Most of you picked up an extra hour of sleep on Sunday (sorry, Arizona and Hawaii). The clocks went back at 2 a.m., you rolled over, and that was pretty much the extent of your involvement.<\/p>\n
Nebraska’s Cooper Nuclear Station had a busier morning. At 4:54 a.m. Central, with the reactor climbing back toward power after a refueling outage, the people in the control room shut it down by hand.<\/p>\n
The plant’s owner, Nebraska Public Power District (NPPD), told the Nuclear Regulatory Commission<\/a> the reactor was at 9 percent power when operators tripped it over “elevated vibrations on the only feedwater pump in service.”<\/p>\n Let’s deal with the scary stuff first, because there isn’t much of it. NPPD reported this as a non-emergency, says the trip wasn’t complex and that everything responded the way it should, and says neither the public nor anyone at the plant was affected.<\/p>\n Scram is the industry’s word for what those operators did. It means driving the control rods into the core in a hurry to stop the chain reaction, and in this kind of reactor, they shoot up from the bottom. A trip like this has to be reported within four hours, and NPPD called it in at 9:20 a.m. Eastern, about three and a half hours later.<\/p>\n Cooper’s a boiling water reactor, and that name is pretty literal. Water boils right inside the reactor vessel, the steam spins a turbine, and a condenser turns it back into water. Feedwater is what that returning water is called, and a feed pump shoves it back into a vessel that’s already under a lot of pressure.<\/p>\n So if that flow stops, the water level starts dropping and the plant has to lean on its backup systems. You can think of it like the water pump in your car starting to howl on an empty stretch of highway. You could keep driving and hope it lasts, or you could pull over while that’s still your decision.<\/p>\n At 9 percent power, pulling over doesn’t cost much, so frankly I think it’s a pretty easy call. NPPD hasn’t explained its reasoning beyond the vibration, though, so that’s just my read.<\/p>\n And should that word “only” worry you?<\/p>\n I don’t think so. The NRC’s training manual for this General Electric reactor family<\/a> describes two feed pumps, each driven by its own steam turbine and rated at 14,000 gallons a minute. Each is rated at 67 percent of the system’s capacity, and the first gets started when reactor pressure reaches about 350 psi. So one pump running at 9 percent power looks pretty normal to me.<\/p>\n Those are family numbers, and I couldn’t find Cooper’s own pump ratings. NRC paperwork on the plant does show it has two steam-driven feed pumps, though.<\/p>\n The same manual says those turbines carry bearing vibration detectors, so my guess is someone was reading a gauge instead of listening for a rattle. NPPD hasn’t said how bad the vibration got. It also hasn’t said whether the trouble is in the pump or in the turbine driving it.<\/p>\n The NRC’s daily status report<\/a> listed Cooper as down for refueling since September 28, and it still showed 0 percent on the morning of October 30. I couldn’t pull a report for October 31. The November 1 edition shows 1 percent.<\/p>\n That 1 percent is a long way short of the 9 percent in the event report, and I can’t fully square them. The status numbers come from the plant, the NRC labels them unevaluated, and they’re collected somewhere between 4 and 8 a.m. Eastern. 4:54 Central is 5:54 Eastern. So my guess is the 1 percent reading came first and the reactor kept climbing. I’m going with 9 for the moment of the trip, because that’s what NPPD told the NRC.<\/p>\n Either way, this reactor was barely awake. Cooper’s licensed for 2,419 megawatts of heat, so 9 percent works out to about 218. The report puts it in Mode 2, the startup stage, and Cooper’s technical specifications don’t call it power operation until Mode 1. Which means I’d be pretty surprised if it was putting electricity on the grid at that point, though the report doesn’t say.<\/p>\n With the rods in, Cooper dropped into what’s called hot shutdown. The chain reaction has stopped, but the water’s still above 212\u00b0F.<\/p>\n NPPD says the water level is being held by the control rod drive system, which probably isn’t what you’d picture as a water supply. That’s the hydraulic system that moves the rods. According to that same training manual, it normally sends about 47 gallons a minute through the rod mechanisms to keep them cool, compared with 14,000 gallons a minute for one feed pump.<\/p>\n So how does a stream that small keep up? There isn’t much left to keep up with. Once the chain reaction stops, the core is only making leftover heat, and a core that’s spent nearly five weeks shut down and then barely come back shouldn’t have much of that. That’s my reasoning rather than NPPD’s, but it fits what the report describes.<\/p>\n NPPD’s site<\/a> calls Cooper the state’s largest single-unit generator, and a May release from the utility calls it the state’s only nuclear generator. The site lists 800 megawatts of net capacity, says the plant generates about 835, and claims that’s enough to cover 385,000-plus residential customers at the peak of summer.<\/p>\n The plant employs 676 people near Brownville, 23 miles south of Nebraska City and close to the Missouri River. It’s a General Electric Type 4 reactor with a Mark I containment.<\/p>\n It’s also been around a while. NPPD lists its in-service date as July 1, 1974, and its license runs to January 18, 2034. The NRC received an application on May 7 to keep it going for longer, and the agency’s published schedule has a decision due in June 2027. I’d be surprised if one shaky feed pump moved that schedule.<\/p>\n If all of this feels familiar, we’ve been here a lot lately. In September, an Illinois reactor got shut off by hand at 4:31 a.m.<\/a> over a false reading on its control rods, and last month a Louisiana reactor tripped itself the night before its refueling outage<\/a>. You can read every one of these reports yourself, which is arguably the best thing about the system.<\/p>\n Nobody’s said. NPPD’s press page had nothing on the shutdown when I checked on Tuesday, and its latest release was dated October 21. The NRC’s November 3 event report didn’t carry an update on this notification either.<\/p>\n The repair could go either way. A bad sensor might only cost a few days, while a damaged bearing could take a lot longer. That’s just me guessing, since we don’t know what they’ve found.<\/p>\n If you want to keep an eye on it yourself, the NRC posts that status report every day, with Cooper listed under Region IV. The November 2 edition had it at 0 percent.<\/p>\n","protected":false},"excerpt":{"rendered":" Most of you picked up an extra hour of sleep on Sunday (sorry, Arizona and Hawaii). The clocks went back … <\/p>\nSo why shut a reactor down over one shaky pump?<\/h2>\n
Cooper had only just woken up<\/h2>\n
So where’s the water coming from now?<\/h2>\n
Cooper’s a big deal in Nebraska<\/h2>\n
So when does it come back?<\/h2>\n