Current Status of the Thermodynamic Data for Technetium and Its Compounds and Aqueous Species

Joseph A. Rard
2005 Journal of Nuclear and Radiochemical Sciences  
The technetium isotope 99Tc is a major fission product from nuclear reactors. Because 99Tc has few applications outside of scientific research, most of this technetium will ultimately be disposed of as nuclear waste. Geochemical modeling of the dissolution of nuclear waste, and of the solubility and speciation of the dissolved radionuclides in groundwaters, is an important part of the Performance Asssessment of the safety of a nuclear waste repository that relies on the availability of a
more » ... lly-assessed thermodynamic database. Such a database for technetium was published in the book Chemical Thermodynamics of Technetium, covering the published literature through 1998. This database is described here, along with a discussion of more recent relevant studies. Gaps in the knowledge of the chemical and thermodynamic properties of technetium are identified here, and recommendations are made for measurements that are required to eliminate these gaps. 1.Introduction Technetium is produced in major amounts by the thermal neutron fission of 235U and 239Pu reactor fuels. According to Lieser,1 the burn up of 1 ton of 3% enriched uranium in a nuclear reactor produces about 1 kg of 99Tc (6.13% fission yield), which is about 10 mass-% of the fission products. Popova et al.2 in 2003 cited an estimate that 60 tonnes of 99Tc have been accumulated from spent reactor fuel. El-Weare et al. cited an estimate that 10% of the 99Tc produced by nuclear reactors has been released into the environment, along with an order of magnitude smaller amount of 99Tc as a relic from the atmospheric testing of nuclear weapons. The metastable isotope 99mTc (produced by neutron irradiation of 98Mo) is used extensively for diagnostic purposes in nuclear medicine;4 99mTc undergoes gamma decay to the ground state 99Tc, with t1/2=6.0 hours, which then occurs in the medical waste. 99Tc is also produced by the spontaneous fission of the 238U present in uranium ores, but the amount formed is so small as to be nearly undetectable.5 Unlike 99mTc, 99Tc and other technetium isotopes have relatively few uses outside of scientific research. Depending on the type of fuel used in the nuclear reactor, the extent of burn up, and other reactor conditions, intermetallic (Mo,Tc,Ru,Rh,Pd) particles may form. These particles are quite difficult to dissolve and usually become part of the highly active waste. The remaining waste is generally dissolved in nitric acid as part of the PUREX process or its modifications, and the dissolved 99Tc is co-extracted with uranium and plutonium as mixed anion complexes such as UO2(NO3)(TcO4)(TBP)2 solvated with tributlyphosphate.
doi:10.14494/jnrs2000.6.3_197 fatcat:esefww4jqvc5xn7a7tk7da2xm4