The use of laser technology to achieve nuclear fusion may prove to be a purely academic exercise unless a method is found to convert the substantial amount of energy that is released into a usable form, reports North-West University School of Mechanical Engineering’s Peet van Schalkwyk.
His comments come on the back of the US National Ignition Facility’s (NIF’s) plans to replicate the extreme conditions needed to achieve not only fusion ignition and burn, but also energy gain, which are two key milestones in the scientific pursuit of fusion energy as a source of electricity.
The NIF, the world’s largest laser, is due to be completed later this year at Lawrence Livermore National Laboratory, in California, and is operated by Lawrence Livermore National Security for the US Department of Energy’s National Nuclear Security Administration.
The NIF’s 192 intense lasers will be used to initiate a nuclear fusion reaction. If successful, the NIF will be the first facility to demonstrate fusion ignition and energy gain in a laboratory setting, reports the February 2009 edition of Popular Mechanics.
“The NIF’s attempt to initiate a fusion reaction with lasers will undoubtedly be a success. What the group is trying to do is not impossible; however, its results will remain purely academic until we have the knowledge to harvest and convert that energy into electricity in an economical way. And I have doubts as to whether this will be possible any time soon,” says Van Schalkwyk.
Nuclear fusion takes place when the nuclei of atoms of elements with low atomic numbers are fused together under tremendous pressure and temperature.
This is the same fusion energy process that makes the stars shine and provides the life-giving energy of the sun. Nuclear fusion produces far greater amounts of energy than nuclear fission, which is the splitting of atoms.
However, a method has not yet been identified for harvesting the immense energy from such a fusion reaction and converting it into electricity.
In the 1950s, scientists believed that human-kind was only a few years from developing a method of harvesting the energy released by nuclear fusion; however, “in nearly 60 years we have not come any closer to achieving that goal”, says Van Schalkwyk.
Lasers and Nuclear Fusion
The force of the collision required to fuse the nuclei of two atoms together and the pressure under which the collision must take place are immense owing to the need to overcome the positive charge on each of the nuclei, which is a repulsive force that drives the nuclei away from each other.
Scientists overcome this repulsive force in a hydrogen bomb by using the detonation of a uranium bomb to drive the nuclei of hydrogen together. However, the destructive capability of such a method makes it impractical for peaceful experimentation.
Popular Mechanics reports that the NIF has determined that laser power will be sufficient to supply the level of energy required to initiate nuclear fusion. It will focus the intense energy of 192 powerful laser beams on a target smaller than a pea filled with the heavy isotopes of hydrogen fuel, deuterium and tritium, fusing, or igniting, the hydrogen atoms’ nuclei.
If successful, the NIF will have produced the first laboratory setting in which the energy released from the fusion fuel exceeds the laser energy used to produce the fusion reaction.
The NIF’s powerful lasers will impinge on the fuel capsule and the laser energy will rapidly ablate, or burn away, the capsule’s outer layer.
Newton’s third law of physics takes the role that the uranium bomb detonation plays in the hydrogen bomb. Conservation of momentum, in which every action has an equal and opposite reaction, forces the remaining material to implode or compress as the capsule’s outer layer is ablated.
The resulting compression of the deuteriumtritium fuel to extraordinarily high temperature, inertial pressure and density, ignites a burning hydrogen plasma, releasing an amount of energy far greater than was required to initiate the reaction.
Deuterium and tritium are the simplest fusion fuels and are derived from water and the metal lithium, a relatively abundant resource. One in every 6 500 atoms on earth is a deuterium atom. They are available worldwide and will continue to be available as long as there is water on earth.
One litre of seawater potentially contains the equivalent energy of 300ℓ of petrol and the fuel from 50 cups of water could offer the energy equivalent of 2 t of coal.
Fusion and nuclear fission offer energy sources capable of satisfying the earth’s need for power for the next century and beyond.
Unlike a nuclear fission reaction, which is a continuous reaction that generates energy, a fusion reaction is a single release of energy that is not continuous. A facility that would harvest the energy of a nuclear fusion reaction and convert it into electrical power would initiate a single reaction with a small amount of fuel, harvest the energy and then begin again with more fuel.
Therefore, concludes Van Schalkwyk, there would be no danger of a runaway reaction or core meltdown in a fusion power plant if man ever reaches that level of advanced technology.