Continuous dissolution of uranium‐aluminum reactor fuels

Abstract

Extensive pilot plant studies of the continuous, mercury-catalyzed nitric acid dissolution of uranium-aluminum alloy materials similar to possible reactor fuel elements were carried out. Marked dfierences were observed in the dissolution rates of cast and wrought alloys. Optimum feed-acid concentrations varied with the type of alloy. At constant acid feed conditions dissolving rates vaned approximately with the cube root of catalyst concentration up to a limiting concentration. The metal dissolving rate was proportional to the 0.8 power of the nitric acid feed rate. A general empirical correlation was developed.Nuclear react.or fucls arc periodically processed to sep:ir:ite the fissiotia hlc material froni :illoying elcnients and froni fission pmduct.a \vliicli may "poison" t.lie retictor if they :ire not removed. Many reactors use uraniiitii-:ilumiiiuni-allo~ fuel elcnients of various compositions and shapes, the spent fuels Iking processcd hy batchwise mcrcury-rata1yzec.l tlissolution in nitric acid follo\veil by separation cJf uranium froni aluminum ant1 fisaiuti products by solvent. extniction. I t w i s anticipated that operating simplifications could he achieved by continuoits, as opposed to l)atch, dissolution. Consequently, estensive pilot plant studies of thc continuous dissolution of uraniumalumiiiuni alloys containing up to about 10 wt. yc uranium wcrc undertaken. Data atid eorrel:it.ions arc given for the portion of these investigations pertaining to the important proccss re1:itionships in contiiiuoua dissolution.OFF GAS NO..NO. METAL FEED R Kg TEST EQUIPMENT AND MATERIALSTlir colitiiiunus tliasolver uacd in the experiinerit:il work w i g a td1 %in. I.D. type-3.17 stninlws stcd pipe with :i rooliiig scctioii :it. ttic top. This ovc*rhend cooliiig sert.ion and a down-draft. condriistrr of standard design w r e used to vondenw the steani in t.he o 8 -g :~~ riairig from ttic dissolver solution. The ucid fcwl wris cont-ixiuourly stnun he:itcd 1.0 near-hiiliiiK tempc.ratuwa: t.hc product w i s w t c r cooled and tlicii (sollwted in :I st:iililew steel drum pl:ic.cd on s platform si-alc!. The. : i d fced rntc W:LS controlled with :L rotahctcr rind metinired I)y liquid-levc4 readings on the frctl t:riik. Inet.runientntion for measuring the ked and product spcc?itic grnvitirs arid tenilrrst.urcs 3t variou9 locat.ions was uvailablc.The reavtor fuel materid t.o he dissolved was simul:itcd by usirig cylintiric*:il ~U K S ubout 1% in. in dinmekr rind 8 in. long. AltlrougIi riot rcprrwnttrtive of M.T.R.-type furl plates. in tlint the surface-to-volume ratio of such h r s is much lower th:rn that of pl:rtcs, this forni of mntcrisl \\'as H convenient. arid rconomical choice!. Extruded 2s alumirium-bar Rtoek tirid cnsl a i d extruded urariium-uluminun~-alloq. slugs sere ueed in thcs experimental work. The slugs wcre loaded periodically through : i pair of gate vulves loc:rtecl at the t.op of the disBoIver. GENERAL CONCEPTSA kclienxitic c1i:igr:im of the continuous dissolver and t.he iriiportant ausiliary equipment i...

Dissolving Uranium in Nitric Acid

“…Equipment. The reaction vessel consisted of a 1-liter round-bottomed flask immersed in a constant temperature 1 Present address, Chemistry Department, Northern Illinois University, DeKalb, 111. bath. The metal samples (cubes or pieshaped wedges weighing 20 to 50 grams) were supported in the acid by a sample holder consisting of two Teflon disks (approximately 1 inch in diameter) between which two opposite corners of a metal sample were clamped and to which a Chromel rod was fastened.…”

Dissolution of Irradiated Murr Fuel Assemblies

“…Rice reports (with limited data) the dissolution of pure aluminum to be zero order with respect to catalyst concentration for low nitric acid concentrations and 0.3 to 0.65 order for higher nitric acid concentrations. Long 20 reports the dissolution rate to vary as the cube root of mercury concentration below 0.005 M but cited continuous dissolution work by Boeglin 17 that is not directly comparable to other data from batch dissolution. In pilot scale tests, Caracciolo 21 reported a single comparison for aluminum-25 wt % U fuel where the dissolution rate increased ~70% when the catalyst concentration doubled from 0.001 to 0.002 M. This author recommends relying heavily on the results of Schlea and assuming that the dissolution rate is approximately linear with catalyst concentration; however, this recommendation should be used with appropriate caution as other researchers suggest that it could overstate the effect.…”

“…17,18 An oxide coating that forms on the aluminum requires the use of a mercury catalyst to achieve sufficiently rapid dissolution rates in nitric acid. Mercuric nitrate, at concentrations ranging from 0.001 to 0.02 M, was used during previous SRS H-Canyon fuel campaigns.…”

Undissolved Uranium Aluminide in Reprocessing of Test Reactor Fuels.

“…At an aluminum dissolction rate comparable to that of ICPP, shown in Table I, the unirradiated ETR fuels were very reactive, and mercury levels required were considerably lower than those used in ICPP flowsheets. Based on the preliminary studies, the ETR-B&W fuel was dissolved for 15 hours continuously at steady state using 6 and mercury concentration produced similar product concentrations for unirradiated ETR-AI and ETR-Syl fuels.…”

“…According to earlier work (6) , the increase in dissolution rate, corresponding to an increase in aluminum concentration in the dissolver p r o d u c t , is negligible above the 8-foot level in an 18-foot dissolver, whereas the increase is very significant at 2, 4, and 6 feet; as a result, the height of the submerged fuel in the dissolver can vary considerably in the upper portion of the 18-foot dissolver without drastically affecting the dissolver produck concentration. In spite of the large fuel surface area of the fuel pieces in the 19-inch dissolver, the fuel height is extremely ilnpcjrtant, and it is necessary to keep the fuel height constant.…”