Man's Next Migration

Traveling Wave Reactor: New Possible Source of Energy and Propulsion for the Goldilocks Mission Ships

Today's nuclear reactors provide clean, efficient and continuous power generation with a lifetime between 30-40 years save for their expensive radioisotope fuels, refueling and maintenance every two years and the amount of radioactive waste produced. A special kind of fission reactor called the traveling wave reactor (TWR) reduces the volume of uranium fuel as well as the need for refueling and maintenance. The TWR  actually uses conventional reactor radioactive by-products and converts it to fuel it can use using only a small amount of enriched uranium to start the fission process. This means that the TWR can even use its own waste products and run theoretically for a hundred years or more before refueling! It can provide the energy necessary to power the Goldilocks Mission ships, specially if the space travel duration reaches hundreds of years.

So how does it work? The name “traveling wave” comes from the process used in the reactor itself. Using depleted uranium core (spent nuclear fuel or any other nonfissile material), a “wave” that moves at a rate of about one centimeter per year travels through the core and slowly transforms it into plutonium which can then proceed with nuclear fission. The principle behind the TWR is nuclear transmutation.

Nuclear reactions lead to the conversion of one element or isotope into another, hence transmutations. All radioactive elements undergo natural transmutation to finally form more stable elements over long periods of time. Transmutation induced in the TWR uses a small amount of energy (small amount of enriched uranium) to change the nuclear structure of the depleted uranium. The nonfissile material is converted into the fuel that the reactor can use. Since this is the case, the reactor itself could also reuse its own fuel by recycling the spent material.
About fifty-thousand tonnes of depleted uranium is produced per year adding to the global stockpile of more than 1.5 million tonnes. More than 400,000 tonnes of toxic radioactive waste is generated each year from existing nuclear power plants. This means that there is a substantially large amount of fuel available for TWRs.

The traveling wave reactor uses liquid sodium as its coolant as opposed to water cooled conventional power plants. The reason for for this is that temperatures inside the TWR core reaches to about 550 degrees Celsius compared to the regular 330 degrees Celsius found in the typical fission reactor. Heat generation is higher which could also mean that more of it could be converted to energy. But since there is so much heat, more space and even more absorbent coolants are needed. The current reactors typically use the heat to drive steam turbines that are attached to electric generators. Building a nuclear power plant in one of the Goldilocks Mission ships would require the ship to be of gargantuan size. Portability of the nuclear reactor is a major concern for it to be used in spaceships. But if new technologies would arise including efficient coolants, and turbines, then the TWR could easily be integrated into the ship.

Aside from steam turbines, direct conversion of heat to electricity (thermo-electricity) is a major alternative to the bulky generators. Special semiconductors sensitive to heat and temperature changes cause electrical currents to flow. The Carnot limit is the stopping wall to this conversion process and even thermoelectric devices only achieve less than a tenth of that limit. However, in a study conducted in 2009 researchers were able to achieve an efficiency as high as forty percent and the calculations conclude that it can reach as much as 90% of the Carnot limit. If this can be achieved, the TWR would certainly find its way into the Goldilocks Mission ships.

The TWR is a unique reactor that can make its own fuel out of what it consumes. It is a revolutionary concept that can make huge changes in the way power is generated from nuclear processes. Much like the space elevator, it has been around since the 1950's, and is just awaiting other technical and engineering hurdles to move out of its way. As the wave of scientific research progresses at an exponential rate, it is only a matter of time until these hurdles are addressed.

References:

Nuclear Power Reactors. World Nuclear Association. Accessed 22-February-2010.
TerraPower: How The Traveling Wave Nuclear Reactor Works. Katie Fehrenbacher. earth2tech.com. Accessed 22-February-2010.
Traveling wave reactor. Wikipedia.org. Accessed 22-February-2010.
TR10: Traveling-Wave Reactor. Matt Wald. MIT Technology Review. Accessed 22-February-2010.
Turning heat to electricity... efficiently. David L. Chandler. Physorg.com Accessed 22-February-2010.