US3825649.Pdf

3,825,649 United States Patent Office Patented July 23, 1974 1. 2 l, . 3,825,649 Thorium-232 - I - Thorium-233 --> PROCESS FORSEPARATION OF PROTACTINUM, 23.5 min. ... THORIUM AND. URANIUM FROM NEUTRON IRRADIATED THORIUM Protactinium-233 -> Uranium-233. Alan T. Gresky, Jouko E. Savolainen, and William T. 5 27.4 day McDuffee, Jr., Oak Ridge, and Russell P. Wischow, Thorium may be subjected to neutron bombardment in Nashville, Tenn., assignors to the United States of varying types of reactors. For example, thorium metal America as represented by the United States Atomic may be inserted as aluminum-encased slugs into a hetero Energy Commission geneous reactor, or slurries of thorium oxide may be cir * * * * * Filed Aug: 7, 1956, Ser. No. 602,686 0 culated about a homogeneous reactor core of an aqueous ?“ '.' : , nt. CI. C01g 56/00; C22b 61/04 uranyl sulfate solution enriched, beyond natural abund U.S. C. 42 . 21 Claims ance, with regard to uranium-235. The chemical processing of neutron bombarded tho - ABSTRACT OF THE DISCLOSURE rium is of prime importance, for any product material lost Protactinium, uranium and thorium are separated from in the chemical processing, in effect, increases the de an aqueous nitric acid solution of neutron-irradiated tho mands upon the efficiency of the reactor system. Further rium containing these elements and fission products by more, in reactor fuel processing, contrary to most chem contacting, under net nitrate deficient conditions, the ical processing operations, relatively great amounts of acid solution with an organic solution of a trialkyl phos unreacted material must be separated from relatively phate in an inert organic diluent, thereby preferentially small amounts of products. This arises from the fact that extracting uranium and thorium into the organic phase nuclear fission products of high neutron-absorption cross while confining protactinium and fission products to the sections compete with the fuel for fission-released neu aqueous phase. After scrubbing the organic phase with a trons. Unless such fission products are removed from the aqueous solution of an inorganic nitrate salt to remove reactor, the maintenance of the chain reaction itself may small amounts of protactinium and fission products, the be threatened. Thus, in actual practice, the fuel and fertile two-phases are separated and thorium and uranium are material must be periodically removed from the reactor separately recovered from the organic phase. for decontamination long before the fuel and fertile ma terial are consumed. In addition to extremely high recov ery of fissionable uranium-233, ideally approaching Ouriyention relates to a process for the decontamina 30 100%, the chemical processing should also achieve ex tion of neutron-irradiated thorium, and more particularly cellent decontamination of uranium-233 and thorium to a process for the separation of protactinium-233, from highly radioactive fission products before prepara thorium and uranium-323 from neutron-irradiated tho tion for reuse in reactors. This is essential for both per 1. sonnel safety and maintenance of good neutron economy. A major factor in the cost of generating electricity Perhaps the most perplexing of all problems associated from nuclear fission is the cost of the fuel. Factors which with the chemical processing of neutron-irradiated tho contribute to low fuel cost and towards which reactor rium is the handling of the highly radioactive protactin designs, seek to approach are low, cost fabrication of fuel ium-233, the parent of uranium-233. This isotope usually elements, high burn-up of fuel before reprocessing is re accounts for greater than 95% of the beta-gamma ac quired, low cost reprocessing, and high thermal efficiency. 40 tivity in the irradiated thorium at the time of withdrawal A concurrent approach in reducing the unit cost of gen from the reactor. The relatively short half-life of protac erating electricity from nuclear fission is to obtain by tinium (27.4 days) would argue for prolonged cooling products of high value which can be credited against other of the irradiated thorium prior to any chemical process 45 ing to minimize losses of potential uranium-233. It is generation costs. A principal effort in this direction is estimated that a cooling period of about 250 days would towards the regeneration of fissionable material from normally permit uranium-233 losses of less than 0.1% “fertile" materials concurrent with the consumption of and would allow decay of the 24.1-day thorium-234 ac nuclear: fuel. Reactors designed for fuel regeneration as tivities which otherwise limit thorium-product purification. well as power production are commonly known as "dual Furthermore, the extreme radioactivity of protactinium, purpose" or “breeder" reactors and the regenerative fis 50 with its consequent shielding and handling problems, pre sionable materials produced by such reactors are the well sents additional argument for longer cooling before chem known plutonium (from uranium-238) and also uranium ical processing. Overcoming all these arguments in favor 233 (from thorium). Depending upon the neutron econ of longer cooling period, nonetheless, is the single, crucial ofny of a particular reactor (the number of neutrons 55 fact of the high inventory charges against fissionable ma available for radiative capture by a fertile material be terials. Thus, the precious and expensive fissionable ura yond the requirements of maintaining the chain reaction) nium-233 and the fertile thorium cannot be permitted to remain dormant and unproductive. Furthermore, and as much or more fissionable material may be produced apart from a uranium-233 breeder program, protactinium as is consumed. Such a breeding program may make re itself is required for basic academic studies, as a tracer actor-produced power- competitive with conventional 60 and as a concentrated beta-gamma source for a host of radiation purposes. Therefore, the chemical process for ply of precious fissionable material will be conserved. In recovering uranium-233 must be prepared to deal with fact, since the world supply of thorium is greater than relatively short-cooled feed material, e.g. 40 days and even the world supply of uranium, the potential exists for ac less, as well as possess flexibility for treating longer-aged tually, increasing the amount of fissionable material by material. - conversion of thorium to fissionable uranium-233, which, The separation of protactinium, thorium and uranium upon recovery, may be used to convert additional thorium presents problems of unprecedented severity. For exam to uranium. : . ." ple, thorium, protactinium, and uranium are immedi Uranium-233 is obtained by the neutron bombardment 70 ately adjacent neighbors in the actinide rare earth series of naturally occurring thorium-232, essentially by the fol of the Periodic Chart of the Elements. Although recog llowing principal nuclear. reactions: nizable differences are present among the rare earths, 3,825,649 .. 3. 4. they are notoriously chemically similar, since they differ trate salt, separating said protactinium- and fission prod only in the number of electrons in their deep, underlying ucts-containing aqueous phase from said uranium- and shells, rather than in their valence electrons which nor thorium-containing organic phase, and thereafter separat mally govern chemical reactions. Furthermore, there is ing said extracted uranium and thorium from each other. scanty and unreliable information available concerning The practice of our invention achieves an excellent sep the basic chemistry of protactinium. Ideally, a protac 5 aration of protactinium, thorium and uranium in a single, tinium recovery process should provide for its separa relatively simple, continuous solvent extraction cycle. A tion relatively early to permit the subsequent chemical single extractant, trialkyl phosphate, in proper volumetric separation and decontamination of thorium and uranium proportion in an inert organic diluent, in combination 233 to be conducted under less shielding and with re O with an aqueous scrub Solution of an inorganic nitrate duced radiation hazards. salt, sharply and efficiently extracts thorium and uranium There are presently available continuous solvent ex from an aqueous nitric acid solution of neutron irradiated traction processes for accomplishing the two-way sepa thorium, while confining protactinium and the preponder ration of plutonium and uranium from neutron-irradiated ance of fission products to the aqueous phase, the net ex uranium. A representative process of this nature is de traction and scrub conditions being nitrate ion deficient scribed in Ser. No. 303,691, filed Aug. 1, 1952 in the The protactinium may be thereafter separated from fission names of T. C. Runion, W. B. Lanham, Jr. and C. V. Elli products or may be permitted to decay to, uranium-233, son for "Process for Separation of Plutonium, Uranium and the fission product solutioni readily concentrated to and Fission Product Values.” In brief, this process consists relatively small volume for convenient storage or recov of the extraction of uranium and plutonium from an aque- is ery of individual radioisotopes. By first separating-protac ous solution with an organic solvent while confining the tinium, which accounts for approximately 95% of the ra fission products to the aqueous solution, followed by pref dioactivity of short-cooled thorium, from thorium and erential stripping of the plutonium and then of the ura uranium-233 in a single

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