Rosatom’s Fuel Division, TVEL, has produced a unique fuel assembly, OS-5, based on uranium-plutonium nitride (SNUP – Smeshannoe Nitridnoi Uran-Plutonievoe) fuel with a liquid metal sublayer. Sodium metal was placed under the steel shell enveloping the uranium-plutonium fuel pellets.
Research and calculations by Russian scientists show that the use of a liquid metal sublayer will improve the characteristics of fuel rods with nitride fuel for fast neutron reactors. It is expected that the temperature of such fuel will be lower while maintaining the coolant parameters. Also, the uranium-plutonium pellet will swell less, provoking possible depressurisation. This will improve both economic efficiency and operational reliability of the fuel.
The production of OS-5 is part of a large-scale programme of work to improve the efficiency of SNUP fuel for the BREST-OD-300 lead-cooled fast neutron reactor, which is being built in Seversk as part of the pilot demonstration power complex being developed under the Breakthrough (Proryv) project intended to demonstrate closed fuel cycle technology.
Since 2014, Rosatom scientists and engineers have been carrying out pilot operation of SNUP fuel in the BN-600 reactor at the Beloyarsk NPP and undertaking post-reactor studies of irradiated fuel rods. As a result, increasingly higher burnup of SNUP fuel is being experimentally confirmed and gradually substantiated.
“The first generation of SNUP fuel for the launch load BREST-OD-300 was justified with burnup at the level of 6% of heavy atoms. Our goal is to stepwise increase the burnup to an average of 12%, said Mikhail Skupov, Deputy Director of the AA Bochvar Research Institute of Inorganic Materials (VNIINM). “To test SNUP fuel to maximum limit parameters in the BN-600 reactor, our scientists have already used a number of non-standard innovative solutions, for example, special removable containers in irradiation assemblies. Fuel rods with a liquid metal sublayer, OS-5, are a revolutionary technological solution and another important step in the development of nitride fuel for fast reactors. With this assembly we expect to achieve the design targets for fuel for fast reactors of the future.”
The OS-5 irradiation assembly was manufactured at the Siberian Chemical Combine (SKhK – Sibirskovo Khimicheskovo Kombinata) in Seversk, in cooperation with colleagues from the Fuel, Scientific and Mechanical Engineering Divisions of Rosatom. After approval from regulator Rostechnadzor, the innovative fuel will undergo pilot operation in the BN-600 reactor at the Beloyarsk NPP in the Sverdlovsk region.
“The work of our scientists to develop SNUP fuel technologies is of strategic importance for the nuclear energy sector of the future,” said Alexander Ugryumov, Senior Vice President for Scientific & Technical Activities at TVEL. “We have experience in operating a fast BN-800 reactor with a full load of mixed oxide (MOX) fuel. Nitride fuel is denser, and therefore potentially more cost-effective. Initially, it was created for reactors with lead coolant – the BREST-OD-300 and subsequent BR-1200. But potentially it can also be used in fast sodium reactors such as the BN-1200M, which also provides an option to use a nitride core. Experience will show which technology is more viable. Our ultimate goal is not only to take advantage of the nuclear fuel cycle closure in fast neutron reactors but also to make these installations as competitive as possible in the electricity and capacity market compared with other types of generation.”
SNUP fuel is a type of nuclear fuel in which fissile material (a mixture of uranium and plutonium) is presented in the form of a nitrogen compound – mononitride – instead of standard uranium dioxide. Such fuel is not yet used in industry. It is being developed for promising fast neutron reactors with sodium and lead coolant. The high density ensures a high fuel consumption and fuel reproduction rate, allowing reactors to be made more compact. High thermal conductivity ensures reliability and temperature resistance of the fuel. During reactor operation, the isotopic composition of the fuel is equalised, which simplifies fuel refabrication.
