Aqueous Chemical Processing

12.1. Improved PUREX Process – Removal of Np, I, and Tc

Neptunium can be effectively separated by a relatively simple modification to the conventional PUREX process. Neptunium can be oxidized to the VI state by nitrous acid and extracted into the organic phase with U and Pu in the first cycle and, later, separated during the second purification cycle of uranium. Neptunium can be transmuted by blending with MOX fuel (homogeneous recycle) or fabricated into special targets (heterogeneous recycle) for irradiation. Unlike neptunium, the other minor actinides, americium and curium cannot be separated by modifying the PUREX process. Processes such as the TRUEX process, described previously, must be designed specifically to remove americium and curium (trivalent actinides) from HLW.

Technetium and iodine can also be separated by reasonable modifications to the traditional PUREX process. During dissolution, 10–20% of the technetium remains as an insoluble residue with the noble metals Ru, Rh, and Pd with the cladding hulls. The technetium that does dissolve in the nitric acid is Tc(VII) as TcO4– which can be

extracted by TBP. Iodine is released as a gas in the dissolution step and is separated by trapping it from the off-gas. A flow diagram showing the removal of Np, I, and Tc from such a modified PUREX process is shown in Figure 13.

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Figure 13: The PUREX process modified for the separation of Np, I, and Tc.

12.2. UREX and UREX+ Processes

A variation of the PUREX process that has been proposed as a possible alternative partitioning scheme for the transmutation of wastes is the UREX process. In this aqueous based process, shown schematically in Figure 14, uranium and technetium are co-extracted from a nitric acid solution containing the dissolved spent commercial LWR fuel with plutonium in the HLW stream with the other minor actinides and fission products. UREX, like the PUREX process, uses tri-butyl phosphate (TBP) as the extractant. However, in the UREX process, uranium is extracted at a high separation factor in one extraction cycle. The distinguishing feature of the UREX process is the addition of a reductant/complexant, acetohydroxamic acid (AHA), to the dissolver solution. The addition of AHA results in the suppression of plutonium extraction and in the retention of certain fission products (e.g., Mo, Zr, and Ru) that would otherwise contaminate the organic phase. Technetium is back-extracted into a high acidity aqueous phase, yielding a relatively pure technetium stream. Uranium can then be stripped from the organic phase. The UREX liquid waste stream or raffinate contains all of the transuranic elements and all of the fission products except technetium, iodine, xenon and krypton. The separation requirements for the UREX process development include recoveries of uranium and technetium at greater than 99.9% and 95% respectively. (Note: Iodine is released as a gas in the dissolution step and is separated by trapping it from the off-gas line.)

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Figure 14: Flow diagram for the UREX process.

The UREX+ process is a series of five solvent-extraction flowsheets that perform the following operations: (1) recovery of Tc and U (UREX), (2) recovery of Cs and Sr (CCD-PEG), (3) recovery of Pu and Np (NPEX), (4) recovery of Am, Cm, and rare- earth fission products (TRUEX), and; (5) separation of Am and Cm from the rare earth fission products (Cyanex 301). The goals of the separation process development for UREX+ include:

• Uranium

o Recovery ≥ 90%

o Purity to allow disposal as low-level waste

o Fission products to meet regulatory limits and TRU < 100 nCi/g (3700 Bq g-1)

o If U is to be recycled, additional separations would be required to increase the purity to the acceptable standard

• Technetium

o Recovery ≥ 95%

o Purity only important for use as transmutation target • Cesium/Strontium

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o Contribution to the heat load in the repository equal to that of all other fission products

o Purity to allow ultimate disposal as low-level waste after decay storage • Plutonium/Neptunium

o Recovery ≥ 99% to allow 100-fold reduction of heat load to repository o Purity to meet mixed-oxide (MOX) fuel specifications

• Americium/Curium

o Recovery ≥ 99.5% to allow 100-fold reduction of heat load to the repository o Purity to meet fast-spectrum reactor lanthanide specification of < 20 mg/g

Uranium plus TRU

• Raffinates from the UREX+ process

o TRUEX, all soluble fission products but Cs, Sr, Tc, I, and REEs o Cyanex 301, RE elements

o Purity requirements are basis for recoveries in other streams

The flow diagrams for steps 2 and 3, and steps 4 and 5, in the UREX+ process are shown in Figures 15 and 16 respectively.

Figure 15: The flow diagram for Step 2 (recovery of Cs and Sr using CDC-PEG, a mixture of chlorinated cobalt dicarbollide (CCD) for cesium extraction and

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polyethylene glycol (PEG) for strontium extraction diluted by phenyltrifluoromethyl sulfone) and Step 3 (for recovery of a pure Pu/Np product) using the NPEX process.

Figure 16: The flow diagram for Step 4 (recovery of Am, Cm, and rare-earth fission products using TRUEX) and Step 5 (separation of Am and Cm from the rare earths

using Cyanex 301).

If UREX+ can be successfully used, it has the advantage that iodine and technetium are captured for safe disposal, uranium is recovered as for recycle or disposal as LLW Class C waste, Np and Pu are recovered together for blending into MOX fuel for thermal reactors, Am and Cu are separated together and can be transmuted in a fast reactor, Cs and Sr are removed from the waste stream going to a repository, greatly reducing the heat load in the waste form, and, finally, the remaining fission products are stored in the repository.

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