Vol 121, No 4 (2017)

Physical startup of the IVV-2 nuclear research reactor at nominal power 5 MW was conducted 50 years ago. Designers, builders, and personnel at the Institute of Reactor Materials needed about 10 years to develop a new reactor – IVV-2M – on the basis of the IVV-2 design. The redesign, which was accomplished in 1974–1982, made it possible not only to increase the neutron-physical parameters of the reactor but also the nuclear and radiation safety. The structural features of the IVV-2M reactor and the basic problems now being solved as well as the prospects for new advancement are shown in the present article.

Data on a shielded-enclosure block, which is part of the scientific-research complex at the Institute of Reactor Materials, are presented. The equipment for the primary post-reactor studies and methods of flaw detection in objects are described. Articles are cut into blanks and samples on in-box remote-controlled machines. The equipment and methods for determining the physical and mechanical properties and the elemental and isotopic composition, analyzing gas mixtures, and performing metallographic, microstructural, and electrochemical studies are listed. The most significant and interesting results and publications on the post-reactors studies of spent fuel and other structural elements of reactor facilities are presented.

Survey information on integral material studies of advanced fuel compositions performed at the Institute of Reactor Materials over a period of 50 years is presented. The influence of the reactor testing conditions on the radiation resistance of different forms of nuclear fuel was investigated: oxide, carbide, nitride, carbosulfide, and some of their compounds. Uranium oxides possessed a different initial microstructure and were doped with the oxides Y₂O₃, TiO₂, Sc₂O₃, and others. Carbide fuel was tested in the form of solid solutions of uranium carbide with Zr, Nb, and Ta carbides. The dimensional, phase, and structural stability of fuel compositions were studied in hot boxes; the electrophysical and mechanical properties, such as strength in compression and bending, Young’s modulus, the shear modulus, and Poisson’s ratio were investigated. The experimental data on the radiation resistance of advanced fuel compositions formed the basis for the validation of the serviceability of different nuclear reactors.

One of the stages preceding the failure of fuel elements in research reactors with uranium-molybdenum dispersion fuel is the formation of bulges in the fuel-element cladding with accumulation of gaseous fission products in them. It is impossible to validate the operational safety of fuel elements with such fuel if the behavior of gaseous fission products under such conditions is not understood. The results of experimental determination of the free volume of bulges and the volume, composition, and pressure of the gaseous fission products in them are presented.

Two directions of modification for increasing the stability of dispersion fuel U–Mo/Al under reactor conditions are examined. The first direction is to introduce Zr, Ti, and Si into the fuel alloy and Si, Mg into the aluminum matrix. The second direction is to create barrier coatings on the surface of fuel particles. Reactor tests of mini-fuel-elements with barrier coatings comprised of Nb, the alloy Zr–1%Nb, and UO₂ around the fuel particles were performed in the IVV-2M reactor. These tests showed a positive effect on the reduction of the swelling of mini-fuel-elements and growth rate of the (U, Mo)Al_x layer. The results of electron-microscopy and the distributions of U, Mo, Zr, Nb, Si, Al, Xe, Kr, and Nd in the components of the fuel compositions U–Mo/Al and U–Mo/Al–12%Si with barrier coatings comprised of niobium and the alloy Zr–1%Nb and UO₂ around fuel particles and without coatings irradiated to burnup 60% are presented.

Electropotential monitoring, used as a fast method for evaluating fuel-element cladding after operation in the BN-600 reactor and in the shielded-enclosure block at the Institute of Reactor Materials during incoming inspection, is described. The plots of changes in the electrical resistance obtained during electropotential monitoring reflect the degree of imperfection of a fuel element and make it possible construct a cutting scheme for subsequent post-reactor material science studies. The particularities of electropotential monitoring of fuel-element cladding made of the steels ChS-68 and EK-164 are shown.

Technological advances associated with the production of radioisotope products in the IVV-2M reactor have been made at the Institute of Reactor Materials. The design of the core and the operating regimes were changed, and the useful neutron load of the reactor was doubled. Novel approaches are presented to the production of high-activity ¹⁹²Ir in a medium-flux reactor, ¹⁴C production by irradiation of aluminum nitride with efficient use of reactor resources and ¹³¹Cs by methods giving 99.99% purity, and the possibility of using barium carbonate with native ¹³⁰Ba content 0.1% as the target material, as well as ¹⁷⁷Lu. The most significant achievement is the implementation of a design for creating an intense neutrino source based on ³⁷Ar for the international project SAGE.

The average volume activity of iodine isotopes is determined, and the ratio of the physicochemical forms of iodine isotopes entering the ventilation system of the first loop of the IVV-2M reactor facility before the emission purification system is investigated. New sorption-filtering materials and a new model for the interpretation of the measurement data is used to determine the physicochemical forms of iodine. It is shown that most of the iodine isotopes are represented in the gaseous form as organic compounds and elemental iodine – 29 and 63%, respectively. The aerosol fraction of the iodine isotopes in the experimental samples of the ventilation system air did not exceed 8% on average.