Boron nitride insulating material

High temperature BN-insulated heaters for use as fuel pin simulators in reactor thermal hydraulic test facility studies comprise a cylindrical housing and a concentric heating element disposed within the housing and spaced apart from the housing to define an annular region therebetween. The annular region contains BN for providing electrical resistance and thermal conductivity between the housing and the heating element. The fabrication method of this invention comprises the steps of cold pressing BN powder at a pressure of 20 to 80,000 psig and a dwell time of at least 0.1 to 3 seconds to provide hollow cylindrical preforms of suitable dimensions for insertion into the annular region, the BN powder having a tap density of about 0.6 to 1.1 g/cm3 and an orientation ratio of at least about 100/3.5. The preforms are inserted into the annular region and crushed in place