Electrical Penetration Assemblies (EPA) allow for the electrical wiring to pass through the wall of the safety containment structure in nuclear reactors. In an interview with Asian Nuclear Energy, Thomas Fink, Head of the Nuclear Safety Division of the German High-Tech group SCHOTT AG, says glass-to-metal seals are best suited for this. Excerpts.
Mr. Fink, SCHOTT AG is known all over the world as a manufacturer of glasses for Ceran cooktop panels, buildings and reflector telescopes, as well as photovoltaic modules, in more recent times. Probably only very few people know that you also supply products for nuclear power plants.
That is correct. We build Electrical Penetration Assemblies or EPAs for short, through which the control and power cables are guided inside the safety containment structure of the reactor. Conductive feedthroughs are an important part of our business. We not only make them for nuclear power plants, but also for liquefied gas tankers, where they seal off the tanks and guide the electrical wiring to the submersible pump inside the tank. We manufacture millions of units of feedthroughs for encapsulating electronic components for sensors and airbag igniters in automobiles or for optoelectronic components for the telecommunications industry, for example.
What does this have to do with glass?
The unique thing about all of our feedthroughs is that the seal between the conductor and the metal housing is made of glass. Regardless of whether this is intended to be an EPA for nuclear power plants or a housing for a small sensor inside the motor, the principle is always the same: a glass ring is inserted between the conductor and the housing and then melted down. By selecting the appropriate coefficient of thermal expansion, the metal housing presses against the glass ring, which in turn presses against the conductor. This bond remains perfectly sealed even with major leaps in temperature.
What advantages do glass-to-metal feedthroughs offer?
Well, to start with, they are hermetically sealed, which means that gases are unable to permeate the seal. Of course, this applies for the hot steam that results from an accident inside a reactor, as well. Our penetrations have a life span of sixty years and more. The earliest glass-to-metal feedthroughs from SCHOTT in nuclear power plants have been in use for over 40 years and they are a still in use, performing very well. This is why we are sure that they actually last longer than 60 years. We owe their long service life in nuclear power plants to the fact that glass, an inorganic material, is radiation resistant. It doesn’t age, neither in response to nuclear radiation, nor shifts in temperature. No other material offers this.
More than 50 nuclear power plants internationally are equipped with EPAs from SCHOTT. Can you tell us more about this?
That’s correct. All of the German nuclear power plants since the 1960s have been equipped with our EPAs and since the German Nuclear Safety Standards are amongst of the highest worldwide this makes us very proud. The last few years have clearly shown that our EPAs are, in fact, superior. The operators of 23 nuclear power plants located outside Germany have replaced their old epoxy EPAs with glass-to-metal sealed EPAs from SCHOTT because they apparently had problems with the seals. This doesn’t surprise me, because epoxy is an organic material that ages noticeably when it is subjected to nuclear radiation or high pressures and temperatures. In the meantime, our EPAs have been installed in 50 nuclear power plants internationally.
If glass-to-metal feedthroughs offer so many advantages, why aren’t other manufacturers offering them too?
Because they don’t have the specialized know-how. One can’t just use any type of glass for glass-to-metal seals. You have to know exactly how the glass will react when it is heated up and cools down, for instance, and how well it will work together with certain metal alloys. Here, there are virtually infinite combination possibilities. We have been manufacturing glass-to-metal feedthroughs since 1941 and our scientists and engineers at our research and test centers are constantly striving to further develop and improve the technology designs. Here, however, it is not only important that one has a command of the materials, but also the way in which the cable connection is constructed. For example, we have developed feedthroughs that can be equipped with up to seven modules, whereby one module can accommodate up to 118 conductor feedthroughs. The wires inside and outside of the safety containment structure can be connected quite easily by plugging them in. This is a lot easier and more flexible than the wiring boxes that epoxy EPAs use. We are also in a position to supply feedthroughs for hermetically sealed fiber-optic transmission lines for use in transmitting optical data.
How do you feel that the market for EPAs will develop?
Nuclear energy is experiencing a boom once again all over the world, whereby India certainly plays the role of a trendsetter. Some 40 new reactors are currently under construction on a global basis. Even nations that are seeking to get out of nuclear energy are now either planning or building new reactors. As a result, the market for EPAs will grow and I am confident that SCHOTT will be able to increase its market share rather quickly.
What makes you so certain?
The development of new reactors is working in our favor. The systems of the new generation, such as Gen 3, 3 plus and 4, as well as future generations, will be placing much higher demands on materials. For instance, in current reactors, temperatures reach around 160 degrees Celsius and pressure is around 5 bar when hot steam escapes as a result of an accident. For the next generation reactors, the temperature can increase to 220 degrees in the event of an accident. The pressure requirements are also increasing considerably: to 9.5 bar and even 80 bar in helium-cooled high-temperature reactors. There is no way that epoxy EPAs can handle values like these, while glass-to-metal EPAs from SCHOTT meet these demands quite easily. They can resist pressures of up to 1000 bar and temperature spans ranging from minus 200 to plus 240 degrees Celsius.
We simulate conditions like these in the laboratory by subjecting all of the EPAs to hot steam before we allow them to be shipped. We test for whether they are hermetically sealed with the help of a mass spectrometer that is capable of detecting even the smallest conceivable volumes of a test gas. The electrical properties of the feedthroughs are yet another important aspect of this test.
Because they offer so many advantages, glass-to-metal EPAs are probably a lot more expensive, aren’t they?
Not at all. Our products are very competitive, especially when we look at this over their entire lifetime, then EPAs from SCHOTT are actually the best offer because the principle “fit and forget” truly applies to them. And maintenance costs are zero; therefore the total costs are lower.