Plutonium oxide systems

As Am is considered to be the first of the lanthanide-like actinides, it would be expected that its oxide system would bear some resemblance to the oxide systems of the lanthanides. For Am, a monoxide, sesquioxide,.a dioxide, and an oxide with an O/M ratio intermediate to that of the sesquioxide and dioxide have been reported. Although the quantity of Am has permitted extensive investigations of its oxide system, the number of studies is still small relative to the efforts that have been extended on the uranium and plutonium oxide systems. In part this is due to the more intense radioactive nature of Am but also to a decreased interest in its chemistry in contrast to those of U and Pu, whose applications in weapons and reactors promoted many detailed studies of those elements. However, the number of studies that have been carried out on the americium oxygen system is still large in comparison to the number of studies with higher actinides. 

Plutonium. Gardner et al. (26) have made a careful high temperature x-ray diflFraction study of the plutonium-oxygen system in the range from room temperature to 900°C. observing diffraction from oxide samples contained in silica capillaries. They review briefly previous work apropos of phase transformations (i.e., thermal and electrical measurements) and construct a phase diagram as shown in Figure 5. 

Markin et al. (38) have determined the e.m.f. of high temperature galvanic cells involving the plutonium oxide-oxygen system. The plots of partial molal free energy of oxygen vs. temperature show a profound change in the composition interval 1.691 and 1.812. In many respects the behavior of PuO is quite similar to CeO. ... 

Plutonium oxides. The phase diagram of the plutonium-oxygen system is shown in Fig. 9.2. The observed compounds are the stoichiometric PujOj and PUO2 and the nonstoichiometric PuOj.sj and Pui. i. PuO has also been shown to exist, but only under extreme conditions. No oxide of higher oxidation state than PuOs has been formed. 

II. International Atomic Energy Agency The Plutonium-Oxygen and Uranium-Plutonium-Oxygen Systems A Thermochemical Assessment, Report of a Panel on Thermodynamic Properties of Plutonium Oxides, Vienna, Oct. 1966, Tech. Rept. Series No. 79, Vieima, 1967. 

Dust control. Small plutonium oxide particulates may become airborne and result in inhalation dose. Particulate size distribution at various stages and aerosol transport are subjects for careful study. Equipment and process designs can be used to control airborne particulate, such as setting up various pressure control zones in ventilation design, installing HEPA filtration systems, and installing area airborne particulate monitors. 

Spill of Pu oxide powder. Spills of plutonium oxide powder by human error or equipment failure are always a safety concern. Examples are dropping a container containing Pu oxide, or leaky seals in the Pu oxide process system. Adequate protection systems to protect against accidental spills or leakage are mandatory. 

The idealized systems c sisted of concentric spheres of tt-phase plutonium metal (d = 19,7 g/cm ), PuOji + HjO watyr solution, iand a i4-cm H]0 reflector. A compromise with reality is made in the assumption of a PuO + HiO solution. The idealizied Solution more nearly represent dissolving operations than the Idealized plutonium/ water systems. The results are directly applicable to many hc d-deslgn problems involving operations with plutonium in metal and oxide form. 

Criticality measurements are needed to provide lor experimental verification of calculated data points and for supplementing the criticality data already available for mixed-oxide systems with plutonium concentrations and enrichments in the range of interest to light-water reactor applications and for low enriched uranium systems. These needs, as identified by the survey, are summarized below. 

Homogeneous mixed-oxide systems. The greatest need exists for PUO2 concentrations in the range of 1 to 10 wt% with plutonium having Pu contents ranging between 5 and 20 wt%,... 

The validation of these data libraries through the calculation of critical experiments has been an ongoing effort. The validation for the uranium-fueled systems was reported at the last ANS meeting. The cross-section validation for mixed-oxide and plutonium-fueled systems has been completed. Particularly valuable in the cross-section validation for structural and neutron-absorbing materials have been the experiments performed at Pacific Northwest Laboratories. ... 

Although a rhombohedral intermediate oxide phase has only been verified for Cm and Cf, in principle it may also exist for Pu, Am and Bk. Of these three, the plutonium oxygen system has been the most extensively studied and a rhombohedral intermediate oxide (e.g., PU7O12) has not been reported. The stability of the dioxide of these three actinides may be contributing to the inability to obtain this partieular intermediate phase (e.g., they are too easily oxidized to the dioxide). A study of the lanthanide systems (see section 1) provides a complete picture of the complexities that these intermediate oxide systems can exhibit. Whether or not these actinide oxide systems are more closely comparable to the lanthanide systems requires more extensive studies. 

BR-5 was the first reactor in the world using sodium as a coolant and plutonium oxide as a fuel (Status of liqued metal cooled fast breeder reactors. Technical Reports Series, No. 246, IAEA, Vienna, 1985, p. 89). The main purpose of the reactor was to gain bumup data on Pu02 and other fuel types, to obtain experience in operation of radioactive sodium systems. 

MO2 ions are formed by the four elements from uranium to ameridum. For uranium, the hexavalent oxidation state is the most stable one. Though easily reduced, it is also prominent in the chemistries of neptunium and plutonium. Ameridum(vi) is a very strong oxidizing agent. As the ameridum isotopes available are quite radioactive, a steady radiation-induced reduction of AmO occurs in aqueous solution, as soon as a strongly oxidizing system is not present [19]. 

The CEFR is a sodium cooled, bottom supported 65 MW(th) experimental fast reactor fuelled with mixed uranium-plutonium oxide (the first core, however, will be loaded with uranium oxide fuel). Fuel cladding and reactor block structural materials are made of Cr-Ni austenitic stainless steel. It is a pool type reactor with two main pumps, and two loops for the primary and secondary circuit, respectively. The water-steam tertiary circuit has also two loops, with the superheated steam collected into one pipe that is connected with the turbine. CEFR s has a natural circuit decay heat removal system. 

The only crystalline phase which has been isolated has the formula Pu2(OH)2(SO )3(HaO). The appearance of this phase is quite remarkable because under similar conditions the other actinides which have been examined form phases of different composition (M(OH)2SOit, M=Th,U,Np). Thus, plutonium apparently lies at that point in the actinide series where the actinide contraction influences the chemistry such that elements in identical oxidation states will behave differently. The chemistry of plutonium in this system resembles that of zirconium and hafnium more than that of the lighter tetravalent actinides. Structural studies do reveal a common feature among the various hydroxysulfate compounds, however, i.e., the existence of double hydroxide bridges between metal atoms. This structural feature persists from zirconium through plutonium for compounds of stoichiometry M(OH)2SOit to M2 (OH) 2 (S0O 3 (H20) i,. Spectroscopic studies show similarities between Pu2 (OH) 2 (SOO 3 (H20) i, and the Pu(IV) polymer and suggest that common structural features may be present. 

The investigation of plutonium chemistry in aqueous solutions provides unique challenges due in large part to the fact that plutonium exhibits an unusually broad range of oxidation states -from 3 to 7-and in many systems several of these oxidation states can coexist in equilibrium. Following the normal pattern for polyvalent cations, lower oxidation states of plutonium are stabilized by more acidic conditions while higher oxidation states become more stable as the basicity increases. 

Despite the problems of direct experimental evaluation of plutonium stability constants, they are needed in modeling of the behavior of plutonium in reprocessing systems in waste repositories and in geological and environmental media. Actinide analogs such as Am+3, Th+, NpOj and UOj2 can be used with caution for plutonium in the corresponding oxidation states and values for stability constants of these analogues are to be found also in reference 20. 

The authoritative documents on plutonium 0 >2) do not include photo-chemical reactions of plutonium in aqueous systems. The first papers in Western world literature on studies that were dedicated to aqueous plutonium photochemistry appeared in 1976 (3, 4 ), even though photochemical changes in oxidation states were indicated as early as 1952 (5,, ]) ... 

The primary reason for studying aqueous plutonium photochemistry has been the scientific value. No other aqueous metal system has such a wide range of chemistry four oxidation states can co-exist (III, IV, V, and VI), and the Pu(IV) state can form polymer material. Cation charges on these species range from 1 to 4, and there are molecular as well as metallic ions. A wide variety of anion and chelating complex chemistry applies to the respective oxidation states. Finally, all of this aqueous plutonium chemistry could be affected by the absorption of light, and perhaps new plutonium species could be discovered by photon excitation. 

Only the obvious studies of aqueous plutonium photochemistry have been completed, and the results are summarized below. The course of discussion will follow the particular photochemical reactions that have been observed, beginning with the higher oxidation states. This discussion will consider primarily those studies of aqueous plutonium In perchloric acid media but will include one reaction in nitric acid media. Aqueous systems other than perchlorate may affect particular plutonium states by redox reactions and complex formation and could obscure photochemical changes. Detailed experimental studies of plutonium photochemistry in other aqueous systems should also be conducted. 

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