Investigation of Coolant Local Hydrodynamics in the Mixed Core of the VVER Reactor

Localizing safety systems are provided to contain radioactive substances in an accident and attenuate ionizing radiation at a modern nuclear power plant. Together with radioactive substances, hydrogen is also retained, which is formed during the decomposition of the primary coolant. The accumulation of hydrogen in the presence of oxygen from the atmosphere in the accident localization zone carries the danger of the formation of flammable and explosive concentrations of these components. Nuclear power plant (NPP) deigns with water-water energetic reactor (WWER) provides for a hydrogen removal system including passive catalytic hydrogen recombiners. The device capacity is confirmed experimentally under reference conditions (lean air-hydrogen mixture, pressure and temperature close to normal, no interference with gas exchange). Capacity is an important safety parameter. In the event of an accident, conditions inside the ealed enclosure of the localizing system of NPP with WWER can differ from the reference ones and affect the capacity. On the basis of calculations, the operation of recombiners with lack of oxygen and with hindered gas exchange has been investigated in the paper. The decrease in capacity with lack of oxygen reaches 50 %, which is mainly caused by an increase in underburning. Compared to the reference conditions, the effect is more pronounced in the event of an accident – 60–70 %. The hindered gas exchange is modeled by a decrease in the height of recombiner traction channel. This case can be reduced to the placement of the device in cramped conditions and the effect of the atmosphere speed inside the enclosure. Regardless of the hydrogen concentration, the operating characteristic of the device remains linear, with a two-fold decrease in height leads to a decrease in capacity by 20 %. The results can be used to substantiate the safety of NPPs with WWER and to review on the safety subtantiation of power units.

Section: работа рекомбинатора в условиях недостатка кислородаunclassified

Section: Introductionunclassified

Аnalysis Catalytic Hydrogen Recombiner Capacity Calculation Taking into Account Conditions Inside Sealed Enclosure of Containment Safety System of Nuclear Power Plants with Water-Water Energetic Reactor

Сорокин

2021

Calculation of Start-Up Time of Passive Catalytic Hydrogen Recombiner of Localization Safety System of a Nuclear Power Plant Equipped with VVER

Сорокін

2022

The hydrogen removal system ensures hydrogen safety. At a VVER nuclear power plant, it consists of passive catalytic hydrogen recombiners. The calculation of devices is of great importance for safety justification, since the complex conditions of an accident at a power unit are not reproducible in experiments. The recombiner consists of a casing and a cassette with catalytic elements, the design of which ensures the passage of a gaseous medium through the device. Upon contact with the catalyst, a chemical reaction of hydrogen and oxygen compounds occurs, accompanied by the release of heat; as a result, the concentration of hydrogen under the shell decreases. The problem is starting from a cold state since the activity of the cold catalyst is low, and the thrust is not observed until the catalyst is heated and a column of warm gas is formed inside the device. The transition from the cold state to the working state takes a certain time, during which the recombiner performance is below nominal. The start-up time is a parameter that is important in terms of safety. The article calculates the start-up time of a hydrogen recombiner with a catalytic block in the form of equidistant parallel catalytic plates. Local cross-sectional averages and transmission coefficients are used, the latter taking into account the influence of free convection and chemical reaction. The gas velocity is determined by the balance of buoyant and resistance forces. The calculated data and the data known from the scientific and technical literature coincide satisfactorily. As a conservative estimate of the start-up time of the recombiner, it is recommended to use the value of 300 s. An increase in temperature practically does not affect the start of the recombiner with an active catalyst, an increase in the concentration of hydrogen accelerates the start, and a decrease in pressure slows it down. The results obtained in the study can be used in the justification of the safety of VVER nuclear power plants and the examination of reports on the justification of the safety of power units.

Autonomous Thermosiphon System of Passive Residual Heat Removal from the Primary Circuit of the Reactor Plant: Features of Operation, Characteristics and Basic Advantages

Sviridenko,

Shevelyov

2023

An autonomous system of passive removal of residual heat (PRRHS) of a reactor installation with VVER designed to ensure the safety of nuclear power plants in an accident with complete long-term blackout is considered. The system provides for the removal of heat directly from the first circuit of the reactor plant (PRRHS R). In order to increase the reliability and safety of the emergency heat sink, heat exchange equipment based on closed-type evaporation and condensation devices – two-phase thermosyphons – has been used in the system. The main feature of such heat exchangers is that their thermosiphon assemblies structurally separate the primary circuit and the auxiliary circuit of the PRRHS, which is removed outside the reactor compartment, and provide safe and efficient heat removal, reduce the risk of radioactive contamination spreading beyond safety barriers. Such autonomous passive systems will provide effective heat removal directly from the primary circuit by changing the chain of successive heat transfer sites from nuclear fuel to the final absorber and excluding from it such elements, as for example steam generators, the condition and operability of which in the emergency process of heat removal have a major impact on the safety of the reactor core. The article presents a diagram of an autonomous heat sink system; also, a description of its operation is given. The main characteristics of the course of the emergency process of removal of residual heat by the autonomous thermosiphon PRRHS R obtained by computational modeling have been considered. The advantages of an autonomous thermosiphon passive system in comparison with a passive heat removal system of a reactor installation with VVER through the second circuit are analyzed. The obtained results are proposed to solve the problems of diversification of passive safety systems of evolutionary reactor plants of nuclear power plants with VVER type reactors.