Electrical Penetration Assemblies for Nuclear Power Plants

Product Description

Approximately 17% of all worldwide electric generation is produced by nuclear power plants. In these plants, controlled nuclear fission is used as a high-energy heat source to turn water into steam, which spins an electricity-generating turbine.

SCHOTT Eternaloc® - Small component. Big impact.

nuclear power plant containment structure
Nuclear power plant containment structures are entirely sealed off constructions designed to safely contain the nuclear reactor. In order to keep the radioactive high-energy source controlled, electricity is needed inside the containment structure to supply thousands of instruments, control panels, electric motors and many other electronic devices.

SCHOTT Eternaloc® Electrical Penetrations, made with Glass-to-Metal Sealing Technology, allow the safe conduction of electricity through the fire-protective, pressure-resistant and hermetically sealed containment walls of nuclear power plants. The performance of electrical penetrations is critical to running core functions within the reactor, and their seals must be strong enough to maintain the pressure boundary integrity of the containment vessel in all accident conditions. In case of an accident, these glass-to-metal sealed nuclear penetrations also prevent steam, pressure and radioactivity from escaping.

Eternaloc® penetrations are sealed by non-aging glass that remains pressure- and vacuum-proof for many decades. All around the world, our glass-to-metal-seals have been proving their durability and reliability in power plants, performing maintenance-free since the early 1960s.

A penetration module for nuclear power plants consists of: 
  • A metal housing with sealed-in conductors
  • The glass-to-metal seal (using compression seal technology)
  • Connections to tail cables (LOCA-proof sealed by shrinking tubes or elastomer sealing)
  • Tail cables (length and type according to customers' specification)
  • Metal housing for protection and support of the cable connections
Our latest generation penetrations use receptive modules with mountable nuclear connectors, which allow smaller construction sizes and easy on-site installation.


Eternaloc® electrical penetrations assemblies (EPAs) provide considerable safety advantages over organic epoxy seals to support the integrity of electrical penetration seals and containment vessels by utilizing inorganic, non-aging glass seals with significant heat- and radiation-resistant properties.

SCHOTT’s Eternaloc® glass-to-metal sealed penetrations have been stringently tested at Wyle Laboratories in Huntsville, Alabama (USA). SCHOTT EPAs have successfully passed an extensive qualification test program per IEEE Standards 317 and 344, including full survivability in conditions analogous to an earthquake reaching a Richter scale magnitude of 12.

Eternaloc® electrical penetrations are already installed in over 100 nuclear reactors worldwide and are qualified for 60 years of use. The performance range of SCHOTT EPAs represents the gold standard in safety: they have been proven to withstand pressures greater than 400 bar (5,800 psi) and temperatures of 400 degrees Celsius (752 degrees Fahrenheit) in previous tests.

In contrast, epoxy seals may break down under extreme temperatures and pressures. This was observed at the Fukushima Daiichi nuclear power plant disaster in 2011. While electrical penetrations using organic polymer seals, such as epoxy, meet the current design basis specifications for electrical penetration assemblies in first and second-generation nuclear power plants, nuclear experts have voiced concern that the effects of severe accidents which have exceeded the design basis conditions could compromise the integrity of the seal.

Eternaloc® hermetically sealed penetration modules:
  • Have high packing density of up to 120 poles at connector solutions and more than 500 pins at flange types, enabling a reduction of the required embedded pipes.
  • Undergo virtually no aging of the used conductor and seal material – other types of penetrations (i.e. epoxy-to-metal bonds) with signs of aging have been replaced by glass-to-metal type penetrations.
  • Include connections between conductor pins of the hermetically sealed feedthrough and cables for use in nuclear power plants. The plugs are easy to connect, allow easy exchange of modules in case of system modification, and offer high stress resistance.


Eternaloc® Glass-to-Metal Sealed (GTMS) penetrations are based on proven technology and are inherently more stable and safe than counterparts made with lesser materials and inferior sealing.

SCHOTT hermetic glass-to-meal sealing technology is already proven in a range of safety-critical applications. These include nuclear submarines, air bags for cars, liquefied natural gas (LNG) vessels, and high pressure and temperature applications in the oil and gas industry.

Within the nuclear industry itself, SCHOTT has 12,000 GTMS EPAs installed in more than 100 active nuclear power plants. This includes a variety of reactor types, such as Pressurized Water Reactors (PWR), High Temperature Reactors (HTR) - also known as Pebble Bed Reactors (PBR), Boiling Water Reactors (BWR) and Fast Breeder Reactors (FBR).

Meeting and exceeding stringent requirements for longevity, some of these EPAs remain in operation -  maintenance-free - after almost 50 years of service. This is due to the inorganic and non-aging properties of the glass-to-metal seal which offers a virtually unlimited lifespan at the pressure boundary.

For Forsmark 3 in Sweden, SCHOTT designed a new generation electrical penetration which can withstand the latest severe accident requirements (e.g. submerged conditions at higher pressure levels and higher temperatures).

Technical Details

Compression glass-to-metal sealed feedthroughs are made of a metal housing, a glass sealant and metal conductors. The preassembled component is heated to a temperature where the glass melts to the metal. During the cooling process, the metal housing contracts at a rate much higher than that of the glass. This compression creates a hermetic seal that offers the safety qualities of being highly temperature and pressure resistant.

The glass preform, metal housing and conductor pins, made of nickel-iron or oxygen-free copper (low-ohmic), are assembled on a fixture. The complete assembly is heated to a temperature where the glass becomes free-flowing and fills the space between the main body and the conductor pin or bolt.

During the cooling process, the glass solidifies at its set point. Upon further cooling, the outer metal housing shrinks more than the sealing glass and the conductor (due to its higher coefficient of thermal expansion), leading to a compressive radial force onto the glass body. This leads to a very high
and tightness even under high temperature or gas pressure and thermal cyclic conditions.

Finally, corrosion-protective layers are deposited on the metal shell; whereas the nickel-iron conductor pins are plated with a gold layer.

The cable tails are connected to the pins by plugs, crimping or soldering and then insulated by shrinking sleeve and potting with elastomer to ensure the electrical function even during extreme environmental conditions (important during Loss of Coolant Accident; LOCA).

Stainless steel housings are mounted to the ends, providing mechanical protection and strain relief at the cable inlet. Junction boxes inside and outside containment provide additional protection for this electrical system. For receptacle modules, the connectors can be provided either cabled according to customer requirements or finished on site.

Besides power and regular control/instrumentation penetrations we also produce coax and
penetrations in single or double barrier versions.

Features / Specifications
  • Degree of Protection: IP 54
  • Temperature Resistance: ≥ 200 °C / 8h
  • Pressure: ≥ 20 bar
  • Radiation Dose: 0.5 x 106 G


Our Eternaloc® penetration modules are manufactured according to KTA 3403 or IEEE317 standards (Standard for Electric Penetration Assemblies in Containment Structures for Nuclear Power Generating Stations).

For more information about our quality assurance process, please refer to this page.
Visola logo
Nuclear Penetration Assembly Case Study

This case study by Visola Electric Insulation Technology Ltd illustrates the use of SCHOTT's EPAs in the PAKS NPP (Hungary) since 1995 with no incidents of failure.
Exhibitions & Events
Exhibition CIOE - China International Optoelectronic Exposition, Shenzhen, China, 09-01 to 09-03-2021
Exhibition Gastech, Singapore, Singapore, 09-13 to 09-16-2021
Electronic Packaging
SCHOTT North America, Inc.

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