Calculation of emergency situations occurring during the conversion of the research reactor at the Institute of Nuclear Physics of the Academy of Sciences of the Republic of Uzbekistan

Abstract

Calculations of emergency situations occurring during the conversion of the VVR-SM reactor at the Institute of Nuclear Physics of the Republic of Uzbekistan are presented. Results are presented for the following situations: motion of a compensating rod (two cases), loss of electric power, and obstruction of a fuel assembly. It is shown that safety is maintained for all the emergency situations which were analyzed and the behavior of all zones which were analyzed is actually identical. The computational results will be used for the conversion of VVR-SM to low-enrichment fuel.This article presents the results of calculations and analysis of emergency situations for a core fully loaded with IRT-3M or IRT-4M fuel assemblies with high-and low-enrichment uranium, respectively, and 16 IRT-3M + 4 IRT-4M fuel assemblies. Four emergency situations are examined: extraction of an automatic rod, ejection of a compensating rod, loss of electric power, and obstruction of a fuel assembly.The VVR-SM core contains six tubular IRT-3M and IRT-4M fuel assemblies, containing tubular coaxial fuel elements with a square cross-section and 1.4 and 1.6 mm thick walls, respectively. The fuel element cladding, which is 0.45 mm thick, consists of the alloy SAV-1. A channel with a control rod or an experimental channel with the outer diameter 28 mm and inner diameter 24 mm is placed in a fuel assembly.The neutron-physical and thermohydraulic methods of calculation and calculation of fuel burnup and fuel assembly power were used, in turn, for thermohydraulic calculations of a stable state and emergency situations [1]. The emergency situations were calculated using the PARET program and a two-channel model [2]. The hottest and average fuel plates (first and second channel) and the associated water flows were chosen for the calculations. Since this program makes it possible to calculate only one type of fuel, which has definite geometric parameters and a composition, the data for a transitional core were obtained separately for IRT-3M and IRT-4M, after which the energy release was summed [3]. The following assumptions were made: accidents occur at the nominal maximum power (10 and 11 MW), the reactor stops at power 12 MW, the time delay between an excursion above maximum power and the lowering of a compensating rod into the core is 0.1 sec for both types of fuel assemblies. The reactor is stopped by lowering three emergency protection rods from the completely extracted position and six compensating rods from the critical position. The emergency protection rods are inserted to a distance of 0.6 m in 0.5 seconds, and a compensating rod is inserted completely at the constant velocity 35 mm/sec. For transient processes caused by an increase of reactivity, the three main pumps continue to operate after the reactor stops and stop feeding the water 4 sec after power loss; the auxiliary pump operates continually, powered by chargeable cells, with water flow rate 100 m 3 /h.