SWELLING OF NEUTRON ABSORBER ON THE BASIS OF INDUSTRIAL AND RECONSTRUCTED QUALITIES B4C AFTER TWO-YEAR IRRADIATION IN THE EMERGENCY PROTECTION RODS

Abstract

The radiation swelling of hot-pressed B4C blocks of industrial and re-fabricated quality with the participation of absorber elements providing emergency protection, up to fast neutron fluence 7⋅1022 cm-2, was investigated. The reason of metal cladding machining deformation of neutron absorber during a two-year exposure period was swelling. The volumetric swelling and the porous parameters of the absorber are measured by hydrostatic weighing and digital analysis of electron microscopic fractograms of fresh splits. The pore allocation and destruction manner of B4C was studied by scanning electron microscopy. It was found that swelling for industrial and re-fabricated B4C, with maximum neutron fluence in the lower part of absorbing elements, culminates 12 and 18 % respectively. The drastic decrease in its swelling was caused by fluence reduction along of absorber rod. Volumetric changes are explained by growth of closed porosity. The maximum swelling of irradiated briquettes for industrial and re-manufactured B4C is 25 and 30 %, respectively, with the same initial values of about 19 and 23 %. The maximum value of closed porosity from industrial and re-manufactured B4C after irradiation is 14 and 21 %, respectively, against the initial values of 4 and 9 %. All detected pores settle at the grain border. The primary bulk of the pores is of technological origin. The size and concentration of pores correlate with an increase of neutron fluence and burnout during irradiation, as well as provoke to formation of microcracks between the crystals and grains of the boron carbide matrix. Nanoscale helium pores with a diameter of more than 100 nm were not detected in the grain body. The open porosity inside maximum swelling zone and mechanical interaction absorber with the cladding is reduced. The fracture nature of the swelling B4C changes from a mixture with a predominance of intragranular splitting to a purely intergranular type under increasing fluence.