ELECTRICAL PENETRATION ASSEMBLIES (EPAS) ARE a small but important component found in all nuclear power plants.
Electrical and instrumentation cables must be fed into the reactor through its containment structure to monitor and control it. The containment walls are fire-protective, pressure-resistant and hermetically sealed. The containment structures are entirely sealed off, to contain the reactor safely and avoid the smallest leakage. The penetration assemblies must provide a leak-tight pass-through for the necessary cabling and also maintain the pressure boundary integrity of the entire containment structure. This is of the highest importance: the most important function of the penetration assembly in the event of a severe accident is to prevent radiation and gas leakage, while allowing monitoring and reactor control functions to remain intact.
During normal operation and accident conditions, the power, control and instrumentation cables have to connect the thousands of instruments, control panels, electric motors and many other electronic devices and systems within the containment structure.
The penetrations are extremely important, so they must be held to the highest standards of safety and reliability. Schott’s glass-to-metal sealing technology allows the safe conduction of electricity and control cables.
Shidaowan
The Shidaowan twin high temperature reactor (HTR) units are small modular reactors (SMRs). SMRs, defined by the International Atomic Energy Agency as reactors under 300MWe, are emerging as a promising new nuclear power option. Their modular designs and interchangeable components allow them to be factory-produced. Additional units can be added should more capacity be required.
Examples of potential applications include remote sites in the far reaches of the world, such as maritime shipping locations and military installations, where a single SMR could power an entire community.
Small modular reactors are different from most operating reactor types because they incorporate sets of electrical penetrations in more challenging areas: in the primary loop, and in the reactor itself.
Schott worked in partnership with Chinergy Co. Ltd and Jiamusi Electric Machine Co. Ltd to help support the safe and cost-efficient operation of the HTR at Shidaowan in China’s Shandong province. The site will ultimately have a series of 210MWe units of this type. Each unit is made of two HTR-PM reactors with a single steam turbine. The first reactor vessel was installed in March 2016.
Schott provided technical support for the teams at Chinergy and Jiamusi Electric Machine. The result was a tailored solution that meets the plant’s specific installation requirements and can withstand the high-temperature and high-pressure environment of the HTR’s primary loop.
Schott recommended the use of Eternaloc penetration assemblies, made with glass-to-metal sealing technology.
Schott says the EPAs it has manufactured for the Shidaowan facility are more robust than counterparts made with organic materials. Historically, the mechanisms that controlled the fuel rods or control rods were on the outside of the reactor vessel. In new HTR designs, they are placed inside. This requires penetrations that can manage high pressures and high temperatures in a smaller area, so there is a need for penetrations that are smaller and can withstand prolonged exposure to high heat and pressure. In HTR applications, glass-sealed penetrations are the only viable feed-through option for the primary loop. Electrical penetration assemblies with polymer seals that have inferior temperature and pressure resistance are simply not feasible alternatives.
One significant innovation of SMRs is that many components can be manufactured very quickly and efficiently and put into operation easily by utilising a strong connector. Advanced EPAs can connect 120 electric compactors together. It is simply plugged in, locked and ready for operation. In contrast, supplying individual wires to a junction box can take hours of strenuous work. By choosing a plug-and-play option the process can be completely streamlined.
The incorporation of glass-to-metal sealed EPAs in the primary loop of HTRs is a promising step forward.
In fact, Schott believes glass-sealed EPAs represent a superior solution for all nuclear power plants. This is because polymer-based seals are organic and naturally age, so their performance degrades over time and there is a need for multiple replacements over the operating lifetime of the reactor.
Glass-sealed penetration assemblies can maintain uncompromised seal integrity for a qualified lifetime of 60 years, reducing both the cost burden and any potential safety risk.
Thomas Fink is Nuclear Safety Division General Manager at Schott
