What does it take for a piece of electrical equipment to meet environmental qualification requirements? The test battery is rigorous. Here are the testing standards . . . .
Several environmental and qualification criteria must be considered when installing critical equipment into a harsh environment. There are eight of them that equipment must consider to become EQ qualified:
- Temperature and pressure – these can vary widely depending on the installed location. For example, at the onset of a LOCA, the rapid introduction of high-temperature, high-pressure coolant into a lower-temperature, low-pressure environment will quickly raise both ambient temperature and pressure. Electrical equipment must be able to withstand this sudden shock as well as elevated conditions for the long-term.
- Humidity – Equipment must be designed to withstand humidity during and following an accident. Things can go from dry to damp in an instant.
- Chemical effects – chemicals are added to coolant for corrosion control and radioactivity management, and they generally remain confined there. But during an accident, these chemicals will be sprayed into the local atmosphere along with the leaking coolant. These chemicals tend to be corrosive and explosive, so protective enclosures made of corrosion-resistant, and even explosion-proof, materials are used to protect electrical equipment (stainless steel, NEMA 4X-certified, etc).
- Radiation – equipment must be rated to function when subjected to normal radiation rates and lifetime total dose at those rates, but it must also be able to sustain the very high doses it will accumulate during and following an accident. The type of radiation matters (alpha, beta, gamma), and that can vary between normal and accident conditions. Electronic equipment is particularly susceptible to radiation exposure because it interferes with digital operations. Getting digital equipment into a high-radiation environment is difficult and expensive.
- Aging – Equipment is aged (or “preconditioned”) before being subjected to simulated accident conditions. This is to test the worst-case scenario of a major accident occurring at the very end of an equipment’s life. It assumes a piece of equipment has already been exposed to perhaps 40 years of high temperatures, high radiation, and a wet environment before being subjected to a major shock in the form of an accident.
- Submergence – If electric equipment is expected to be submerged, it will obviously need to be specially designed to withstand this. It’s not uncommon for cables to be submerged.
- Synergistic effects – equipment is often tested to the variables listed here independently of each other because of testing limitations (it may be hard to locate a test chamber that will test all of the variables at once), but depending on the equipment and conditions the effects of multiple conditions applied simultaneously may have a greater effect than when they are applied one at a time. It’s a classic case of the whole being greater than the sum of its parts. This phenomenon must be considered and documented when it’s “believed to have a significant effect on equipment performance.”
- Margin – it is a requirement that margin be applied to account for expected uncertainties. Two that the NRC lists are production variations and instrument uncertainties.
The equipment’s ability to withstand the environmental conditions outlined above is usually demonstrated through testing. Individual units are tested in a lab as representative samples of an entire model line to prove the line can withstand the conditions it’s rated for. The purchased equipment will then be linked to the tested samples by a test report.
The regulations allow analysis to be conducted as an alternative to testing, but even the analysis must be combined with some amount of test data to justify conclusions. Because of the rigor and scrutiny involved, testing is usually preferred (though admittedly expensive).
Accelerated aging can be achieved by subjecting equipment to high temperatures or high doses of radiation for a short time to simulate exposure at lesser conditions for a long time. It can also be subjected to wear-testing (perhaps by switching contacts or flipping switches thousands of times to simulate a lifetime of use) and mechanical vibration tests.
As the IEEE puts it, it is the “degradation with time (aging), followed by exposure to the environmental extremes of temperature, pressure, humidity, radiation, vibration, or chemical spray resulting from design basis events which presents a potential for causing common cause failures of Class 1E equipment.”
Equipment can even be naturally aged. Though this is the most accurate means of aging a test sample, it’s understandable why samples aged to the end of their natural life may not be readily available.
AND THE STANDARDS ARE . . . .
In practice, these numerous qualification tests take the form of two primary standards that the NRC has endorsed:
IEEE 323 is for qualifying Class 1E equipment, and IEEE 383 is for qualifying Class 1E cables.