3.4.1 Compliance with Code Requirements
The main components of an NPP with the RBMK-type reactors is: the nuclear reactor itself, equipment of the MCC (including MCP, pipelines, devices), separator drums, turbines, generators and transformers.
Principal Components of the Main Circulation Circuit
The design power of the Ignalina NPP the RBMK-1500 is 1500 MW, though as noted previously after the Chernobyl event it was reduced to 1370 MW. The steam generation capacity of the nuclear reactor is determined by the nominal steam flow rate to the turbines, by a pre-set temperature in the turbine condenser, and all thermal loads for the internal requirements of the NPP, and requirements needs of the industrial site and the local residential settlement. The final calculated value of steam generation capacity is increased by a margin of at least 3 %.
The RBMK-1500 reactor has two independent main forced circulation circuits. Each MCC consists of four main circulation pumps, two separator drums and the necessary pipes and associated equipment
At the Ignalina NPP type CVN-8 main circulation pumps are used. The shaft of the pump has a flywheel to increase the MCP slow-down period to 120-130 s by loss of power of electrical motor. A moment of inertia of the CVN-8 is 9400 kgm2, and the total moment of inertia of the pump complex is 13150 kgm2.
The MCPs provide the main circulation circuits with the required flow and the pressure head. As a rule, parameters of the pump correspond to the characteristics of the off-site network and have a head margin of 2-4 %.
Internal pipe diameters of the MCC of Ignalina NPP are 0.02-0.9 m, and the flow rates are 2-12 m3/s.
Selection of profile for the MCC pipelines and the strength calculations is performed in accordance with code rules for calculation of strength for the reactor equipment of the NPPs [21,23].
Principal Equipment of the Turbine Plants
An RBMK-1500 plant is equipped with two type K-750-65 turbines. Although the installation of two turbines in the NPP is somewhat expensive, it nevertheless enables to utilize the capacity of the unit more effectively, because in the RBMK-type reactors it is possible to do the core loading without reactor shutdown. Moreover, the availability of two turbines in one unit makes it easier to do general turbine repairs (during operation), which normally need a considerable amount of time.
Flow of the main feedwater pumps must correspond to the steam generation rate of the nuclear reactor with a margin of 10 %. In case of failure of one pump, the rest would provide the reactor operation by nominal capacity of all steam generating equipment. Three main feedwater pumps for each circulation loop are used at the Ignalina NPP based on the so-called 3 x 50 % principle.
Each unit of the Ignalina NPP is provided with six AFWPs. The capacity of each pump is sufficient to provide removal of residual heat from the core. Approximate flow of one AFWP is 2-4 % of the nominal feedwater requirement. Pipeline systems, as well as electrical cables to auxiliary feedwater pumps, are laid independently from the corresponding lines of the main feedwater pumps.
For reliable removal of thermal energy from the reactor during startup and shutdown of the unit special steam-relief valves are installed to the steam discharge to the turbine condensers (SDV-C), to the fifth pool of the accident confinement system (SDV-A) and to the deaerators (SDV-D). These steam discharge valves have a safety margin of 100 %. Their capacity is determined to provide an appropriate cool-down rate for the reactor. The removal of residual heat in the core as well as the heat, which is accumulated by equipment and coolant is taken into account.
Feedwater, which is directed to the reactor, should be well deaerated. To this end, the use of deaerators in thermal schematic was foreseen. All aerated water flows (drainage, overflows from drainage tanks, condensate overflows, etc.) are fed to the deaerators if they have a temperature exceeding 50 oC. In the opposite case, they are directed to the condensers of the turbines. Total capacity of the deaerators is selected based on the maximum feedwater flow with a margin not less than 10 %. Two deaerators are provided for each turbine plant. Feedwater margin in the tanks of main deaerators provides for one unit operation with full power during a time period not less than 180 s.
For water supply to the separator drum in case of emergency cool-down of the reactor a permanent margin of feedwater in special tanks is foreseen. Two such tanks are available, their capacity can provide sufficient coolant to sustain an NPP operation at full power of at least 300 s. On the other, hand an operation at power equal to 2 % from nominal (for removal of residual heat) should not be less than 10 hours. Loss of feedwater is supplanted by de-mineralized water.
Three condensate pumps are used for each turbine, each pump providing 50 % of the nominal flow. The head of the condensate pumps is designed to overcome the hydraulic resistance from the turbine condensers to the deaerators, including the water pressure in the deaerators.
3.4.2 Qualification Tests and Analyses
Operation of installed equipment and systems is permitted only after stringent control measures are taken and equipment acceptance tests are completed. Acceptance and pre-operational tests of installed equipment and systems usually begin 8-12 months before start-up of the NPP. They are performed by the constructors in cooperation with the customer. For the installing of the NPP equipment the following tests were performed:
Individual Tests
Tests of Individual NPP components and equipment consist of the following:
Post-installation Flushing of the MCC and Turbine Hall Equipment
The equipment is flushed by "cold" and "hot" coolant to ensure a thorough cleaning of the internal cavities of the Ignalina NPP equipment and pipelines. Flushing by circulation of coolant in the MCC was performed in two steps: first, high velocity water was used to flush separate sections, the flushed water being directed to waste disposal. Second, the MCC was flushed with cold water at temperatures up to 100 oC and with hot water at temperatures 250-270 oC.
A large portion of the NPP equipment, systems and pipelines are subjected to high velocity water flushing:
During high velocity flushing of individual elements and pipelines it is necessary to ensure that the flushing velocity is 1.5 time higher than nominal design values, but not lower than 2.5 m3/s.
A criterion that the "cold" flushing has been performed successfully is a water transparency of not less than 95 %. Hot flushing of the MCC is provided to ensure cleanliness of the internal cavities of equipment and the pipelines. Water is heated in the MCC by the operation of the MCP. During hot circulation flushing each half of the MCC is in turn connected to two MCP’s. The water temperature is maintained at 250 oC at a flow rate of about 8 m3/s. The water is cleaned continuous by mechanical filters and special grid filters, which are temporarily used in the group distribution headers.
Effectiveness of hot flushing is controlled not only by transparency, but also by monitoring the presence of corrosion products and chemical admixtures in the water. According to code, a transparency of the flushing water of at least 95% is required, corrosion products must decrease to o concentration of less than 10-3 g/kg, chloride - less than 10-4 g/kg, and the total hardness of water must be less than 10-6 g(equiv.)/kg. Oil and mechanical admixtures in that water are not permitted after hot circulation flushing.
A characteristic of the NPP with RBMK - type reactors is that the main quantity of metal corrosion products (up to 90%) in the MCC are generated by reaction occurring with feedwater. Therefore, the condensate-feed circuit and other auxiliary systems of the turbine room are subjected to careful cleaning from post-installation contamination as well as from corrosion products. Mechanical cleaning, high velocity flushing and chemical flushing of all the main and auxiliary systems of the turbine room are used for this purpose. For high velocity flushing of the main pipelines of the condensate-feed circuit, the water velocity has to beat least 3-4 m3/s. Effectiveness of the cleaning procedures are controlled mainly by requiring water transparencies of at least 90 %.
Several equipment items and pipelines of the condensate-feed section require a more extensive cleaning procedure. Therefore, together with water flushing, a procedure using a chemically active (acidic) wash of selected pipes and equipment is performed. For this purpose a flushing solution at a flow rate of 0.2-0.3 m3/s is circulated by special acid-resistant pumps. The procedure employs tanks for storage of solutions of chemical reagents and temporary pipelines. After the chemical flushing a special flush using ammonia is performed to neutralize acidic residues.
Acclimatization of Equipment at Hot Conditions
The RBMK equipment is operated under hot conditions only after completion of the complex testing program assuring the quality of the installed and manufactured components and systems. During the hot acclimatization of the reactor equipment and systems, the hydro-dynamical characteristics of the reactor and main circulation circuit, reliability of reactor control and protection systems, as well as other systems, which provide safe and reliable operation of the reactor are tested. Average duration of the warm-up of the NPP equipment is about 10-15 days. From the results of the warm-up period conclusions regarding the quality and reliability of installation of the equipment and pipelines are made. This includes the assessment of the readiness of the reactor and all its service systems including the loading of the nuclear fuel to the core of the reactor.
Startup of the Reactor
Startup procedures of the RBMK-1500 reactor include the determination of flux and power distribution of the initial core loading and the evaluation of the main neutronic and thermal characteristics of the reactor. In order to provide reliable control and to ensure nuclear safety during the loading of fuel assemblies into the core a highly sensitive CPSs is used in addition to the regular control and protection system. This CPS makes it possible to control the neutron flux and the reactor reactivity level, and provides an emergency shutdown if necessary.
Fuel assemblies are loaded in several stages. During the first stage criticality of the reactor is achieved. At room temperature this consists of 23-24 fuel assemblies, which have an initial enrichment of 2 %. In the second stage the number of fuel assemblies is increased to 916 and 154 additional absorbers and 56-60 CPS rods are inserted into the core. In the subsequent stages the initial core is loading is completed. During the determination of the neutron flux distribution of the initial core loading the power is on the order of 10-5-10-4 % of the nominal value. At this low power the neutron-physical characteristics of the reactor, the effectiveness of the CPS rods and the reactivity margin are established. The composition of the initial core loading of the RBMK-1500 reactor consists of 1445-1455 fuel assemblies and 230-240 additional absorbers.
During the first stage single fuel assemblies are loaded into the reactor. In the subsequent stages it is feasible to load two assemblies. Total time for full scale core loading is about 20-25 days.
During startup of the reactor several measurement programs are carried out. This includes measurement of the following parameters:
The start-up phase of the RBMK-1500 reactor is completed by transferring control of low-power level to the CPS. This is a power level at which automatic controls can become effective and amounts to about 1 % of the nominal thermal power. At this power level the temperature reactivity coefficients are determined by temperature variation in the range of 100-150 oC. Heat-up of water in the MCC is accomplished by utilizing four MCPs ( two by two at each half of the MCC).
Start of Power Generation and Testing of Reactor at Power
Testing of NPP equipment and systems for power operation is accomplished at power levels from 1 to 10 % of the nominal reactor power. Initiation of power generation for the RBMK-type reactors proceeds in several stages each of which encompasses a gradually widening range of tests.
Before completion of the first stage of energy startup, an adjustment and trial of the fuel channel failure detection system is performed. During the first energy startup stage, the main pipelines are cleaned by steam, which is generated in the reactor. To carry out the proper steam blow-through steam flows of up to 95-100 kg/s must be generated at a pressure in the separator drums of about 1.2-1.4 MPa. Therefore, the MCC water is heated to 80-100 oC by operation of the MCP, and subsequently fission power is increased to up to 8-10 % of the nominal power. The rate of temperature increase in the MCC must not exceed 10 oC/h.
Blow-through of steam through the main pipelines is performed in the following sequence: separator drum, main pipeline, main steam gate valve, pipeline from the main steam valve to the emergency regulating valve, temporary blowdown pipeline, discharge to atmosphere. The blow-through of the main pipelines is maintained foe 15-20 minutes, and the total time for blowdown of all the pipelines is 15-20 hours. To provide the necessary amount of steam, the deaerators, separator drums and tanks are filled with 3500-4000 m3 of de-mineralized water.
The second stage of the startup involves adjustment of the main and auxiliary equipment of the reactor and turbine hall at powers of up to 10 % of the nominal as well as an adjustment of steam discharge valves, emergency steam reception systems, safety valves at the separator drum and bubble condensers. At this stage the reactor power is varied from 2 to 10 % from the nominal.
The third start-up stage involves a test startup of the reactor accompanied by testing of individual turbine generators under a relatively small load (75-100 MW).
A complete testing program of the NPP equipment and systems is carried out during 72 hours at specific power by means of nominal parameters in all circuits.
Approach to Design Power
Testing of the NPP at design power is performed in several stages, beginning from 10 % of the nominal power and subsequently in steps of 20-30, 40-45, 55-60, 75, 85-90 and 100 % of the nominal thermal power. At each stage, acceptance tests of the thermal-hydraulic components and electronic equipment are carried out.
The safety and reliability of reactor operation at a given power level is checked. This includes the following tests and procedures: