A Study of the Firing and Corrosion Characteristics of Boron Silicide

The element boron, with its high thermal neutron absorption cross section, is a very suitable nuclear control material. However, since elemental boron is unstable in most reactor coolant environments, it is necessary to use the element in the form of a stable compound. The problem has been to find a chemical compound which has a high boron concentration and also is stable in the presence of reactor coolant environments. Boron silicide is a relatively new chemical compound and has shown strong potential for meeting the above requirements as a reactor control material. Due to the large number of different types of nuclear reactors it was not feasible to evaluate the stability of boron silicide in every type of reactor coolant. The purpose of this thesis program was to study the stability of boron silicide in high temperature water for water cooled reactors. The study consisted of a) fabricating green pellets from the boron silicide powder, b) firing the green pellets in air to produce well bonded dense bodies and c) corrosion testing the fired pellets in high purity saturated water at various temperatures to determine their corrosion resistance. The boron silicide powder was pressed into green pellets with the use of a hand operated pellet press. A general pressing curve was developed shoving the relationship between pellet density and pressing pressure with the use of a binder additive. The pellets were fired in air to oxide bond the powder into a dense sound body. The firing characteristics of the material were determined and a suitable firing procedure was developed to give optimum bonding. The fired pellets were corrosion tested in high purity water at various temperatures and was found to be dependent on the degree of particle bonding and fired properties of the pellets. The pellets fired at 2000 F and refired at 2500 F had good corrosion resistance to water up to 350 F. But at 500 F the corrosion resistance was considerably reduced. Based on the results of this study it would not be feasible to use boron silicide in a water cooled reactor without a suitable protective cladding. However, the material has sufficient corrosion resistance to prevent catastrophic failure of a control rod should a penetration develop in the cladding and expose the material to the coolant water