|Fig. 1: Thorite Thorium Silicate: Will thorium power cars of the future? Source: Wikimedia Commons|
Based on the current rate fuel consumption, scientists estimate that the world's oil reserves will be depleted between 2052 and 2094.  Diminishing fossil fuel deposits indicate that a shift toward other energy options is necessary to fuel increasing transportation demand. Nuclear energy appears a promising alternative to oil because it is relatively plentiful compared to fossil fuels. About 12 billion pounds of readily-extractable uranium are on Earth, of which 890 million are deposited in the United States.  These reserves can generate power for up to 5 billion years, assuming breeder reactors are used.  Nuclear power therefore has the potential to be the solution to the world's energy and transportation crises.
The economic advantages of powering a vehicle with nuclear fission can be demonstrated with a few simple estimations.
Knowing that the energy released per gallon of gasoline is 1.18 × 105 Btu/gallon and assuming that the average American uses about 1000 gallons of gasoline per year, the energy required to power a typical passenger vehicle is [4,5]
Assuming the lifetime of a car is about 15 years and knowing that fission of one pound-mass of U-235 releases 2.89 × 1010 Btu of energy, the mass of uranium required to power a car for its lifetime is 
|15 years × 1.18 × 108
2.89 × 1010 Btu/lb
|= 0.061 lbs|
The price of uranium as of December 2011 was $52 per pound, so the cost of fuelling a nuclear fission vehicle for fifteen years is 
The lifetime fuel costs of uranium are less than one gallon of gasoline. The U.S. average cost of gasoline in the same month, December 2011, was $3.258 per gallon. 
In 1957, Ford Motor Company revealed the concept model for the Nucleon, a passenger car powered by a compact atomic fission reactor in the vehicle's trunk. Motivated by nuclear energy's efficiency and reduction in harmful emissions compared to the traditional internal combustion engines, engineers modeled the Nucleon's power system after existing nuclear marine propulsion reactors in submarines. Designers hoped to scale down this technology for use in cars, using uranium fission to heat a steam generator and convert water into high-pressure steam. A set of steam turbines would then provide torque and electricity for the vessel. The reactors powering the Nucleon were designed to be interchangeable, allowing for replacement of depleted reactors after an estimated 5,000 miles and customization for high-torque or high-efficiency, depending on the user. Due to engineering setbacks and concerned public opinion on atomic power, the Ford Nucleon never was developed to market. 
Practical design flaws kept the Ford Nucleon from ever being developed past a concept model and are important considerations for the future of nuclear-powered vehicles. These flaws included the large size of a nuclear reactor, which must fit in the trunk of the car, and the heavy mass of necessary radiation-shielding materials, most commonly lead. The development of smaller reactors was envisioned to allow a nuclear-powered engine to fit in a compact space and the Nucleon was designed with the passenger cab suspended beyond the vehicle's front axle to counterbalance large mass of the reactor. Overcoming the inefficiency of supporting and driving around the power system's tremendous weight proved an insurmountable obstacle. 
While smaller nuclear reactors and lighter shielding, such as graded-z shielding, have been developed since 1957, the possibility of car crashes is another serious consideration for whether nuclear vehicles should be pursued. The recent Fukushima Daiichi nuclear power plant disaster raises the question of safety, especially how robust a nuclear power system can be under mechanical impact. Earthquake and tsunami disasters damaged the Daiichi plant's cooling system and containment vessels, causing a INES Level 7 accident with serious overheating and radiation leakage.  These events bring reasonable doubt as to whether nuclear-powered passenger vehicles could ever be safe enough for use on the roads and have generated distrust and opposition among the public against nuclear power.
While a nuclear-powered vehicle is still a distant dream, recent technological developments have made its engineering and realization more conceivable. Several private companies have created small thorium reactors, which are designed to be more compact than conventional thermal nuclear power plants. In these thorium reactors, an accelerator-driven system produces a proton beam using a particle accelerator. The proton beam produces excess neutrons when aimed at thorium and the thorium nuclei and neutrons result in uranium-233, an isotope that releases energy in fission.  General Electric released its Cadillac World Thorium Fuel Concept at the 2011 Chicago Auto show, which included a theoretical a thorium-powered engine from Laser Power Systems.  Using atomic energy indirectly, such as powering a electric vehicle using the electricity generated in nuclear fission, also remains a possibility.
© Claire Durkin. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
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