Uranium fissionable isotope

Different combinations of stable xenon isotopes have been sealed into each of the fuel elements in fission reactors as tags so that should one of the elements later develop a leak, it could be identified by analyzing the xenon isotope pattern in the reactor s cover gas (4). Historically, the sensitive helium mass spectrometer devices for leak detection were developed as a cmcial part of building the gas-diffusion plant for uranium isotope separation at Oak Ridge, Tennessee (129), and heHum leak detection equipment is stiU an essential tool ia auclear technology (see Diffusion separation methods). 

The First Reactor. When word about the discovery of fission in Germany reached the United States, researchers thereafter found that (/) the principal uranium isotope involved was uranium-235 (2) slow neutrons were very effective in causing fission (J) several fast neutrons were released and (4) a large energy release occurred. The possibiUty of an atom bomb of enormous destmctive power was visualized. 

The Natural Reactor. Some two biUion years ago, uranium had a much higher (ca 3%) fraction of U than that of modem times (0.7%). There is a difference in half-hves of the two principal uranium isotopes, U having a half-life of 7.08 x 10 yr and U 4.43 x 10 yr. A natural reactor existed, long before the dinosaurs were extinct and before humans appeared on the earth, in the African state of Gabon, near Oklo. Conditions were favorable for a neutron chain reaction involving only uranium and water. Evidence that this process continued intermittently over thousands of years is provided by concentration measurements of fission products and plutonium isotopes. Usehil information about retention or migration of radioactive wastes can be gleaned from studies of this natural reactor and its products (12). 

Natural uranium consists mostly of and 0.711 wt % plus an inconsequential amount of The United States was the first country to employ the gaseous diffusion process for the enrichment of the fissionable natural uranium isotope. During the 1940s and 1950s, this enrichment appHcation led to the investment of several bUHon dollars in process faciHties. The original plants were built in 1943—1945 in Oak Ridge, Teimessee, as part of the Manhattan Project of World War II. 

Uranium-235 is the most important uranium isotope for nuclear fuel. Uranium-238, although not fissionable itself, can be converted into the fissionable plutonium-239 in a breeder reactor by the following nuclear reaction ... 

Chain reactions do not occur to any great extent in naturally occurring uranium ore because not all uranium atoms fission so easily. Fission occurs mainly in the isotope uranium-235, which is rare and makes up only 0.7 percent of the uranium in pure uranium metal (Figure 4.23)- When the more abundant isotope uranium-238 absorbs neutrons created by fission of a uranium-235 atom,... 

Constructing a fission bomb is a formidable task. The difficulty is in separating enough uranium-235 from the more abundanr uranium-238. Scientists took more than 2 years to extract enough of the 235 isotope from uranium ore to make the bomb detonated at Hiroshima, Japan, in 1945. To this day, uranium isotope separation remains a difficulr process. 

The paper of 1939 [1 ], On the Chain Decay of the Main Uranium Isotope, studies the effects of elastic and non-elastic neutron moderation and concludes that chain fission reactions by fast neutrons in pure metallic natural uranium are impossible. The 1940 paper, On the Chain Decay of Uranium under the Influence of Slow Neutrons [2 ], is classic in the best sense of this word its value is difficult to overestimate. The theoretical study performed showed clearly that the effect of resonance absorption of neutrons by nuclei of 238U is a governing factor in the calculation of the coefficient of neutron breeding in an unbounded medium it was concluded that a self-sustained chain reaction in a homogeneous natural uranium-light water system is impossible. 

The element uranium is the element used for almost all fission processes. It has two natural isotopes. One of them is 238CI which, constitutes 99.3% of uranium ore, and the other is 235CJ, which constitutes 0.7% of uranium ore. Fissionable nuclei such as 235CJ and 239Pu are called fissile. Nuclear fission reactions occur... 

Although the fission products could be recovered as byproducts from the waste from spent nuclear reactor fuel, special-purpose neutron irradiation of highly enriched uranium (isotopically separated uranium-235) followed by chemical separation is the normal production method. The major products, molybdenum-99 and iodine-131 with fission yields of 6.1 and 6.7 percent, respectively, have important medical applications. Mo-99,... 

Natural uranium contains close to 0.72% by weight of the only fissionable isotope and more than 99% non-fissionable U, and a trace quantity of U. For uranium to be useful as the fuel to the nuclear reactor, the level needs to reach 1 to 5% (more often 3 to 5%) while most of the nuclear weapons and submarines require a concentration of at least 90% The separation of those uranium isotopes, very similar in properties, can not be effected by chemical means. 

Pu by neutron capture of the U in natural uranium. Chemical separation of Pu from the uranium and fission products would be easier than separating the isotopes of natural uranium to produce weapons grade... 

Altogether there are three outstanding fissionable materials, or nuclear fuels the Big Three, plutonium-239, natural uranium-235, and the synthetic uranium isotope, U-233. 

Some of the neutrons released in the controlled chain reaction strike the nuclei of non-fissionable U-238 atoms. In this case, the U-238 captures the neutron and becomes a heavier uranium isotope, U-239, which eventually decays to produce plutonium-239. 

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. 

We are now in position to derive equations that will give the degree uf bumup nuclear fuel can experience before it ceases to be critical. First, we must determine how the concentration of each nuclide that affects the neutron balance changes with time. We consider fuel that at time zero contains N s atoms of U per cubic centimeter, atoms of U, and no other uranium isotopes, plutonium, or fission products. This fuel is then exposed to a thermal-neutron flux 0(0, which may be a function of time. The variation in concentration of each nuclide in this fuel with time is obtained as follows. 

The other full-scale applications of the Thorex process have been to separation of from thorium irradiated at the U.S. Atomic Energy Commission s production reactors at Savannah Rivet and Hanford. As the object of these irradiations was to produce of high isotopic purity for use in the first core of the LWBR, the bumup to which the fuel was exposed was low, and the concentrations of uranium and fission products in the irradiated thorium were much lower than will exist in power reactor fuel irradiated to full bumup. Nevertheless, the successful separation of uranium and thorium from each other and from fission products is significant confirmation of the workability of the Thorex process. 

Attempts have been made to determine cumulative yields for some of the short-lived krypton and xenon isotopes by gas sweeping a solution of uranium undergoing fission. However, quantitative removal of inert gases... 

Early Work. The irradiated fuel, upon discharge from the reactor, comprises the residual unbumt fuel, its protective cladding of magnesium alloy, zirconium or stainless steels, and fission products. The fission process yields over 70 fission product elements, while some of the excess neutrons produced from the fission reaction are captured by the uranium isotopes to yield a range of hew elements—neptunium, plutonium, americium, and curium. Neutrons are captured also by the cladding materials and yield a further variety of radioactive isotopes. To utilize the residual uranium and plutonium in further reactor cycles, it is necessary to remove the fission products and transuranic elements and it is usual to separate the uranium and plutonium this is the reprocessing operation. 

It should be mentioned that the uranium isotope 235, when bombarded with neutrons, undergoes fission, liberating more neutrons in what can become a self-supporting chain reaction with a very large liberation of energy. This is the principle of the atomic bomb and the production of nuclear energy. The fusion of protons and neutrons to form helium nuclei at the enormous temperature of an atomic bomb explosion is the basis of the principle of the hydrogen bomb. ... 

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