Ageing of electrical systems and I&C

Electrical components and I&C are replaceable and the required perfor­mance of these commodities can be ensured via maintenance and scheduled replacement. The qualified condition of the electrical and I&C equipment has to be ensured.

Full scope ageing studies had been prepared for the Paks NPP for the fol­lowing electrical and I&C items:

1 Equipment of electric power and transmission systems:

• Bus cabinets

• Overhead-line towers, medium- and high voltage insulators

• LV and HV cables of power supply systems

• Cables for containment electrical penetration

• Cable joints and assemblies

• Enclosed electrical equipment

• Battery packs.

2 Equipment of the technological systems:

• Fixtures for transmitters

• Impulse pipes and assemblies

• Operation monitors

• Relay boards

• Cables for E, I&C equipment

• Cables of containment electrical penetration for E, I&C

• Cable joints and assemblies

• Terminal boxes.

The basic issue at all VVER plants regarding electrical and I&C equip­ment is the lack of or insufficient environmental qualification. Lack of initial qualification of the VVER equipment was recognized in the 1980s at all VVER-440/213 plants as well as at VVER-1000 “small series” and VVER-1000/320 models.

Establishing the initial qualification is understood as a current licensing basis requirement at all VVER plants. This consists of the following steps:

• Definition of environmental parameters characteristic of the installation site.

• In the case of safety equipment, definition of environmental parameters characteristic to the installation site under accidental (loss-of-coolant) conditions.

• Definition of accelerated thermal and radiation ageing test parameters.

• Performing laboratory tests with the above parameters (accelerated thermal and radiation ageing, radiation exposure with accident condi­tion and simulation of loss-of-coolant conditions).

• Performance checks on tested samples to verify conformity with accep­tance criteria.

The maintenance of qualified condition of the cables for harsh environmen­tal conditions is a critical issue at VVER plants.

In regard to the cables, the technical task of qualification is rather dif­ficult. For example at the Paks NPP there are 130 000 cables and among them, several hundred types. The first necessary measure related to the cables was to develop a comprehensive database, instead of having the cable sheets on paper. The database identifies for each cable the safety clas­ses, types and routes. The environmental conditions to which the particular cables are exposed are identified in the database. It also shows whether these safety-related cables are affected by the harsher conditions after accidents. Examples of ageing mechanisms of important cables are shown in Table 8.2.

Table 8.2 Examples of ageing mechanisms of NPP cables

Item

Site of

Mechanism of

Worst

degradation

degradation

consequences

XLPE I&C cables

Cover and core

Thermal ageing;

Crack/loss of

in harsh

isolation

change of

function under

environment

the material

loss-of-coolant

properties due to heat or irradiation.

condition

6 kV PVC power

Metal structure

Humidity

Decreased

cables in

of cables

penetration;

isolation

channels/

corrosion of

resistance/loss

humidity/

metal structure

of function/

Cable connection

Corrosion of metal

Humidity/chemical

Increased transit

in harsh

structure

corrosion of joints

resistance of

environment

connectors

In its current state, the database at the Paks NPP covers approximately 19 000 safety-related cables. The database allows the formation of commod­ity groups for cables. Currently there are 45 commodity groups related to safety cables. A similar approach is implemented as an example at VVER plants in Ukraine. For each group of cables, a particular sample cable is identified which is under worst-case condition. The condition of the sample cable is monitored during the operation.

The VVER plants replaced the frequently criticized, obsolete I&C sys­tems. At the Paks NPP, nearly all safety-related I&C systems have been replaced: the reactor protection system, Engineered Safety Features Actuation Systems (ESFAS) protection system and load sequencer pro­gramme of diesel generators. The new system is a digital one (Siemens TELEPERM XS) with multiple redundancy and diverse software features, and physical separation of hardware of different trains. The reactor protec­tion logic was also reviewed and modified to assure diverse physical signals for detecting each postulated initiating event and to eliminate unnecessary input and output signals. Similar reconstruction programmes have been implemented in Slovakia and at Russian plants entering into an extended period of operation.

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