Irradiation behaviour of boron carbide in WWER-1000

Rod Cluster Control Assemblies (RCCAs) with rodlets made from vibropacked boron carbide powder are used in WWERs-1000. They are used as automatic control rods (CRs) and safety rods (SRs). RCCA CRs and SRs have been examined after operation periods of 2-4 years and 2-7 years, respectively. The examination of the rodlets shows absence of the corrosion effects, external mechanical influences and cladding wear. The 10B burnup was uneven along height and radius of the rodlets. This is a typical characteristic of RCCAs used as SRs. Considerable values of 10B burnup are observed at a length of 1000-1500 mm for CRs and, at 300-400 mm for SRs. Not more than 5% of released gas penetrates to the rodlet gas plenum and its work is inefficient. The main part of the produced helium is located in the bottom part of the rodlets as a free gas or inside the boron carbide grains. The gas pressure under the cladding was 1.0-1.7 MPa. Under irradiation the plasticity of the cladding decreases according with fast neutron fluence along the rodlets. At a burnup of 20-25%, the powder core forms coagulated sintered substance filling the cladding, but its deformation is not observed yet. Appreciable values of rodlet cladding swelling are observed at burnup of about 45-50%. The rate of 10B burnup in RCCAs varies in the different reactors. The SRs of the Zaporozhe Nuclear Power Plant and the Kalinin Nuclear Power Plant have an average burnup for one year accordingly 2.5 and 5% after ending operation. The similar distinctions are characteristic for CR. Carried out investigations allow to consider the main criterion of resource opportunities of the RCCAs based on a boron carbide is maximal average on section burnup of isotope 10B, which should not exceed 45-50%. There are methods to increase the lifetime of RCCAs based on boron carbide up to 20-25 years. (author)