Conventional wisdom holds that a breeder reactor produces more fuel than it uses. This idea is fascinating to some, but confusing to a whole group of rational people. It sounds too much like science fiction or Madison Avenue hype.
Actually, a breeder reactor is not magic, but it is a marketing exaggeration to state that a breeder can create fuel. A breeder is simply a more efficient way to use fuel that already exists. It does not create new fuel from nothing; it merely converts certain heavy metals – uranium and thorium – from isotopes that will reluctantly fission into isotopes that will readily fission.
More than a Spark
In order to understand breeder reactors, it is useful think of a fire analogy. Most people have had the occasionally frustrating experience of lighting and tending a fire at some time during their lives.
A controlled fire is usually started by putting a match or spark to material that is readily ignited, like twigs, paper, or lighter fluid. This material is often known as kindling. Inexperienced fire tenders may even start to believe that this material is the only stuff that is useful since other materials need more than a spark to make them burn.
A kindling fire, however, will not last very long without a steady supply of new fuel, so an experienced fire designer/operator will surround the kindling with branches, logs and maybe even some damp wood that will burn more slowly if exposed to enough heat from the kindling. This material can provide more heat over a longer period of time.
In the same way, only a very small portion of potential reactor fuel will readily fission. This fissile material – analogous to kindling – is the 0.7% of natural uranium that is U-235. It takes very little effort to make U-235 fission, a single low energy neutron will usually be sufficient.
Natural Safety Feature
It is a good thing that most combustible materials will not ignite very easily. If it were not for this natural safety feature, it would be easier to light a fire, but there would be a greater hazard from uncontrolled combustion.
If natural uranium had a significantly higher concentration of U-235, it would be easier to build a reactor. However, many natural uranium deposits would have sufficient material to sustain a natural chain reaction. Since getting close to an unshielded fission reactor can be deadly, it seems that God knew what he was doing when he made sure that only a small portion of uranium was fissile.
The Art of an Efficient Fire
Early reactor builders, as smart as they were, were rather inexperienced in the art of designing and building nuclear fission reactors. They were also in a hurry to make something work before their sponsors lost interest. They started out by building machines that burned mostly kindling, just barely scorching the major portion of the potential fuel.
Most reactors currently in operation or under construction use less than 1% of the potential energy of the natural uranium that is initially mined to supply them. Some of the potential energy is left behind in the depleted uranium piles at the enrichment factories while another large portion is known as “spent” fuel.
As a result of many decisions and much misinformation generated during the last forty years, even the next generation of commercial reactors are slated to be reactors whose kindling is so well dispersed that it does a poor job of getting the more abundant U-238 to produce heat.
At AEI, we believe that focusing construction efforts on burner reactors is a mistake similar to making corn bread from seed corn. Because it is now understood that the supply of uranium in the world is extensive, there will not be any immediate or near term consequences from the mistake. At the current rate of consumption, burners will be viable for many decades.
The purpose of this issue of AEI is to discuss breeder misconceptions and some of the details of a development program whose accomplishments are essentially unknown. It is our hope that we will contribute to a growing understanding of the issue of breeder reactors and their advantages and disadvantages.