Fission neutron transmutation products

Neutron transmutation of long-lived radioactive minor actinides by the fission process—which entails producing energy and simultaneously turning them into shorter-lived nuclides—is being intensely analyzed in the technical community. Also being proposed is the neutron transmutation of selected long-lived fission products. 

Major issues of radiation effects on V-aUoys are radiation embrittlement at relatively low temperature, and irradiation creep at intermediate temperature. Void swelling is known to be quite small if the alloy contains Ti. He embrittlement is a key issue determining the high-temperature operation limit in fusion neutron environments where 5—10 appm He are produced by transmutation during the irradiation to 1 dpa. However this may be a minor issue for fission neutron environments where the production rate is much lower because the cross-section of He-producing reactions is small when the neutron energy is below 10 MeV. 

Potential fusion appHcations other than electricity production have received some study. For example, radiation and high temperature heat from a fusion reactor could be used to produce hydrogen by the electrolysis or radiolysis of water, which could be employed in the synthesis of portable chemical fuels for transportation or industrial use. The transmutation of radioactive actinide wastes from fission reactors may also be feasible. This idea would utilize the neutrons from a fusion reactor to convert hazardous isotopes into more benign and easier-to-handle species. The practicaUty of these concepts requires further analysis. 

Nuclear power reactors cause the transmutation of chemicals (uranium and plutonium) to fission products using neutrons as the catalyst to produce heat. Fossil furnaces use the chemical reaction of carbon and oxygen to produce CO2 and other wastes to produce heat. There is only one reaction and one purpose for nuclear power reactors there is one reaction but many puiposes for fossil-burning furnaces there are myriad chemical processes and purposes. 

Neutron emission immediately following p transmutation ( ff -delayed neutron emission) is observed for many neutron-rich nuclides, such as Br and many fission products. Delayed neutron emission is very important for the operation of nuclear reactors (chapter 10). 

Fission of uranium was discovered by Hahn and Strassmann in their attempts to produce transuranium elements by irradiation of uranium with neutrons followed by p transmutation of the products. Instead of the expected transuranium elements they found radioactive products with appreciably lower atomic mass such as Ba, indicating the fission of the uranium nuclei. 

Fission of heavy nuclei always results in a high neutron excess of the hssion products, because the neutron-to-proton ratio in heavy nuclides is much larger than in stable nuclides of about half the atomic number, as already explained for spontaneous hssion (Fig. 5.15). The primary fission products formed in about 10 " s by fission and emission of prompt neutrons and y rays decay by a series of successive / transmutations into isobars of increasing atomic number Z. The final products of these decay chains are stable nuclides. 

For use in nuclear weapons, the concentration of °Pu in the plutonium should be low, because the presence of this nuclide leads to the production of appreciable amounts of neutrons by spontaneous fission if the concentration of °Pu is too high the neutron multiplication would start too early with a relatively small multiplication factor, and the energy release would be relatively low. Higher concentrations of " Pu also interfere, because of its transmutation into " Am with a half-life of only 14.35 y. To minimize the formation of " °Pu and " Pu, Pu for use in weapons is, in general, produced in special reactors by low bum-up (<20 000 MWth d per ton). 

The fission products and Cs can be transformed into shorter lived or stable products by charged particle or neutron irradiation. Charged particle irradiation would be very expensive, and irradiation by reactor neutrons would produce almost as much fission products as are destroyed. Therefore the use of intense accelerator driven spallation neutron sources for transmutation by n-irradiation has been suggested. If controlled thermonuclear reactors (CTR) are developed, their excess neutrons could be used for Sr transformation, but less efficient for Cs. 

In the long term ( 600 y) the actinides dominate the risk picture. Continuous neutron irradiation of the actinides finally destroys all of them by fission (cf. Fig. 16.3). The annual production of americium and curium is 5 kg in a 1000 MW LWR, but considerably less in a FBR. Thus if pins of these elements are inserted in a FBR, more americium and curium is destroyed than formed it is estimated that 90% will have been transformed into fission products after 5-10 y. In the future, CTRs could be used for the same purpose. As an alternative it has been suggested to leave the americium and curium in the uranium returned in the LWR cycle. Wastes from transmutation processes will contain some amoimt of longlived nuclides, thus a safe final repository is still needed. 

Some of the MA nuclides (Np, Am, Cm) contained in residual waste from reprocessing have extremely long-term radio toxicity. Means of reducing the radio toxicity of the MA nuclides are presently under investigation. The MA nuclides could produce useful energy if converted into short-lived fission products by neutron bombardment. From this standpoint, a nuclear reactor provides the obvious means for transmutation of MA nuclides. Among the various nuclear reactors, a fast reactor is considered to have the greatest potential to transmute MA effectively, because of its hard neutron spectrum. 

In the reactor transmutation studies on long-lived radioactive waste, nuclear data for MA nuclides and fission products are of primary importance. However, nuclear data for many MA nuclides are still not known to the desired accuracy. Accurate experimental data of neutron cross section for MA are indispensable to establish MA transmutation technology by reactors. Accurate neutron cross section data of RE nuclides become necessary for designing the MA burning core. The data, however, are quite inadequate both in quality and in quantity. 

In studies of Actinide and Fission Product transmutation interest has been focused on accelerator driven subcritical systems which are of particular interest if one considers fast actinide burners where the fissile isotopes of Neptunium, Americium, and Curium have a considerably smaller fraction of delayed neutron emitters (compared to the more common fuels U-238 and U-235), a small Doppler effect and a possibly positive coolant void coefficient. This poses a particular problem of control since the fraction of delayed neutrons is essential for the operation of a nuclear reactor in the critical state. 

The P/T process will be coupled after an improved PUREX process that puts all technetimn, iodine, and neptunium into the waste fraction or into special fractions. Thus, the waste will contain fission products and minor actinides (americium and curium). The process will probably be a solvent extraction process although molten salt systems are also studied as an alternative. The main issue will be to obtain pure Am and Cm fractions for subsequent destruction, i.e., fractions that do not contain any lanthanides. Some of the lanthanides, which are chemically very similar to trivalent actinides, have very high neutron cross sections. Therefore, they must be removed to make actinide burning possible. In some cases, it may also be desirable to transmute some long-lived fission products, e.g., Tc and l, to more shortlived nuclides. 

The fission products that are the most important long-term risks associated with spent fuel are technetium, (exists only as Tc), iodine, and possibly cesium. See O Table 61.3 for a list of other long-lived fission products. The transmutation process for fission products is basically the capture of a neutron (or several neutrons) followed by beta decay until a less toxic (shorter-lived) or stable isotope is formed. For technetium (i.e., for P decaying Tc with T1/2 = 2.13 x 10 years) and iodine (i.e., stable and P decaying with Ti/2 - 1.6 X 10 years), the transmutation can be accomplished in thermal reactors through the following pathways ... 

For both technetium and iodine, capture of a single neutron followed by P decay results in the production of stable isotopes of ruthenium and xenon, respectively. Further, as the irradiation proceeds, additional neutrons can be captured but these products are also stable. In practice, the transmutation of technetium and iodine in conventional thermal reactors will be difficult for several reasons. First, because these are major fission products, they are produced in high yield and thus result in large inventories that must be separated fi-om the HLW and transmuted. Second, the thermal neutron capture cross sections are not particularly large and therefore much higher neutron fluxes are required to significantly reduce the inventory in a reasonable amount of time. Third, the available neutron flux from conventional LWRs is too small (lO neutrons cm s ) to achieve the required level of transmutation in a reasonable length of time. 

Although some of the long-lived fission products have thermal neutron capture cross sections suitable for transmutation in reasonable irradiation times, others do not and would require reactors with very high neutron fluxes to reduce inventories significantly. As a result, dedicated reactors with large thermal fluxes and/or dedicated accelerator-driven systems... 

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