Thousands of independently acting entrepreneurs would have answered it for us, had they been given the chance.

Neither the pressurized water reactor (PWR) nor the boiling water reactor (BWR) - the mainstays of the U.S. nuclear power industry - are adaptable to smaller scale, mobile applications. A variation of the CANDU reactor principle would come closer by using low-boiling-point compounds such as CFCs to spin a turbine, but even that is a stretch for anything smaller than a large bus. (Not to say that it couldn't be done if technology is given a chance.)

But why worry about atomic-powered anythings? What we're currently using is working out pretty well, isn't it? True, but let's look for a moment at how we fuel our vehicles. And though I don't think we are in danger of "running out of oil" any time soon, it is logical to assume that the energy cost of obtaining oil will increase as the oil-bearing strata become more and more difficult to access.

If we take your politically incorrect sports utility vehicle out on the open road with one pound of gasoline in it, the heat energy content of the fuel will propel you about three miles. By comparison the heat energy in one pound of plutonium would take you some 5,420,000 miles down the road - using the same efficiency figure as the gasoline engine. [The efficiency would be lower using today's technology. But even at half or a quarter the already-low efficiencies of internal combustion engines, the incredible heat content of many radioactive isotopes makes for unbelievable comparisons to all fossil fuels.]

Since this is about thirty times more than the typical mechanical life of a vehicle, it is likely that any nuclear-powered vehicle would be fueled for life at the factory.

All well and good, you might say. But what about the lack of adaptability of power-plant technology to mobile vehicles? Good question. There are other nuclear technologies, besides trying to shrink a power plant, that would be interesting to explore. One of these is the radioisotope thermo-electric generator or RTG.

The RTG is based on a very simple physical principle known as thermoelectricity. ["Simple" in practice, as anyone can connect different types of wires together; the theory, known as the Seebeck effect, is a little more complicated.]

If you take two wires of different materials and connect their junctions in a loop, a current will flow when the temperature of the "hot junction" is greater than that of the "cold junction." In the RTG, a radioisotope supplies the heat, while a "heat sink" - such as you might find on the rear of a high-powered stereo amplifier - cools the cold junction. Any number of junctions can be connected in series (known as thermopile) to produce whatever voltage is desired, while connections are paralleled to increase the current flow. The inherent low voltage of the device can be increased with a dc-to-dc or dc-to-ac converter. [Transformers, used on alternating current circuits, do not work for direct current. A "dc-to-dc" converter chops the dc, making it appear to be ac, transforms it to a different voltage, and then rectifies it back to dc.]

An obvious advantage of the thermo-electric generator is its total lack of moving parts - since electrons don't count.

The Manhattan project scientists had inadvertently discovered this "warmness" of the plutonium 239 isotope. [Actually all radioactive isotopes generate heat as a byproduct of decay - but both the rate and the type of decay emissions are important. You really wouldn't want to cozy up with a strong gamma ray emitter.]

Project experimenters supposedly used the plutonium received from the Hanford reservation - gleaned and refined at an almost unbelievable price - as hand warmers. While alpha particles lose their energy too quickly to penetrate the skin, these atomic "shot-puts" collide with and agitate atoms with which they come in contact, hence the feeling of warmth.

As we might expect, the rate of decay of the isotope has much to do with an isotope's heat-producing potential. While bomb-grade plutonium 239 (with its 24,110-year half-life) is okay for hand warmers, another plutonium isotope (atomic weight 238, with a half-life of only 87.7 years) is the candidate of choice for our extraterrestrial deep-space probes. Plutonium 238 can really kick atoms around, to way beyond the boiling point of water. [Spacecraft power supplies operate at approximately 800 degrees Fahrenheit. RTG temperatures can exceed 1,300 degrees Fahrenheit.]

This is not new technology. All deep-space probes must have some sort of nuclear power supply, as none of the alternatives are able to supply usable amounts of power for the years it takes to complete these missions. Batteries are out of the question for even short missions, and solar panels don't work well, since the energy available drops off as the square of the distance from the sun. A ten-by-ten-foot collector for Earth-Moon operations, for instance, would swell to tennis court size for missions to Jupiter, and blossom to the equivalent of more than two football fields for exploration of Neptune. Moreover, Earth-based solar cells are not easy to mount efficiently, even with a solid terra firma foundation. How about trying to maneuver football-field sized collector banks - structures and deployment mechanisms - in a zero-gravity environment? [Fuel cells would be find, except that the weight of the fuel - and its containers - would't allow for much else on the voyage. We might want to note, thanks to the science eduation given by the mission and movie Apollo 13, most of us are now well aware that fuel cells must carry their own oxidizer - which, in the case of oxygen, was not readily available in interlunar space.]

While the RTG has been the only practical choice for deep-space missions, anti-nuclear propagandists have portrayed it as a hazard to the entire human race because of its use of plutonium fuel. The misguided protesters wring their hands over seventy-two pounds of plutonium that they contend might somehow be released into the atmosphere and over the effect that might have on humankind. However, they totally ignore the fact that two to three tons of various vaporized (and hence, breathable) plutonium isotopes were injected into the biosphere by the Nagasaki bomb and the hundreds of above-ground tests just after World War II; yet the last time I looked, the human race was still alive and kicking.

While spacecraft have shown the reliability and longevity of the RTG, why haven't there been applications in transportation utilizing this technology? [Another very successful use of the plutonium RTG was in pacemakers. From 1973 through 1987, 155 radioisotope-powered pacemakers were implanted in a Newark Beth Israel Medical Center study. With a half-life of eighty-seven years, the nuclear devices outlasted battery operated devices - which required surgery for re-implantation - by many years and were ultra-reliable. And although "it has been shown beyond any reasonable doubt that there is no increased risk of malignancy in this group of patients" few, if any, new nuclear devices are being installed. Why? It's our good friend, the Linear No-Threshold hypothesis. See "The Nuclear Pacemaker: Is Renewed Interest Warranted?" American Journal of Cardiology, Oct. 1990.]

It certainly doesn't require a rocket scientist to conceive of an RTG automobile that would have both a generator and auxiliary batteries available for acceleration and hills - yet would recharge itself, both while driving and while sitting all day in a parking lot. But if you remember the story about the Goianians, you may have already considered the possibility of being stoned whenever you pulled your Plutoniumobile out of the garage - not to mention having to deal with swarms of bureaucrats from every imaginable protective agency who would be on the spot to make sure no alpha ray is loosed on the public. With incentives like these for the buyer, entrepreneurs are not exactly standing in line to enter this market.

Want to get 5,420,000 miles to the pound? Me too. Sorry to say that's never going to happen, because the long-standing and difficult problem of squeezing actual energy from potential energy is fraught with some inconvenient impossibilities. But if we are to approach the theoretical limits of physical science, it will take an understanding of the real dangers of radiation, and getting the government out of the policing business. Plus, no doubt, many billions of dollars in research and development costs; but that's what capitalists do: invest their money to make profits from producing things that cause our lives to be more satisfying.

Oh, and not to worry. Manufacturers of nuclear-powered vehicles are not going to fry their customers with gamma rays any more than Campbell's would put botulin toxin in the soup.

It's not good for business.