Commissioning of Boron Enrichment Plant
In FBTR and PFBR boron carbide (B4C) having different levels of enrichment in 10B is used in control rods because of its high neutron absorption and high temperature stability. Natural boron consists of two stable isotopes viz. 10B and 11B with natural abundance of 18.8 atom percent of 10B and 81.2 atom percent of 11B. The thermal neutron absorption cross-section for 10B and 11B are 3837 barn and 0.005 barn respectively, whereas natural boron has an absorption cross-section of 752 barn. For 1 MeV energy neutrons, the absorption cross-section is 1 barn. Boron being reciprocal energy absorber with no resonance peaks and because of higher cross-section of 10B for the reaction 10B(n,a)7Li, 10B enriched specific compounds are used for control rods, neutron counting, neutron capture therapy. The control rod material for FBTR requires 90% of 10B. The necessity of effective reactivity control to have smaller core size in Fast Breeder Reactors (FBRs) and the various other favorable nuclear properties of 10B in reactor control applications dictate enrichment of this isotope in specific boron compounds.
Amongst the various processes used for separation of isotopes like chemical exchange methods, distillation, ion exchange chromatography, gaseous diffusion, gas centrifuge and laser isotope separations, distillation and ion exchange chromatography are industrially viable methods for separation of isotopes of lighter elements. For enrichment of isotopes of boron on industrial scale, these two processes are mainly used. The processes like HPLC and laser separations are still under development stage.
In the process of distillation, DME-BF3 complex is distilled in a packed distillation column operating under vacuum and the conditions of total / partial reflux. The preferential exchange affinity of 10B in the liquid phase is taken advantage of and the complex gets gradually enriched in 10B in the reboiler. Depending upon the amount of product and the enrichment required, the distillation column is designed and the required numbers of columns are inter-connected to get the desired enrichment.
In ion exchange chromatographic process for separation of 10B isotope, resin in ion exchange column is loaded with natural borate ions either as boric acid or any of its complexes. The process of displacement chromatography is used for enrichment of this isotope.
The distillation process has many disadvantages like inflammability and corrosive nature of the feed material, requirement of vacuum and uninterrupted power supply. On the other hand, due to flexibility of operation in ion exchange process and ease of availability of feed material, a plant has been designed and setup in IGCAR to produce enriched boron 10B to 90% to meet the requirements of FBRs. The salient details of the plant are shown in Fig. 1.
Fig.1 : Salient details of Boron Enrichment Plant
Studies have been undertaken in Chemical Technology Section to enrich 10B isotope of boron using ion exchange chromatography. In this process for separation of 10B isotope, a strong base type II resin in ion exchange column is first converted to hydroxyl form. After thorough washing with DM water, it is loaded with natural borate ions either as boric acid or any of its complexes. The process of displacement chromatography is used for enrichment of this isotope and displaced with strong acid. Borate band is moved from one column to the other in a battery of ion exchange columns. During the band movement, the rear end of the borate band is enriched in 10B isotope due to its higher exchange affinity compared to 11B isotope. The exhausted resin in chloride form is then regenerated to hydroxide form and put back in service to facilitate continuous band movement. The product is eluted out of the band as enriched boric acid solution.
After achieving the desired enrichment, the front end of the band is connected to reject column and the rear (enriched) end of the band is quantitatively transferred to product column. An equivalent amount of boric acid is introduced into the band, at the location of matching isotopic composition to achieve the desired length of borate band. These operations are repeated at the end of every batch.
Operation of the plant based on this process mainly involves preparation of feed chemicals, ion exchange chromatographic operations, regeneration and product purification. NaOH is used as regenerant for conversion of the resin from chloride to hydroxyl form and HCl is used for the movement of borate band. The major cost factor for the operation of the plant based on this process is determined by the consumption of these chemicals.
Fig.2: Estimation of HETP from profile data
In further R&D support for the operation of the plant, a resin which changes its color depending upon the pH of the solution, has been developed in this laboratory. This resin is useful for visual monitoring of the band movements as the resin shows different colors depending on the nature of the solution. The changes in the color of the resin when present in equilibrium with solutions of boric acid, NaOH, HNO3, HCl or water are shown in Fig.3. The performance of this resin has been found to be satisfactory when incorporated in Engineering Experimental Setup. The resin probes shall be incorporated in the plant in due course and will be useful in the operation of the plant.
B.K. Sharma and C. Anand Babu)