Experimental Studies of Actinides in Molten Salts

LA-10340 UC-4 Issued: June 1985 LA—10340 DE85 015368 Experimental Studies of Actinides in Molten Salts James G. Reavis DISCLAIMER This report was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor any agency thereof, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsi- bility fcr the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents 'hat its use would not infringe privately owned rights. Refer- ence herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recom- mendation, or favoring by the United States Government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof. Los Alamos National Laboratory Los Alamos, New Mexico 87545 !IT"ISUnOS OF THIS DOCUMENT IS ONLIMfTW t> EXPERIMENTAL STUDIES OF ACTINIDES IN MOLTEN SALTS by J. G. Reavis ABSTRACT This review stresses techniques used in studies of molten salts containing multigram amounts of actinides exhibiting intense alpha activity but little or no penetrating gamma radiation. The preponderance of studies have used halides because oxygen- containing actinide compounds (other than oxides) are generally unstable at high temperatures. Topics discussed here include special enclosures, materials problems, preparation and purification of acticide elements and compounds, and measurements of various properties of the molten volts. Property measurements discussed are phase relationships, vapor pressure, density, viscosity, absorption spectra, electromotive force, and conductance. I. INTRODUCTION portant differences. The actinides are radioactive, and work with them requires special health protection. A A. Perspective particular hazard of working with plutonium and en- riched uranium is that a critical mass may be assembled The importance of the actinides to society in the next inadvertently. 100 years can hardly be overemphasized. The com- Many of the techniques and examples discussed in munications media have publicized widely the potential this report apply equally well to work with nonactinides destmctiveness of energy release from actinides- Less and have been described elsewhere, but it is hoped thai widely known is that the useful energy derivable from there will be sufficient new material in this discussion to economically recoverable actinides (uranium and reward the reader. thorium) is 5 to 20 times as great as the energy derivable Although 15 elements (including actinium) are in the from all economically recoverable fossil fuels.1 This fact actinide series, only about 6 have been used in molten establishes the economic importance of developing effi- salt studies. The list of anions in molten salt studies is cient processes for preparing thorium, uranium, pluto- similarly limited. The combinations found in an ex- nium, and their compounds. Beyond that economic tensive compilation25 of phase diagrams is listed in importance, the study of the actinides is technically Table I as an example of the cation/anion combinations fascinating. The actinide series is analogous to the studied at high temperatures. In addition to the oxides, lanthanide series, with similarities and differences be- fluorides, and chlorides listed, !4 other phase diagrams tween homologs and interesting and unexplained were given for bromides, sulfates, oxychlorides, phos- changes in properties during progression through the phates, silicates, molybdates, and tungstates of the ac- respective series. tinides. These 14 other diagrams are included in the last The actinides are chemically active metals, only column of Table I. This distribution is typical of that of slightly less active than the alkaline-earth and rare-earth actinide-anior. combinations in other molten salt elements. Molten salt techniques in studies of actinide studies. Because oxides are not generally classified as compounds often resemble those used in studies of rare- salts, this report deals primarily with halides of the most earth and alkaline-earth compounds, but there are im- abundant actinides simply because other compounds isolated by 1950. The other five actinides were dis- TABLE I. Actinide/Anion Combinations Listed in a covered after 1950. Compilation of Phase Diagrams Actinide Oxide Fluoride Chloride Total Cm 1 0 0 1 C. Availability of Actinides Np 1 0 0 3 Pu 4 3 9 16 Chemical studies (including molten salt studies) of Th 36 11 0 53 the actinides are limited by the restricted availability of U 34 30 10 80 the elements. Besides govemmentally imposed regulations, there are certain physical problems, such as limited rate of creation and isolation of the elements, coupled with short half-lives. Only microgram quantities of transcalifornium elements will exist in the have not been studied at high temperatures. One reason foreseeable future. Another problem is intense radiation for this lack is that the oxyanion compounds of actinides emitted by most actinides and their daughter elements, are often unstable. requiring shielding and containment Such facilities are available in oniy a few commercial and university laboratories outside the small number of government labo- B. Historical Perspective ratories built specifically for studies of actinides. Order- of-magnitude values of isolated and purified supplies of Only four actinides (actinium, protactinium, the most abundant actinides and naif-lives of their most thorium, and uranium) exist in nature in concentrations useful isotopes are listed in Table II. The actinides not detectable without the most sophisticated techniques. listed in Table II are available only in microgram or Actinium was discovered in 1899 by Devierne and, submicrogram quantities and probably will never be apparently, independently in 1902 by Geisel but was not studied extensively by molten salt techniques. Studies of isolated in pure form until 1947, when milligram quan- actinium, berkelium, and californium will be limited tities were separated from neutron-irradiated radium.6 because of unavailability and because of their intense Thorium, discovered by Berzelius in 1828, was used radioactivity. The radioactivity of americium and commercially before 1900. Protactinium was dis- curium discourages their study. Protactinium is unique covered in 1918 and was first isolated in milligram in that it is naturally occurring and has a long half-life amounts in 1927. Uranium, discovered in 1789, was but is very expensive to recover because the richest well known to scientists before 1900. The remaining mineral source contains only a few parts per million of the element. About 125 g of the element was isolated actinides are all synthetic and were unknown before 7 about 1940. Plutonium was first isolated in visibu. from about 60 tons of ore by workers in the UK. This quantities as an oxide in September 1942. Neptunium, quantity is sufficient for experimentation using ordinary amencium, curium, berkelium, and californium were techniques; however, its high cost dictates conservation. TABLE II. Availability and Half-Lives of the Most Abundant Ac- tinides Element Quantities Available Prevalent Isotope Half-Life, Yr Ac milligrams 227 22 Th megagrams 232 1.4 X 1010 Pa grams 231 3.2 XW U megagrams 238 43 X 10» Np kilograms 237 2.1 X 10* Pa megagrams 239 2.4 X104 Am grams* 241 458 Cm grams* 244 18 Bk milligrams 249 0.86 a milligrams 249 360 *It is planned that kilograms of compounds of ameridum and enrinm will be isolated, bat batch sizes of only 10 g or less are amenable to studies of molten salts without extensive radiation shielding. n. SPECIAL ENCLOSURES REQUIRED FOR AC- Selecting the enclosure to be used for an actinide TINIDES project is mostly left to the experimenter and his em- ployer. This is not to say that the experimenter has A. Health and Safety Standards much freedom in this matter. Managers of various laboratories apparently differ widely in interpreting re- Anyone wishing to conduct experiments with ac- gulations. The nature of the enclosure required is de- tinides or actinide compounds must face governmental termined in part by regulations, in part by the nature of regulation of their possession and use. The ex- the operation, and in part by the amount of actinide perimenter may have to deal with the International involved. Aqueous chemistry experiments involving Atomic Energy Agency, the United States Nuclear Regu- micrograms of even very highly active members of the latory Commission and Department of Energy, state series are performed in chemical hoods, whereas larger agencies, and local government agencies.8"10 Some of the quantities of the same element in powder form must be regulations limit mere possession of these elements, handled in an enclosure of much higher integrity. much less their use. The various regulatory agencies will Within certain guidelines the safety of each series of insist that certain guidelines be obeyed before operating experiments must be evaluated independently to deter- licenses are issued, and these guidelines will dictate mine the enclosure or hood requirements. tradeoffs between amounts of actinides to be used and the complexity of the facility that must be constructed to handle the material. These guidelines dictate irradiation C. Gloveboxes levels

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