If you’ve ever used a pressure cooker, you know the noise it makes when you let the steam out. It’s a loud, angry hiss, and it’s usually enough to clear the dog out of the kitchen.<\/p>\n
Slovensk\u00e9 elektr\u00e1rne, the utility that runs all five of Slovakia’s working reactors and generates 66% of the country’s electricity, just gave the people around its Mochovce nuclear plant a heads-up about something along those lines.<\/p>\n
In its October 2 notice<\/a>, the company said it’s testing the safety valves that let steam out of the steam generators on Mochovce 4, its newest reactor, and that those tests are “usually accompanied by loud noise.”<\/p>\n So why now?<\/p>\n Mochovce 4 is finally climbing toward full power. Engineers started the climb at 3:33 p.m. local time on September 21, after Slovakia’s nuclear regulator signed off, according to Slovensk\u00e9 elektr\u00e1rne<\/a>. The unit’s a 471-megawatt VVER-440, a pressurized water reactor built to a Soviet design. That’s the same basic kind of reactor that runs at most nuclear plants in the US, and the last time we checked in on this one, its 349 fuel assemblies had just gone in<\/a>.<\/p>\n Branislav Str\u00fd\u010dek, the company’s chairman and CEO, called the start of the climb “one of the final major milestones before Unit 4 is put into operation.” He’s right, though it’s a pretty long final stretch.<\/p>\n For the same reason you don’t take a brand-new engine to redline on the way home from the dealership. Slovensk\u00e9 elektr\u00e1rne plans to raise the reactor’s power in seven stops. At every stop, its engineers put the reactor, the turbines, the generators and the safety systems through dozens of checks. The regulator then reviews the results and decides whether they can move up to the next level. The first stop, at 5 percent, is basically a systems check. Nothing goes out to the grid yet.<\/p>\n The 20 percent stop matters most if you’re waiting on electricity. At that point, there’s enough steam coming out of the steam generators to spin up the turbine generators and synchronize them with the Slovak grid, and the first megawatt-hours can start flowing out.<\/p>\n Synchronizing means getting a generator in step with the grid before you connect the two. The generator has to match the grid’s voltage and frequency (50 cycles a second in Europe, 60 in the US), because closing the breaker while they’re out of step can wreck equipment.<\/p>\n From there, the reactor climbs through four more stops between 35 and 90 percent for what the company calls dynamic tests, which look at how the plant’s systems behave together under different conditions. Then comes the final exam, a run of 144 hours straight at 100 percent (that’s six full days) to show the unit hits its design numbers.<\/p>\n Slovakia’s Nuclear Regulatory Authority<\/a> says the operator can’t even ask for an operating permit until every one of those tests is done, evaluated and written up. It’s a pretty slow way to switch on a power plant, and frankly, I wouldn’t want it done any other way.<\/p>\n Figures: Slovensk\u00e9 elektr\u00e1rne, including its published commissioning output data.<\/p>\n The chain reaction in a reactor like this one happens inside its pressure vessel, the thick steel container that holds the fuel and the water flowing through it. Slovensk\u00e9 elektr\u00e1rne’s specs put the vessel at about 38.7 feet tall and 11.6 feet across. \u0160koda JS, the Czech company that built both of the plant’s new reactors, made it. A vessel of this design weighs about 237 tons empty, going by figures the Czech nuclear regulator publishes for this model at the Dukovany plant.<\/p>\n That vessel’s where the reactor first went critical, at 10:10 p.m. on August 6. Critical sounds scary, but all it means is that the chain reaction’s keeping itself going, with each uranium atom that splits setting off about one more split. Every working reactor runs critical, all day, every day.<\/p>\n Running flat out, the reactor makes 1,375 megawatts of heat, and the plant turns that into 471 megawatts of electricity, a bit over a third. The water that carries the heat away is kept under so much pressure that it stays liquid at well over 500\u00b0F. It runs out to the unit’s steam generators, where it boils water in a separate loop that never touches the fuel. That steam spins the turbines.<\/p>\n Slovensk\u00e9 elektr\u00e1rne didn’t put a number on it. It doesn’t give dates or times for the tests either.<\/p>\n As for the valves, they’re basically pressure-relief valves for the steam generators, the same idea as the little weighted valve on top of a pressure cooker. If the pressure inside climbs too high, they pop open and dump steam until it drops. From the looks of it, testing them means making them do exactly that on purpose.<\/p>\n The company calls it a standard operation. It runs the same tests whenever one of its other reactors comes back from a scheduled refueling and maintenance outage, which every one of them goes through at least annually. Conventional power plants do them too, since the law requires regular checks of the safety devices on any pressure vessel.<\/p>\n And the steam in question comes from that separate loop, the water that never touches the fuel. Honestly, a loud valve test seems like a pretty reasonable thing to give the neighbors a heads-up about.<\/p>\n Reactors usually make the news for coming off the grid, like the two French reactors shut down by hand before jellyfish could clog their seawater screens<\/a>. Mochovce 4 hasn’t even gotten on yet.<\/p>\nWhy not go straight to 100 percent?<\/h2>\n
There’s a 39-foot steel vessel in the middle of all this<\/h2>\n
How loud is loud?<\/h2>\n
So when does the grid get its first megawatt?<\/h2>\n