Ok, training time out requested. What is a breeder reactor?
In this case I am talking about a thorium breeder reactor. Thorium is not the primary fuel it is the target to generate the fuel in this case U-233. Not anything new even in commercial reactors a percentage of the total power per fuel cycle is plutonium from activated U-238.
Thorium as a nuclear fuel
Thorium, as well as uranium, can be used as a nuclear fuel. Although not fissile itself, Th-232 will absorb slow neutrons to produce uranium-233 (U-233)a, which is fissile (and long-lived). The irradiated fuel can then be unloaded from the reactor, the U-233 separated from the thorium, and fed back into another reactor as part of a closed fuel cycle. Alternatively, U-233 can be bred from thorium in a blanket, the U-233 separated, and then fed into the core.
In one significant respect U-233 is better than uranium-235 and plutonium-239, because of its higher neutron yield per neutron absorbed. Given a start with some other fissile material (U-233, U-235 or Pu-239) as a driver, a breeding cycle similar to but more efficient than that with U-238 and plutonium (in normal, slow neutron reactors) can be set up. (The driver fuels provide all the neutrons initially, but are progressively supplemented by U-233 as it forms from the thorium.) However, there are also features of the neutron economy which counter this advantage. In particular the intermediate product protactinium-233 (Pa-233) is a neutron absorber which diminishes U-233 yield.
Over the last 40 years there has been interest in utilising thorium as a nuclear fuel since it is more abundant in the Earth's crust than uranium. Also, all of the mined thorium is potentially useable in a reactor, compared with the 0.7% of natural uranium in today's reactors, so some 40 times the amount of energy per unit mass might theoretically be available (without recourse to fast neutron reactors). But this relative advantage vanishes if fast neutron reactors are used for uranium.http://world-nuclear.org/info/inf62.htmlA more traditional breeder reactor would be the abandoned Clinch River Breeder reactor were deleted uranium would be the target material to produce plutonium used for fuel in the inner core.
The reactor would have been rated at 1000 megawatts (MW) of thermal output, with a net plant output of 350 MW (electrical) and a gross output of 380 MW.
The reactor core was designed to contain 198 hexagonal fuel assemblies, arranged to form a cylindrical geometry with two enrichment zones. The inner core would have contained 18% plutonium and would have consisted of 108 assemblies. It would have been surrounded by the outer zone, which would have consisted of 90 assemblies of 24% plutonium to promote more uniform heat generation.
The active fuel would have been surrounded by a radial blanket consisting of 150 assemblies of similar, but not identical, design containing depleted uranium oxide; outside of the blanket would have been 324 radial shield assemblies of the same overall hexagonal geometry.
The primary (green) and secondary (gold) control rod systems would have provided overall plant shutdown reliability. Each system would have contained boron carbide. The secondary rods were to be used only for SCRAM, and would have been required to be fully withdrawn before startup could be initiated.EBR 2 was intended to not only breed more fuel than it used but allow reprocessing on site.
The fuel consists of uranium rods 5 millimeters in diameter and 13 inches ( 33 cm ) long . Enriched to 67% uranium-235 when fresh, the concentration dropped to approximately 65% upon removal. The rods also contained 10% zirconium. Each fuel element is placed inside a thin-walled stainless steel tube along with a small amount of sodium metal. The tube is welded shut at the top to form a unit 29 inches (73 cm) long. The purpose of the sodium is to function as a heat-transfer agent. As more and more of the uranium undergoes fission, it develops fissures and the sodium enters the voids. It extracts an important fission product, caesium-137, and hence becomes intensely radioactive. The void above the uranium collects fission gases, mainly krypton-85. Clusters of the pins inside hexagonal stainless steel jackets 92 inches ( 234 cm ) long are assembled honeycomb-like; each unit has about 10 pounds (4.5 kg ) of uranium. All together, the core contains about 680 pounds (308 kg ) of uranium fuel, and this part is called the driver.
The EBR-II core can accommodate as many as 65 experimental sub-assemblies for irradiation and operational reliability tests, fueled with a variety of metallic and ceramic fuels—the oxides, carbides, or nitrides of uranium and plutonium, and metallic fuel alloys such as uranium-plutonium-zirconium fuel. Other sub-assembly positions may contain structural-material experiments.http://en.wikipedia.org/wiki/Experimental_Breeder_Reactor_IIAside from the breeder function the safety design was very interesting.
The expansion of the fuel and structure in an off-normal situation causes the system to shut down even without human operator intervention. In April of 1986, two special tests were performed on the EBR-II, in which the main primary cooling pumps were shut off with the reactor at full power (62.5 megawatts, thermal). By not allowing the normal shutdown systems to interfere, the reactor power dropped to near zero within about 300 seconds. No damage to the fuel or the reactor resulted. This test demonstrated that even with a loss of all electrical power and the capability to shut down the reactor using the normal systems, the reactor will simply shut down without danger or damage. The same day, this demonstration was followed by another important test. With the reactor again at full power, flow in the secondary cooling system was stopped. This test caused the temperature to increase, since there was nowhere for the reactor heat to go. As the primary (reactor) cooling system became hotter, the fuel, sodium coolant, and structure expanded, and the reactor shut down. This test showed that an IFR type reactor will shut down using inherent features such as thermal expansion, even if the ability to remove heat from the primary cooling system is lost.http://ebrh.com/ebr_II.htm
