Nuclear Chemistry-State
of the Rrt for Teachers
Aspects of Nuclear Waste Disposal of Use in Teaching Basic Chemistry Gregory R. Choppin Florida State University, Tallahassee, FL 32306-3006
Currently the United States has 109 nuclear power plants in operation in 33 states. Nationally, about 20% of our electricity is produced by these plants. The percentage of nuclear-oroduccd clectricitv is hiehest in Vermont. 76'b. which compares with the 80% for France as a nation. Because energy is tied closely to economic development, predictions for such development and population growth led to the projection by the International Institute for Applied Statistical Analysis (Vienna) that energy consumption would increase by 2030 A.D. by between 190% and 260%. The low figure was obtained by assuming that the economy will grow only 2% per year and that the current practice of energy-efficient measures continues ( I ) . Since that prediction in 1980, global use has already exceeded this predicted lower rate by 7%. This indicates at least a doubling of energy demand in about thirty years. Because fossil fuel reserves may become depleted or their use restricted by environmental concerns, increased need for nuclear power seems inevitable. The growth of nuclear power has been slowed in the US. by public and governmental concerns about the safety of nuclear plants as well as the ability to handle and dispose of the nuclear wastes produced. To ensure safety in both power production and waste disposal, we must attract a proper portion of the better, technologically oriented students to this field. Moreover, it is equally important to provide a basic understanding of nuclear poweFto the general ~ublic.who include our next eeneration of students. In this article, various aspects of nuclear waste disposal are discussed for their value in providing pedagogical examples, both for illustrating chemical principles and for showing the practical value of chemistry in this important field.
-
~
-
826
Journal of Chemical Education
The Nature of the Stored Material
Selection of a Solid for Permanent Storage Energlr Changes in Crystalline and Noncrystalline Solids The choice of method for permanent storage of nuclear wastes can be used to illustrate the differences between crystalline and noncrystalline solids. It is believed that storage of radioactive wastes in crystalline solids would be less efficient in retarding.release of the radioactive substances to the surroundings than storage in glass. In crystalline solids, the radioactive decay reactions would transform the radioactive atoms in the lattice to atoms of a different element. Moreover, radioactive reaction results in release of much more energy than that released from chemical bonds. The result is new atoms with an initial +8 to +12 charge state and lattice imperfections, yielding a lattice with higher energy than that of the normal crystalline pattern. Such imperfections increase the solubility of the crystals, thereby increasing the release of the residual radioactivity. In contrast, becausc glass 1s a noncrystalhnc supcrn~olcd liauld. no lattice exists. Thus. lattice lmoerfectlonsare not As a result, the naturally "imperfectnglass can "store" much more of the radioactivity without significant increases in solubility and thus also without release of the radioactivity. At present, the most radioactive fraction of the waste is included in borosilicate glass within sealed metal canisters. Far-Field Behavior These considerations concerning appropriate storage assume eventual contact between the stored material and water in the repository. Permanently dry storage sites
Figure 1. A schematic presentation of a deep geological repository for permanent storage of nuclear wastes. would never allow release of nongaseous radioactive elements in the absence of explosive forces (e.g., from earthquakes or volcanoes). Choice of dry, deep, underground sites free from geological hazards is a primary factor in establishment of nuclear waste repositories. The volume within the repository (Fig. 1) into which radioactivity might he released is called the near field. The aqueous phase in this volume is likely to he restricted to rock fissures or to small saturated inclusions in salt beds. I t also may he hot (perhaps as high as 150 'C) and subject to radiolysis. Such conditions make it difficult to discuss such systems in beginning chemistry courses because the conditions are not common to the students'everyday experiences. When the dissolved radionuclides migrate slowly from this near-field volume, they enter the far field, which represents the normal surroundings. The far field lies beyond the region that is directly perturbed, for example, by the heat or radiolytic effects of the repository Calculations indicate that such near-field intrusion could take 10,000 to 100,000 years or even much longer. I n such a time period, the vast majority of the radioactivity will have been spent. The heaviest elements (e.g., plutonium) and the longest lived fission products (e.g., technetium or iodine) will remain. Of these, plutonium offers the most interesting pedagogical opportunities.
some of these are reviewed briefly to show how they may serve a s examples in beginning chemistry courses.
Kinetics and Mechanism Plutonium can exist as a cation in four different oxidation states (111-VI). In oxidation states I11 and N, in strong mineral acid solution, plutonium i s present a s the simple cation. In oxidation states V and VI, however, it exists as the linear plutonyl cations, PuOz+ and Pu0z2+.The redox
Redox Potentials for Pu
The Chemical Diversity of Plutonium
Plutonium has a diverse set of chemical properties a s shown by Figure 2 (2). Many different chemical reactions could occur if plutonium were released to the environment. Redox, complexation, precipitation, absorption, and ionexchange are all possible. I n the remainder of this article
I Figure 2. Potential diagram with Elnvalues for plutonium at pH and pH 8. Volume 71 Number 10 October 1994
827
P" Figure 4. The variation of Am(lll) species as a function of pH Figure 3. The variation of U O species ~ as a function of pH in a surface water under normal atmospheric pressure. chemistry of plutonium is made quite challenging by the similar half-cell potentials for these states (see Fig. 2). Which state or states are present in a particular environmental system depends on the oxic (or anoxic) state of the water and the wncentration of plutonium. For example, in laboratory solutions of pH P 9 , plutonium at tracer levels M) exists as KOz+.However, if the concentration of plutonium or the pH increases, PuOZ2+ or Pu(N) bewmes dominant. This behavior reflects the redox disproportionation of Pu(V). The reactions are 2Pu02++ 4Ht = Pn4++ P u 0 F + 2H20 (1)
Reactions 1 and 3 are slow because they require breaking F'u-0 bonds, whereas reaction 2 is much faster because only electron transfer is involved. This set of reactions can be used in discussions of redox to illustrate the different kinetics for mechanisms involving bond breakage (slow) and electron transfer (fast). They also can be used to show the role of kinetic effects in nature: PuW) is the form found in natural waten where the hydrogen . . ion concentration is very low (pH 7-8) and plutonium is present at about M. This concentration is important due to the secondwder dependence on PuOzi of the disproportionation in eq 1. Oxidation State Analogues Periodicity Due to the relative ease of change in the oxidation states of plutonium, it has been difficult to develop analytical methods to determine which oxidation state predominates at very low concentrations (10-i2-1~16M) in environmental systems. To overwme this problem, .chemistshave Equilibrium Constants for Am(lll)
Species
828
log OH!%
Species
Journal of Chemical Education
log C%n
relied on the periodic table. From our confidencein chemical periodic relationships, it is possible to choose a redoxstable cation predicted to behave much like plutonium in a particular oxidation state. For example, the trivalent 4f elements should behave almost identically to Pu(II1). Thus, Nd(II1) or Eu(II1) is used as a Pu(II1) analogue. Although uranium has the same oxidation states as plutonium, uranium is quite stable as U0z2+.The same multiplicity of oxidation states exists for neptunium as for plutonium and uranium. However, NpW) as Np0z2+is much more stable than Pu02'+ and can be expected to have almost identical properties. Consequently, it is used as the more stable analogue of PuOzi. It is more difficult to find a satisfactory analogue of Pu(N). Usually, Th(N) is used, but the differencein ionic radii (mclv, = 11.0 nm, rhcrv,= 10.1nm)leads to small but sienificant differencesin chemical behavior. .. Study of the environmental behavior of these oxidationstate analoeues allows us to predict the behavior of plutonium in eaeh of the differen
