Electrical Penetration Assemblies for VVER Type Nuclear Power Plants

Role and behaviour of electrical penetration assemblies in a nuclear power plant

 

Electrical penetration assemblies (shortly EPAs) in a nuclear power plant are installed into the concrete wall of the containment. Their function is to make an electrical connection between inboard and outboard sections of power, instrumentation and control cables. Consequently, EPAs should meet all the requirements of cables, both in normal operation and under LOCA (Loss of Coolant Accident) conditions. Neither the high pressure of hot steam nor the  intensive gamma irradiation may cause damage in their operation. They should withstand the impact of decontamination too. (Cleaning by acidic and alkaline solvent.). Their lifespan should fit the designed operation time of the reactor.

On the other hand, EPAs should meet the requirements of hermetically sealed containment wall as well. The rate of their gas leakage must remain below the level defined by relevant standard, to prevent an escaping radioactive by-products into environment. Furthermore they shall be fire resistant and earthquake proof. Their gamma screening must be as effective as that of the concrete wall.

 

Design of electrical penetration assemblies for VVER type nuclear power plants

 

Thirteen sorts of low voltage EPAs were developed and are manufactured by VISOLA Ltd for Paks NPP / Hungary. They have a common design. The principal drawing is shown in Fig. 1.

The components of EPAs are assembled in a thick walled stainless steel casing tube equipped with two flanges. The casing tube comprises three main components:

  • A hermetic cable penetration module (in most of the cases with 2 barriers). It joints to the front flange with Viton O-rings and a metal C-ring in the interface.
  • Cable tails connected to the penetration module.
  • A gamma-shielding system which consists of two parts filling in the free space around penetration and between cable tails.

The penetrations we apply are the so called glass-to-metal sealed ones, made by Schott Electronic Packaging GmbH. They are maintenance-free products,  widely used all over the world. However, they had to be adopted to the special requirements of soviet designed, VVER type reactor, where all penetrations are equipped with individual gamma-shielding. It was developed by VISOLA Ltd.

The radiation shielding is manufactured of a special synthetic resin with heavy additives having a density of 4.59 g/cm3. The gamma-ray attenuation properties of the material were measured in the leakage gamma-spectrum of a nuclear reactor. A dummy cable penetration assembly was also tested. Several EPAs – designed for concrete walls with different thickness – were investigated by computer simulation. It was proved that the selected designs provide better gamma-radiation shielding than the wall where they are installed.

 

Figure. 1:  Principal drawing of low voltage electrical penetration assemblies

 

In order to avoid any ingress of humidity, or moisture from containment into the free space of the EPA along cables, they are sealed longitudinally. The end-flange and the integrated metallic components of the cable glands are made of stainless steel. All sealing and cables are LOCA-proof.

The cable jointing box covering the end-region of each instrumentation and control EPA provides a further barrier able to release the effects of moisture, high temperature and increased pressure in accident. It is also made of stainless steel, resistant to chemical impact of decontamination.

After installation of an EPA the penetration module is filled with N2 gas. The internal space of the penetration is connected to a pressure-gauge for continuous leak rate testing.

 

Features of  EPAs

  • The whole family of  EPAs have a common design with many identical components.
  • The sensitive parts are protected by a heavy-duty casing tube.
  • EPAs are factory assembled and tested. There is no quality risk at installation on site.
  • The mechanical installation requires only welding of two steel plates to the wall lining.
  • Penetration is fastened only by screws and if it’s needed can be replaced without welding.
  • EPAs are type tested and meet requirements of  IEEE-317, IEEE-323, IEEE-383, ...
  • The main insulation of penetration is glass with negligible ageing, confirmed by experiences.
  • Double sealed penetrations are applied for the sake of continuous leak rate monitoring
  • Sealed cable junction boxes of I&C EPAs provide mechanical protection as well.

 

Available EPAs of this design

  • Low voltage power (EHKA): 4x200 A, 8x100 A, for wall thickness 800-1500 mm
  • Low voltage I&C (MHKA): 42x4 mm2, 118x1.5 mm2, for wall thickness 800-1500 mm
  • Coaxial EPAs (KHKA): 7 cables, 50 Ω impedance, for wall thickness 800-1500 mm

 

Reference

 

Paks NPP / Hungary. 345 pieces delivered.

Since 1995 in service. No failure occurred.