Material testing research of emergency rods of fast neutron reactors BOR-60 and BN-600 and problems of its service time increasing

For the reactors on fast neutrons in system of control and protection are used safety rods (SR), made on base enriched by isotope 10 B boron carbide in kind of hot pressed pellets. The service time of such rods is limited and after operation in reactor during appointed service life they are replaced by new rods. The fulfilling safety rods are subject to utilization and are stored in storage pools. During 1992-1993 in State Scientific Centre Research Institute of Nuclear Reactors researches of safety rods, fulfilling in reactor BOR-60 (1 rod of design 1057-0) and BN-600 (4 rods of a design 1663) and long time storing in pools of storage, were conducted. It was shown, that in rod SR 1057-0 after 537 effective days of operation in reactor BOR-60 and 18 years of storing in pool of storage was undamaged of absorbing pins, the majority of pellet B4 C have satisfactory conditions and sizes. Simultaneously essential decrease of the mechanical characteristics of absorbing pins claddings and external cover tube of rod was shown. Absorbing pins of rods design 1663, fulfilling various time in the reactor BN-600, not had of damages. Significant quantity of pellets B4 C have save integrity. Low extension part of a rod, fulfilling 330 effective days, had appreciable bent because of non-uniformity neutron influence, and the material had low fracture toughness. The absorbing pin cladding have reduced the plastic characteristics, The conclusion was made, that the resource of fast reactors safety rods is limited by decrease of the mechanical characteristics of constructional details at low part of rods. Thus the measures on increase service time of the safety rods should first of all provide use as constructional materials radiation resistant steels and alloys. Improvements in should also be introduced a design of rods, having excluding of regular influence of their low part at operation