I have been asked on a number of occasions whether there will ever be a nuclear powered car. The idea of powering a car by nuclear fission certainly seems to have some obvious appeal. The energy density of the fuel for a nuclear reactor is so great that it would be possible to build a car that never needed to be refueled, having a reactor core capable of long outlasting the useful lifespan of the car itself. The average life expectancy of a car is about 11 years and many accumulate 200,000 miles or more. However, a nuclear reactor could, in principle, power a car for decades. Thus, only those who kept their car well past the point of it being a classic would ever need to be concerned about running low on fuel.
Despite the appeal of never needing to buy fuel or worry about mileage, there are a number of reasons why a small nuclear reactor just does not work well as the power plant for a road vehicle. In principle, a nuclear reactor could be made small enough, but doing so push the inherent limitations of fission and result in an exceptionally expensive and problematic vehicle.
Of course, in a sense, all vehicles are really nuclear powered. Fossil fuels contain energy that is derived from ancient plant material, thus being an indirect way of using solar energy, which, of course, was produced by a nuclear reaction. Electric cars also use nuclear energy, whether or not they are connected to a nuclear-powered grid, since all energy on earth had to come from a nuclear reaction at some point.
Powering vehicles with nuclear fission is thus entirely possible if the fission reactors are used to provide grid electricity for charging electric car batteries or if the energy from the reactors produces synthetic fuel or even hydrogen. This is a much more economical and realistic means of having the energy for transportation generated by nuclear power than having everyone drive around with a reactor under their hood.
Of course there are other types of nuclear energy aside from fission. Unfortunately, none would actually be likely to work for an automobile.
Nuclear reactors are a really great way of producing energy for static applications and for ships and submarines. However, for automobiles, they turn out to be a lot more trouble than they are worth. In 1958, Ford did create an illustration of what a nuclear-powered car would look like, dubbing it the Nucleon, but the Nucleon was more of a publicity piece than a viable transportation concept. No land vehicle propelled by a nuclear reactor has ever been built. The Soviet Union designed several small, transportable nuclear power plants, and the US Army even built a prototype of a road-transportable reactor, but these were not propelled by the nuclear reactors.
For one thing, the regulatory hurdles to getting a nuclear reactor into an automobile are likely to be insurmountable. All nuclear power reactors must conform to strict safety and security guidelines. In the case of an automobile, it is probably not a bad thing that safety regulations would prevent it from taking to the road. The core of an operating nuclear reactor does indeed become very radioactive, and if a road accident lead to a containment breach, the decay of short-lived isotopes could produce lethal levels of radiation exposure to the cars occupants.
Because of the radiation produced by an operating reactors core, all reactors that operate near people must also have substantial shielding. For static applications, this really is not a problem. Just having a large amount of water around the reactor provides ample shielding from both gamma rays and neutrons. Concrete also works well, and in circumstances where a lesser volume is desired, dense metal like lead can be used. This is also no problem for ships and submarines, which need ballast anyway. However, to build a truly portable reactor, as much be in a car, the amount of shielding required presents a real problem. At a minimum, the reactor core would need to be surrounded by a combination of heavy materials to block gamma rays and some kind of neutron shield, which might be composed of large amounts of water or an organic material like polyethylene.
The US ran into the problem of portable shielding when trying to develop nuclear-powered aircraft in the 1950′s. As part of the program to develop a nuclear powered aircraft, a B-36 bomber was modified to carry and operate a nuclear reactor while in flight. Even by modern standards, the B-36 is a massive aircraft. It had the capacity to carry a payload of well over 30 metric tons. Yet even despite this huge capacity, the sheer weight of sheidling needed to maintain minimum crew safety proved a problem. In order to make flights possible at all, designers eventually decided to forgo most of the shielding on the reactor itself and instead shield the crew compartment.
The image to the left shows the crew compartment of the NB-36H. It was surrounded by multiple layers of lead and weighed no less than 11 tonnes. In addition to the shielding around the cabin, large tanks of water and boron were placed in the front and middle bombays to shield against neutrons and gamma radiation. The reactor was placed in the rear bombay, as far back from the crew as possible. Even the massive B-36 struggled to get off the ground with so much additional weight.
All of this was necessary for a relatively small nuclear reactor. The reactor flown on the NB-36H only produced one megawatt of power. This is only a few times larger than the output that would be required to power an automobile.
Providing adequate shielding for an automobile would be no easier. It would not be possible to place the reactor a significant distance away from passengers and shielding the passenger compartment, but not the reactor, would not be an option. Additionally, it would have to be robust enough to survive a highway accident or other mishaps.
Another problem with putting a nuclear reactor in a car is the sheer size of the reactor. There is a limit to how small a reactor can be built, because it must have enough nuclear material to achieve critical mass. Exactly how much is required depends on the type of fuel and how enriched it is. For most reactors, a minimum of several tons of fuel is required to achieve criticality. In large power reactors, a typical core may be hundreds of tons. Reactor size can be reduced, quite dramatically, by using extremely highly enriched fuel, but in addition to such fuel being extremely expensive, it would mean weapons grade material being used in every car.
There are any number of other problems that a nuclear-powered automobile would present. Nuclear reactors require cooling for some time after they are shut down, due to decay heat. Providing adequate cooling could be a major problem if the car is kept in a closed garage. If the reactor were powered up and shut down as often as car engines are, it could become unstable or be susceptible to xenon poisoning. Other concerns range from maintenance to decommissioning and disposal, and if nothing else, it would be extremely expensive.
Radioisotope thermal generators:
In principle, if you wanted to build a nuclear -powered car, this just might be the way to do it. In fact, one does exist, but unfortunately, it does not carry passengers and it’s busy driving around mars. Indeed, radioisotope thermal generators have proven themselves to be highly reliable and rugged power sources for spacecraft and have been used in other applications where remote power is required for long durations.
In an RTG heat is produced not by nuclear fission, but by the decay of a radioisotope. Because of this, only very high energy particle emitters are suitable for RTG’s, as their decay produces a significant amount of heat. Most RTG’s used for space applications use plutonium-238, but a variety of other isotopes, including strontium-90 and polononium-210 have been used successfully.
RTG’s are inherently simple and therefore reliable. Isotopes can be chosen which decay primarily by particle emission and produce only weak gamma emissions. This dramatically reduces the need for shielding. Since the RTG does not need things like control rods or a moderator, it can be simpler and smaller. Also, because it does not need reach critical mass, it can be as small as desired.
However, RTG’s do have a number of drawbacks, which would make them unsuitable for automotive use. One of the biggest is that they cannot be turned off and their power level cannot be decreased or increased at will. This presents a number of problems. For one, it means that the RTG requires constant cooling that is not diminished when the vehicle is turned off. The power output of the RTG must be high enough to power the vehicle during times of high power consumption, meaning that excess power is produced most of the time. This could be reduced by adding a storage battery, allowing for additional power during acceleration or other times it may be needed, but it would not solve the issue entirely. Even with a storage battery, the RTG would need to produce enough power to keep the battery from running down during extended periods of driving.
In order to produce enough power, a very large amount of radioisotope would be required. Exactly how much depends on the energy of the isotope’s particle emissions and its half-life. Isotopes with short half-lives have the advantage of producing more power per gram m than those with long half-lives. For example, polonium-120, which has a half-life of only 138 days produces 140 watts per gram. Plutonium-238 has a half-life of more than 87 years and requires more than a kilogram to produce the same amount of power. However, while the plutonium-238 would easily power a car for years without any noticeable reduction in power output, while the polonium-210 would produce noticeably less power after a month and would need to be replaced at least every few months to a year.
Most RTG’s have used thermoelectric generators to convert the heat from radioactive decay into electricity. However, these have a very low efficiency, typically well under 15%. It’s possible that this could be replaced with a thermal engine, such as a sterling engine, and thus achieve efficiencies of upwards of 30%. However, in either case, a substantial amount of fuel would be required.
The cost alone of these radioisotopes makes the idea impractical. Any automobile powered by an RTG would need an extremely large isotope source costing many millions of dollars or possibly even hundreds of millions of dollars. Even if production of radioisotopes were increased dramatically, the costs are nowhere near practical for a car.
Aside from that, there are other problems. Such a massive amount of radioactive material is inherently dangerous and highly radiotoxic. In an accident or other mishap, if the isotope were ejected, it would be red hot. Since decay cannot be “turned off” there is no way to avoid this. Repairs and servicing would also be complicated by this issue. Inadequate cooling or blocking of radiators could cause overheating, meltdowns or a fire.
Non-thermal nuclear batteries:
There are methods that exist for converting ionizing radiation to electricity without the need to first convert it to heat. As such, much greater efficiencies are at least theoretically possible. Most such systems are currently only experimental or have only been built in relatively small sizes.
Betavoltaic batteries can convert beta emissions directly to electricity, but they tend to be of limited efficiency and have suffered problems with degradation of the semiconductor material used to produce the usable voltage. Because of this, relatively low energy beta emitters such as tritium are preferred. However, these isotopes produce less energy per gram. Thus far, betavoltaics have seen limited applications.
Optoelectric nuclear batteries use a radioactive source to produce light by irradiating a phosphorescent material which then illuminates a photovoltaic cell. This can reduce the problems of materials degradation, but again, it has limitations, due to the efficiency of photovoltaic the limited amount of light that can be produced by a given amount of most phosphorescent material. In most designs, this limits practical applications to low power devices. A very high power version was developed at the Kurchatov Institute in Russia, but it requires a very large amount of highly radioactive gas in a pressurized vessel.
Other types of energy converters have been experimented with over the years. Self-charging capacitors have shown some promise as a way of converting particle radiation into usable voltage. Several years ago, nuclear chemist Paul Brown developed a method for converting particle emissions into electrical current by decelerating the particles within the magnetic field of a resonator. The device showed extremely high efficiency and test versions were able to develop very high current output for a short period of time. Unfortunately, the system suffered from materials degradation and instability that proved insurmountable and prototypes were only able to function for a very short period of time.
Assuming a non-thermal energy converter of sufficient size to power a car could be produced, it might reduce some of the problems associated with an RTG, but it would not eliminate them. Even if the amount of radioactive material could be cut by 75%, it would still constitute an enormous, expensive and potentially dangerous. With current and foreseeable systems, extremely large stacks of converters would be needed. Cooling would likely remain an issue, even if a lesser one.
Aside from the hydrogen bomb, there are currently no methods of producing artificial nuclear fusion that have been able to produce a greater amount of energy than they consume. There are ongoing efforts to develop fusion power, but most of these are nowhere near the point of commercial deployment and have numerous technical hurdles to overcome.
It is possible to build a fusion apparatus that is small enough to be easily accommodated in an automobile. For example, electrostatic-confinement fusion systems like the Farnsworth Fusor can fit on a desktop. However, such devices are inherently limited to relatively small amounts of fusion and cannot produce anywhere near a surplus of energy.
Most current fusion research focuses around fusion reactors like the tokamak or inertial confinement fusion. A cornerstone of these concepts is that they may be able to generate more energy than they consume if they are made large enough. Hence, they would never be able to be scaled down to a size for a car. One novel approach, known as polywell fusion, may work at smaller sizes, but would still have to be much too large for a car in order to produce energy. (and once again, it needs to be stressed that none of these have actually proven a viable energy source)
Even if a fusion reactor could usable energy, there are other problems besides the size of the reactor. Fusion reactors tend to be very complex and expensive, requiring extensive cooling, special materials and numerous subsystems. Fusion also produces intense radiation. Most forms of nuclear fusion produce large amounts of high energy neutrons, which would require substantial shielding. High energy collisions also produce x-ray radiation, also requiring shielding.
Thus, even if some breakthrough made fusion energy possible, it is unlikely to every be suitable to automobiles.
Induced Gamma Emissions:
The concept of induced gamma-ray emissions as a means of energy storage is currently an area of great controversy in the scientific community. If it is possible at all, this represents a branch of nuclear energy that is far different than fission, fusion or decay. It is based on the concept of inducing the release of energy from a nuclear isomer through stimulation by external radiation, such as x-rays.
Nuclear isomers are composed of atoms whose nuclei have entered a highly energized state. In most cases, an atom which enters this state will shed the energy rapidly, within less than a millionth of a second. However, in a few cases, isomers can exist with extended half-lives, a few even being measured in years. Isomers should not be confused for isotopes. They have the same number of protons and neutrons, but are denoted by the letter m to indicate that a given isotope exists in a higher energy state, as a nuclear isomer. A common example is technetium-99m, which decays with a half-life of about six hours into technetium-99. The energy is given off in the form of a gamma ray.
Nuclear isotopes decay at a given rate. The half-life cannot be directly changed. The only way to accelerate the decay of a radioisotope is to transform it into some other isotope by bombarding it with particles. However, in principle, it is possible to stimulate the release of energy from a nuclear isomer, although done so in a manner that produces a significant energy gain has yet to be demonstrated. If this were possible, an isomer containing a huge amount of energy could be stimulated to release that energy rapidly, rather than over a very long period of time.
There are very few isomers that are stable enough to be stored long term and have high enough energy densities to be used as an energy source. One of the most attractive is an isomer of hafnium-178Dallas. In 1998, a team at the University of Dallas reported that they had successfully demonstrated the ability to trigger gamma emissions in Hf-178m2 by means of external x-rays. These experiments have not been independently verified and a great deal of controversy has arisen about whether it is indeed possible.
If this is indeed possible (which it might not be) then a hypothetical power system would work by using a small x-ray machine or radioisotope to expose a sample of Hf-178m2 to radiation. This radiation would trigger the release of much more energetic radiation from the isomer, thus resulting in a very large net gain in energy. In principle, it could store orders of magnitude more than any chemical compound, though less than fission or fusion. However, it has been suggested that making this work would require impractically high energy x-rays or that the losses associated with generating the necessary photons may make end up making an unworkable energy source.
Even if this could be made to work, it would be highly unsuitable for an automobile. For one thing, the energy is released in the form of high energy gamma rays, thus necessitating a great deal of shielding. The amount of gamma radiation that could be produced by such a reaction is high enough to present great concern over its use by anyone with a drivers license.
An isomer like Hf-178m2 is also necessarily very expensive. Such isomers are created by some nuclear reactions, but not in the quantities necessary for use as a power source. In order to create that much of the isomer, Hf-178 would first have to be extracted from natural hafnium and then converted to Hf-178m by energizing it with a particle accelerator. Since the energy would all have to be added to the material, it would not actually be an energy source, but rather an energy storage medium, albeit a very dense one.
Because particle accelerators are very inefficient, only a small amount of the energy used would actually end up being stored in the Hf-178m2 and even less of it would be usable after accounting for the thermal engine it would need to power and the additional expense of running an x-ray source. Thus, the usable energy from the isomer ends up coming at enormous expense, making it much more expensive than conventional fuels, even if only a tiny amount would be needed for the entire lifespan of the car.
Our conclusion is that the utilization of nuclear isomers for energy storage is impractical from the points of view of nuclear structure, nuclear reactions, and of prospects for controlled energy release. We note that the cost of producing the nuclear isomer is likely to be extraordinarily high, and that the technologies that would be required to perform the task are beyond anything done before and are difficult to cost at this time
Sorry, but all known methods of producing energy by nuclear means turn out to be wholly unsuitable for use in a car. Baring a breakthrough in direct energy conversion or small-scale aneutronic fusion, it will remain impossible. Even if such a breakthrough were to occur, it is highly unlikely that it would be economically or technically appealing to power the vehicle directly by nuclear means.
This entry was posted on Friday, January 4th, 2013 at 7:52 pm and is filed under Good Science, History, Misc, Nuclear. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.
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