Medidas y Reglas de Prevención de Accidentes en Laboratorio de

The actinide elements are characterised by the filling of the seven 5f orbitals (except Actinium and Thorium which are 6d filled only). There are 15 chemical elements with atomic numbers, 89 (actinium) through 103 (lawrencium) as shown in Figure 1-29.

Figure 1-29 – Period 7, The Actinide Series, taken from the periodic table [108]

The actinides are all radioactive elements. Actinium, thorium, protactinium and uranium are the only four actinides that have been found naturally in the environment, the others are artificial, being produced by particle accelerators or in nuclear reactors [11], [109]. The elements within the actinide series have several common properties [109]:

 Most elements (heavier that U) were discovered by synthetic means  All actinide isotopes are radioactive

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 Many have a large number of oxidation states. For example Pu can exist in aqueous solution simultaneously in four oxidation states (+3 to +6, with +7 exhibiting very different redox potentials)

 Actinium and the elements americium through lawrencium are similar in many respects to the lanthanides (elements that fill the 4f subshell). Elements thorium through to neptunium have some properties similar to those of the d-block transition elements.

Pu exhibits properties similar to both the lanthanides and d-block transition metals, presenting some unique challenges in the study of its chemical behaviour, as will be illustrated below.

1.5.1.1

Oxidation States and Aqueous Electrochemistry

Actinides in aqueous solution have several different oxidation states, as shown in Table 1-2: Table 1-2 – Known oxidation states of the actinides and species in solution. The bold number

represents the most stable oxidation state in solution of each element [110], [111].

Ac Th Pa U Np Pu Am Cm Bk Cf Es Fm Md No 2 2 2 2 2 2 3 3 3 3 3 3 3 3 3 3 3 3 3 3 4 4 4 4 4 4 4 4 4 5 5 5 5 5 6 6 6 6 7 7

Oxidation states up to +7 have been identified for some elements (Neptunium (Np), Plutonium (Pu) and Americium (Am)). This multivalent behaviour leads to very complex redox behaviour. For example, Pu has a wide variety of oxidation states ranging from +3 to +6 such as Pu3+_{, Pu}4+_{, PuO}

2+ or PuO22+ all of

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The aqueous speciation of the actinides in aqueous solution is shown in Table 1-3. Table 1-3 – Actinide speciation in aqueous solution [110], [111]. Oxidn

State

89 90 91 92 93 94 95 96 97 98

+3 Ac3+ _Th3+ _Pa3+ _U3+ _Np3+ _Pu3+ _Am3+ _Cm3+ _Bk3+ _Cf3+

+4 Th4+ _Pa4+ _U4+ _Np4+ _Pu4+ _Am4+ _Cm4+ _Bk4+ _Cm4+

+5 PaO2 UO2+ NpO2+ PuO2+ AmO2+

+6 UO22+ NpO22+ PuO22+ AmO22+

7 NpO25- _PuO25-

The oxidation state of an actinide may alter due to a change in redox potential. The redox potential can be influenced by chemical composition of the solution, concentration of dissolved O2 or pH [107].

Having discussed the general chemistry of the actinides, we now describe in more detail the solution chemistry of the three most likely to be encountered actinides in nuclear reprocessing streams: Uranium, plutonium and neptunium.

1.5.1.2

Uranium Solution Electrochemistry

Natural uranium occurs in three main isotopes, 234_{U (0.0055% wt.),} 235_{U (0.72% wt.) and} 238_{U (99.27%}

wt.). Globally, the fissile isotope 235_{U provides the most commonly used energy source of nuclear reactors}

and atomic weapons [112].

Uranium exists in aqueous solutions in the +3, +4, +5 and +6 oxidation states. U3+ _{is a powerful reducing}

agent which is slowly oxidised to U4+ _{in anoxic conditions and rapidly in the presence of oxygen. U}4+ _is

regarded as a ‘stable’ species of U solution, but is slowly oxidised to UO22+ by air. UO2+ has a short-lived

existence in solution, it is most stable in the pH range 2-4. UO2+ is prone to disproportionation at any pH

to U4+ _{and UO}

22+ via the following reaction [110], [111]:

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The general electrochemical behaviour of uranium in aqueous solutions is dominated by the reduction of the hexavalent ‘uranyl’ ion, UO22+. The uranyl ion is the most stable oxidation state and therefore difficult

to reduce.

Figure 1-30 gives the oxidation potentials for uranium in pH 0 aqueous solution (akin to what would be expected in high nitric acid concentrations).

Figure 1-30 –Redox potentials (vs. SCE) of uranium in aqueous solutions at pH 0 [111], [113].

1.5.1.3

Plutonium Solution Electrochemistry

Plutonium exists in five oxidation states in aqueous solution: Pu(III), Pu(IV), Pu(V), Pu(VI) and Pu(VII) which occur as the hydrated ions Pu3+_{, Pu}4+_{, PuO}

2+, PuO22+ and PuO53- respectively. Tetravalent

plutonium is the most stable oxidation state [114], [115].

Figure 1-31 gives the oxidation potentials for plutonium in pH 0 nitric acid solution.

Figure 1-31 –Redox potentials (vs. SCE) of plutonium in a HNO3 solution of pH 0 [113]. Pu(III) is unstable at pH 0 and can be oxidised by a variety of oxidants to Pu4+_{. It can also be oxidised to}

Pu(IV) by the α radiation produced by plutonium isotopes. Pu(IV) is stable in concentrated acids, but in mild acids (free of complexing agents) Pu(IV) disproportionates to Pu(III) and (PuVI). Oxidation of Pu4+

in aqueous solutions produces hexavalent plutonium, PuO22+.

UO₂2+ _UO 2+ UO4+ UO3+ -0.795 0.205 -0.157 0.025 PuO22+ PuO2+ Pu4+ Pu3+ 0.669 0.943 0.675 0.756 0.809

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Pentavalent plutonium, PuO2+, is only stable between pHs of 2 and 6, a pH value unlikely in strong nitric

acid environments; thus its disproportionation, stoichiometrically analagous to that of uranium, is not considered here further. However, Hexavalent plutonium, PuO22+, while stable in strong acidic solutions

(see above) is slowly reduced by the products of the radiolysis of water by the α-radiation produced by plutonium isotopes. In an acid solution, in the absence of complexing agents, the disproportionation of Pu(IV) follows the reaction [114]:

3𝑃𝑢 + 2𝐻 𝑂 ⇄ 2𝑃𝑢 + 𝑃𝑢𝑂 + 4𝐻 (1.40)

The disproportionation is temperature dependent and the rate at which it occurs is proportional to the concentration of H+ _[115].

The overall reaction of Equation (1.40) may be further split into two stages. The first stage involves two Pu(IV) combining to generate Pu(III) and Pu(V). The formation of Pu(V) is slow because it involves the formation of a Pu=O bond. In the second stage, the Pu(V) produced in the first stage reacts with Pu(IV) to produce Pu(III) and Pu(VI). This requires only an electron transfer which occurs rapidly.

2𝑃𝑢 + 2𝐻 𝑂 ⇄ 𝑃𝑢 + 𝑃𝑢𝑂 + 4𝐻 (1.41)

𝑃𝑢𝑂 + 𝑃𝑢 ⇄ 𝑃𝑢 + 𝑃𝑢𝑂 (1.42)

Disproportionation is complete when reactions (1.41) and (1.42) have reached equilibrium [114], [115].

1.5.1.4

Neptunium Solution Electrochemistry

In aqueous solution, neptunium exists as ions in all oxidation states from 3+ to 7+. The stability of these ions is strongly affected by pH, oxidants and reductants, complexing agents and the concentration of Np itself.

In the absence of complexing agents, Np3+_{, Np}4+_{, NpO}

2+ and NpO22+ exist as hydrated ions. Np3+ is

quickly oxidised to Np4+ _{by air. In aqueous solutions of low acidity Np}3+ _andNp4+ _{form insoluble}

hydroxides where, once again, Np(III) is oxidised to the more stable Np(IV) by oxygen. In acidic solutions the pentavalent and hexavalent Np ions act as Lewis acids and form dioxo species, NpO2+ and

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NpO2+ disproportionates to Np4+ and NpO22+ through the following reaction:

2𝑁𝑝𝑂 + 4𝐻 ⇄ 𝑁𝑝 + 𝑁𝑝𝑂 + 2𝐻 𝑂 (1.43)

The extent of the disproportionation is dependent on high solution acidity and high NpO2+ concentration

[116].

Figure 1-32 gives the oxidation potentials for neptunium in pH 0 aqueous solution.

Figure 1-32 –Redox potentials (vs. SCE) of neptunium in aqueous solutions at pH 0 [116].