In the Classroom
Radioactivity, Radiation, and the Chemistry of Nuclear Waste†
W
Nick Zevos Department of Chemistry, State University of New York College at Potsdam, Potsdam, NY 13676;
[email protected] Environmental science is of great interest to many students. A number of articles have appeared on this subject in this Journal and suggestions have been made to incorporate environmental chemistry content into the chemistry major (1). The nuclear waste produced by commercial reactors and the Department of Energy (DOE) is of great environmental concern, and the permanent storage of the waste is a very active area of investigation. The ongoing issue of cleaning up federal sites that played a central role in the production of plutonium is receiving a great amount of attention. The Hanford and Savannah River sites in particular pose major environmental problems in the USA. The nuclear waste at these sites is the result of years of plutonium production. Some of the waste at the Hanford site dates back to World War II; most of it is a result of the Cold War arms race. It is now estimated that it will take about four years to “clean up” (or more accurately, stabilize) the site, at a cost of about one billion dollars a year. While the DOE will continue to play a critical role, most of the waste treatment will be carried out by private industry under contract with DOE, and many of our current chemistry students will be involved in the cleanup. Nuclear chemistry has all but disappeared from the course offerings of the undergraduate chemistry major. Yet the use of radioactive isotopes in medicine and biochemistry is still very common. The topic of nuclear waste disposal has been presented in this Journal (2). We have developed a course that is similar to the one described by Williams (3) in its focus on nuclear waste. Our emphasis is on the waste generated from the production of plutonium at DOE sites such as Hanford, WA, and Savannah River, GA. It is popular with both science and nonscience students who must take a physical science course as part of their undergraduate requirement. The Course
Introductory Material While traditional nuclear chemistry courses present a mix of material from nuclear physics and radiochemistry, our aim is a one-semester course that will give students a background for understanding the chemical processes that occur in highlevel nuclear waste. Since general chemistry is a prerequisite for the course, the general chemistry textbook serves as an introduction to the necessary background material, which covers nuclear structure, energy barriers, radioactivity (emission of particles), nuclear fission, chain reactions, and the production of isotopes. Isotopes are linked to the periodic table, with an emphasis on isotopic abundance and the uses of isotopes. This background material leads to discussion of the production of isotopes and the building of nuclear reactors. A number of publications that provide historical material can be obtained from the DOE and may be made available to †
Presented in part at the 15th Biennial Conference on Chemical Education, Waterloo, ON, August 1998.
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students. The number of DOE publications on nuclear waste is very large. Many of them may be obtained from the Environmental Management Information Center (1-800/736-3282). A few are listed below. Gosling, F. G., and Terence R. Fehner. Closing the Circle: The Department of Energy and Environmental Management, 1942–1994; DOE History Division, Draft, Mar 1994. Bates, Mary Ellen, and Kathryn G. Norseth. Site History of Savannah River. Department of Energy History Division, Jan 1993. Belanger, Dian O. Site History of the Oak Ridge Reservation. Department of Energy History Division, Jan 1993. Dudgeon, Ruth A. Site History of the Pantex Plant. Department of Energy History Division, Jan 1993. Gerber, Michele Stenhejem. The Hanford Site: An Anthology of Early Histories. WHC-MR-0435. Richland, WA: Westinghouse Hanford, Co., Oct 1993.
Portions of these publications are given to the students as reading assignments. They provide a historical background to the production of plutonium and related nuclear projects. Students gain an insight into the initial work of the Atomic Energy Commission, the expansion of the effort during the Cold War years, and the cleanup that is now being carried out by the DOE.
Interaction of Ionizing Radiation with Matter This second portion of the course includes safety and health issues associated with radiation and radioactive materials. We focus on the radiation chemistry of water and aqueous solutions. A considerable amount of time is devoted to explaining qualitatively how charged particles (electrons, protons, etc.) and photons interact with atoms and molecules. Emphasis is on the chemistry that occurs—what chemical species are generated from the deposition of the particle energy. The reactions that occur within nuclear waste tanks are excellent examples of radiation-induced reactions. Included here are some recent experimental results. Because of the complex composition of the tank waste a detailed model of the chemistry is not possible, but even a simplified model provides a good picture of the complexity of the tank chemistry. Vitrification of Nuclear Waste and Other Topics The final section of the course is devoted to vitrification of the nuclear waste and to applications that involve radioactive isotopes or radiation chemistry. Topics covered depend on student interest. Past topics included food irradiation, the use of radioactive tracers, and the use of radiation in medicine. Background Information
Tank Radiation Chemistry Concern about the chemical reactions within storage tanks emerged very quickly. Although an effort was made to
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In the Classroom
recover and reuse chemicals used in the separation process, substantial quantities of radioactive isotopes remained in the waste. Therefore, the waste is continuously subjected to radiolysis, which produces a spectrum of products resulting from a variety of mechanisms. Since the waste contains high concentrations of nitrates and nitrites, the possibility of nitration of the organic components is a chief concern. This concern was reinforced by an incident in the Soviet Union in 1957. There was a tremendous explosion in one of the waste storage tanks, which was described in a Russian report in 1989 and circulated by the International Atomic Energy Agency (IAEA) (4 ). The explosion occurred in a waste tank located near Kyshtym, USSR, that contained sodium acetate and sodium nitrite waste from weapons production. It released a large quantity of radioactive material, contaminating a very large area and resulting in the removal of land from normal use and the relocation of a portion of the population. There were similar concerns in the United States. At the Hanford site (5), the waste generated from the production of plutonium was stored for a number of years in singlewalled tanks. Although the tanks were continuously monitored, human error resulted in a large leak that produced a substantial loss of liquid and the contamination of a large volume of soil beneath the storage tank. The tank known as 106-T had been leaking for 51 days, emitting about 2,500 gallons of waste per day. The total loss was estimated at 115,000 gallons. Fortunately, the tank was located well within the interior of the Hanford Reservation. Similar incidents occurred at other sites, for example, Savannah River and Idaho Falls. The DOE contractors that ran the sites dealt with them as they occurred. There was a lack of knowledge of the radiation and thermal chemistry that occurs in the waste storage tanks. A detailed study by the radiation chemistry group at Argonne National Labs (6–8) provides the background for this highly complex radiation chemistry problem.
This is an excellent point in the course to reinforce and expand certain concepts that were introduced in general chemistry. These include free radicals, redox properties, valence states of metal ions, and the determination of pKa’s of acids. Bonding, atomic structure, and elementary chemical kinetics are important throughout the course.
Radiation Chemistry of Water and Aqueous Solutions Radiation chemistry is introduced by a discussion of energy transfer by collisions and ionization in gases, liquids, and solids. This explains how ionizing radiation interacts with water, in particular, to form highly reactive chemical species. The radiolysis of water and aqueous solutions has been studied extensively. The principal species generated are well known (10, 11). In a solution, the radiation interacts with the species that has the highest concentration. Using water as an example,
Redox Properties of Free Radicals The reaction of the primary radicals (Hⴢ, e᎑[aq] + ⴢOH) with dissolved solutes to produce secondary radicals is developed at this time. For some students, this is the first mention of the reactivity of free radicals. Discussion centers on the abstraction, addition, and electron-transfer reactions of the primary radicals with organic and inorganic substrates; redox properties are also mentioned (15). Hundreds of secondary radicals have been produced and studied. Many of them provide an insight into the mechanism of chemical reactions in waste storage tanks. Recent research on organic electron transfer reactions has focused on one-electron redox potentials (16 ). Tables of standard reduction potentials of free radicals have been created and the basic rules that apply to using standard reduction potentials in general chemistry apply here. Radicals may be generated from a large number of organic compounds and compounds of biochemical interest (e.g., porphyrins, ascorbic acid, riboflavin, and DNA bases). This material provides a good basis for discussing health effects of ionizing radiation.
radiation
H2O → e᎑(aq) + H+ + OHⴢ
(1)
The nature of the primary species is strongly dependent on the pH of the solution. In acidic solutions the reducing species produced in reaction 1 will be hydrogen atoms; in alkaline solutions it will be a hydrated electron. In alkaline solutions, the oxidizing radical or hydroxyl radical exists as O᎑. The reactivity of these species is well documented (10, 11). The primary radicals may dimerize Hⴢ + Hⴢ → H2 (2) OHⴢ + OHⴢ → H2O2 or react with one of the dissolved species in the solution.
(3)
Radical Ionization The primary radicals produced by ionizing radiation are very reactive. The hydroxyl radical is a strong oxidizing species (E ° = 2.8 V) and in alkaline solutions it undergoes ionization (12). ⴢOH Oⴢ– + H+ (4) The reactivity of the ionized and unionized forms differ kinetically. For example, the hydroxyl radical will oxidize Fe(CN)64᎑ but Oⴢ– does not. Our general chemistry students carry out an experiment to determine the pKa of a weak acid. Review of that experiment is very instructive at this point. Similarly, the primary reducing species may exist as a hydrogen atom or as the hydrated electron, and these differ greatly in reactivity (13). For example, in reaction with an aqueous solution of chloroacetic acid, the hydrogen atom produces hydrogen by an abstraction reaction, whereas the hydrated electron reacts with the chlorine atom to produce chloride.
Unusual Valence States The hydrated electron is capable of reducing a large number of solutes (E ° = ᎑2.77 V). A number of unusual metal ion oxidation states have been produced by the reaction e᎑(aq) + Mn+ → M(n-1)+
(5)
The effects of metal ion complexation are discussed. Although many kinetic arguments are mentioned here, they are not central to the presentation. The article by Sellers (14 ) is an excellent source of material for unusual valence states.
Secondary Radicals Generated in Storage Tanks The Organic Tanks Safety group at Hanford has studied the radiation chemistry in nuclear waste storage tanks using
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simulated ion compositions. A typical composition is given in Table 1. The composition of the simulants is based on analytical analysis of the tanks and the operating records from various Hanford plants. Since nitrate and nitrite are present in very high concentrations, the following reactions take place readily. The reaction of the hydroxyl radical with nitrite is shown in reactions 6 and 7: (6) NO2᎑ + ⴢOH → NO2 + OH᎑ or, at high pH, O᎑ + NO2᎑(aq) → NO2 + 2OH᎑
(7)
Reaction 6 has a diffusion-controlled rate constant (k = 1 × 10 10 M᎑1 s᎑1) and the reaction generates nitrogen dioxide. Reaction 7 takes place at a slower rate and also produces NO2; this reaction will dominate in the tanks because of their high alkalinity. The overall result is that both forms of the oxidizing species produce nitrogen dioxide. The primary reducing species, the hydrated electron that is generated by the ionizing radiation, reacts very readily with nitrate ion to generate a double-charged intermediate: NO3᎑ + e᎑(aq) → NO32᎑
(8)
Both the absorption spectrum and the charge on this species have been established (17 ). At high concentrations of water, this dianion rapidly decays to produce nitrogen dioxide: NO32᎑ + H2O → NO2 + 2OH᎑ (k9 = 5.5 × 104 M᎑1) (9) Therefore, both the oxidizing and reducing radicals initially produced by the ionizing radiation are converted to nitrogen dioxide, which is a secondary free radical. Some of the NO2 will dimerize to N2O4 and a steady-state concentration of NO2 and N2O4 is generated. 2NO2
N2O4
(10)
This equilibrium system has been studied extensively in the gas phase. In an aqueous solution the dimer N2O4 reacts with water to produce nitrite and nitrate: N2O4 + H2O → NO2᎑ + NO3᎑ + 2H+
(11)
This reaction has received much attention by environmental chemists because it may be the primary path for the production of acid rain. The reported rate constant for reaction 11 is 1.8 × 108 M᎑1 at 25 °C (18, 19). This reaction regenerates some of the nitrite and nitrate that were consumed by the reaction of the primary radicals produced by the ionizing radiation. The hydrolysis competes with the reaction of NO2 with the other solutes in the tank waste. This is an example of the generation of secondary radicals, in this case NO2 (electron dot formulas are instructive). The reactions of NO2 with the other components of the tank waste (e.g., the chelators employed to remove fission products) and a pathway to some common products found in the tank are discussed.
Vitrification It is apparent that storage in a liquid waste tank is not a permanent solution to the problem. The reactive chemistry and the mobility of the waste material causes wall reactions that ultimately cause the tanks to rupture. Permanent storage must involve solidification to immobilize the radioactive isotopes and reduce the volume of the waste (9).
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Table 1. Composition of Simulant SIM-PAS-95-1c Species
Amount in Amount in Mixture/ Species Mixture/ (mg/g) (mg/g) 16.9
PO43᎑
0.507
3.1
SO4
2᎑
0.506
17.3
Bi2+
0.419
HOCH2CO2᎑ 11.3
Ce3+
0.377
NO3᎑
109
Ni2+
0.099
107
2+
0.080
HEDTA3᎑ 4᎑
EDTA
Citrate3᎑
+
Na
Ca
OH᎑
39.3
Mn2+
0.068
NO2᎑
37.9
Cr3+
0.044
Cl ᎑
0.004
Fe3+ 3+
2.61
2+
0.003 0.003
Al
1.493
Pd
F᎑
1.281
Rh3+
Pb2+
1.066
Ru4+ H2O
0.003 640
NOTE: The composition of this simulant is based on B plant chemical inventories. Organic Tanks Safety Program FY97, Waste Aging Studies.
Certain conditions must be met before the solids are ready for final disposal. They must be: 1. Solid and not in a powder form or a form that is readily dispersible 2. Insoluble or trapped in a matrix that is insoluble 3. Thermally stable with good heat conductance 4. Radiation stable
The current treatment of the waste involves reducing the liquid waste to a solid. This results in the decomposition of the nitrates and nitrites, and the metal ions are converted into oxides. The solid obtained from this process is not suitable for storage and requires further treatment. It is usually converted into borosilicate glass and surrounded by a suitable container. The glass usually contains about 20 wt % of fission product oxides. The amount is dependent on how long the waste was stored before treatment. Short cooling times mean a higher content of fission products, which could cause the glass to melt and crystallize. The glass matrixes containing the nuclear waste are usually collected into stainless steel cylinders. Ceramic, graphite, Al2O3 and a variety of other coatings have been used, depending on the history of the waste and the type of storage area that will be used. These coatings must provide protection against corrosion in the final repository.
Storage Site Selection After the waste is immobilized and encapsulated, it must be placed in a site for permanent storage. The storage site should provide long-term isolation of waste (20). It should have geologic stability, characterized by Absence of earthquakes and volcanic activity Absence of surface waters; low ground-water flow; and general dryness Good heat conductivity to disperse the heat generated from the radioactive decay
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In the Classroom
The site should also be remote and guarded against future development by humans. Several countries have evaluated a number of sites, such as underground repositories in granite, basalt, and salt formations. The former Federal Republic of Germany stored low-level waste in an abandoned salt mine. In Sweden, construction of the Aspo Hard Rock Laboratory Oskarshamn (21) began in 1990 and the facility provides information on site selection to a number of commercial utilities that employ nuclear reactors. Selection of a final storage site is one of the more difficult problems of nuclear waste disposal.
Cleanup at Hanford The decontamination process at the Hanford, WA, site continues. It was disappointing when the DOE canceled the billion-dollar contract with BNFL, the U.S. subsidiary of British Nuclear Fuels Ltd. (22). Former Secretary of Energy Bill Richardson then announced that Bechtel Washington had been selected to design, construct, and commission a new wastetreatment and immobilization plant to treat the 54 million gallons of tank waste that is a by-product of World War II and the Cold War (23). Course Summary The students are interested in the history of the nuclear industry that produced plutonium, which they learn about by reading the assigned DOE articles. There is a continuous interchange between the historical and chemical parts of the story. The chemistry is at a level slightly beyond general chemistry. The students respond well to the radiation chemistry because it provides them with a picture of how radiationinduced radicals react with various components in the waste tanks, food, and living systems. This material also exposes them to some interesting areas of active research. The students seem very comfortable with this, as indicated by their evaluations of the course. Students are encouraged to submit questions related to nuclear phenomena to the class. They have found articles in a variety of publications related to nuclear and radiation chemistry. The Internet is a great source of information, some of which reinforces the chemistry that is part of the course. The course includes a visit to the nuclear power plant in Oswego, NY. The center provides a guided tour and a demonstration of the daily operation of the plant. A large display area is devoted to the history of power generation. Much of the display is devoted to nuclear power, and the students are able to revisit many of the topics that were part of the course. This course was originally an upper-division elective primarily for chemistry majors. Over the years the number of students taking the course decreased drastically and it was restructured into a course for nonmajors. Three other nonmajor chemistry courses are offered by the chemistry department at SUNY Potsdam. One of them (CHEM 100) has no prerequisites and is very popular. The other two (Environmental Chemistry and Forensic Chemistry) require one semester of any college science. The nuclear/radiation course has one year of general chemistry as a prerequisite and also has the lowest enrollments of the four. Introduction of the nuclear waste component increased the enrollment in the course from 5–6 students to 12 students.
Conclusion The chemistry of nuclear waste addresses an important environmental problem that attracts and interests undergraduate students. Our course has been received very well. Being interdisciplinary it attracts students with majors in biology and geology, as well as chemistry and physics. Nuclear power is experiencing new life. A number of nuclear power plants have recently been relicensed. With the growing demand for electrical power, continued utility growth is predicted and nuclear power plants will continue to contribute to the generation of electrical power. Acknowledgments I wish to thank Donald Camaioni for a stimulating sabbatical year at Pacific Northwest Laboratories, where I was involved with the chemistry that occurs in nuclear waste. I also would like to thank Dan Meisel, Notre Dame Radiation Laboratories, for providing the support for that sabbatical. W
Supplemental Material
A list of topics covered in the course, additional references, and further details of radiation and storage tank chemistry are available in this issue of JCE Online. Literature Cited 1. 2. 3. 4.
5. 6.
7.
8.
9. 10. 11. 12. 13. 14. 15.
Hartman, I. S.; Soltzberg, L. J. J. Chem. Educ. 1995, 72, 981. Hoffman, D. C.; Choppin, G. R. J. Chem. Educ. 1986, 63, 1059. Williams, D. H. J. Chem. Educ. 1995, 72, 971; 1986, 63, 1059. Drozhko, E. G., et al. Experience in Radioactive Waste Management at the Soviet Radiochemical Plant and the Main Approaches to Waste Reliable Confinement Development; Ministry of Nuclear Power Engineering and Industry, released through IAEA, undated. Gillette, R. Science 1973, 181, 728. Meisel, D.; Diamond, H.; Horwitz, E. P.; Jonah, C. D.; Matheson, M. S.; Sauer, M. C. J.; Sullivan, J. C. Radiation Chemistry of Synthetic Waste; Argonne National Latoratory: Argonne, IL, 1991; ANL Report 91/40. Meisel, D.; Diamond, J.; Horwitz, E. P.; Jonah, C. D.; Matheson, M. S.; Sauer, M. C. J.; Sullivan, J. C.; Barnabas, F.; Cerny, E.; Cheng, Y. D. Radiolytic Generation of Gases from Synthetic Waste; Argonne National Latoratory: Argonne, IL, 1991; ANL Report 91/41. Meisel, D.; Jonah, C. D.; Kapoor, S.; Matheson, M. S.; Sauer, M. C. J. Radiolytic and Radiolytically Induced Generation of Gases from Synthetic Wastes; Argonne National Latoratory: Argonne, IL, 1993; ANL Report 93/43. Choppin, G. R.; Rydberg, J. Nuclear Chemistry Theory and Applications; Pergamon: New York, 1980. Radiation Chemistry: State-of-the-Art Symposium. J. Chem. Educ. 1981, 58, 83–173. Spinks, J. W. T.; Woods, R. J. An Introduction to Radiation Chemistry, 2nd ed.; Wiley: New York, 1976. Rabani, J.; Matheson, M. S. J. Am. Chem. Soc. 1964, 86, 3175. Hart, E. J.; Anbar, M. The Hydrated Electron; Wiley: New York, 1970. Sellers R. M. J. Chem. Educ. 1981, 58, 114. Neta, P. J. Chem. Educ. 1981, 58, 110.
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In the Classroom 16. Additional one-electron reduction potentials may be found at the Notre Dame Radiation Laboratory (NDRL), Radiation Chemistry Data Center Web site; http://www.rcdc.nd.edu (accessed Feb 2002). See Wardman, Peter. J. Phys. Chem. Ref. Data 1989, 18, 1637–1755; a table of 700 one-electron couples in aqueous solution is presented. 17. Meisel, D.; Camaioni, D. M.; Orlando, T. M. In Nuclear Site Remediation: First Accomplishments of the Environmental Science Management Program; Eller, P. G.; Heineman, W. R., Eds.;
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18. 19. 20. 21. 22. 23.
ACS Symposium Series 778; American Chemical Society: Washington, DC, 2001; pp 342–361. Park, J. Y.; Lee, Y. J. Phys. Chem. 1988, 92, 6294–6302. Lee, Y.; Swartz, S. E. J. Phys. Chem. 1981, 85, 840–844. Choppin, G. R. J. Chem. Educ. 1994, 71, 827. Banwart S.; Wickberg, P.; Olsson, O. Environ. Sci. Technol. News 1997, 31, 510A–514A. Johnson, J. Chem. Eng. News 2000, 78 (May 15), 13. Johnson, J. Chem. Eng. News 2000, 78 (Oct 2), 39.
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