Determination of uranium and O/U ratio by the gravimetric technique

Introduction

The determination of uranium by the gravimetric technique is widely used in the industry. In our experience it is the most precise and easiest to use method for the determination of uranium.

Theory

Pre weighed uranium dioxide samples are oxidized to U₃O₈ in air at 900^o ± 25^o C. The method is not specific for uranium and corrections must be made for impurities. Differences in atomic weights also require corrections. The basic chemical reaction that takes place is

900^oC
3UO₂ + O₂ → U₃O₈

U₃O₈ is a stoichiometric compound and the percent uranium can be calculated by using the appropriate gravimetric factors and correction.

Equipment needed

• Analytical balance

• Platner mortar
  

• Crucibles, quartz or platinum

• Desiccator
  
  

Procedure

Figure 1 lists the basic, analytical steps in this technique. The procedure is simple and utilities basic good laboratory practices, as required by any accurate gravimetric technique.

Calculations

Figure 2 shows the basic calculations used in this analysis. You will not that the weight of the ignited sample is corrected for the weight of nonvolatile impurity oxides to give the true weight of U₃O₈.

WEIGHT SAMPLE INTO CRUCIBLES WHICH HAVE BEEN COOLED AND STORED IN A DESICCATOR FIVE TO TEN GRAMS ARE OPTIMUM. IGNITE SAMPLE IN A MUFFLE FURNACE AT 900o ± 25o C FOR A MINIMUM OF FOUR HOURS (CONSTANT WEIGHT). COOL IN A DESICCATOR AND RE-WEIGHT. U3O8 IS A STOICHIOMETRIC COMPOUND AND CAN BE USED TO CALCULATE THE PERCENT URANIUM THE METHOD IS VERY ACCURATE IF APPROPRIATE CORRECTIONS ARE MADE.

Fig.1 Basic analytical steps

FORMULA %U = F{W2-(W2xImx10-6)} x 100 W1 WHERE F = 3 x ATOMIC WT URANIUM 3 x ATOMIC WT URANIUM +(8x15.9994) W2 = WEIGHT U3O8 AFTER IGNITION W1 = WEIGHT OF SAMPLE TAKEN Im = NON VOLATILE IMPURITY OXIDES EXPRESSED AS PARTS OXIDE PER MILLION PARTS U3O8 O/U RATIO = MOLE OXYGEN / MOLE URANIUM

ig 2. calculations

• Impurity corrections. Nonvolatile impurities in the UO₂ will be converted to higher oxides during the ignition step. The impurity content of each element is determined, usually by emission spectroscopy. Appropriate factors are applied to convert the weight of the elements oxide before ignition to the weight of the ignited sample U₃O₈. This is important step for accurate work. The effect of not correcting for impurities is shown in Fig.3 For the sake of this discussion, assume a UO₂ sample has only iron as an impurity at the concentration shown. During reduction to UO₂, the iron is present as FeO. During oxidation, it is converted to Fe₂O₃. If no correction are mede for this impurity, the error will be as shown with the results being reported high. It is obvious that at higher levels, the bias becomes significant.

• Isotopic Corrections. A smaller bias can be introduced if corrections are not made for differences in atomic weight caused by differences in enrichment or ²³⁵U content. Figure 4 shows the errors that will be introduced if the gravimetric factor is based upon the atomic weight of natural uranium-commonly called a reference book factor. In this figure, it is assumed there is no ²³⁶U and the ²³⁴U remains constant over the range of ²³⁵U contents shown. In actual practice, this would not be the case, however, for the sake of illustration, it is acceptable. Note thet the errors are listed as percent from natural and olmost all are negative values. The negative value will result in a positive bias. Positive values will result in a negative bias.
  

Iam sorry is not shown of Fig.3 and Fig.4

Literature

• Sharkey R.W.,”Analytical Method Used at Model Facillity”, ABB Combostion Engineering, Hematite.

• ASTM C-696 paragraphs 16-22.