The Ignalina NPP is a 2 unit RBMK-1500 reactor. This is one of the most advanced versions of the RBMK design series. There are presently RBMK-type plants at Ignalina, Chernobyl, Leningrad (Sosnovy Bor), Kursk and Smolensk, Table 1.2. All operating RBMKs represent three generations of reactors having significant differences with respect to their safety design features. The six first generation units were built all RBMK-1000s. Of the ten second generation units, eight were RBMK-1000s and two were RBMK-1500s. Unit 2 at Ignalina contains safety features beyond those other second generation units. One third generation RBMK-1000 unit has been built at Smolensk, which is Unit 3. And one third generation unit is currently under construction at Kursk. The principal design differences between RBMK generations are summarized below.
Table 1.2 Status of the RBMK plants
Unit |
Generation |
Status |
Ignalina 1 |
2 |
operating |
Ignalina 2 |
2 |
operating |
Chernobyl 1 |
1 |
operating |
Chernobyl 2 |
1 |
closed down |
Chernobyl 3 |
2 |
operating |
Chernobyl 4 |
2 |
closed down |
Leningrad 1 |
1 |
operating |
Leningrad 2 |
1 |
operating |
Leningrad 3 |
2 |
operating |
Leningrad 4 |
2 |
operating |
Kursk 1 |
1 |
operating |
Kursk 2 |
1 |
operating |
Kursk 3 |
2 |
operating |
Kursk 4 |
2 |
operating |
Kursk 5 |
3 |
under construction |
Smolensk 1 |
2 |
operating |
Smolensk 2 |
2 |
operating |
Smolensk 3 |
3 |
operating |
First generation units were designed and brought on line in the early-to-mid-1970s, before new standards on design and constructions of nuclear power plants issued in 1973 were introduced in the Soviet Union. Units brought on line since late 1970s and early 1980s are generally grouped as second generation RBMKs. These plants were designed and constructed with updated safety standards issued in 1982. The third generation units were originally designed in accordance with these updated safety standards, and to a large extent already comply with the latest safety regulations issued in 1988 after the Chernobyl accident.
The unmodified first generation units do not comply with current safety requirements in a number of respects. Therefore, they operate under such a mode which provides for the operation of reactor at a reduced capacity to reduce power density and linear rating of the fuel, increase the volume of primary circuit pipework that is inspected in service, implement measures for beyond DBA management and to improve the reliability of primary circuit isolation valves. The main advantages of the second generation units over the first generation units are the accident confinement system and advanced functionality of the emergency core cooling system. The second generation units are very similar to the third, the most obvious difference being in the two- storey design of ACS pressure suppression pool of these units. The main factors needed to bring the second generation plant into line with latest regulations of 1988 include seismic safety, improved fire protection and upgrading of safety systems. Ignalina NPP safety upgrading program is discussed in Section 1.5.
Fig. l.5 Simplified Ignalina RBMK-1500 heat flow diagram:
1 - reactor; 2 - fuel assembly; 3 - steam separator; 4 - turbine; 5 - generator; 6 - condenser; 7 - condensate pump; 8 - deaerator; 9 - feedwater pump; 10 - main circulation pump
Fig. 1.6 Simplified schematic of the main circulation circuit:
1 - steam separator; 2 - downcomers; 3 - suction header; 4 - suction piping of the MCP; 5 - MCP tanks; 6 - pressure piping of the MCP; 7 - bypass between headers; 8 - pressure header; 9 - GDH with flow limiter, check valve and mixer; 10 - water piping; 11 - channel to core; 12 - fuel channel; 13 - channel above core; 14 - steam-water pipes; 15 - steam pipelines
In many respects, the Ignalina NPP is quite similar to it predecessors. It belongs to the category of �boiling water� reactors, a simplified heat flow diagram of which is provided in Figure 1.5. The detail reactor flow diagram of the Ignalina NPP is given in Appendix 2. The reactor cooling water, as it passes through the core, is subjected to boiling and is partially evaporated. The steam-water mixture then continues to the large steam separator (3), the elevation of which is greater than that of the reactor. Here the water settles, while the steam proceeds to the turbines (4). The remaining steam beyond the turbines is condensated in the condenser (6), and the condensate is returned via the deaerator (8) by the feed pump (9) to the water of the same steam separator (3). The coolant mixture is returned by the main circulation pumps (10) to the core, where part of it is again converted to steam.
The Ignalina NPP uses a RBMK-type channelized reactor. This means that each nuclear fuel assembly in this type of an RBMK-type reactor is located in a
separately cooled fuel channel (pressure tube). There are a total of 1661 of such channels and the cooling water must be equally divided among that number of feeder pipes. Past the core, these pipes are brought together to feed the steam-water mixture to the above-mentioned separator drums.
Each reactor is divided into two halves. Each half is cooled by a main circulation circuit provided with flow from three operating main circulation pumps. A fourth MCP on each side of the reactor is normally in standby mode. The MCPs feed common pressure header of each side of the reactor. Each pressure header provides flow to 20 group distribution headers, each of which in turn feeds 38-43 pressure tubes. The reactor fuel assemblies are contained within these pressure tubes. Core exit piping connects pressure tube to one of four steam separators. The steam from both sides of the reactor is combined in the main steam lines prior to entering the two high pressure 750 MW turbines. Liquid is recirculated to the MCPs through downcomer pipes from the separators to a common pump suction header on each side. Feedwater is provided to the steam separator to make up for the steam flow to the turbines. Figure 1.6 provides a schematic of the RBMK-1500 main circulation circuit.
