Dissolution in nitric acid

Starch nitrated by Brown and Miliar (Ref 10) by dissolution in nitric acid and subsequent precipitation with the help of sulphuric acid, was stabilized by washing in ether and chloroform... 

The reflux of aqueous Pu(IV) solutions containing <6 M HNO3 produces polymer precipitates that are resistant to subsequent dissociation and dissolution in nitric acid. Eapid aging of the Pu(IV) polymer to form a PuC -like structure is responsible for the unusually stable polymer. Comparative studies under nonreflux conditions show that polymer does not form at concentrations of HNO3 >3 M. 

Scheme 13 Formation of nitrato-carborane via dissolution in nitric acid.

In order to make use of thorium as a nuclear resource for power generation, development of efficient separation processes are necessary to recover 233U from irradiated thorium and fission products. The THORium uranium Extraction (THOREX) process has not been commercially used as much as the PUREX process due to lack of exploitation of thorium as an energy resource (157,180). Extensive work carried out at ORNL during the fifties and sixties led to the development of various versions of the THOREX process given in Table 2.6. The stable nature of thorium dioxide poses difficulties in its dissolution in nitric acid. A small amount of fluoride addition to nitric acid is required for the dissolution of more inert thorium (181). 

Cases have been observed where the isotopic line absorption profiles completely overlap, e.g. boron-10 and -11 in a krypton-filled lamp at 249.7 nm [244]. Hannaford and Lowe [245] later showed that this was caused by an unusually large Doppler half-width induced by the fill-gas, and, if neon is used, the 208.9 and 209.0 nm lines can allow the determination of boron-10 and boron-11 isotope ratios. The 208.89/208.96 nm doublet was found to be more useful than the 249.68/249.77 nm doublet. Enriched isotope hollow-cathode lamps were used as sources. A sputtering cell was preferred to a nitrous oxide/acetylene flame as the atom reservoir, as it could be water-cooled to reduce broadening and solid samples could be used, thus avoiding the slow dissolution in nitric acid of samples of boron-10 used as a neutron absorber in reactor technology. 

The dissolution in nitric acid is accompanied with the formation of nitrogen oxide ... 

Uranium and plutonium carbide fuels have also been investigated and their dissolution in nitric acid results in the formation of organic acids as well as In the case of (Uo.gPuo.2)C... 

Will and Lenze s studies, published in 1898, reflect an increasing appreciation of this vital problem of adequate stabilization of the nitrate ester. They fractionated starch nitrates (produced by the inferior technique of initial dissolution in nitric acid) by means of acetone-alcohol, and the fraction insoluble in alcohol was purified by several boilings in alcohol. A maximum nitrogen content of 14.04% was reported. This figure is possible but, taking into account the analytical accuracy of the time, was probably an inaccurately high value. 

At about the same time, Saposchnikoff attempted a molecular-weight determination on starch nitrate produced by dissolution in nitric acid, and arrived at the figure of 1845— agreeing best with the formula C36H430is-(N03)i7. This formula, wherein the DP is 6, is not unlikely, as the conditions used probably effected degradation. In Britain, Arnold and his associates were granted patents for the nitration of potato starch in mixed acids. The product was probably worthless, owing to the naive assumption that it was freed of acids by currents of fresh water. ... 

The first step in the conventional process for refining manium is dissolution in nitric acid. When the concentrates have been produced by chemical leaching and are in the form of diuranates, dissolution proceeds rapidly and leaves little solid residue. When the concentrates have been separated mechanically and are in the form of the original uranium mineral, dissolution may require more concentrated acid, higher temperatures, longer times, and addition of oxidants such as MnO. Also, filtration to remove undissolved residues is usually required. In either case, dissolution produces an aqueous solution of uranyl nitrate hexahydrate U02(N03)2 6H2 0, containing excess nitric acid and variable amounts of nitrates of metallic impurities present in the concentrates. 

Electrolytic dissolution in nitric acid has been used at the Savannah River [B22] and Idaho Qiemical Processing plants [AlO, All] to dissolve a wide variety of fuels and cladding materials, including uranium alloys, stainless steel, aluminum, zircaloy, and nichrome. The electrolytic dissolver developed by du Pont [B22], pictured in Fig. 10.4, uses niobium anodes and cathodes, with the former coated with 0.25 mm of platinum to prevent anodic corrosion. Metallic fuel to be dissolved is held in an alundum insulating frame supported by a niobium basket placed between anode and cathode and electrically insulated from them. Fuel surfaces facing the cathode undergo anodic dissolution in a reaction such as... 

Confusion about the composition of kupfernickel remained even after the mineral had been described. In 1726 the German chemist I. Link studied the mineral and established that its dissolution in nitric acid yields a green colour. He concluded that the mineral was most probably a cobalt ore with admixtures of copper. When Swedish miners found a reddish mineral which, being added to glass, did not produce a blue colour, they named it cobold that had lost his soul . It was also one of the nickel minerals. 

The head-end which comprises aU operations from the receipt of the UNE with its conversion to a feed suitable for solvent extraction, usually by dissolution in nitric acid... 

Comparable analyses of a BWR fuel rod having experienced almost identical heat ratings but a lower burnup (about 20 MWd/kg U), yielded a fraction still retained in the fuel of about 50% (Bleier et al., 1984). The reasons for the differences in retention between PWR and BWR fuels have not yet been completely clarified. Presumably they are due to a higher density and lower open porosity of the BWR fuel analyzed, the impact of which on H2O mobility in UO2 fuels is well known. As for the chemical state of tritium in the fuel there is no difference between the two fuel types, i. e. in analyzing BWR fuel virtually the whole tritium inventory is also detected as HTO after dissolution in nitric acid. 

Purification of the precipitated concentrates is often carried out by dissolution in nitric acid followed by extraction with tributyl phosphate (TBP) dissolved in kerosene or n-hexane. The extraction is based on the formation of a complex between the uranyl nitrate and TBP according to the equation ... 

In outline, reprocessing was based on disassembly of the fuel assemblies to individual pins, single pin cropping, dissolution in nitric acid and operation of a Purex-type flowsheet in a chemical separation plant equipped with pulsed mixer settlers. The modified dissolver cell and solvent extraction plant were recommissioned late in 1979 and were tested in reprocessing of the final batches of DFR fuel. The new head-end facility, to disassemble the PFR fuel assemblies and to crop the pins, and the new waste treatment facilities were commissioned in 1980 reprocessing of the first batches of PFR fuel began later that year. 

More recently different types of fuel have been compared according to the following parameters thermal properties, compatibility with sodium and potential for dissolution in nitric acid. This reached the following conclusions [7.88] ... 

As far as reprocessing in the U/Pu fuel cycle is concerned, several chemical separation techniques have been proposed and developed in the past few decades. The most efficient process to date remains the PUREX process (Plutonium and Uranium Recovery by Extraction). This process uses nitric acid HNO3 and organic solvents to dissolve and extract selectively U and Pu, resulting in two separate product streams (U on one side and Pu on the other side of the process chain). As far as reprocessing in the Th/ U fuel cycle is concerned, THOREX (Thorium Oxide Recovery by Extraction) technology must be used, also based on dissolution in nitric acid and solvent extraction (however, with special care for the extraction of Pa, for the separa-tion of U and U, and for the dissolution of thorium dioxide in pure nitric acid). 

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