70 LABORATORY OF NUCLEAR ANALYTICAL METHODS A NOVEL METHOD FOR THE SEPARATION OF URANIUM, PROTACTINIUM AND THORIUM BY CATION EXCHANGE CHROMATOGRAPHY Rajmund S. Dybczyński, Marta Pyszynska, Ewelina Chajduk With a recent increased interest in the develop- (from THOREX solution [9]) in the system: Dowex + ment of nuclear power plants including also pos- 50WX8[H ] – HNO 3 aq. were determined by batch sible utilization of thorium fuel cycle it appeared equilibration at room temperature and are pre- that the existing status of nuclear data is not satis- sented in Fig.1. Pa was determined radiometri- factory. As one of the authors stated: “nuclear data cally using 233 Pa obtained by irradiation of 50 !g of are very inconsistent and there is a need of con- thorium nitrate (standard Th solution for ICP-MS, sid erable improvement drawn by means of new Perkin-Elmer, in the reactor Maria at a neutron measurements, theoretical model calculations and flux of 10 14 n cm –2 s –1 for 50 min and dissolution in empirical systematics” [1]. Experiments in which concd. HNO 3 (4 ml) and 1 ml H 2O2). Count rate thorium oxide (ThO 2) irradiated with neutrons in measurements of the aliquots of the solution, after a nuclear reactor will be analyzed for isotopic com- the equilibrium was established, were performed position of uranium formed in nuclear reactions, with the aid of a gamma-ray spectrometer with a belong to the actions whose necessity was formu- well type HPGe detector (Canberra) (of 255 cm 3 lated above. To achieve this aim, after irradiation nominal volume, 55% relative efficiency, resolu- the three main elements i.e. uranium, protactin- tion 2,15 keV at 1332 keV). 312 keV and 300 keV ium and thorium should be separated to enable photopeaks were measured in this experiment and accurate measurement of uranium isotope ratios in subsequent elution runs. by ICP-MS. U and Th elemental determinations were per- When selective separation of a multicomponent formed using an ICP-MS (Perkin Elmer ELAN mixture of inorganic ions is required, extraction or DRC II). Uranium was determined at the mass ion exchange chromatography seem to be a logical number 238 and thorium – 232; as internal standard choice. While many methods for the separation of In-115 was used. The operating conditions typical- uranium and thorium can be found in the literature ly applied to the determinations were: RF power [2-5], the data for adequate separation of protac- – 1000 W, nebulizer gas flow rate – 0.95 L min –1 , tinium from these elements are much more scarce. plasma gas flow – 15 L min –1 , auxiliary gas flow – In addition, as follows from our experience, several 1.2 L min –1 , lens voltage – 6.75 V. of the published procedures are difficult to repro- duce. The reason for that is probably difficult chem- istry of Pa(V) and, in particular, its tendency to hydrolyze. According to Kirby [6], in most of aque- ous mineral acids, the solutions of Pa(V) are un- stable and change with time, the exceptions being solutions in sulfuric and hydrofluoric acid. This is the reason why many published separation methods employ anion exchange chromatography with hy- drofluoric acid or its mixtures with other mineral acids as eluents for the separation of Pa(V) from other elements. For example, Th(IV), Pa(V) and U(VI) were separated by stepwise elution from strongly basic anion exchanger Dowex 1X10[Cl –]. Th(IV) was eluted first with 10 M HCl, followed by Pa(V) eluted by a 9 M HCl – 1 M HF mixture and finally U(VI) was eluted with 0.1 M HCl [7]. Fig.1. Weight distribution coefficients, , of U, Pa and Th From the standpoint of the determination of iso- in the system: Dowex 50WX8[H +]-HNO aq. tope ratios in uranium formed as a result of neu- 3 tron irradiation of thorium oxide, this elution se- As can be inferred from Fig.1, there is little quence is not the most advantageous. chance for successful separation of the three ele- An attempt was made to devise a new chro- ments in this system by elution with HNO 3 solu- ma tographic method for the separation of U(VI), tions of varying concentration although separation Pa(V) and Th(IV) which would avoid using hy- of Pa from Th by elution with, e.g. 1-2 M HNO 3 drofluoric acid in the elution procedure and in should be possible. To elute U from the cation ex- which uranium would be eluted first. Cation ex- change column as the first element, drastic lower- change systems were considered by us as an obvi- ing of its electric charge would be required. In con- ous alternative to anion exchange, although some nection with that we have tried a selective complex- authors openly discouraged the use of cation ex- ation of uranium with Glyoxal Bis-(2-hydroxyanil) changers because of the strong tendency of Pa(V) (GBHA). This compound has been known as a to hydrolysis [8]. Weight distribution coefficients, reagent selective towards some divalent elements (amount per g of dry ion exchanger [H +] /amount giving with them colored complexes. GBHA was per ml of the solution) of U(VI), Pa (V) and Th(IV) used for spectrophotometric determination of LABORATORY OF NUCLEAR ANALYTICAL METHODS 71 uranium [10], and calcium [11,12]. To our best at elevated temperature, some gas bubbles may knowledge it has never been used so far, as a com- form in the column adversely affecting the regu- ponent of eluent solution in ion exchange chro- larity of the eluent flow. Usually, also small (a few matography. percent of the total Pa load) leakage of Pa was Preliminary experiments have shown that selec- ob served in the first 1-3 fractions of the effluent. tive elution of uranium, while leaving all or most of The magnitude of this leakage may depend on the Pa and Th on the column, is possible with mixtures sub tle differences in sample preparation, final of alcoholic GBHA solution and appropriate aque- concentration of HNO 3 in which the sample is ous electrolyte solution. Micro columns with inter- loaded onto the column, time elapsing between nal diameter of ca. 3 mm equipped with a jacket sample preparation and loading and tempera- through which water from a thermostat was circu- ture. As follows from the literature, in moderately lated, were employed. The resin bed rested on a acidic solutions Pa may exist in a variety of ionic 2+ + glass wool plug and the flow rate was regulated by forms like: Pa(OH) 3 , Pa(OH) 4 [8], but also + 2+ a peristaltic pump. [Pa(OH) 3NO 3] , [Pa(OH) 3NO 3] etc. [13]. Com- Two variants of the method enabling separa- plex equilibria existing between these forms which tion of U-Pa-Th mixture in trace quantities were are difficult to predict and control may be respon- devised. sible for the above-mentioned leakage. First variant: The mixture in dilute HNO 3 solu- The second variant consists in: + + tion is loaded onto a Dowex 50WX8[H ] column • the use of resin in the [NH 4 ] form, equilibrat- (100-200 mesh), washed shortly with the same di- ed with the (3+1) eluent (3 parts of 1.5 mg of lute HNO 3 and eluted with the mixture of solution GBHA/1 mL of C 2H5OH solution plus 1 part of of GBHA in ethyl alcohol + 0.2 M aqueous solu- aq. 0.2 M CH 3COONH 4; tion of CH 3COONH 4. The elution of uranium is • sample preparation by evaporation of the relatively slow at room temperature ( cf. Fig.2A) U-Pa-Th mixture plus 2 mg of H 3BO 3 with and faster at elevated temperatures (Fig.2B). At concd. HNO 3 followed by evaporation two or o 60 C, more than 90% of uranium is eluted in 35 three times with 0.15 M HNO 3 to dryness or to column volumes. Most of Pa can be eluted with 2 a micro-drop and final dissolution in the (3+1) M HNO 3 or 0.5 M HCl + 1 M NH 4CNS, and fi- eluent solution. nally Th is eluted with 4-5 M HNO 3. There are In these conditions most of uranium is rapidly some drawbacks associated with this procedure. eluted even at room temperature and still better GBHA in acidic medium undergoes changes, what at 60 oC, ( cf. Fig.3). A B Fig.2. (A) Separation of microgram amounts of U and Th and trace amounts of Pa by stepwise elution. Column: 3.3 cm x 0.0687 cm 2 Dowex 50WX8[H +] (100-200 mesh); temperature: 25 oC; sequence of eluents shown on the diagram, flow rate: ~0.6 cm min –1 . (B) Separation of of U and Th and trace amounts of Pa by stepwise elution. Column: 5.0 cm x 0.0678 cm 2 Dowex 50WX8[H +] (100-200 mesh); temperature: 60 oC; sequence of eluents shown on the diagram, flow rate: ~2.2 cm min –1 . is manifested by a change of the color into brown, Over 97% of U is eluted within 40 column which presumably may be due to some condensa- volumes (90% in 10 column volumes) at 60 oC ( cf. tion (?) reaction of the reagent. The resulting color- Fig.3B). Pa is then eluted with 2 M HNO 3 and ed substance adsorbs on the resin. At the same rapid elution of Th can be effected with 0.005 M time, especially when the separation is carried out H2SO 4 + 0.5 M (NH 4)2SO 4 solution [13].