IGNALINA NUCLEAR POWER PLANT

The Ignalina nuclear power plant contains RBMK-1500 water-cooled graphite-moderated channel-type power reactors.

The RBMK-1500 reactor is the largest power reactor in the world. The thermal power output of one unit is 4800 MW, the electrical power capacity is 1500 MW. The Ignalina nuclear power plant, like all the stations with RBMK reactors, has a direct cycle configuration - saturated steam formed in the reactor proper by passing the light water through the reactor core is fed to the turbine at a pressure of 6,5 MPa. The light water circulates over a closed circuit.

The first stage of the nuclear power project comprises two 750 MW turbines. Each generating unit is provided with a fuel handling system and unit control room. The turbine room, waste gas purification and water conditioning rooms are common for all the units. Ignalina NPP generates about 74% of electricity consumed in Lithuania.

TECHNICAL DATA ON RBMK-1500 REACTOR

REACTOR DESIGN

The main structural element of the reactor, a graphite stack with fuel channels, absorber rods and surrounding metal structures, is housed in a concrete vault. The vertical graphite stack columns contain fuel channels and control rod channels. The graphite stack is carried by a welded steel structure resting on a concrete foundation. On top the graphite stack is spanned over by an upper steel structure resting on the annular water tank of the biological shield.

A welded shell enclosing the graphite stack, as well as the upper and bottom steel structures form a sealed reactor space. To prevent graphite oxidation and to improve heat transfer from graphite to fuel channels, the reactor space is filled with a helium-nitrogen mixture. Provision is made to replace the fuel channels and control rod channels on the shutdown and cooled reactor. The fuel channels are tubes whose lower and upper portions are fabricated from corrosion-resistant steel, while the central part is made of Zircalloy. The split graphite rings in the channels provide thermal contact with the graphite bricks of the stack. Suspended in the fuel channel is a fuel assembly bank. The fuel assembly bank consists of two fuel assemblies.

Each fuel assembly contains 18 fuel rods in the form of sealed Zircalloy tubes which are filled with uranium dioxide pellets. Light water coolant is fed into the lower end of the fuel channels. From the fuel channel the coolant is fed into the separators. To improve heat exchange, the upper fuel assembly carries special intensifying grids. Removal of irradiated fuel elements, their handling and charge of fresh elements are performed on load by means of a refueling machine mounted in the central room. The biological shield is made of carbon steel, serpentine crushed stone and gravel, concrete, sand, water.

FUEL LOADING SYSTEM

Fuel is charged and discharged by means of a refueling machine while the reactor is on load. The main element of the refueling machine is a casque with a biological shield designed to take the working pressure within the fuel channels and equipped within mechanisms serving the following functions:

The refueling machine is provided with two systems of precise positioning over the fuel channel-optical/television and contact.

The casque is mounted on a bogie moving along a bridge rail-bound in the central room. The refueling machine is controlled from the operator's room which is located behind the wall of the central room.

TURBOGENERATOR SETS

Each unit contains two K-750-65/3000 turbines with 800 MW generators. The turbines are double-flow tandem machines (one high-pressure cylinder and four low-pressure cylinders) with reheat. The rotor speed is 3000 rpm. The three-phase 50 Hz generators with hydrogen and water cooling are connected to the outdoor substation.

The turbines are controlled by computer-based control system ASUT-750.

STATION CONTROL AND MONITORING SYSTEMS

The control and monitoring systems provide reliable and safe operation of the major equipment and maintain stable process parameters. 

Functionally the monitoring and control systems comprise:

•  reactor control and protection system;
• control and protection system of reactor process equipment ;
•  control and protection system of turbogenerator and outdoor switchgear;
• functional group control system;
• refueling machine control system.

Most of process parameters are monitored by a data logging system. Control data are displayed on the unit control board using VDU, visual and recording instruments, various announciation windows and indicators, mimic diagrams and printers. The station is controlled from the unit control board.

General control and coordination of the operators work are responsibilities of the shift chief or his deputy. The station incorporates provision for multiple protection of process equipment initiates operation of various kinds of protective gear providing controlled reduction of the reactor power at a rate of 2-4 per cent/s to the safe level. The reactivity scramming which brings the reactor power down to zero, is applied in rare cases.

REACTOR CONTROL AND PROTECTION SYSTEM

The control and protection system is intended for reliable follow-up of the reactor performance and its safe operation. The system provides start-up, automatic maintenance of power at the set level, allows control of energy distribution along the radius and heightwise of the core, compensates for fuel burn-up, provides protection of the reactor under emergency conditions.

The control and protection system is built of fail-safe and redundant devices using integrated circuits to receive and process signals from various sensors, as well as to present the reactor status information to the operator. The reactor power release and its distribution are controlled by 211 carbide boron rods placed in the control channels and moved by individual servomotors mounted on the top of the control channels. The control rods are cooled with water from a special loop.

Out of the total numbers of rods, 40 ones are used for energy distribution control through the height of the active zone of the reactor. 24 rods perform the function of prompt emergency safeguard introduced into the active zone within 2.5 seconds under definite emergency situations. The remaining rods are unified and serve the function of reactivity scramming, automatic maintenance of the reactor power release at the set level, control of energy distribution over the core radius.

REACTOR PROCESS MONITORING SYSTEM

The reactor process monitoring system provides the operating personnel with information and inputs data into the control and protection system.

The reactor process monitoring system consists of the following functional elements:

•  data logging system which provides follow-up, processing and presentation of the data;
•  self-contained energy release control system which provides measurement, control and indication of energy release in the reactor channels'
•  self-contained system monitoring tightness of fuel assembly cladding and providing measurement, control and indication of coolant activity rise:
•  system monitoring integrity of the fuel and control channels and providing measurement of temperature and indication of relative humidity of gas pumped through the gas paths of the core;
•  system monitoring coolant flow in the reactor channels;
•  system monitoring temperature of the main and auxiliary equipment of the reactor.

The data logging system is configured in a three-level hierarchy using computers SM-1M and SM-2M and interface facilities.

The energy release monitoring and control system includes energy release detectors providing inertialess measurement of neutron flux density along the radius and height of the core, and the equipment to process information and signals on the control board.

The system monitoring tightness of the fuel assembly claddings includes scintillation gamma-spectrometer sensors, equipment, to ensure operation and movement of sensors in the intertube space of the steam lines, and facilities for processing and output of data.

The system monitoring the coolant flow through the reactor channels consists of tachometric transducer, and equipment affording frequency-to-analog signal conversion.

The system monitoring the temperature of the reactor equipment contains mainly heat-resistant cable heat-electric transducers.

RADIATION SAFETY

The RBMK-1500 reactor is provided with special elements and systems ensuring radiation safety of the nuclear power plant and the environment both under normal operating conditions and in the emergency cases. The radiation safety and doze control systems include:

•  highly reliable computerized control and protection system;
•  reactor scram system;
• accident isolation system;
•  fuel rod cladding tightness monitoring system;
•  special facilities for gaseous effluent purification;
•  liquid radio waste discharge, processing and hold-up system;
•  computerized radiation doze control system;
•  computerized system monitoring gaseous emissions and waste discharges;
• facilities for environmental radiation dose control.

The system monitoring tightens of fuel rod cladding specially designed for the RBMK-1500 reactors and applying modern techniques for detection of faulty fuel rods and computer-based data logging provides the core radiation control. The computerized radiation doze control system at the nuclear power plants with reactors of the RBMK-1500 type is provided with facilities monitoring radiation exposure of all components and systems of the station.

All this helps to maintain the radiation conditions at a safe level by implementing the purposeful actions (removal of leaky fuel assemblies, decontamination, replacement and repair of the equipment. To reduce emissions of noble radioactive gases. A two-stage system is used for cleaning gaseous and aerosol effluents discharged through a 150 m high stack into hold-up chamber. When noble gases pass through it, their activity is reduced due to natural decay.

The second stage-activity suppression facility purifies and reduces activity of noble radioactive gases by the method of dynamic sorption using the radiochromatographis char columns. To reduce radioactive aerosol emissions at the nuclear power plants with the RBMK-1500 reactors provision is made for purification facilities absorbing aerosols by special filters. The nuclear power plants with the RBMK-1500 reactors use a closed-circuit water supply system. Liquid radioactive effluents undergo special treatment. Radioactive discharge into air and water is monitored continuously using instruments of the computerized radiation dose control system.

The external radiation exposure surveillance service at the nuclear power plant with the RBMK-1500 is equipped with instruments to analyze concentration of radionuclides in the elements of the environment. The health physics laboratory is provided with facilities and sampling methods, dozimetric, radiometric, spectrometric instruments for objective assessment of the radiation conditions in the environment.

SPENT NUCLEAR FUEL STORAGE

The important area of a nuclear power plant safety is storage of the spent nuclear fuel. From the beginning of Ignalina NPP operation the spent nuclear fuel is stored under a layer of water in special pools placed in the same buildings, as reactors.

It is a temporary way of storage, therefore the international competition for the spent nuclear fuel storage was announced, the victory in which was gained by the German company GNB. In 1993 Ignalina NPP and the German company GNB signed the contract on the delivery of 20 CASTOR and 40 CONSTOR steel containers for the storage of the spent nuclear fuel. The total cost of the contract is about 30 mln DM.

According to this contract in year 2001 the company GNB should deliver 18 containers of CONSTOR type (22 have already been received) and the contract will be executed. The first container CASTOR was sent to the storage site constructed nearby Ignalina NPP on May 12, 1999. Some part of the spent nuclear fuel has already been placed in all available containers of CASTOR type (20 containers) and was taken to the spent nuclear fuel storage site.

"Cold" and "hot" tests of the containers of CONSTOR type were executed and the license of Lithuanian Nuclear Power Safety Inspectorate (VATESI) for their operation is expected. The weight of the empty container is about 70 tons, with spent nuclear fuel - about 84 tons. The container is located on a special platform at the site. One of the important works connected with the future decommissioning of INPP Unit 1 is the unloading and location of the spent nuclear fuel in the storages.

The Ignalina NPP experts affirm that the available number of the containers will not solve the problem of the spent nuclear fuel, basing on the evaluation that in the case of Unit 1 shutdown at the end of year 2004, and Unit 2 shutdown in year 2010, 350 containers in addition would be required. The spent nuclear fuel can be stored in containers CASTOR and CONSTOR for 50 years, then it is necessary to take it out to the final burial place, but in Lithuania such place is not stipulated yet.