Fission-Product Radioactivity

For irradiation in a constant neutron flux, the activity of any fission-product nuclide can be evaluated from the equations in Chap. 2. When fissions occur at a constant rate and when neutron-absorption reactions in the fission product and its precursors can be neglected, the activity of a nuclide with relatively short-lived precursors can be evaluated by applying Eq. 

N — atoms of long-lived fission product present after cooling for a time Tr = irradiation time, s Tc = cooling time, s 

When the half-lives of a fission product and of its decay precursors are short compared to the irradiation time (Fi/j Tr), the fission-product nuclide reaches saturation prior to the end of the irradiation. Its saturation activity per unit of reactor power is a constant, so that 

The saturation activity is conveniently expressed in curies per watt of thermal power, or 

Practical irradiation periods for fuel in power reactors are in the range of about 1 to 4 years. Most of the fission-product nuclides reach saturation in this period. An example is 8.05-day I, which is formed in 2.93 percent of U fissions. Its saturation activity is 0.023 Ci/W. 

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Lanthanum fission product, radioactive, tracer experiments, early work. 7, 10... 

The character of the radioactive debris from a land surface explosion is determined largely by the extent of mixing between the extraneous debris injected into the cloud and the fission product radioactivities. Within the early cloud there is a well developed toroidal circulation (5), which is clearly evident in the case of air bursts and large yield surface bursts. In low yield surface explosions it may be obscured quickly by the dirt cloud and by rapid damping of a systematic circulation. 

Assume that a reactor is operated for a very long time so that all fission products reach saturation activity. On the average there are three radioactive decays in each fission-product decay chain. Calculate the fission-product radioactivity at saturation in units of curies per watt of thermal power from fission. 

Contacting equipment used in the extracting section must have low holdup to minimize solvent degradation from the intense fission-product radioactivity. Here, centrifugal contactors or pulse columns are preferred to mixer-settlers. In the scrubbing section and in the balance of the solvent extraction plant, mixer-settlers are often used. 

The first nuclear test took place in the United States in July 1945. Since then a number of other nations have tested nuclear devices both above and underground. The total amount of fission which took place in the atmosphere is estimated to be a little more than 200 megatons (MT). Most of the explosions occurred in the Northern Hemisphere, and most of the material produced was injected initially into the stratosphere. Careful and continuous monitoring and inventorying of the atmosphere since 1958 has indicated half-residence times for the fallout particulates of about one year in the stratosphere and about one month in the troposphere. Measurements in the stratosphere and upper troposphere showed that the fission product radioactivity was in or on particles with a diameter of. 03 jum or less. ... 

If desired, the fission products, radioactive and stable, and the 94 produced by normal operation of the pile can be recovered after a predetermined exposure of the uranium in the slurry to high neutron densities by removing the slurry, separating the uranium therefrom, and extracting the desired elements as referred to above. The... 

Fission product distribution Radioactive waste activity distribution... 

Safety. A large inventory of radioactive fission products is present in any reactor fuel where the reactor has been operated for times on the order of months. In steady state, radioactive decay heat amounts to about 5% of fission heat, and continues after a reactor is shut down. If cooling is not provided, decay heat can melt fuel rods, causing release of the contents. Protection against a loss-of-coolant accident (LOCA), eg, a primary coolant pipe break, is required. Power reactors have an emergency core cooling system (ECCS) that comes into play upon initiation of a LOCA. 

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). 

Water as coolant in a nuclear reactor is rendered radioactive by neutron irradiation of corrosion products of materials used in reactor constmction. Key nucHdes and the half-Hves in addition to cobalt-60 are nickel-63 [13981 -37-8] (100 yr), niobium-94 [14681-63-1] (2.4 x 10 yr), and nickel-59 [14336-70-0] (7.6 x lO" yr). Occasionally small leaks in fuel rods allow fission products to enter the cooling water. Cleanup of the water results in LLW. Another source of waste is the residue from appHcations of radionucHdes in medical diagnosis, treatment, research, and industry. Many of these radionucHdes are produced in nuclear reactors, especially in Canada. 

Spent Fuel Treatment. Spent fuel assembhes from nuclear power reactors are highly radioactive because they contain fission products. Relatively few options are available for the treatment of spent fuel. The tubes and the fuel matrix provide considerable containment against attack and release of nucHdes. To minimi2e the volume of spent fuel that must be shipped or disposed of, consoHdation of rods in assembhes into compact bundles of fuel rods has been successfully tested. Alternatively, intact assembhes can be encased in metal containers. 

The primary issue is to prevent groundwater from becoming radioactively contaminated. Thus, the property of concern of the long-lived radioactive species is their solubility in water. The long-lived actinides such as plutonium are metallic and insoluble even if water were to penetrate into the repository. Certain fission-product isotopes such as iodine-129 and technicium-99 are soluble, however, and therefore represent the principal although very low level hazard. Studies of Yucca Mountain, Nevada, tentatively chosen as the site for the spent fuel and high level waste repository, are underway (44). 

Sepa.ra.tion of Plutonium. The principal problem in the purification of metallic plutonium is the separation of a small amount of plutonium (ca 200—900 ppm) from large amounts of uranium, which contain intensely radioactive fission products. The plutonium yield or recovery must be high and the plutonium relatively pure with respect to fission products and light elements, such as lithium, beryUium, or boron. The purity required depends on the intended use for the plutonium. The high yield requirement is imposed by the price or value of the metal and by industrial health considerations, which require extremely low effluent concentrations. 

At least 21 tellurium isotopes are known, with mass numbers from 114 to 134. Of these, eight are stable, ie, 120, 122—126, 128, 130. The others are radioactive and have lifetimes from 2 min to 154 d the heaviest six, 131m, 131,132, 133m, 133, and 134, are fission products (see Radioisotopes). 

Cesium isotopes can be recovered from fission products by digestion in nitric acid, and after filtration of waste the radioactive cesium phosphotungstate is precipitated using phosphotungstic acid. This technique can be used to prepare radioactive cesium metal or compounds. Various processes for removal of Cs isotopes from radioactive waste have been developed including solvent extraction using macrocycHc polyethers (62) or crown ethers (63) and coprecipitation with sodium tetraphenylboron (64). 

A reactor core s fission product inventory is the primary source of radioactivity from which the public is protected by the following independent barriers ... 

In 1962 the report, TID-14844 was published presenting analysis and assumptions coneeming the behavior of containment (essentially Hazard State 2). The TID report postulated the release of all of the noble gas, 50% of the iodine, and 1% of the radioactive solids to the containment. In addition, TBD-14844 provided assumptions for containment leakage (the TMI-2 containment is intact) and for atmo.spheric transport of the fission products. These results form the basis for Regulatory... 

Computer sensitivity studies show that hole size strongly affects the fraction of fission products released from the containment. The failure location determines mitigation due to release into another building in which condensation and particulate removal occur. The quantity released depends on the time of containment fails relative to reactor vessel failure. If containment integrity is maintained for several hours after core melt, then natural and engineered mechanisms (e.g., deposition, condensation, and filtration) can significantly reduce the quantity and radioactivity of the aerosols released to the atmosphere. 

The main drawback to nuclear power is the production of radioactive waste. Spent fuel from a nuclear reactor is considered a high-level radioactive waste, and remains radioactive for a veiy long time. Spent fuel consists of fission products from the U-235 and Pu-239 fission process, and also from unspent U-238, Pu-240, and other heavy metals produced during the fuel cycle. That is why special programs exist for the handling and disposal of nuclear waste. 

Stanley G. Thompson joined my group on October 1, 1942 and it fell to his lot to discover the process that was chosen for use at Clinton Laboratories (in Tennessee) and the Hanford Engineer Works (in the state of Washington) for the separation of plutonium from uranium and the immense intensity of radioactive fission products with which it was produced in the nuclear chain reactors. Again I turn to my journal to tell the story ... 

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