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       The “TKR” test facility(«fuel channel break») is designed for investigation of the response of the system of RBMK fuel channels in the graphite stack to an accident involving a break of a fuel channel under the maximally realistic conditions. The TKR test facility is a key testing facility in the complex of the different scale test facilitys for modeling of the events, associated with the break of fuel channels in the channel-type reactor core.
       The test facility includes the graphite stack module (operating section, containing a full-scale fragment of the RMBK-1000 stack), heating systems of the pipelines, graphite stack, fuel channels and metalwork of the graphite stack module, water treatment installation (WTI) for injection of the steam-water mixture at required flow rates into the central channel of the graphite stack module during simulation of the fuel channel break accident. The test facility also includes: assembly bay (for assembly of the components before their installation into the reactor graphite stack module and for storage of tools, instruments, and large components), fuel channel preparation site (for inspection of fuel channels and for installation of instruments), and suspension site (for storage of the fuel channels).
The graphite stack module is a core simulator of the TKR test facility, containing a full-scale fragment of the RMBK-1000 reactor space - from the bottom plate of the top metal structure to the top plate of the bottom metal structure. It consists of 5x5 cells (25 fuel channels) surrounded by a row of columns with the reflector cooling channels. The graphite stack fragment includes standard fuel channels, standard graphite blocks and graphite «solid contact» rings. The central channel of the graphite stack module is a model of the broken fuel channel. Depending on the assumed nature of the failure of the central channel, either an intact channel, or a channel with a preliminarily introduced defect can be installed for the experiment.
       The 24 peripheral channels simulate the intact fuel channels. The coolant flows through the peripheral channels during heat-up of the graphite stack module. During the experiment, the conditions (coolant temperature and pressure) corresponding to those in the RBMK-1000 fuel channels are maintained in the peripheral channels. Maximum pressure in the test facility fuel channels is 10 MPa.
       The facility is equipped with the state of the art research automation and instrumentation and control systems. The measurements to be performed at the test facility are divided into two types: in-process (operating) and research. In-process (operating) measurements ensure a preset test facility regime, control of working medium parameters and safety operation of the equipment. These are measurements of pressure, temperature, flow rates and coolant level, force of current, voltage and electrical power of heating elements. There are more than 100 channels in all with sampling frequency 20 Hz
       Research measurements make it possible to register thermohydraulic, geometrical parameters and parameters of process deformation mechanics proceeding in the TKR stack during the experiment. These are measurements of pressure, temperature, coolant flow rates, temperature of graphite block surfaces and different test facility elements, measurement of FC deformation and displacement of graphite block columns. There are more than 200 channels in all with sampling frequency 190 Hz.

Purpose of the TKR test facility and main lines of experimental studies

Assessment of the possibility of multiple fuel channel breaks during accidents for the currently operational RBMK reactors.
Determination of stresses and deformations in the tubes, displacements of the graphite stack components, distribution of pressure and composition of the gaseous mixture during its spreading through the stack gaps, temperature distribution in the stack.


MRS module	ТКR testbed

Coolant – water-steam
Elevation scaling factor – 1:1
Number of fuel channels – 25
Number of the graphite columns – 45
Maximal operating pressure in the FC – 10 MPa
Maximal coolant temperature – 300 °C
Coolant flow rate in the heat-up mode – up to 30 m³/sec
Maximal coolant flow rate through the break – 150 kg/sec

Results

On the 15th of December 2004, the first full-scale experiment with the emergency channel break in the middle of the graphite stack containing 25 graphite columns with fuel channels was carried out after a series of preliminary experimental studies. The data obtained from the experiment define integral processes and phenomena following up FC break.
In June 22, 2005 the second full-scale experiment with the overheating and with the break of the central emergency channel in the graphite stack of MRS was performed. The test was carried out in the conditions close to most dangerous in respect of rupture of the channels surrounding the emergency one. After the emergency channel break and during the coolant effluence to the graphite stack, all channels were remaining their integrity. It was confirmed experimentally that multiple fuel channel rupture of RBMK reactor is impossible in an accident with a single fuel channel overheating and its break.

The cartogram of the residual deformation of the graphite stack after the second full-scaled experiment

TKR-F

The TKR-F test facility is a fragmentary installation, designed for experimental studies of the RBMK fuel channel breaks. The tested core of the test facility is a model of the zirconium part of the channel in a graphite column, with the linear elevation scaling factor of 1:2. During the experiments, the scenario of overheating and failure of a fuel channel is reproduced on the test facility in order to determine the failure conditions, character and consequences.

Purpose of the TKR–F test facility and main lines of experimental studies

Determination of the conditions of thermo-mechanical failure of the fuel channel with or without incipient defect in different conditions of constrained deformations and additional mechanical effects;
Determination of the parameters of deformation and breakdown of the graphite stack for different fuel channel failure modes;
Refinement of the methodology for test preparation and conduct for testing thermo-mechanical failure of the central fuel channel of the full-scope TKR test facility for a given break size.

Coolant – water-steam
Height scaling factor – 1:2
Number of fuel channels – 1
Number of graphite columns – 1
Maximal operational pressure in the FC – up to 10 MPa
Maximal coolant temperature – 350 °C
Coolant flowrate in the heat-up mode – up to 4 kg/sec
Emergency tube heating rate – up to 10 °C/sec

Results

      After accomplishment of the test facility integrated testing programme in December 2001, the first research experiment with thermo-mechanical failure of RBMK fuel channel model was made.
      In 2002, two experiments with thermo-mechanical failures of channel models were made, with two different designs of the thermite heater. Thereafter, the refinement of the thermite heater design was continued with a series of six experiments with steel channels.
      In 2003, three experiments were made with different designs of the thermite heater at the steel channel. Then, a series of five experiments was conducted on the core simulator with a T-joint and a diaphragm, in order to assess the effects of the thermite heaters of different designs for the dynamics of the processes around the channel after the tube failure. In July 2003, an experiment was performed with a thermo-mechanical failure of a model of RBMK channel with the graphite «solid contact» rings within the graphite column, pre-heated by an external heat source to 600 °C (RBMK conditions). In September 2003, an experiment was performed with a thermo-mechanical failure of a channel model with the graphite rings in the cold graphite column, heated by steam only (conditions of the TKR test facility). The fuel channel failure did not occur due to high heat transfer from the channel to the graphite stack through the rings. In November 2003, an experiment was performed with a failure of the fuel channel with heat-insulating rings located in the thermite heater zone added in order to reduce heat losses. The failure was successfully achieved, and its type was quite similar to the failure type obtained for the tube in the column, pre-heated to 600 °C.

The TKR-F (graphite)

The TKR-F (graphite) test facility deals with the experiments on the flow of steam-water mixture (coolant) through graphite gaps and interaction of coolant with graphite block surface.
The tested core of the test facility includes a channel tube with a T-joint and a diaphragm, two or four vertically located graphite blocks with controlled gaps. Blocks are heated up to the specified temperature by means of electrical hetaers. The coolant is supplied in the gap between blocks through the channel and a flat nozzle.

Purpose of the TKR-F (graphite) test facility and main lines of experimental studies

- studying of process of steam-water mixture distribution through narrow gaps considering effects of cooling, wetting and phase transfers
- studying of heat exchange processes and processes of steam generation in graphite stack gaps during coolant discharge


TKR-F (graphite) testbed

Results

In 2003, a series of tests with breaking diaphragms was conducted and dependence of steam impact force to graphite blocks on steam pressure and flow rate, gaps size and graphite temperature was defined.
In 2004, the experiments with effects of coolant flow rate and void fraction, gaps sizes and graphite temperature on hydraulic pressure losses in graphite blocks gaps were performed. In the course of the experiments, the tools were selected and methods of detectors installation were developed, techniques for measuring coolant parameters in graphite stack gaps were refined. The resulted data were used in preparation and performance of a full-scale experiment on the TKR test facility.

TKR-M

TKR-M test facility is a diminished model of reactor stack module designed for the investigation of mechanical response of a row of columns to static and dynamic force under different conditions of fastening. The core simulator of the test facility is a model of a row comprising five fuel channels of RBMK graphite stack scaled 1:6.47 (a fuel element tube of RBMK-1000 reactor is used by way of FC). In the course of experiment, a mechanical load is applied to one brick in the first column in the row, perpendicularly to FC axis. Strain in FC models and displacement of graphite brick models in the line of applied force are measured.

The purpose of TKR–M test facility and lines of experimental studies

studying static and dynamic bending of channel tubes package in graphite stack under transverse and longitudinal loads of different magnitude and location;
studying propagation of mechanical disturbances in the row of channel tube columns, obtaining efficient characteristics of fluid hardness;
studying percussion of graphite bricks under dynamic forces, and effect of friction between the bricks on the inflection shape and flexibility of the column;
obtaining data for designing a simulator for mechanical resistance of the missing part of the reactor stack for TKR test facility.

Results

In 2003, the following experimental studies were performed at the TKR-M test facility:

experiments on investigation of deformation characteristics of a single column and FC without graphite block models under static and dynamic loads;
experiments with investigation of a system of 2 columns deformation under static and dynamic loads;
experiments on detection of dependence of a group of 2 or 3 FC deformation nature in graphite block simulators on the value of applied static axial load in the FC model;
in 2005-2006 test facility’s modernization and series of experiments aimed at simulation processes and effects, which typical of RBMK graphite stack after a long period of exploitation, were carried out.

Prospects

The results of the experimental studies performed on the complex of TKR test facilitys were presented and approved at a meeting of the scientific technical council (STC) of Concern «Rosenergoatom» in 11.10.2006. In particular the STC recommended further investigation in the following areas and issues:

investigations of process of single FC break at rated power due to channel pipe’s defect; investigation of influence of such break on the probability of the FC multiple break;
Influence of changes of the mechanical properties and geometry of the stack’s elements and of the FC pipes during exploitation on possibility of FC multi-break emergence;
investigations of process of swelling and cracking of the graphite blocks and of the significant deformation of the graphite stack under influence of the neutron flux; and investigations of this processes’ influence on probability of FC multi-break emergence in the “mature” stack;
investigations of accidents with the draining of groups fuel channels.

In 2009 investigations including a full-scale experiment on breaking of FC of the RBMK at coolant rated parameters in consequence of a defect of the FC will be carried out on the TKR test facility. These investigations are necessary for the justification of the RBMK safety after long exploitation of fuel channels, resulted in their embrittlement and corrosion cracking.


TKR-M test facility	Scale — 1:6.47 Number of fuel channels (FC) — from 1 to 5 Number of graphite columns — from 1 to 5 Maximum static load — 1000 N