Objectives. The objectives of fuel dissolution are (1) to bring the uranium and plutonium in the fuel completely into aqueous solution; (2) to complete the separation of fuel from cladding; (3) to determine as accurately as possible the amounts of uranium and plutonium charged to reprocessing; and (4) to convert uranium, plutonium, and fission products into the chemical states most favorable for their subsequent separation.

Reactions. Because the Purex process requires that the elements to be separated be present in aqueous solution as nitrates, the dissolvent is always nitric acid. The principal reactions that take place are

3U02 + 8HN03 -+ 3U02(N03)2 + 2NO + 4H20

and U02 + 4HN03 -* U02(N03)2 + 2N02 + 2H20

Ordinarily, both reactions take place to some extent, with the first dominant at acid concentrations below 10 M and the second at higher concentrations [S8]. In principle, formation of gaseous reaction products could be avoided by addition of oxygen directly to the dissolver:

2U02 + 4HN03 + 02 ->• 2U02(N03)2 + 2H20

This process is known as “fumeless dissolving” and is used in European plants. Practically, small amounts of nitrogen, nitrogen oxides, and gaseous fission products are also formed. Reference [07] gives an example of fumeless dissolving.

Plutonium in oxide fuel dissolves as a mixture of tetravalent and hexavalent plutonyl nitrates, both of which are extractable with TBP. Neptunium dissolves as a mixture of inextractable pentavalent and extractable hexavalent nitrates.

Most of the fission products go into aqueous solution. However, at high bumups above

30,0 MWd/MT, some elements such as molybdenum, zirconium, ruthenium, rhodium, palladium, and niobium may exceed their solubility limits and be present as solids.

In the solution, americium, curium, and most of the fission products are in a single, relatively inextractable valence state. Iodine and ruthenium are important exceptions. Iodine may appear as inextractable iodide or iodate or as elemental iodine, which would be extracted by the solvent and react with it. Ruthenium may appear in any valence state between 0 (insoluble metal) and 8 (volatile ruthenium tetroxide) and, at valence 4, may form a number of nitrosyl ruthenium (Ru^NO) complexes of varying extractability.

An important objective of dissolution and the preconditioning of feed solution prior to extraction is to convert these fission-product elements into states that will not contaminate uranium, plutonium, or solvent in subsequent solvent extraction.

Dissolution rates. Uranium dioxide dissolves more rapidly than Pu02 or Th02. The time required for dissolving more than 99.5 percent of the U02 from unirradiated stainless steel-clad U02 pellets was found to be 40, 70, and 110 min for ;-, 1-, and 2-in chopped lengths, respectively [W4]. Irradiated fuel usually dissolves faster, probably because of cracking during irradiation. These tests were made with from 150 to 200 percent excess of 10 M HNO3 at a temperature just below boiling. The instantaneous dissolution rate in 8 M HN03 is about one-half that in 10 M acid. Because extensive foaming results when fuel is added directly to boiling nitric acid, the preferred procedure in a batch dissolver is to add U02 to cold acid, then bring the solution to just below the boiling point, with adequate cooling available to deal with heat evolved from chemical reaction and radioactive decay.

The rate of dissolution of Pu02 in nitric acid is slower than U02 and depends on the plutonium/uranium ratio, the methods used to fabricate fuel, and the conditions of irradiation. At one extreme, plutonium produced at low concentration in U02 by transmutation dissolves almost as rapidly as the associated U02. At the other extreme, plutonium present as Pu02 mixed mechanically with U02 without proper sintering dissolves much more slowly and less completely than U02. Plutonium present as a solid solution (U, Pu)02 at the concentration of 20 to 25 percent used in breeder-reactor fuel dissolves at an intermediate rate.^

In all cases, however, dissolution of irradiated fuel in nitric acid leaves some plutonium associated with undissolved fission products. This plutonium can be leached from the residue with mixed nitric and hydrofluoric acids or with mixed nitric acid and ceric nitrate, Ce(N03)4 [U2]. Residue from irradiated mixed U02-Pu02 fuel was 99.94 percent dissolved in 4 h by treatment with 4 M HNO3-0.5 M Ce(IV). Ceric nitrate is preferred to HF in the secondary dissolution step because cerium is already present as a fission product, and its addition does not complicate subsequent solvent extraction. Use of Ce(IV) in the primary dissolution step is undesirable because it would convert all plutonium to the less extractive hexavalent state and would volatilize much of the ruthenium as Ru04.

Separation from cladding. After reaction of fuel with acid has been completed, the resulting solution and any suspended fine particles are drained from the coarser cladding fragments. The cladding is washed, first with dilute nitric acid and then with water. The cladding is checked by gamma spectroscopy to establish removal of adherent fuel Tand then discharged for packaging as radioactive waste. The fuel solution, possibly containing suspended particles, is clarified by centrifugation. Centrifuged solids are accumulated and periodically leached as described above for recovery of plutonium and uranium.

Accountability measurements. The dissolver solution and washings are collected in a calibrated accountability tank and mixed thoroughly. The volume and density of the solution are measured as accurately as possible, and samples are taken for determination of uranium and plutonium concentrations. This is the first point in reprocessing at which a quantitative measure of input amounts can be made. Even here, measurement is difficult because of the intense radioactivity.

tDissolution of Pu02-U02 fuel is also discussed in Sec. 6.8.

Conditioning of feed. Before solvent extraction, the concentrations of nitric acid and uranyl nitrate are brought to the desired values by addition of water and/or nitric acid, as required. Preferred concentrations are HN03, 2 to 2.5 M U02(N03)2, 1.2 to 1.4 Af.

It is usually considered desirable to bring all plutonium to the most extractable, tetravalent state, although this step was not found necessary at West Valley. Sodium nitrite was formerly used for this purpose, but N204 or hydroxylamine is now favored because each adds no nonvolatile material to the aqueous phase. With N204, hexavalent plutonium is reduced:

Puvi022+ + N204 + 2H+ ->■ Pu4+ + 2HN03 Any trivalent plutonium that might be present would be oxidized:

4Pu3+ + N204 + 4H+ -*■ 4Pu4+ + 2NO + 2H20

Prevention of criticality. In dissolving fuel obtained from irradiating material more enriched than natural uranium, precautions must be taken to prevent accumulation of a critical mass in the dissolver. Three general methods are (1) use of subcritical geometry, (2) control of fissile material concentration, or (3) addition of a soluble neutron absorber with the dissolver solvent. For subcritical geometry, dissolvers have been built as thin slabs or long cylinders of subcritical diameter. A good example of subcritical geometry combined with concentration control is the dissolver used in the West Valley plant of Nuclear Fuel Services, Inc., shown in horizontal cross section in Fig. 10.6. Fuel baskets were 7 ft high and 8 in or less in diameter. The basket diameter selected for a particular fuel was one that limited the concentration in the 3-in

annulus and 10-in cylinders after dissolving to 60 percent of the critical value. Nuclear interaction between the cylinders was prevented by addition of 0.5 w/o natural boron to the concrete which provided 30-in separation between cylinders.

The Barnwell plant of Allied-General provides an example of use of soluble poison. There it is proposed that 5.6 g natural gadolinium, as nitrate, per liter be added to the nitric acid solvent. At the design concentrations of plutonium and uranium in dissolver solution, this will prevent criticality even with fully enriched 235 U (Prob. 10.1).

Dissolution equipment. Dissolution equipment, termed dissolvers, must provide for (1) adding fuel and dissolvent; (2) removing the product solution, undissolved solids, and gaseous effluents;

(3) maintaining proper contact between fuel and dissolvent; and (4) controlling the dissolution rate. Dissolvers may be characterized by the mode of fuel addition as either batch or continuous, or by their shape as column, slab, annular, or pot. The first three shapes are used for geometric control of criticality. Of the many types of dissolver that have been used, only a few examples can be described.

Batch pot dissolvers have been widely used, especially for low-enrichment fuel. The big advantage of batch operation is simplification of charging fuel and discharging residues. A disadvantage is the variable reaction rate, which is highest at the start when a large quantity of fuel is present, and which becomes much smaller toward the end when most of the fuel has been dissolved. The dissolution rate can be made more uniform by varying the concentration of dissolvent during the cycle. Dissolver product from a previous cycle, partially saturated with uranium, may be charged at the beginning of a cycle, to produce the most concentrated solution. When the reaction slows down, this solution may be replaced by fresh nitric acid, to produce a partially saturated solution to be used as solvent at the beginning of a subsequent cycle. Finally, at the end of a cycle, the residue in the dissolver would be washed with water to remove the remaining acid and fuel solution.

A batch dissolver typically is provided with heating coils to bring the solution to the desired temperature, cooling coils to remove the heat of reaction when it is most rapid, corrosion-resistant baskets or other containers to hold the fuel undergoing dissolution and retain cladding hulls at the end of the operation, and a cover to prevent escape of steam, nitric acid vapors, and volatile fission products and lead them to a condenser and fission-product traps. For ease of placement and removal, the cover may be sealed by a flanged ring that dips into a trough containing a sealing liquid. Recirculation of liquid through the dissolver is sometimes used to provide more uniform conditions and increase reaction rates. Product solution and undissolved sediment are withdrawn from the bottom. For dissolvers using nitric acid, heavy-gauge stainless steel has satisfactory corrosion resistance.

The volume of a batch of fuel to be charged to a batch dissolver may be evaluated from the product of the desired fuel reprocessing rate and the time required to complete the dissolving cycle. The cycle time may be estimated from small-scale experiments that simulate the geometry and time-temperature-concentration variations in the production dissolver, with a substantial allowance for inability to mock up accurately all relevant conditions of a production dissolver.

Continuous dissolution is especially advantageous when fuel and cladding are to be dissolved completely, as there is then no problem in removing undissolved solids from the dissolver. In such a case, fuel may be charged continuously at the top, dissolvent may be fed continuously, and dissolved solution removed continuously. The volume of undissolved fuel in the dissolver adjusts itself automatically so that the rate of solution balances the rate of addition. The big advantages over batch dissolution are smaller dissolver volume, more uniform product solution composition, steady gas evolution rate, and smaller and more efficient absorption system. It is estimated [B12] that the volume of a continuous dissolver may be from one-tenth to one-twentieth that of a batch dissolver of the same average dissolving rate.

This is especially advantageous for enriched fuels, where criticality limits dissolver dimensions.

The overall dissolution rate in a continuous dissolver is controlled by the metal feed rate, the temperature, the concentration of feed solution, and its flow rate. The metal composition of product solution at steady-state operation is just the ratio of the metal mass feed rate to the solution volume feed rate.

In a continuous dissolver with no solid residue, the solid necessarily flows downward. Liquid flow may be either down or up. With liquid downflow the dissolver is sometimes called a trickle dissolver. With liquid downflow it is preferable to remove off-gases at the bottom, to prevent flooding. With liquid upflow the dissolver is sometimes called a flooded dissolver. Off-gases separation from liquid is simpler with liquid upflow than with downflow.

When a solid residue such as cladding hulls remain, design of a continuous dissolver is much more complicated. Provision must be made for washing dissolver solution from the residue and for discharging the residue without escape of off-gases. A number of possible design concepts for continuous dissolvers have been tested by Oak Ridge National Laboratory [G15, 012].