Nuclear Basics
I have been asked to provide a layman's understanding of how a reactor works and how spent fuel elements are handled.
Commercial nuclear reactor plants are essentially, not unlike steam boiler power plants, with the exception that the heat which generates the steam which drives the turbine-generator unit, is produced by the fission of the uranium nuclear fuel. rather that a boiler powered by oil or natural gas.
For example, my first assignment in the industry was in 1965 when I was employed by the Bechtel Corporation as a mechanical start-up engineer at the San Onofre Nuclear Plant near San Clement, Ca. Of our six mechanical engineers, the Chief engineer and one other, were graduates of the California Maritime Academy at Vallejo, Ca., another from the U. S. Maritime Academy at Kings Point, N. Y., another from the New York Maritime Academy at Fort Schyler, N. Y., and then two of us from the Naval Academy. Both the Westinghouse Nuclear system and his Westinghouse Turbine erector Chief Engineers, were also Cal. Maritime graduates. My last two seagoing assignments in the Navy were as Chief Engineer on oil fired boiler ships. Maritime boiler/turbine plants differing mainly, only in size, compared to stationary power plants.
Control rods containing neutron absorbers are driven in and out of the core controlling the heating process and steam production.
Feedwater supplied to the reactor core, passing through the core region, is flashed to steam, thereafter, it is transported to the turbine where it gives up the energy contained in the steam through turbine blades. The steam exhausted from turbine enters a large chamber, called a condenser, where it passes over tubes cooled by sea water. The condensed cooled steam is returned through condensate and feedwater piping back to the reactor to continue the closed loop process.
The reactor core is usually comprised of three regions of uranium fuel rods packaged into fuel bundles, of different percentages of uranium 235, averaging about 3%.
In general, a third of the core is exchanged annually, the fuel removed, being placed in a spent fuel pool for continued cooling and covered with about 10 feet of water which provides the necessary shielding for the protection of plant workers.
Cooling of the spent fuel is provided by a closed loop system with the heat transfered to sea water through an heat exchanger.
As I remember, spent fuel pools were designed to contain 1-1/3 cores. This allows for the 1/3 core to cool sufficiently before being shipped off-site to a recycling facility. This also provides for the complete forced off-loading of a core load in the case of the need for work on the reactor vessel.
"If you take a fuel rod bundle out of a reactor and put it in a pool, you have to leave it for five years before you can take it out. They don't produce a lot of heat, but it is unrelenting," said Richard Lahey, who was General Electric's head of safety research for boiling water reactors when the company installed them at Fukushima.
