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How Accurate Are The Instruments in Nuclear Reactors? About This Podcast

Accurately measuring the reactor water level in a nuclear power plant is critical to safe operation, yet nuclear power reactor water monitoring systems do not work correctly. What would happen today if your car’s speedometer read 60 miles per hour, but in actuality, you might be driving at 40-mph or even 95-mph? Listen to today’s Fairewinds Energy Education podcast as Dave Lochbaum from the Union of Concerned Scientists and researcher Lucas Hixson discuss the dangerous dilemma reactor operators face when a reactor has an emergency shutdown and operators simply do not know if the reactor has enough water to keep it cool! http://fairewinds.org/podcast/flat-so…
Today we’re doing a special show about reactor water level monitoring. We have as our guests nuclear researcher Lucas Hixson and Dave Lochbaum, nuclear expert with the Union of Concerned Scientists. This is not one of our typical podcasts in that this is a very technical podcast, but it’s for you geeks out there. A lot of people have written in to us and asked for this kind of material, and even if you’re a layperson I think that you will really, really find this interesting and it’ll give you insight into how difficult it is to operate a nuclear power reactor.

we’re going to be speaking about what is called the Reactor Water Level Monitoring System at nuclear power plants. Water is used in the nuclear reactor as a critical neutron moderator and coolant. Water levels in a nuclear reactor are not monitored directly, but rather through an indirect monitoring system, which incorporates a reserve tank which is termed a reference leg. There have been some reported flaws with this cooling system throughout the years, some of the most notable being brought forth by Paul Blanche in the early 1990’s.

PODCAST TRANSCRIPT

DL: Yes. Paul found some problems with the level instrumentation used in boiling water reactors like Fukushima in Japan and Pilgrim and Browns Ferry here in the United States. As you mentioned, in that type of reactor, the water boils right in the reactor vessel. It’s difficult to measure the level of water that’s vigorously boiling. If you imagine a pot of boiling water on the stove, you see all that froth level at the top, what is the level of water in the pot? So what boiling water reactors do is use the reference leg, which is just a non-boiling column of water, and compare the pressure or the weight of that water to the weight of water in the reactor. And we can judge the density of the water in the reactor vessel easier than we can determine its actual height. And we can use that differential pressure between what the weight of the water in the reactor is versus the weight of the water in that reference column to determine what the level of the water in the reactor vessel is. If you look at a bottle of soda pop and you shake it up and then crack the top, the water level — the beverage level jumps from a nice low level to spewing out the open top. Because the non-condensable gases inside that soda have become freed by the agitation. Likewise, what Paul noted was that if the pressure of a reactor vessel were suddenly to drop, as it could happen during an accident, the non-condensable gases inside the water can cause the water in the reference column to all of a sudden change dramatically as bubbles come out of that water due to the pressure drop, which is similar to cracking a soda pop. Its pressure drops and the bubbles form. We hadn’t accounted for that in the water level instrumentation. As those bubbles formed under that situation, the indications of level to the operator could become vastly wrong — several feet, dozens of feet wrong. And the reactor core is only 12 feet tall and if the level instrumentation is off by 20 feet, you’ve got a big problem.

LH: Is there any other method for operators to determine the water level if the reactor water level monitoring system is not providing accurate data?

DL: The operators are provided about five sets of water level instrumentation for boiling water reactors. They’re calibrated at hot conditions, high pressure, high temperature, as well as cold conditions where the reactor is shut down and the reactor vessel’s head is off, the water is less than 212 degrees. The problem is that during an accident, you go from high temperature, high pressure to high temperature, low pressure as this pipe breaks and water flows out. The operators must choose amongst these five sets of instrumentation to figure out which one is most accurately monitoring the conditions at that moment; and that indication will shift from instrument to instrument, and it’s the operator’s guess as to which one’s providing the most accurate indication. And when you have 100 tons of reactor core to deal with… a wrong guess comes at a high cost.
continued http://tinyurl.com/monv35o