This pa per pres ents the re sults of an anal y sis of a uni ver sal cool ing sys tem for the core of research re ac tors built on the pas sive prin ci ple of nat u ral con vec tion. A 3-D model, tech no log ical and de sign di a grams of the re ac tor in stal la tion are pro vided, along with ex am ples of numer i cal eval u a tion of tran sients dur ing the op er a tion of the cool ing cir cuit in nor mal and emer gency modes to sub stan ti ate the pos si bil ity of us ing such a cool ing sys tem in re search reac tors of small and me dium power. The prin ci pal fea ture of the de scribed pas sive sys tem is the ab sence of not only ac tive el e ments, such as cir cu la tion pumps and shut-off and con trol valves from the cool ing cir cuit, but also of pas sive el e ments with mov ing parts, such as a check valve. The cool ing cir cuit in cludes only ves sels, pip ing and a heat exchanger. The ab sence of el e ments with me chan i cal mov ing parts can sig nif i cantly re duce the like li hood of equip ment fail ures and im prove the re li abil ity of such a cool ing sys tem while also re duc ing its cost. The ver sa til ity of the pro posed sys tem al lows it to be used for a wide range of re search re ac tor plants with var i ous ca pac i ties, which are now a days be ing de vel oped de signed to carry out pro grams in var i ous ar eas of re search and ap plied us ages re lated to nu clear tech nol o gies.

И з в е с т и я в у з о в • Я д е р н а я э н е р г е т и к а • № 4 • 2 0 1 7 17

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Achieving high neutron fluxes in research pressurized water reactors is directly related to the intensity of the coolant flow through the core and the pressure in it, which provides an increased saturation temperature and a margin to critical heat flux. Therefore, it is practically impossible to provide very high neutron fluxes in pool-type reactors, especially in the case of downward movement of the coolant in the core. At the same time, vessel-type research reactors (for example, SM-3 and HFIR) make it possible to achieve neutron flux densities up to 4 × 1015 n/(cm2 s), but at the same time, the risks of core degradation in case of violations in the heat removal system become quite high. The proposed concept of a heat removal system for a high-flux reactor facility combines the increased reliability of safe heat removal from the core and the convenience of handling irradiation cells, for example, in the production of isotopes. The concept provides for the location of a compact core in a pressurized vessel and the placement of a neutron reflector around the vessel in the reactor pool. Cooling of the reactor core in the housing and the irradiation channels in the neutron reflector is carried out by different systems of forced circulation of the coolant. At the same time, at the shutdown reactor, after opening the natural circulation valves, safe heat removal from the reactor core and the neutron reflector can be carried out by the water of the reactor pool. However, even with a complete failure of all forced circulation circuits, the evaporation of water from the surface of the pool makes it possible to safely remove the residual heat from the fuel assemblies and from the irradiation devices in the cells of the reflector.

The production of the artificial 51Cr neutrino source with activity > 3 MCi for the experiment BEST is presented. This procedure consisted of making a 50Cr target and irradiating it with thermal neutrons in a nuclear reactor SM-3. The production of a target in the form of disks with a thickness of 4 mm and a diameter of 84 and 88 mm included enrichment (to 96.5% in 50Cr) of natural chromium in the form of oxyfluoride by gas centrifugation, electrolytic reduction and refining of metallic chromium, as well as the formation of chromium disks by spark plasma sintering. Simulations of various source geometries, neutron flux and nuclear transmutation were carried out to validate the design of the source, the irradiation device and the transport container, the required chemical purity of the target and the irradiation schedule in the reactor. The calculated activity of the source after 75 effective days of irradiation was 3.55 MCi. The activity of the source was measured by the calorimetric method and amounted to 3.41 MCi at the time of its delivery to the Baksan Neutrino Observatory. This is the most intense chemically pure neutrino source ever produced.