Light Water Breeder Reactor: Adapting A Proven System
At 12:30 am, on August 26, 1977, the operators at the Shippingport Atomic Power Station began lifting the central modules of the experimental breeder reactor core into the blanket section. At 04:38 am, the reactor reached criticality. During the next five years, the core produced more than 10 billion kilowatt-hours of thermal power – equivalent to about 2.5 billion kilowatt hours of electrical power – with a current retail value of approximately $200 million.
It showed no signs of approaching the end of its useful life. It was obvious from the core performance that the reactor was at least a very efficient converter with a long life core. However, in October, 1982, the reactor was shut down for the final time under budgetary pressures and a desire to conduct the detailed fuel examination needed to determine if breeding had actually occurred.
A report on the experiment was quietly issued in 1987. The core contained approximately 1.3% more fissile material after producing heat for five years than it did before initial operation. Breeding had occurred in a light water reactor system using most of the same equipment as used for conventional reactor plants.
New Fuel Source
Instead of using uranium-plutonium fuel like a liquid metal fast breeder reactor, the light water breeder reactor used uranium-thorium. In a process very similar to the one that produces fissile plutonium from U-238, it is possible to produce a fissile isotope of uranium, U-233, from thorium 232.
The advantage of this combination from a technical point of view is that U-233 produces more neutrons if fissioned by a low energy (thermal) neutron than does U-235. This characteristic means that more excess neutrons are available to convert fertile material. In a carefully designed and constructed reactor, uranium-thorium reactors have enough excess fission neutrons to overcome the parasitic neutron absorptions inherent in a water cooled and moderated reactor.
Recalling a fire analogy, even wet wood can be made to burn if you have enough high quality, carefully arranged dry wood to overcome the heat lost to absorption in the water.
In order to minimize neutron absorptions in the water coolant, the designers went through a process they called “squeezing out the water”. They redesigned the fuel elements to make the clearances between the fuel pins tighter. Since water is also the means of moving the heat from the reactor to some place where it can be of use, this process has its limits.
The designers chose a seed and blanket core configuration. In this type of reactor, the fissile material is concentrated in the central core region while the fertile material surrounds the central core region including the top and bottom. Most of the neutrons produced in the central core are used to sustain the chain reaction, while most of those than leak out at the boundary are either reflected back into the fissile material or absorbed by fertile material.
The designers also decided to develop a new form of reactivity control that did not depend on putting neutron absorbing control rods into the core. Every neutron lost to a control rod over the life of the core was one less neutron available for converting thorium into uranium.
The seed elements were movable and less than a critical mass if not reflected. If the seed was lowered, more of the neutrons at the boundary would leak out, causing reactivity to drop. If the seed was raised into the blanket region, neutrons would be reflected back into the fissile material by the blanket allowing the core to reach a critical mass.
Why No Follow-up?
The light water breeder reactor was a technical success. It demonstrated a sophisticated way to more effectively use a proven technology and to make better use of natural resources. It even demonstrated a way to significantly reduce the volume of high level nuclear waste per unit of electrical power output.
Unfortunately, the program leaders were not focused on factors that make new innovations successful in the market. The following weaknesses prevented commercial success.
- There was little effort to promote the technology. Knowledge of the program is rare even within the nuclear industry. There is little chance of an unknown idea – particularly one with as much potential impact as a light water breeder reactor – becoming a new technical standard.
- The core engineers did not pay enough attention to production difficulties. The assembly of the core modules required a great deal of manual labor including 2,000 precise measurements for each module. This effort implies a high production cost even if raw materials are used more efficiently.
- There was no effort to develop other uranium-thorium reactors in an effort to help spread the fixed cost of fuel material production.
- The program was viewed as Admiral Rickover’s pet project.
Professional rivalry or ingrained hard feelings against Rickover probably helped seal the fate of the program. By the time the experimental core was shut down, the Secretary of the Navy had already declared his intention to retire Rickover. By the time that the core had been analyzed, Admiral Rickover was dead and many of his strongest political supporters were either retired or dead.
Dead End or New Direction?
It may be that the light water breeder reactor is not a viable alternative to conventional light water reactors. It seems that there is an abundant supply of fissile materials and that the higher costs of core manufacture will not be overcome without a significant automation effort. However, it is useful to know that efficient conversion – even breeding – is not only possible in a thermal reactor without the use of liquid metals, it has been actually demonstrated in a large scale experiment.
In a future issue, AEI will discuss a new design for a thermal breeder reactor designed to be less costly to manufacture and operate.