Plutonium breeding

In thermal reactors fueled with plutonium, the number of neutrons produced per neutron absorbed is less than 2.0 and breeding is impossible. For U, on the other hand, this number is substantially greater than 2.0, and breeding is practicable in a thermal reactor. In fast reactors, the number of neutrons produced per neutron absorbed is close to the total number of neutrons produced per fission, so that breeding is possible with both and plutonium. Breeding as here defined is not possible with U, because there is no naturally occurring isotope from which can be produced. 

Fresh mixed-oxide fuels consist of a UO2 matrix with a content of 3 to 4% fissile plutonium, with the plutonium concentrated in the master-mixture grains. During irradiation, fission product and transuranium elements are built up in the same manner as in uranium fuel the newly formed transuranium elements are homogeneously distributed in the fuel matrix with the exception of the preferential plutonium breeding in the outermost zone of the pellet. The plutonium constituent has little effect on the chemical conditions in the fuel therefore, both types of fuels are quite similar with regard to the chemistry environment of the fission products under the operating conditions of light water reactors. 

The situation is quite different for fast neutron spectmm reactors where burnable poisons do not exist but excellent neutron economy allows for plutonium breeding to be conducted internal to the core lattice itself to produce new fissile materials that can be bred in-situ to completely compensate reactivity loss with bum-up. 

Mill Wastes. The uranium-containing wastes from milling are mounded and covered with earth. This earth cover prevents erosion and delays for decay the 14-hour radon gas, the gaseous decay product of uranium. These mill waste repositories are located near the mines and mills and are not a very different hazard from the original naturally occurring uranium deposits. The depleted uranium from the enrichment operations is stored in cylinders as uranium hexafluoride for future use in the uranium-plutonium breeding cycle. Other uranium wastes from enrichment and fuel fabrication go to the low-level repositories. 

The role of the reactor may be either as a converter, which produces some plutonium by neutron absorption in uranium-238 but depends on uranium-235 for most of the fission, or as a breeder, which contains a large amount of plutonium and produces more fissile material than it consumes. Breeding is also possible using uranium-233 produced by neutron absorption in thorium-232. 

Breeder reactors were developed to utilize the 97% of natural uranium that occurs as nonfissionable U-238. The idea behind a breeder reactor is to convert U-238 into a fissionable fuel material, plutonium. A reaction to breed plutonium is... 

Bose-Einstein Condensate phase of matter that is created just above absolute zero when atoms lose their individual identity Boyle s Law law that states volume of a gas is inversely related to its pressure Breeder Reactor type of nuclear reactor that creates or breeds fissionable plutonium from nonfissionable U-238 Buckministerfullerene Cg, allotrope of carbon consisting of spherical arrangement of carbon, named after architect Buckmin-ister Fuller, Invertor of geodesic dome Buffer a solution that resists a change in pH... 

One of the fascinating features of fission power is the breeding of fission fuel from nonfissionable uranium-238. Breeding occurs when small amounts of fissionable isotopes are mixed with uranium-238 in a reactor. Fission liberates neutrons that convert the relatively abundant nonfissionable uranium-238 to uranium-239, which beta-decays to neptunium-239, which in turn beta-decays to fissionable plutonium-239. So in addition to the abundant energy produced, fission fuel is bred from relatively abundant uranium-238 in the process. 

Plutonium-239. Plutonium-239 represents a fortuitous phenomenon. Whereas U-235 is the only significant fissile nuclide in nature, its major isotope, U-238, captures a neutron to produce another fissile nuclide, plutonium-239. A substantial amount of the energy produced during the life of uranium fuel is produced by the conversion of U-238 to Pu-239 which subsequently fissions. This process provides the basis for the nuclear breeding cycle. 

A similar set of processes has been partially developed for the thorium-uranium system but is not discussed here because it is not expected to be employed in the next several decades. The important feature of the thorium cycle is that it could be used to achieve breeding (to produce more fissionable material than is consumed) in thermal reactors, but nuclear as well as chemical factors have frustrated this development (for more information, see Reference 22). The increasing cost of the natural uranium supply for the ura-nium/plutonium cycle may, several decades in the future, justify development of the thorium cycle. 

Two engineering system demonstrations were performed to reduce the uranium-from-ore requirements of LWRs recycle of the plutonium and conversion to the thorium-uranium cycle to achieve thermal breeding. The demonstration phase of the plutonium recycle development was carried out in seven power reactors. Several LWRs originally were started up on the thorium-uranium cycle, and a light... 

That is the advantage of fission. Its drawback is the deadly radioactivity it generates, particles whose mass, from one type of reactor, is almost equal to the mass of the fuel consumed. Waste from a fission reactor typically requires thousands of years before it breaks down into biologically safe levels. Fission reactors are also relatively inefficient. They can use but a single isotope (atoms of an element that have the same number of protons but a different number of neutrons) of uranium, U-235, which makes up less than 1 percent of natural uranium ore. (More than 99 percent of natural uranium is nonfissionable U-238.) So-called fast breeder reactors might overcome the supply limitation by breeding fissionable fuel from U-238. But the fuel it produces from the uranium is plutonium, the same stuff that was inside the Nagasaki bomb—not an ideal by-product in a politically unstable world. 

On the other hand, liquid metal-cooled fast reactors (LM-FRs), or breeders, have been under development for many years. With breeding capability, fast reactors can extract up to 60 times as much energy from uranium as can thermal reactors. The successful design, construction, and operation of such plants in several countries, notably France and the Russian Federation, has provided more than 200 reactor-years of experience on which to base further improvements. In the future, fast reactors may also be used to burn plutonium and other long-lived transuranic radioisotopes, allowing isolation time for high-level radioactive waste to be reduced. 

The Integral Fast Reactor would also be capable of breeding plutonium which could be used as nuclear fuel. This type of reactor was seen as the key to a nuclear future. Liquid sodium is a volatile substance that can burst into flames if it comes into contact with either air or water. An early liquid sodium-cooled breeder reactor, the Fermi I, had a melting accident when 2% of the core melted after a few days of operation. Four years later when the reactor was about to be put into operation again a small liquid sodium explosion occurred in the piping. 

In a typical breeder reactor, nuclear fuel containing uranium-235 or plutonium-239 is mixed with uranium-238 so that breeding takes place within the core. For every uranium-235 (or plutonium-239) nucleus undergoing fission, more than one neutron is captured by uranium-238 to generate plutonium-239. Thus, the stockpile of fissionable material can be steadily increased as the starting nuclear fuels are consumed. It takes... 

The critical mass of fissile material required to maintain the fission process is roughly inversely proportional to the neutron-absorption cross section. Thus the critical mass is lowest for plutonium in thermal reactors, larger for the uranium isotopes in thermal reactors, and much greater in fast reactors. For this reason, as well as others, thermal reactors are the preferred type except when breeding with plutonium is an objective then a fast reactor must be used. 

Nuclear properties of Pu become more favorable for breeding if fission is carried out with fast neutrons, with kinetic energies of the order of 2 X 10 eV. In such a fast reactor the TJ for plutonium is around 2.3, and breeding with U (or natural uranium) is possible. 

---