Fissionable isotopes, production

Fjeld RA, DeVol TA, Goff RW, Blevins MD, Brown DD, luce SM, Elzerman AW (2001) Characterization of the mobilities of selected actinides and fission/activation products in laboratory columns containing subsurface material from the Snake River Plain. Nucl Tech 135 92-108 Fleischer RL (1980) Isotopic disequilibrium of uranium alpha-recoil damage and preferential solution effects. Science 207 979-981... 

The half-life of 244Pu (8.2 X 107 years) is short compared with the age of the earth (4.5 X 109 years), and hence this nuclide is now extinct. However, the time interval (a) between the element synthesis in stars and formation of the solar system may have been comparable with the half-life of 244Pu. It has been found recently in this laboratory that various meteorites contain excess amounts of heavy xenon isotopes, which appear to be the spontaneous fission decay products of 244Pu. The value of H calculated from the experimental data range between 1 to 3 X 108 years. The process of formation of the solar system from the debris of supernova is somewhat analogous to the formation of fallout particles from a nuclear explosion. 

Radioxenon production in reactors is thought to arise from two pathways, first via xenon emitted from cracks in fuel rods, and secondly from fission of uranium on the exterior of fuel rods or in cooling water. Radioxenon released from medical isotope production comes... 

Fermi s work made two developments possible (1) the exploitation of nuclear fission for the controlled generation of energy in nuclear reactors and (2) the production of a slow- and fast-neutron fissionable isotope of plutonium, as an... 

Reproduced (adapted) from JINR Report R7-86-322, Domanov VP, Timokhin SN, Zhuikov BL, Chun KS, Eichler B, Chepigin VI, Zvara I, Search for spontaneously fissioning isotopes of elements 107-110 in the products of the interaction of 235U +40 Ar, 1986, with permission from Joint Institute for Nuclear Research. 

In addition to fissionable isotopes ( U, or plutonium) and fertile isotopes ( U or thorium), spent fuel from a reactor contains a large number of fission product isotopes, in which all elements of the periodic table from zinc to gadolinium are represented. Some of these fission product isotopes are short-lived and decay rapidly, but a dozen or more need to be considered when designing processes for separation of reactor products. The most important neutron-absorbing and long-lived fission products in irradiated uranium are listed in Table 1.4. 

After the discovery of uranium radioactivity by Henri Becquerel in 1896, uranium ores were used primarily as a source of radioactive decay products such as Ra. With the discovery of nuclear fission by Otto Hahn and Fritz Strassman in 1938, uranium became extremely important as a source of nuclear energy. Hahn and Strassman made the experimental discovery Lise Meitner and Otto Frisch provided the theoretical explanation. Enrichment of the spontaneous fissioning isotope U in uranium targets led to the development of the atomic bomb, and subsequently to the production of nuclear-generated electrical power. There are considerable amounts of uranium in nuclear waste throughout the world, see also Actinium Berkelium Einsteinium Fermium Lawrencium Mendelevium Neptunium Nobelium Plutonium Protactinium Rutherfordium Thorium. 

The total amount of plutonium formed in various reactors is givoi in Table 21.4. The old gas-graphite reactors and heavy water reactors are the best thermal plutonium producers. They have therefore been used in weapons fabrication. The fast breeder reactor is also an efficient Pu producer. Whereas thermal reactors (except at very low bumup) produce a mixture of odd and ev i A Pu isotopes, a fast breeder loaded with such a mixture, by a combination of fission and n-capture increases the relative concentration of Pu isotopes with odd A in the core and produces fairly pure Pu in the blanket. H ice the combined Pu product from core and blanket elements has a much higher concentration of fissile Pu isotopes than plutonium from a thermal reactor (the fast breeder not only produces more Pu than it consumes but also improves Pu quality, i.e. increases the conc tration of fissionable isotopes). The LWR and AGR are the poorest plutonium producers. 

The core delayed neutron fraction () ) varies from 0.0065 at beginning of cycle in the Initial core to 0.005 at the end of an equilibrium cycle. This average delayed neutron fraction was obtained by weighting the P s of each of the three major core fission Isotopes, U-235, Pu-239, and U-233, by their relative contribution to the neutron production rate. For example, the relative production rate contribution from U-235 varies from 100% at the beginning of the initial cycle to 57% at the end of an equilibrium cycle. The relative production rates of these three nuclides are given In Table 4.2-13a. 

Sources of radiation in fresh fuel are plutonium isotopes, products of decay of the plutonium isotopes, and impurities of products of fission in the regenerated plutonium. As a result, the gamma and neutron radiation dose on a surface of fresh fuel bundles generated by fuel from weapon plutonium exceeds by more than an order of magnitude the appropriate dose capacity for FB from uranium fuel. Moreover, capacity of dose on a surface of FB with regenerated plutonium exceeds on an order of magnitude the dose capacity for FB with weapons plutonium. 

The rate of production of the originating fragment of toe 135 chain is a function of the neutron density in which the fissionable isotope is immersed, and therefore dependent upon the power at which reactors of given type are operated. The radioactive xenon 135 is produced with a noticeable effect on the reaction a few hours after the reaction is started and the effect is, of course, greater as the neutron density is increased and maintained. The xenon 135 effect on toe operation of high power reactors when the xenon remains in the reactor can be summarized as follows. 

However, other radioactive fission products (in addition to those having gaseous stages of decay) with high cross-sections, such as samarium, are produced, and will cause a small neutron loss even if the solution is purified once a day. However, this loss can be tolerated and daily purification will also remove corrosion products before they can build up to any substantial loss factor and permit the replacement of the amount of fissionable isotope destroyed by fission. 

A HaO solution containing the fissionable isotope which has been reacted and therefore contains solid fission products, is pumped from the converter and made 1 N in HNO3 and 10 N in NH4NO3. It is then charged to the center of a continuous counter-current extraction column. A suitable water-immiscible solvent, such as hexone or dibutyl carbitol, is charged to the column at the bottom and passes up through the water phase. 

The fissionable isotope passes from e water solution into the organic solvent, which is separated at the top of the column. The water solution containing the fission products is passed out of the bottom of the column into waste. The organic solvent containing the fissionable isotope is next charged to a similar continuous... 

Non-productive neutron absorption in the absorption zone, however, has a more serious effect. For example, protactinium is an intermediate stage between the thorium and the uranium isotope desired, and as far as is presently known is not fissionable. This element can cause a neutron loss in two ways. First, a neutron loss by 0Q the neutrons which Pa absorbs, and second, by formation of an element decaying into instead of into a known fissionable isotope. This effect however can be kept to a minimum by extracting the Pa from the slurry at sufficiently frequent intervals to reduce the ab-55 sorption by the Pa to about. 5 percent of the absorption by the thorium. 

To obtain a more complete conversion of one fissionable isotope to another, it has been proposed to utilize a substantially pure fissionable isotope in solution or dispersion in a liquid moderator for the neutronic reaction, and then form the new fissionable isotope separately by absorption of leakage neutrons produced as a by-product of the reaction. 

The fissionable isotope passes from the water solution into the organic solvent, which is separated at the top of the column. The water solution containing the fission products is passed out of the bottom of the column into waste. The organic solvent containing the fissionable isotope is next charged to a similar continuous counter-current extraction column at the bottom and re-extracted into a water solution of the composition used in the pile. The columns are identical in construction, and the water layer from the bottom of the second column can be pumped directly back into the reactor or can be passed through further purification cycles as above, if needed. 

A primary object of the present invention is to provide a breeder system wherein a nuclear fission chain reaction is utilized to produce fissionable material at a rate 10 greater than the rate of consumption of fissionable material within the chain reacting composition. This is accomplished by neutron bombardment of fertile material adapted to undergo nuclear reaction productive of fissionable material as hereinafter described. Fertile iso-15 topes as herein defined are isotopes such as and U238 which are converted to thermally fissionable isotopes, and Pu 39, respectively, by nuclear reaction under neutron bombardment. These fertile isotopes are fissionable by fast neutrons and substantially nen-fission-20 able by slow neutrons (below about 1000 e.v.) and absorb neutrons fast or slow to undergo the above-mentioned nuclear reactions. 

For the development of nuclear energy for military purposes in World War II, the Manhattan Project of the U.S. Army Engineers Corps required large quantities of D.O, highly enriched and the fissionable isotope of uranium, 235y most difficult task was the production of kilograms of 90% U from the natural abundance of 0.7%. Many processes were... 

Dose consequence calculations for mixed fission products that are typical of the isotope production target show the dose at 3000 m is approximately 0.015 mrem for each curie released (Naegeli 1999, Mitchell and Naegeli 1999), or a maximum of 0.08 mrem for an unmitigated release of 5 curies of respirable material released during a fire. These calculations were done using the methodology described in Section 3.4.1. 

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