Thorex process

Np, and fission products. The Thorex solvent extraction process is generally used to reprocess spent Th-based fuels. As in the Purex process, the solvent is TBP diluted in an appropriate mixture of aliphatic hydrocarbons. Figure 12.9 shows the Thorex process flow sheet used by Kuchler et al. [41] for reprocessing high-burn-up thorium fuel. 

If reprocessing of spent Th fuels is needed in the future, many drawbacks of the Thorex process can be avoided by using A, A -dialkylamides instead of TBP. It is known from studies of Pu(IV)-U(VI) separation with amides that U(VI)-Th(IV) separation in acidic medium is easier than with TBP. The advantages of amides over phosphates could probably be considered more freely for Thorex than for Purex plants because no Thorex plant has been built yet. The extraction behavior of Pa(V) has not been yet investigated but in view of the general chemical properties of the two extractants, smaller distribution ratios are expected with the A, A -dialkyl-amides. 

Fig. 12.9 Two-stage acid Thorex process for highly irradiated fuels. Numbers in the frames indicate stage number, whereas numbers on the lines indicate flow volumes relative to the feed volume (DOD and FP are dodecane and hssion products). 

Table 12.10 Performance of Two-Stage Thorex Process and a Uranium Purification Cycle...

The uranium and thorium ore concentrates received by fuel fabrication plants still contain a variety of impurities, some of which may be quite effective neutron absorbers. Such impurities must be almost completely removed if they are not seriously to impair reactor performance. The thermal neutron capture cross sections of the more important contaminants, along with some typical maximum concentrations acceptable for fuel fabrication, are given in Table 9. The removal of these unwanted elements may be effected either by precipitation and fractional crystallization methods, or by solvent extraction. The former methods have been historically important but have now been superseded by solvent extraction with TBP. The thorium or uranium salts so produced are then of sufficient purity to be accepted for fuel preparation or uranium enrichment. Solvent extraction by TBP also forms the basis of the Purex process for separating uranium and plutonium, and the Thorex process for separating uranium and thorium, in irradiated fuels. These processes and the principles of solvent extraction are described in more detail in Section 65.2.4, but the chemistry of U022+ and Th4+ extraction by TBP is considered here. 

More recently a flowsheet has been developed which employs 30% TBP/OK as the solvent.349-446 447 This involves the use of an acid feed to the first cycle to assist in zirconium decontamination and suppress hydrolysis. An acid-deficient partition cycle then follows in which the U-Th separation is effected. A pilot plant (JUPITER) has been constructed at Julich in Germany to process Th02/U02 fuel using this flowsheet. Although a complete separation of thorium, uranium and FPs is possible using TBP in the Thorex process,448 alternative approaches... 

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). 

Even though in the THOREX process 233U can be preferentially recovered from irradiated thorium fuel by using an extraction flowsheet based on 5% TBP n-dodecane as the extractant, further lowering of the concentration of TBP in the solvent has certain advantages in terms of reduced co-extraction of thorium and fission products (195, 196). Ramanujam et al. reported a sequential precipitation technique... 

Bond, W.D. Thorex process, In Science and Technology of Tributyl Phosphate, W.W. Schulz, K.P. Bender, L.L. Burger, and J.D. Navratil (Eds.), Vol. 3, Applications of Tributyl Phosphate in Nuclear Fuel Processing, CRC Press, Boca Raton, FL (1990), pp. 225-247. 

Zimmer, E., Borchardt, J. 1986. Crud formation in the PUREX and THOREX processes. Nucl. Technol. 75 332-337. 

Solvent extraction can be carried out in pulsated extraction columns, in mixer-settlers or in centrifuge extractors. Organic compounds such as esters of phosphoric acid, ketones, ethers or long-chain amines are applied as extractants for U and Pu. Some extraction procedures are listed in Table 11.11. The Purex process has found wide application because it may be applied for various kinds of fuel, including that from fast breeder reactors. The Thorex process is a modification of the Purex process and has been developed for reprocessing of fuel from thermal breeders. 

Liquid-liquid extraction (LLE) systems using neutral phosphorus-based organic compounds have been the subject of extensive study since Warf (1) first reported the use of tributyl phosphate, TBP, as a useful extractant for cerium(IV), uranyl and thorium nitrates. After more than twenty years, liquid-liquid extraction systems (such as the Purex and Thorex processes) employing TBP dissolved in a suitable diluent versus an aqueous HNO3 phase remain the most widely accepted systems for reactor fuel reprocessing. 

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