This invention relates to the nuclear art and to the art of separating isotopes and it has particular relationship to the separation of selected isotopes of zirconium to improve the efficiency of nuclear reactors. Zirconium is used predominantly for parts or components of nuclear reactors such as cladding for fuel, thimble tubes, grids, pressure tubes (CANDU reactor), fuel plates, cladding liners and the like. Zirconium contains nuclei which absorb neutrons without producing corresponding fissions. This parasitic absorption reduces the overall efficiency of a reactor and its capability of using the quantity of the nuclear fuel loaded into the reactor which it could use in the absence of this neutron absorption. In addition, the minerals from which zirconium is derived contain hafnium which has a very high neutron absorption cross section. In the processing of these minerals to derive zirconium not all the hafnium can be removed and the efficiency of a reactor is reduced in accordance with the quantity of residual hafnium which is retained in the zirconium of its parts. It is an object of this invention to improve the efficiency of nuclear reactors by reducing absorption of neutrons without resulting fissions by the zirconium components or parts of the reactor.
The naturally occurring isotopes of zirconium, and their neutron cross sections and abundances are shown in the following Table I:
TABLE I __________________________________________________________________________ Isotopes in Natural Zirconium Cross Section (Barns) Product, Cross Section (Barns) Product, Isotope Abundance, A For Thermal Neutrons, .delta..sub.T .delta..sub.T .multidot. A For Fast Neutrons, .delta..sub.F .delta..sub.F .multidot. __________________________________________________________________________ A .sup.90 Zr 0.515 0.03281 0.01690 0.01565 0.00806 .sup.91 Zr 0.112 0.5367 0.06011 0.1604 0.01796 .sup.92 Zr 0.171 0.08601 0.01471 0.03285 0.00562 .sup.94 Zr 0.174 0.03012 0.00524 0.01979 0.00344 .sup.96 Zr 0.028 0.01294 0.00036 0.1996 0.00559 Total 1.000 0.09732 0.04121 __________________________________________________________________________
Table I shows that the effective thermal neutron-absorption cross section of the zirconium could be reduced from its natural value of 0.09732 barns to 0.03281 barns, a factor of about three, if only the isotope .sup.90 Zr were present. Correspondingly the fast neutron absorption cross section would be reduced from its natural value of 0.04121 barns to 0.00806 barns if only .sup.90 Zr were present, a reduction by a factor of five. The maximum yield of a process to separate .sup.90 Zr would be 51.5%. Alternatively, if .sup.91 Zr were removed from the natural mix of isotopes and the remaining isotopic mix of .sup.90 Zr, .sup.92 Zr, .sup.94 Zr, and .sup.96 Zr were utilized, the thermal and fast neutron absorption cross-section would be 0.0419 and 0.0256 barns, respectively. The maximum yield of a process to remove .sup.91 Zr would be 88.8%. If .sup.94 Zr were to be separated it would have attractive thermal and fast neutron absorption cross sections of 0.03012 and 0.01979 barns, respectively, but the maximum yield would be only 17.4%. Such separations are within the scope of equivalents of this invention.
Computer calculations based on existing codes for specific thermal reactors have shown that if .sup.90 Zr were separated and used in a pressurized water reactor having 4 loops, fuel cost savings of 7% would result. Similar calculations relative to removing .sup.91 Zr and using the remaining isotopes in the reactor have shown that a 5% fuel cost saving would result. In either case the fuel cost saving is a substantial amount of money per reactor core loading, such that if all of the saving were assigned to the cost of performing the necessary isotopic separations, it would amount to $115 per pound of Zircaloy-4 alloy for the .sup.90 Zr separation case, and $80 per pound of Zircaloy-4 alloy for the .sup.91 Zr removal case. These figures are based on fuel costs of 50.cent./MBTU.
The ratio of Hf to Zr in zirconium bearing minerals varies widely, typically between 1% and in excess of 10% by weight. However, the commercial ores used for deriving zirconium for nuclear reactor parts have a hafnium content amounting to between 1 and 2% of the zirconium content. Since hafnium has a high thermal neutron absorption cross section of 103 barns and in addition has a series of resonances in the range of 1 to 200 eV, it is necessary to remove it during the processing of the zirconium ore to zirconium metal. This is currently done in the United States by means of a liquid-liquid extraction process involving the preferential extraction of hafnium from a hafnium thiocyanate complex into methyl isobutyl ketone (hexone). This so-called hexone-thiocyanate process accounts for 14% of the capital costs and 20% of the operating costs of a plant which converts beneficiated ore to intermediate metal product (mainly tube blanks), reflecting the difficulty of separating the two metals, which have very similar chemical properties. The current specification of Westinghouse Electric Corporation for the hafnium content of reactor-grade zirconium is 100 ppm (by weight) maximum, while actual values range from .about.30 ppm to .about.87 ppm.
Separation of hafnium from the zirconium which is used to make nuclear-reactor parts has an important advantage in addition to improving the efficiency of a nuclear reactor. The separated hafnium has important uses. It is used in nuclear reactors as a neutron absorbing material for control rods. It is also used as an alloying additive in a number of alloys, for example superalloys, which serve non-nuclear purposes.
Based on the above factors it is an object of this invention to improve the efficiency of a nuclear reactor by enriching the zirconium used in forming nuclear-reactor parts in isotopes having low neutron-absorption cross-sections or depleting this zirconium in isotopes having high neutron-absorption cross section and further by separating and recovering the hafnium from this zirconium and particularly from the depleted zirconium.