An Introduction to the Purex Process .

IntroduCtion allowing for the decay of short-lived fission products is termed as spent fuel reprocessing. During the operation of nuclear reactors, 235u component of the natural fuel is fissioned, Process Requirements generating neutrons and a host of highly radioactive fission products. Simultaneously, the fertile 238u Fuel reprocessing differs from conventional captures some of the neutrons and yields 239Pu, anew chemical processing due to the radioactive nature of. and useful fissile nuclide. the materials being processed. The equipment have to be installed behind massive concrete shielding 2 ~W + /,n ~ Fission products + 2.5 ~n + 194 MeV (sometimes as much as 1.5 M .thick) to provide built-in arrangement for separation and protection of the operating personnel from equipment with high 238U I 239lJ ~- 239 R- 239n radiation field. Stringent air ventilation and exhaust 92 + on ~ n : > 93Np ~d 94t"U 23 .5 mtn. 2 .3 ay requirements are to be met in the entire plant to The accumulated fission products in the fuel protect the operating personnel and the environment hinder the operation of the reactor system from from getting contaminated due to air borne radiation and neutron economy points of view. radioactivity. The processes and equipment used are modified to suit remote operation and maintenance. Becau~e of this, as the burn up (expressed as MWD/tonne) of the fuel goes high, the spent fuel has Another important difference between to be replaced with fresh fuel in the reactor to traditional and nuclear chemical engineering is the . continue its operation. ' need to provide a design that precludes the possibility The spent fuel discharged from reactors of accidentally producing a self-sustaining nuclear contains significant quantities of fissile nuclides, chain reaction- the condition known as criticality­ 235 when lar~ concentrations or quantities of fissile mainly the unutilized U and the newly formed 39 235 233 239Pu which can be re-used. The process of isotopes ( Pu, U or U) are handled. Safe separating the fissile and fe1tile materials (Pu and U) operation of a reprocessing plant is generally from the . fission products in the spent fuel after achieved by criticality control techniques like mass, volume, and concentration control of fissile materials or geometry control of the equipment used.

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Fig. 1 Flow scheme of Purex process

Purex~ • Final U purification cycle • Final Pu purification cycle World over, Purex process is followed for the • Pu and U reconversion to their oxides I I separation and purification of uranium (U) and (Pu) from irradiated spent fuels after The first three steps together are called dissolving the fuels in nitric acid. Purex process is head-end treatment steps meant to bring the fuel into essentially a solvent extraction process which uses dissolved aqueous form suitable for Purex process. Tri-n-butylphosphate (TBP) diluted with dodecane The aim of the co-decontamination and partitioning as solvent to extract and further separate U and Pu step is to extract and separate U and Pu together from from highly radioactive fission products. The U and the rest of the fission product impurities and then Pu products are further purified and converted to further separate and purify U and Pu from each other. oxides, while fission product · waste solution is In the subsequent steps, U and Pu are purified further neutralized and stored for subsequent treatment and individually before conversion to their oxide forms. disposal. The general flow scheme used is given in Fig. I . The extent of purification or dec