Patent Number: 
Section: description

The strontium-89 production method is based upon a unique ability to effect not only the final radioisotopes, but also its precursors produced as a result of the nuclear transformation of products in the decay chain of elements with mass 89 occurring in a nuclear solution reactor. The decay chain is Se89xe2x86x92Br89xe2x86x92Kr89xe2x86x92Rb89xe2x86x92Sr89. A liquid fuel nuclear reactor having a uranyl sulfate water solution (UO2SO4) core is used in the present invention. Uranium-235 and/or uranium-233 can be used as fissionable material in the fuel solution of uranyl sulfate. The Russian Argus reactor was the particular reactor used. It used 90% enriched U235 in a concentration of 73.2 g/l in the water solution. The uranyl sulfate water solution volume (pH=1) was 22 liters. It can be brought up to its rated power of 20 kW in 20 minutes. The thermal neutron flux density in the central channel is 5xc3x971011 neutrons/cm2s. Homogenous solution fuel reactors have a number of advantages over hard fuel reactors. They have large negative temperature and power reactivity effects, which provides for their high nuclear safety. The core design is much simpler. There are no fuel element cladding spacers and other parts reducing the neutron characteristics. Solution preparation is much cheaper than fuel element production. Solution fuel loading (pouring) is much easier too, and makes it possible to change the fissionable material concentration in fuel or solution volume if necessary. There can be no local over-heating provoked by power density field deformations in the core of the solution reactor, thanks to good conditions for heat transfer. These reactors are simple and reliable in operation and do not require a large staff for their operation. A number of radioactive inert gases are produced in uranyl sulfate solution reactor during its operation, including the desired krypton-89. The majority of these gases leave the solution in the gas phase, accumulating above the liquid surface. The process by which this takes place is based on xe2x80x9cradiolytic boiling.xe2x80x9d Gas bubbles containing water vapor and hydrogen form in the tracks of fission fragments. The vapor is condensed within about 10xe2x88x928 seconds and a gas bubble forms having a radius of about 10xe2x88x925 cm. Fission fragments either get into the gaseous bubble during its generation or afterwards by diffusing from the solution. They then migrate to the surface of the fuel solution. The radiolytic gas bubbles rise to the surface in only a couple of seconds, making it possible to remove relatively short-life radioisotopes, such as krypton-89. Bubbling the fuel with an inert gas can speed up this process of removal of fragment gases. Krypton-89, along with small quantities of other fission fragment elements are produced at the same time. The main chains of fission products"" decay resulting in strontium radionuclides whose gaseous precursors have a half-life of more than one second are shown in FIGS. 1A to 1C. One of the fission products is krypton-89 (Kr89), a radioactive isotope of the inert gas, krypton, preceding strontium-89 in the decay chain of fission products with an atomic mass of 89. It has a half-life of 3.2 minutes, decaying to rubidium-89. Rubidium-89 decays with a half-life of 15.4 minutes to the desired strontium-89. Other isotopes of krypton, however, also bubble to the surface, including the highly undesirable precursor to strontium-90, krypton-90. Krypton-90 decays in 33 seconds to rubidium-90 and in 2.91 minutes to strontium-90. Because krypton-89 and krypton-90 are gases and because of the differential in half-life of the two isotopes, it is relatively easy to separate the two. There is no such possibility in the core of a typical nuclear reactor in which the fissionable material, e.g., U235, is a hard oxide or metal enclosed in the cladding of fuel elements. Other radioactive components with half-lives short compared to krypton-89 can also be readily separated. The high productivity of this method is primarily the result of: (1) the large cross-section of the decay reaction (n,f) of up to 600-800xc3x9710xe2x88x9224 for thermal neutrons for such nuclei as U235, U233, or Pu239; and (2) the ability to remove the krypton-89 from other gaseous end products of the reaction due to differential decay. For a unit target, this method is about 1000 times more efficient than the prior art. Because the half-life of krypton-89 (190.7 seconds) is significantly longer than that of krypton-90 (32.2 seconds), it is possible to decrease the content of strontium-90 in the mixture to about 10xe2x88x924 atomic percent, providing for high radioisotope purity in the strontium-89. The method of strontium extraction via a continuous gas loop is illustrated in FIG. 2. The process is begun after the transitional processes bound up with the reactor start-up are finished (about 20 minutes). Referring to FIG. 2, valves 3 and 9 are opened and a gas pump 5 is turned on. Gas from above the fuel solution is moved to a delaying line 4. The delaying line is designed to keep the gas from arriving at the precipitation device 7 for the time necessary for krypton-90 to decay to strontium-90, thereby removing it from the gas mixture. Rubidium and strontium isotopes that have not precipitated in the delaying line settle in the filter 6. The diameter of the delaying line pipe is determined by the condition of laminar gas flow in the pipe. The pipe""s length is determined by the delay time for a preset gas flow rate. (If the gas flow rate is about 2 l/min, a delay time of ten minutes is achieved when the pipe inner diameter is 10 mm and the pipe length is 255 meters. If the diameter were 20 mm, a delay line length of 64 meter would give a 10-minute delay.) A ten minute delay yields a radionuclide purity (Sr90/Sr89) of about 3xc3x9710xe2x88x928. After going through the delaying line, the gas arrives at the strontium-89 precipitation device 7. The precipitation device is another pipe whose diameter and length are designed for a delay period sufficient for the remaining krypton-89 to decay to strontium-89. This would be about 30 minutes at a gas flow rate of 2 l/minute. Those isotopes of rubidium and strontium, which have not precipitated in the precipitation device, pass through it and settle in the filter 8. The gas, less those fission fragments that have precipitated out or otherwise been removed, is returned to the reactor. After the cycle of strontium-89 production is completed, the valves 3, 9 are closed. Strontium-89 deposited in the precipitation device and in the filter 8 are subsequently extracted. The circulating gas flow removes water vapor from the fuel solution. The initial part of the gas pipe 10 shown in FIG. 2 is inclined so that water vapor is condensed on the pipe wall and the water runs back into the reactor vessel by gravity preventing fuel solution water loss. A trap 11 is indicated in FIG. 2 at the entrance to the gas loop to hinder non-gaseous fission fragments moved by the gas flow over the fuel solution from getting into the gas loop. If the precipitation rate of strontium-89 is high, most of it will accumulate in the precipitation device 7. An acid solution can then be used to wash out strontium-89 from which it is subsequently extracted and subjected to radiochemical purification. If the precipitation rate is low, most of the strontium-89 will accumulate in the filter 8. This filter can consist of thin, fine nets of stainless steel. The strontium-89 can then be extracted by pumping an acid solution through the filter. Alternatively, a removable filter could be used with extraction of the strontium-89 being done at a later time.