Process for the separation of uranium from a radioactive feed solution containing technetium

A process for the separation of uranium and plutonium as well as fission ducts including technetium from a nitric acid feed solution (fuel solution) in which said solution is treated with an extraction agent in an organic solvent to charge said agent with U, Pu and fission products including technetium by the counterflow process. The charged extraction agent containing U, Pu and fission products including technetrium is treated with a washing solution and subsequently with a reducing agent for the separation of the uranium from the plutonium and from fission products including technetium not previously washed out. To improve the separation of uranium and obtain a cleaner uranium end product, the treatment of the organic solvent containing said charged agent with reducing agent is effected in one or more stages by the crossflow process instead of by the counterflow process. A single- or multi-stage pretreatment with reducing agent by the crossflow process may be provided in advance of the counterflow treatment with reducing agent.

The invention is concerned with a process for the separation of uranium 
from a radioactive feed solution containing uranium, plutonium and fisson 
products. 
BACKGROUND OF THE INVENTION 
In the reconditioning of spent nuclear fuels, the fuel (uranium and 
plutonium oxide) and the fission products are dissolved in nitric acid. 
After filtration, the addition of nitric acid or water, and concentration, 
the solution is fed to a liquid-liquid-counterflow-extraction process 
known as the PUREX process for the separation of uranium, plutonium and 
fission products (cf. Kerntechnik, 1978, No. 2, pages 77-79). In this 
extraction process tri-n-butylphosphate (TBP) is the extraction agent in 
the form of an organic solution of TBP in kerosene for uranium and 
plutonium (designated below by the chemical symbols U and Pu). 
THE PRIOR ART 
The nitric acid feed solution containing U, Pu and fission products is fed 
into an extraction column at the top while the lighter organic extraction 
agent is fed in from the bottom. On the way up, the extraction agent 
becomes charged with U and Pu. In doing so, traces of fission products are 
also extracted with them. In order to separate these fission products, the 
charged extraction agent subsequently runs through a washing procedure 
with nitric acid in a washing column. The stream containing the fission 
products leaves the column at the bottom as refined product. The organic 
product flow containing U and Pu is fed as feed solution into a separation 
column. The Pu, present in the organic phase in the tetravalent or 
hexavalent state, is reduced by a flow of reducing agent (uranium (IV) 
nitrate and hydrazine nitrate) to organically poorly-soluble Pu (III) and 
backwashed into the aqueous phase. U remains undissolved as U(VI) in the 
organic phase. The aqueous product flow of Pu leaves the separation column 
at the foot while the organic product flow of U escapes at the head and is 
fed as feed solution into a backwash column at the bottom. Through an 
aqueous backwash solution (nitric acid) fed into this backwash column from 
the top, the uranium is backwashed out of the organic into the aqueous 
phase. 
The aqueous uranium product flow leaves the column at the foot and the 
spent extraction agent escapes at the head and after running through a 
solvent wash may be fed back into the cirucit. 
The columns employed today are usually pulsated, i.e., rapid pulselike 
motions are superimposed upon the flows in order to bring the two phases 
(product flow and extraction agent or reducing agent) into intensive 
contact. Mixer-settlers may also be employed but they are mechanically 
more complicated than the columns. 
In order to achieve the best possible separation after the first extraction 
cycle described, further purification cycles must usually be provided, 
which increases the expense. 
In the case of the uranium (IV) nitrate and hydrazine nitrate mainly used 
today as reducing agent, a heavy excess of U (IV) is necessary in order to 
achieve a quantitative separation of the uranium. Hydrazine represents a 
stabilizing agent by functioning as "HNO.sub.2 -catcher". That is, 
HNO.sub.2 oxides Pu.sup.3+ to Pu.sup.4+ which is soluble in the organic 
phase. 
The fission products T (tritium), Zr (zirconium), Tc (technetium) and Np 
(neptunium) cause special problems; Tc in particular being a problem in 
the extraction, because a not insignificant part becomes extracted with 
the uranium and plutonium. 
Since for enrichment there must be available the purest possible uranium 
end product, a uranium end product contaminated by technetium may not be 
acceptable. A further disadvantage of the technetium lies in the catalytic 
action in the separation column. Through catalytic action hydrazine is 
destroyed. This is extremely undesirable since it produces instabilities 
in the process. As regards the catalytic action reference may be made to 
the publication ISEC, Munich 1986, page I-137 to page I-142. 
THE INVENTION 
The object of the present invention consists in improving the process so 
that the disadvantages described are avoided, the separation of uranium 
and plutonium in particular is improved and a cleaner uranium end product 
is obtained. 
In accordance with the invention, the residence time in the separation 
phase, in the crossflow mixer-settler stage, or the individual crossflow 
mixer-settler stages is significantly reduced. By reducing the reaction 
time between the technetium (Tc) and the reducing agent (U(IV) nitrate and 
hydrazine nitrate) Tc may be reduced and practically quantitatively 
removed from the organic phase. At the same time Pu is reduced to and 
largely backwashed into the aqueous phase. Because of the short reaction 
time the undesirable destruction of the hydrazine caused by the catalytic 
effect of the technetium is minimized. Through the shorter reaction times 
in the crossflow process the following advantages are achieved in 
accordance with the invention: 
avoidance of undesirable secondary reactions injurious to the extractive 
separation; 
higher yield by the reducing agent; 
reduction in waste; 
considerably cleaner separation of uranium; 
increase in the chemical and hydraulic stability of the process; 
possibility of reduction in the amount of hydrazine added in the separation 
stage. 
DETAILED DESCRIPTION 
The invention will be explained in greater detail below with the aid of a 
diagrammatic embodiment represented in the attached drawing.

A feed solution (fuel solution) containing U, Pu and fission products is 
fed into an extraction column (HA) 2 at the top. From the bottom in the 
opposite direction to the feed solution flows a specifically lighter 
organic extraction agent (HAX) which is preferably an organic solution of 
tri-n-butylphosphate (TBP) in kerosene. On the way up the extraction agent 
becomes charged with U and Pu as well as with fission elements such as Tc. 
The extraction column 21 connects to a wash column (HS) 4 into which the 
charged extraction agent is fed from the bottom and subjected in 
countercurrent flow to a washing procedure with nitric acid which is fed 
in from the top in order to wash extracted fission products out of the 
organic phase. The nitric acid wash solution from the bottom of column 4 
is fed into the extraction column 2 at the top. The combined aqueous flow 
containing the extracted fission elements leaves the extraction column 2 
at the bottom as a refined product (highly active waste, HAW). 
The organic product flow (HAP) containing U and Pu and some fission 
products designated "problem elements" among which is technetium, flows 
out at the head of the wash column 4 and is fed as feed solution to a 
separation station designated generally at 6. The product flow first of 
all flows through a crossflow mixer-settler 8 which is represented here as 
four-stage unit. In each stage the organic flow running continuously 
through the stages is only briefly treated in crossflow by a reducing 
agent (uranium(IV) nitrate and hydrazine nitrate) which is subdivided into 
separate partial streams each of which acts upon only one extraction stage 
of the crossflow mixer-settler 8. 
The aqueous solution charged with Pu and Tc is discharged from each stage 
of the crossflow mixer-settler 8, the concentration of the extracted 
substances Pu and Tc decreasing continuously from stage to stage. 
The organic product flow containing uranium and possibly some residual 
small proportions of Pu, after leaving the crossflow mixer-settler 8, is 
fed into the bottom of a separation column 10. The specifically heavier 
reducing agent flows from the top in the opposite direction to the organic 
product flow. The reducing agent charged with Pu leaves the separation 
column at the bottom, while the organic product flow now containing 
practically only U flows out of the separation column 10 at the top and is 
fed to further stations for the recovery of the clean uranium end product 
foreseen for the enrichment process. 
In the drawing, the separation station 6 includes the crossflow 
mixer-settler 8 as a stage before the separation column 10. But the 
separation station 6 may also consist only of the crossflow mixer-settler 
8. The crossflow mixer-settler may be made single- or multi-stage, 
depending upon the existing conditions.