The revised plan

3.2. OVERVIEW OF PYROCHEMICAL REPROCESSING IN INDIA

salt. The chopped pieces of SNF pins are loaded into the anode basket of the electrorefining cell. A solid metal rod or molten cadmium serves as the cathodes. A layer of molten cadmium is maintained at the bottom of the electrorefiner to collect any deposit falling off the cathode.

During the electrorefining process, the elements whose chlorides are highly stable are easily oxidized and are transferred to the electrolyte phase as their chlorides. They are not easily reduced and so they remain in the electrolyte phase. Noble metals whose chlorides are the least stable do not get oxidized and so they remain in the anode basket. Fuel materials, uranium and plutonium and the minor actinides whose chlorides are of intermediate stability are transported to the anode through the electrolyte, are reduced and then deposited on the cathode.

FIG. 16. Schematic diagram of the molten salt electrorefining process.

When a solid rod is used as the cathode, uranium alone is deposited, whereas when liquid cadmium is used as the cathode, uranium and plutonium are co-deposited along with MAs.

The complete pyroprocess flowsheet is shown in Fig. 17.

FIG. 17. The complete pyroprocess flowsheet (a) Pyroprocess flowsheet–I (b) Pyroprocess flowsheet–

II.

Uranium metal deposited on the solid cathode is occluded with salt, which is then distilled to recover the metallic uranium; this step is known as the ‘consolidation step’. Similarly, the consolidation step for the cadmium cathode is the distillation of cadmium to recover uranium, plutonium, and MAs.

The LiCl-KCl electrolyte salt is purified from the FPs by passing it through a zeolite column and then recycled. The zeolite containing the salt and fission products are heated with boron aluminium silicate glass to form a glass bonded ceramic waste form for the salt.

Prior to the removal of salt for purification using zeolite, the actinides in the salt (6 wt%) must be removed and added to the next batch. It is done by an ‘actinide draw down’ step involving equilibration between the salt containing actinides and Li-Cd alloy at 773 K. This is an essential step for achieving a recovery of >98% for actinides.

The noble metal fission products along with the cladding hulls are left behind in the anode in the electrorefining process. These are melted and cast with 15% Zr to form a metal waste form.

Thus there are two waste forms in the pyroprocess, namely ceramic and metal waste forms.

3.2.4. Electrorefining process

A laboratory scale argon atmosphere facility for carrying out studies on molten salt electrorefining has been operating at the Indira Gandhi Centre for Atomic Research (IGCAR) centre for more than ten years. The facility is a train of five interconnected glove boxes maintained under high purity argon atmosphere with less than 20 ppm each of moisture and oxygen. Studies have been carried out on all the unit operations of the uranium alloys electrorefining process. Separation factors of typical fission products (Ce, Pd and Zr) have been determined using U-10 wt% Zr and U-Ce-Pd alloys as the anodes. Studies are now in progress with Pu based alloys.

A code based on thermochemistry, ‘PRAGAMAN’, has been developed for the numerical simulation of the electro transport behaviour of U and Pu in the electrorefining cell.

Based on the experience gained with laboratory scale studies, a demonstration facility has been set up and commissioned. Electrorefining studies of uranium have been carried out at 1 kg level.

The laboratory scale facility will continue to be used for studies on plutonium bearing alloys.

The experience gained in the operations of these facilities will give vital inputs for the design of a pyrochemical reprocessing plant that can be used for the trial reprocessing of metallic fuel pins to be irradiated in FBTR, as well as the metallic SNFs of FBTR.

It is proposed to start the design of a pyrochemical reprocessing plant for the SNFs from FBTR based on the laboratory scale experience with Pu alloys and engineering scale experience with U alloys.

3.2.5. R&D needs for pyroprocessing technology

For the development of pyroprocessing technology, the following R&D is required:

 Engineering scale processing of U alloys in the demonstration facility;

 Laboratory scale studies on Pu alloys with simulated fission products using different cathode materials, such as Cd and Bi;

 Development of advanced electrorefiners and an advanced consolidation set up for the recovery of actinides from cathode deposits;

 Development of actinide drawdown processes, scaling from laboratory scale studies to plant scale studies — both salt/ Li-Cd alloy equilibration process are included, as well as alternate processes;

 Development of ceramic and metal waste forms, with characterization and leachability studies using simulated fission products;

 Development of electro kinetic model for electro transport properties;

 Thermodynamics studies on alloy and salt systems.

3.2.6. Conversion of actinide oxides to metals

The technology for the conversion of actinide oxides to the respective metals in tonnage quantities is essential to transition from a thermal reactor fuel cycle to a metal fuelled FBR fuel cycle. Currently, the conversion is carried out by calciothermic reduction of the actinide fluorides. In recent years a simpler electrochemical reduction process has been reported, which is being developed in many countries.

A schematic diagram of the process is shown in Fig. 18. In this process, the actinide oxide (in the form of a pellet) is used as the cathode of an electrolytic cell using LiCl or CaCl2 as the electrolyte. When LiCl is used as the electrolyte, 2–3% Li2O is added to the salt. Platinum is used as the anode. When a potential of ~3V is applied, the actinide oxide cathode is converted to the metal and oxygen evolves at the anode. The exact mechanism is still being debated.

FIG. 18. Schematic diagram of the electrochemical reduction process.

3.2.7. Electroreduction of uranium oxides

Studies have been initiated to investigate the electroreduction of uranium oxide in molten LiCl-Li2O electrolyte at 923 K. The solid oxide served as the cathode, and platinum as the anode in the electro-reduction cell. Cyclic voltammetric (CV) experiments were carried out to test the electrochemical stability of different electrode materials like glassy carbon, platinum, stainless steel, molybdenum, and tantalum in the molten salt system. These experiments were also used to obtain the safe electrochemical window for electro-reduction work with the platinum anode.

The CV experiments were carried out in molten LiCl containing 1–3 wt% Li2O, and the results showed that melt compositions with 2–3 wt% Li2O are ideal for the electro-reduction work.

Preliminary electro-reduction experiments carried out with solid UO2 and U3O8 powder electrodes, in both controlled potential and controlled current conditions, yielded partially reduced uranium oxides as well as uranium metal in different experiments. The experiments revealed that a moisture free electrolyte melt is a pre-requisite for the successful reduction of

the oxide to the metal and it is necessary to control the anodic potentials below platinum dissolution to avoid the damage of the electrode. Few experiments were carried out with graphite anode, but the electrolysis product was found to be uranium carbide.

3.2.8. R&D needs for the Indian programme

 Laboratory scale studies on direct electrochemical reduction of uranium oxides to optimize the concentration of Li2O in LiCl, to arrive at the best cathode material and the density and form of the oxide material best suited for 100% conversion;

 Studies on PuO2 reduction at laboratory scale;

 Scaling up the process.