Plutonium experimental procedure

Sampling and Measurements. The determination of dissolved actinide concentration was started a week after the preparation of solutions and continued periodically for several months until the solubility equilibrium in each solution was attained. Some solutions, in which the solubilities of americium or plutonium were relatively high, were spectrophotometrically analyzed to ascertain the chemical state of dissolved species. For each sample, 0.2 to 1.0 mL of solution was filtered with a Millex-22 syringe filter (0.22 pm pore size) and the actinide concentration determined in a liquid scintillation counter. After filtration with a Millex-22, randomly chosen sample solutions were further filtered with various ultrafilters of different pore sizes in order to determine if different types of filtration would affect the measured concentration. The chemical stability of dissolved species was examined with respect to sorption on surfaces of experimental vials and of filters. The experiment was performed as follows the solution filtered by a Millex-22 was put into a polyethylene vial, stored one day, filtered with a new filter of the same pore size and put into another polyethylene vial. This procedure was repeated twice with two new polyethylene vials and the activities of filtrates were compared. The ultrafiltration was carried out by centrifugation with an appropriate filter holder. The results show that the dissolved species in solution after filtration with Millex-22 (0.22 ym) do not sorb on surfaces of experimental materials and that the actinide concentration is not appreciably changed with decreasing pore size of ultrafilters. The pore size of a filter is estimated from its given Dalton number on the basis of a hardsphere model used in the previous work (20). 

Figure 5 shows a comparison of U(IV) concentration profiles realized experimentally for the mixer settler and the pulsed column. The U(IV) profile in the columns shows an inventory of about a tenfold stoichiometric excess for the aqueous phase, relative to the Pu profile to be expected from LWR fuel. The U(IV) production rate in the column can easily be increased to an extent higher than the feed rates of externally produced U(IV) normally required in the conventional reduction column. Therefore one can at least expect equally good results for the U/Pu separation with the electro-reduction column as with the normal procedures. This is also confirmed by experiments in the USA which resulted in the installation of an electro-reduction column in the AGNS Plant at Barnwell. In these experiments even with high acid concentrations (2 M HNO in the aqueous strip, BXS) high plutonium decontamination factors have been achieved (17). 

The thrust of the experimental program at ICPP was to find a separation procedure that would separate plutonium, americium, and curium from high-level first-cycle raffinate (see Table I) and leave behind the cladding elements, salting agents, and the bulk of the fission products. Fission-product lanthanides, because of their similar valence and ionic size, would be expected to follow americium in nearly any simple separation scheme. Americium and curium are present in ICPP waste as trivalent ions while plutonium is most likely present as both Pu(IV) and Pu(VI). Any separation scheme must be applicable to all these ionic actinide species. 

The approach-to-critical procedure is employed in each critical mass determination, and the rod worth is simul- -taneously determined for each etqieriment. The control rod worth, in terms of solution volume, varies from about 60 ml t high plutonium concentrationsy to about 560 ml (at low plutonium concentrations) as shown in Fig. 1. When the control rod is started into the solution (sphere. not full), the flux is observed to rise, then foil off as ex-pe ed. This is because the first portion of the rod is worth more as a volume displacement (improved geometry) than as a neutron absorber. The effect is estimated to te about 5f positive reactivity experimentally, and a perturbation calculation provides an estimate of 4.3f Ak/k. 

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