Boron isotopes, separation

Examination of possible systems for boron isotope separation resulted in the selection of the multistage exchange-distillation of boron trifluoride—dimethyl ether complex, BF3 -0(CH3 )2, as a method for B production (21,22). Isotope fractionation in this process is achieved by the distillation of the complex at reduced pressure, ie, 20 kPa (150 torr), in a tapered cascade of multiplate columns. Although the process involves reflux by evaporation and condensation, the isotope separation is a result of exchange between the Hquid and gaseous phases. 

Boron trifluoride is also employed in nuclear technology by uti1i2ing several nuclear characteristics of the boron atom. Of the two isotopes, B and B, only B has a significant absorption cross section for thermal neutrons. It is used in " BF as a neutron-absorbing medium in proportional neutron counters and for controlling nuclear reactors (qv). Some of the complexes of trifluoroborane have been used for the separation of the boron isotopes and the enrichment of B as (84). 

Laser isotope separation techniques have been demonstrated for many elements, including hydrogen, boron, carbon, nitrogen, oxygen, sHicon, sulfur, chlorine, titanium, selenium, bromine, molybdenum, barium, osmium, mercury, and some of the rare-earth elements. The most significant separation involves uranium, separating uranium-235 [15117-96-1], from uranium-238 [7440-61-1], (see Uranium and uranium compounds). The... 

B is a powerful neutron absorber and has been employed in reactor control rods, neutron detectors, and other applications. Cascades based on exchange distillation of boron-ether complexes have usefully large a s and were used for 10B/UB isotope separation by the US DOE. Exchange distillation takes advantage of the fact that condensed phase/vapor phase separation factors can be enhanced (as compared to liquid/vapor a s) by association/dissociation equilibria in one or the other phase. At the normal boiling point (173 K) the VPIE for... 

Kakihana H, Kotaka M, Satoh S, Nomura M, Okamoto M (1977) Fundamental studies on the ion-exchange separation of boron isotopes. Bull Chem Soc Japan 50 158-163 Kieffer SW (1982) Thermodynamics and lattice vibrations of minerals 5. Applications to phase equilibria, isotopic fractionation, and high-pressure thermodynamic properties. Rev Geophys Space Phys 20 827-... 

For production of uranium compounds suitable for use in nuclear reactors or for isotope separation, further chemical procedures are applied, as indicated in Fig. 11.9. Nuclear purity means that the compounds are free of nuclides with high neutron absorption cross section, i.e. free of boron, cadmium and rare-earth elements. Selective extraction procedures are most suitable for this purpose. Uranyl nitrate hexa-hydrate (U02(N03)2 6H2O UNH) is obtained by concentration of solutions of U02(N03)2, and ammonium diuranate ((NH4)2U207 ADU) by precipitation with ammonia. 

This review deals with studies performed at the Oak Ridge National Laboratory concerning the chemical fractionation of boron isotopes between BF3 and its molecular addition compounds. This research resulted in the development of a new separation process which was superior to methods previously employed. The work also led to a theoretical explanation of the exchange reaction which accounted for the anomalous concentration of boron-10 in the molecular addition compound, and the observed variations of the isotopic equilibrium constants as a function of different donors. The model predicted maximum isotopic equilibrium constants for the exchange reaction which were consistent with the experimental data. It also predicted the behavior of the other boron halides. 

T Taturally occurring boron contains 19.8% boron-10 and 80.2% boron-11. The absorption cross section of the natural product for thermal neutrons is 752 barns for pure boron-10 and boron-11, the corresponding values (8) are 3837 and 0.005 barns, respectively. Thus, isotopically pure boron-10 is five times more effective as a neutron shield than natural boron. In view of this difference, it is not surprising that a demand arose, very early in the nuclear era, for separated boron isotopes. 

Thus, the fractionation of boron isotopes between boron trifluoride and its molecular addition compounds may be explained in terms of unique characteristics of the boron and fluorine atoms. The model presented here adequately describes the direction of enrichment as well as the magnitude of the equilibrium constant. It accounts for observed variations in the size of fractionation factor for different donors as well as for different substituents on the same donor. The model correctly predicts the isotopic behavior of other boron halides when these are substituted for BF3 in the exchange reaction. Finally, the proposed model provides insight into the design of a practical chemical exchange system for the separation of boron isotopes. 

Uranium (ca. 20% is used as the fuel, but mainly with 239pu in the form of a UO2/PUO2 mixture. The breeding blanket consists of depleted uranium from isotope separation plants or from reprocessing plants for spent nuclear fuels. Axially movable boron carbide absorbers are distributed in the fuel zone for shutting down purposes. The uranium utilized can be ca. 100 times better utilized than e.g. in light-water reactors. 

Tobin J. Marks (NAS) is the Charles E. and Emma H. Morrison Professor and Vladimir N. Iptieff Professor of Chemistry at Northwestern University. Through landmark synthetic, mechanistic, and thermodynamic investigations, he and his students opened a new portion of the Periodic Table to organometallic chemistry. He has also made major advances in solid state, polymer, bioinorganic, and boron hydride chemistry and in photochemical isotope separation. He received his B.S. from the University of Maryland and his Ph.D. from Massachusetts Institute of Technology. 

Because there is no separation of boron isotopes in the equilibrium vsqmrization of the complex, the overall separation factor in the combined process of evaporation and dissociation is... 

C4. Conn, A. L., and J. E. Wolfe Large Scale Separation of Boron Isotopes, paper presented at 132nd Meeting of American Chemical Society, New York, Sept. 12, 1957. 

Kl. Kilpatrick, M., et al. Separation of Boron Isotopes, National Nuclear Energy Series, vol. 

Nl. Nettiey, P. T., D. K. Cartwrig)it, and H. Kronberger The Production of Boron by Low-Temperature Distillation of Boron Trifluoride, Proceedings of the International Symposium on Isotope Separation, Interscience, New York, 1958, p. 385. 

An ingenious approach to this problem is called boron neutron capture therapy (BNCT). This technique brings together two components, each of which separately has minimal harmful effects on the cells. The first component uses a compound containing a stable boron isotope ( °B) that can be concentrated in tumor cells. The second component is a beam of low-energy neutrons. Upon capturing a neutron, the following nuclear reaction takes place ... 

If low-cost methods for isotopic separation of boron are developed, these salts may be used as primary salts in the AHTR, a fast reactor, or a fusion machine. Boron-11 has a very low nuclear cross-section (0.05 bam). 

The neutron absorption cross-sections of any liquid salt for reactor applications must be low to avoid excessive parasitic capture of neutrons. For thermal and intermediate neutron spectrum reactors, this probably eliminates chloride salts with their higher nuclear cross sections, even if the high cross section Cl is removed. Only fluoride salts are candidates. A wide variety of atoms have low cross sections however, the realistic candidates are also restricted by the requirements of thermodynamic stability to ensure viable materials of construction for the container. Table XXVI-5 shows the primary salt options and their cross sections. If either lithium or boron is used as a salt component, isotopically separated lithium and boron are required to have a salt with a low absorption cross section. 

Some lanthanides, such as gadolinium (48,800 b), europium (4,570 b), or samarium (5,600 b), have neutron absorption cross sections that are higher than boron (760 b) or even (3,800 b), which makes materials containing lanthanide elements potentially attractive neutron absorbers. Additionally, whereas isotopic separation increases the cost of 6 to several 1000/kg, the increased industrial usage of these rare-earth elements has reduced their cost to the 25 to 40/kg range. 

Abundance sensitivity becomes critical when measuring extreme isotope ratios (>100 000) or when neighbouring elements are present in the sample at high concentrations. In ICP-MS this occurs for isotope ratio measurements of elements such as U and Th (extreme ratios) or boron isotope ratio measurements in the presence of high carbon contents (tailing of onto "B). Specially designed filters (electrostatic filter, quadrupole lenses) can increase the abundance sensitivity up to 2 x 10 (see Chapter 2, section 2.2.1 for further details). In the case of boron, a simple matrix separation can help. In the majority of isotope ratio measurements however abundance sensitivity effects are negligible. 

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