Materials testing aspects of fuel elements development for lead-bismuth cooled fast reactor SVBR - 100

Specific operating conditions of SVBR - 100 reactors developed in Russia as modular sources of the electric power for regional needs put forward a complex of requirements to structural and fuel materials of fuel elements. Necessity of maintenance, first of all, of high thermal stability, a low level of radiation creep, long structure-phase stability and, especially, long corrosion resistance of cladding materials in Pb-Bi coolant to such requirements. At the first stages of the SVBR reactor technology learning it is supposed to use the fuel on the basis of uranium dioxide with the subsequent transition to dense fuel within the limits of developed strategy of a closed NFC. In view of requirements on duration of fuel lifetime in the SVBR core, special demands on gas evolution, heat conductivity, crack resistance characteristics are also made to fuel. At present, the base structural materials for SVBR fuel elements are the compound alloyed 12% chromium-silicon ferritic-martensitic little swelling steels. Investigations carried out in IPPE for the last years have shown that long corrosion resistance of such steels in the Pb-Bi coolant is provided with formation in the coolant on a surface of the fuel element cladding the protective barriers on the basis of the compound alloyed oxide with the Me3O4 spinel structure. The spinel chemical compound varies permanently, moreover, during the tests spinel is enriched with chromium and silicon. Reached to the beginning of 2009 a resource of steel corrosion tests in non-isothermal circulating stands at the temperature of 600 deg. C has exceeded 35 000 hours. Any corrosive-erosive damages of the fuel cladding pipes were not observed. According to electron-microscopic investigations, the cladding steel after tests has the structure-phase composition close to initial one, and after 35 000 hours of tests at 600 deg. C has not practically undergone to processes of ageing. For the present time test proceeds, and the prospective integrated resource of corrosion stand tests will make 60 000 hours to 2012. By present time the high radiation resistance of steel under neutron irradiation is confirmed by tests of the steel samples in the fast reactors BR-10, BOR-60, BN-350 and BN-600. The maximum reached damaging dozes amount to 60 dpa at irradiation temperatures of 340 - 670 deg. C. It is supposed to use tablets of the modified uranium dioxide as the fuel in the SVBR reactor fuel elements. Such fuel developed in IPPE with application of new technologies allows to fabricate the fuel tablets with the increased heat conductivity that will allow to reduce a temperature gradient in the fuel core and to decrease the thermomechanical interaction of the fuel with the cladding in the transient regimes. Adding of the microadditions to the fuel allows to produce the tablets with the grain size of 20-40 microns that can provide the gas evolution reduction in the fuel element. These technologies allow to produce the modified oxide fuel both on the dry and on the wet scheme. In the report the basic directions of the materials testing researches for the nearest years are presented. These ones will allow to confirm experimentally the declared characteristics of the efficiency of the core fuel elements of the fast reactors with the Pb-Bi coolant and also will allow to perform verification of the developed strength and thermomechanical fuel element computation codes