VVER Systems

The first Soviet designed VVER reactor based on PWR technology was a 265 MW plant commissioned at Novovoronezh in 1964. The first generation of VVER reactors were of 440 MW capacity (VVER-440/230) and 10 such plants were built in Russia and Eastern Europe. These plants had relatively limited safety features and were followed by a series of 14 second-generation plants (VVER-440/213) with much improved safety features. Two 445 MW plants were also built at Loviisa in Finland in 1977. These included a strong steel-lined reinforced concrete containment, with ice compartments.

The 440 MW VVER reactors have many features in common with Western style PWRs but they also have some differences. For example, they include six coolant loops with horizontal steam generators that are of generally smaller capacity than Western designs. The core lattice is hexagonal, with typical enrichments of 2.2-3.6% uranium-235.

Plant

PWR

BWR (Handbok

VVER (IAEA,

RBMK

PHWR (1997

Magnox (1997

AGR

(George and

over processainband

TC/RER/9/004,

(Alemenas et al.)

World Nuclear

World Nuclear

(IAEA

Board, 1987)

vid storningar

1994)

Industry

Industry

Publications)

I svenska

Handbook,

Handbook,

kokarreaktorer,

1987)

1997)

1997)

Model

Westinghouse

Internal pump

440/213

LWGR

CANDU

Gas

Gas

4-loop

Reference

Sizewell B

Forsmark 3

Bohunice V2,

Ignalina 1 & 2

Darlington 1-4

Wylfa 1 & 2

Hartlepool

plant

3 &4

1 & 2

Nominal

1245

1190

440

1500

935

570

666

electrical

output

Coolant

Water

Water

Water

Water

Heavy water

Carbon dioxide

Carbon dioxide

Moderator

Water

Water

Water

Graphite

Heavy water

Graphite

Graphite

Fuel

Oxide

Oxide

Oxide

Oxide

Oxide

Metal

Oxide

Coolant pressure

15.8

7.0

12.3

7.0

10.6

2.8

4.2

Coolant outlet

325

286

297

284

313

370

675

Containment

Steel-lined

Pressure suppression

Reinforced

Reinforced

Reactor

pre-stressed

pre-stressed

concrete

concrete

building

concrete

concrete

with inner

with liner

steel liner

 

Подпись: Present Generation Reactors 9

The system pressures are somewhat lower than for PWR ~ 12 MPa. Control rods are also hexagonal and replace a whole core assembly.

Third-generation VVERs of 1000 MW design are also operational, the first being built in 1981 in the Ukraine. There are around 18 plants in operation. These include the most advanced safety features among the VVER designs, including sealed containments. Figure 1.3 shows the Temelin 1 reactor in the Czech Republic.

The VVER-440/230 plants have a number of design limitations. In particular, there are limited emergency core cooling systems and large pipe guillotine breaks were not included in the design basis. These plants do not include a strong containment to enclose the reactor. There were limitations in the control instrumentation and in the design of control rooms with limited protection for operators in the event of a large release of radioactivity. These shortcomings have been recognised by the IAEA and other bodies, e. g. the Organisation for Economic Co-operation and Development (OECD) and the European Commission (EC), and improvements have been made. Nevertheless significant safety issues remain for these types of reactors (Lederman, 1995).

The VVER-440/213 plants are much improved in their design and safety concept. For example, their safety systems can mitigate the large guillotine break, they also have sealed containments (or confinements) to localise accidents via a suppression system. The VVER-440/213 plants have been in operation since the early 1980s. They have shown

image006

good availability and have a good safety record (e. g. in terms of radiological safety and event frequency).

VVER 1000 plants are designed consistently with standard international practice. As for Western plants, they employ the well-established defence-in-depth concept and include redundancy, diversity, physical separation and fail-safe principles in design. However, the standards of manufacture and construction of some units have been questioned. There have also been questions on the power stability, instrumentation and control room operation of these units.

As for other PWRs, VVERs are also refuelled off-line.

1.2.3 RBMK Systems

RBMK graphite moderated reactors were the first nuclear power plant designs developed in the former Soviet Union. The first plant was built at Obninsk in 1954 but the first units to provide a significant power capability were commissioned at Beloyarsk (Unit 1 ; 102 MW in 1964 and Unit 2 ; 185 MW in 1976). This led the way to the development of twin 1000 MW designs and two 1500 MW units (the latter at Ignalina in Lithuania), see Figure 1.4.

image007

Figure 1.4. Ignalina 1 RBMK-1500. 1: graphite stack; 2: fuel channels; 3: group distribution header; 4: pressure header; 5: main circulation pump; 6: suction header; 7: drum separator; 8: condensation tray cooling system. Compartments: I, II: reinforced compartments (left — and right-hand sides) enclosing the major components of the main circulation circuit (main circulation pumps, suction headers, pressure headers and downcomers);

III: reinforced steam removal corridor; IV: towers; V: under-reactor compartment; VI: compartments of the lower water piping. Source: Dundulis et al. (2003).

RBMK reactors employ a direct cycle boiling water pressure tube concept, which was favoured because it avoided the problem of fabricating large pressure vessels. The pressure tubes pass through the graphite core, which is about 12 m high comprising the graphite blocks. The blocks are penetrated by the Zircaloy alloy pressure tubes, each about 88 mm internal diameter and 4 mm thick.

In the 1000 MW design, there are 1663 channels, each containing two fuel assemblies, 3.64 m long. The fuel assemblies consist of 18 pin clusters. Each pin contains about 2% enriched uranium dioxide pellets in Zircaloy alloy cladding, 13.6 mm outside diameter and 0.825 mm thick.

The light water coolant is at about 7 MPa pressure. The inlet temperature of the water is 270°C, the quality of the existing steam water mixture is about 14%.

The coolant system consists of two identical loops. These loops feed into two steam drums. Each loop has four primary circulating pumps, of which one is usually for standby. The dry steam is passed to one of two 300 rpm 500 MWe turbine generators. After purification, the condensate is returned to the steam drums via electrically driven feed pumps.

About 5% of the heat is dissipated in the graphite. This is transferred to the fuel channels via graphite rings, which allow good heat transfer between the pressure tube and the graphite blocks. The graphite temperature should not exceed 700°C.

One important feature of the RBMK is that it has a positive void coefficient. Clearly, the net effect of the positive void coefficient and negative fuel temperature coefficient is an extremely important factor, which will depend on the power level. The RBMK therefore is a sensitive reactor to control. At full power, the negative fuel temperature coefficient dominates, but at low power this is not true.

Channels for the control and shutdown rods also pass through the graphite blocks. For the reactor control and protection there are 211 solid absorber rods that are divided into rods with different operations of control.

The RBMK is refuelled at full load.

The primary circuit is contained in a series of compartments that perform the function of a containment in the event of an accident. Each compartment has a design pressure of about 0.45 MPa.

As a consequence of the Chernobyl accident, a number of modifications have been added on other units. These include, improved rate of control rod insertion, automatic shutdown systems to prevent low-power operation and also the problem of positive void coefficient has been mitigated by the fitting of fixed neutron absorbers together with increased fuel enrichment.

There are 15 RBMKs in operation but there is still international concern over the safety of these reactors. Nevertheless, these plants account for relatively high percentages of the total nuclear generating capacity in Russia and Lithuania (NB. in the Ukraine, all Chernobyl plants are now shut down).

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