Mass spectrometry in the development of control rod materials

Boron carbide having different levels of enrichment in Boron-10 isotope is used as control rods in FBTR and PFBR because of its high neutron absorption cross section and high temperature stability. In the case of FBTR, the control rod material requires a 90% of ¹⁰B isotope, where as PFBR requires a 65 % of ¹⁰B isotope. The natural boron consists of two stable isotopes ¹⁰B and ¹¹B with an abundance of 19.9 and 80.1 atom percent respectively. A plant has been designed and set up in IGCAR to produce enriched ¹⁰B isotope of 90% to meet the requirements of FBRs. In the plant, ion exchange chromatographic process of separation is used to get ¹⁰B isotope. Natural boric acid is used as the starting material and the borate ion is moved from one column to another in a battery of ion exchange columns using HCI as the displacing agent. Engineering experiments resulted in enriched boric acid with different levels of enrichment in ¹⁰B isotope. In order to check the level of enrichment, it is essential to analyse the samples at various stages of the experiments. The enrichment obtained in the products was ascertained using an Inductively Coupled Plasma Mass Spectrometer. Recently, the plant has produced boric acid with 65% enrichment of ¹⁰B isotope. The enriched boric acid is then used to produce elemental boron (¹⁰B) using the electrochemical means and then the boron carbide (¹⁰B₄C). It is also essential to know the impurity levels of the products obtained in every step, to meet the control rod grade specification of the boron carbide. The chemical assay of the enriched boric acid and also the enriched elemental boron were also carried out using ICP-MS along with the various other analytical techniques. The talk will highlight the method used by us in getting the isotopic ratio of the samples with different enrichment of ¹⁰B considering the fact of non-availability of Standard Reference Materials in the wide range ¹⁰B enrichment. (author)