Page 196 - Radiochemistry and nuclear chemistry

180 Radiochemistry and Nuclear Chemistry

H. + 0 2 ~ .HO 2
9 HO 2 + Fe 2+~FO 3+ + HO 2-

HO 2- + H +-~H20 2
H202 + Fe 2+ ~ Fe 3+ + OH- + .OH

9 OH + Fe 2 + --~ Fe 3 + + OH-

The yield of Fe 3+ is given by the equation

G(Fe 3+) = 2G(H202) + 3[G(eaq- ) + G(.H) + G(.HO2)] + G(.OH) (7.12)

Figure 7.7 shows radiation yields for oxidation of Fe 2 + in acidic solution as a function of
the LET value (see also w
When the concentration of the aqueous solution is greater than ~. 0.1 M, the solutes may
undergo direct radiolysis. The products of the radiolysis of the solute can react with water
itself or with the radiolytic products of water. Irradiation of solutions containing sulfate,
sulfite, or sulfide ions with fast neutrons yields radioactive phosphorus, through the
32S(n,p)32p reaction, almost exclusively as orthophosphate. However, depending on the
redox conditions of the solution, reduced species of phosphorus also appear in minor
amounts. Somewhat in contrast to this, slow neutron irradiation of solutions of NaHPO 4
yields phosphorus in many different oxidation states; thus P( + 1), P( + 3) and P( + 5) appear
in species such as hypophosphite, phosphite, and o-phosphate. These species are all rather
stable in aqueous solutions, and have been identified through paper r analysis.
In general, it may be assumed that slow neutron irradiation of solutions of oxyanions
changes the central atom to another (usually a more reduced) valence state through
n,y-reactions. For example, while manganese is in the Mn(+ 7) state in MnO4-, neutron
capture leads to the formation of 56Mn2 + species. However, if the product valence state is
unstable at ambient solution conditions, it may be immediately oxidized to a more stable
higher valence state.
The self radiolysis of a solution may change the chemical equilibria of the solution
components. For example, the c~-decay of plutonium decomposes water; in a solution
containing 1 mole of 239pu, ca. 0.01 mole of H202 is produced per day. This hydrogen
peroxide can react with the plutonium to form a precipitate of plutonium peroxide. To avoid
this precipitation, nitrite ions are added to the solution to react with the hydroxyl radicals
formed by the radiolysis and to eliminate the H202.

7.8. Organic compounds

As discussed above, the chemical consequences of radiolysis depends on the physical state
and the molecular composition of the irradiated material. Two properties, the dielectric
constant and the electron mobility, are of great importance for the fate of the ion pairs (the
radical cations and the electrons) formed on ionization.