While initially covered by CNN, it has subsequently been covered by the print media as well, including the New York Times and Reuters. We know that the Taishan reactor plant, a French-designed pressurized water reactor, apparently has reported elevated levels of noble gases in the reactor coolant. According to the CNN story, Framatone, the French firm that helped design the site’s reactors, contacted the US for help with understanding readings coming from the reactor. Specifically, the elevation of noble gas concentrations in the reactor coolant. So, let’s see what this lone fact can tell us – where the noble gases come from and what their presence might mean.
First, a little background.
Nuclear reactor fuel is comprised of small pellets made of uranium oxide and clad or wrapped with a zirconium alloy. Fuel pellets are then loaded into tubes to form fuel rods which are, in turn, arranged into fuel assemblies loaded into the reactor core. These assemblies are then bathed in a coolant that both slows the neutrons and transfers the heat of fission to ultimately generate electricity. (A more detailed description of how a nuclear reactor works can be found here.)
Every uranium atom that fissions splits into two radioactive atoms, fission products. Atoms rarely split exactly in half, so these fission products have different masses, clustering around 95 atomic mass units (AMU) and 135 AMU. It turns out that there are several noble gases, elements that rarely react with other elements, specifically krypton and xenon, that have AMUs in the range of 95 and 135. There are other radionuclides in that same mass range; several nuclides of cesium and iodine in the upper production peak, strontium, technetium, rubidium, molybdenum, and so forth in the lower production peak.
The radioactive noble gases are products of nuclear fission within the reactor. What are they telling us?
The presence of fission product noble gases
One thing their presence tells us is that the cladding wrapper has failed in some manner. Zirconium is a fairly tough metal, and it does a great job of containing the fission product gases. Fuel cladding contains traces of uranium, called “tramp uranium,” that is capable of fission, and those fission products, including the noble gases, end up in the coolant. There are always trace levels of fission products in the reactor coolant. Not only that, but the level of fission product, measured by its radioactivity in the reactor coolant, can change as reactor power goes up and down.
If the cladding gets even a slight crack then these noble gases can escape into the reactor coolant. To separate a “leak” from the routine “trace amounts,” we need to compare the activity of the fission products in the reactor coolant to the reactor’s power history – and then to see if there’s a significant increase over what’s normal. If so, then it can indicate that the cladding has cracked.
It gets a little more complicated
If the crack is tiny, there might be only low, albeit still somewhat elevated, levels of noble gas in the coolant. If the crack is more significant or if it starts to grow over time, then some of the volatile radionuclides – mostly cesium and iodine – might begin to get into the coolant as well. But unless there’s major fuel damage – a meltdown, for example – we’re not likely to see any other elements.
It’s reasonable to wonder what might happen next…and the answer is that there’s no way to know with the information I’ve been able to find. I can make any number of guesses, each based on its own set of assumptions and each likely to be wrong. And in all honesty, if I guessed correctly, it would be as much luck as skill. So, we’ll have to revisit the question when we get more information.
It’s also reasonable to wonder if this is going to escalate into something serious. That’s something else we just don’t know at the moment. But we can make a little better guess here. The Taishan reactor is a Western design, unlike Chernobyl, and it doesn’t seem to have suffered any major shocks, as in the case of Fukushima’s reactors. This means that the reactor plant and its containment are likely to be intact and should be up to the task of keeping radioactivity out of the environment and away from the local population. There’s still likely to be a somewhat elevated level of radioactive noble gas discharged from the reactor plant, but it’s not likely to pose a risk to people living in the area.
This is an evolving story and we don’t yet know what the conclusion will be. I’ll keep an eye on the news and will update as I learn more.
There has been an update on the reactor as of June 17th. The Chinese National Nuclear Safety Administration (NNSA) announced yesterday that the increase in noble gases in the reactor coolant was “caused by damage to the cladding of a small number of fuel rods,” affecting “less than 0.01 percent” of the 60,000 rods in the reactor core – no more than six fuel rods.
The BBC notes that noble gases are “often used in situations where scientists do not want chemical reactions, for instance, in nuclear reactors or lighting.” The phrasing makes it sound as though the fuel rod is loaded with noble gases to prevent corrosion of the fuel pellets and the fuel rod – this is at odds with my understanding of how these components are manufactured and neglects the presence of fission product noble gases.
I suspect that BBC made an error on this point, especially since the Chinese now appear to be acknowledging that some of their fuel rods have been damaged. Other stories discuss changing radiation dose limits (which might be called for in the event of a release of fission product gases). So, unless I learn more that suggests the fuel rods of this plant are loaded with noble gas, it still seems most likely that the noble gas in question comes from uranium fission and indicates the presence of relatively minor damage to some of the plant’s fuel rods.
