The Czech utility ČEZ spent Friday morning sharing some pretty good news with reporters. Its Temelín nuclear plant is switching to fuel from Westinghouse, the American reactor company, after eight years of analysis and testing, and its other nuclear plant, Dukovany, is lined up to follow once its own testing and licensing are done.
By that evening, one of Dukovany’s four reactors was on its way offline.
At 8 p.m. local time on October 30, operators started bringing the power down on unit 3. Its first turbine came off the grid at 12:42 a.m. on Saturday, October 31, and the second one followed at 2:06 a.m. That’s 7:42 and 9:06 p.m. Friday on the East Coast, if you were up watching the grid in Central Europe for some reason.
The cause is a small leak in one valve, which staff caught during a routine check, according to plant spokesperson Radek Doležal. He told the Czech news agency ČTK, in a report carried by oEnergetice, that the leak has no effect on nuclear safety, and the plant’s other three reactors kept running at 100%.
So why switch off a whole reactor over one small leak?
Some fixes can’t be done hot
The State Office for Nuclear Safety, or SÚJB, is basically the Czech version of the NRC. It said the unit would be disconnected from the grid and cooled down so the repair could be carried out, which tells you this isn’t a job anyone wanted to try with the reactor running.
Dukovany’s reactors are pressurized water reactors, a Soviet-designed model called the VVER-440. In a reactor like that, the water carrying heat away from the core is kept under enough pressure that it stays liquid well past the temperature where it’d normally boil. You don’t want anybody loosening fittings on plumbing in that state, so the plant brings the temperature and pressure down first.
Now, I can’t tell you exactly why this particular valve needs a cold reactor. The word ČEZ used is “armatura,” which in Czech power-plant speak covers valves and the hardware bolted around them. The company hasn’t said which valve it is, which loop it sits on, or how much leaked, and SÚJB’s notice doesn’t get into it either.
Frankly, this is about as boring as a leak story at a nuclear plant gets, and boring is what you want here. Somebody found a small problem during a routine check, and the plant took the unit offline to fix it rather than nursing it along until the next scheduled outage.
It’s also a slower, calmer kind of shutdown than the one we covered at an Illinois reactor that was shut off by hand at 4:31 in the morning in September. That one, Dresden 2, went from full power to zero in one move. Dukovany 3 took more than six hours to wind down, on a schedule the regulator had posted earlier that day.
There’s one quirk in the paperwork, though. SÚJB’s notice calls this a planned shutdown, while ČEZ and the Czech press call it unplanned. Both are kind of right. The power-down was planned in the sense that it was scheduled for 8 p.m. that Friday. It just wasn’t planned months ahead, the way refueling outages are.
Why there are two timestamps
A VVER-440 doesn’t feed one giant turbine. It feeds two, each with its own generator, and ČEZ lists each Dukovany unit at 512 megawatts across its pair. When a unit comes down, operators ease the reactor’s power off over several hours and take the turbines off the grid one at a time. Unit 3’s two came off 84 minutes apart.
We saw the same setup running the other direction in Slovakia last month, when a brand-new reactor from the same family sent its first electricity to the grid on one of its two turbines. Dukovany is the older cousin. Its four reactors came online between 1985 and 1987, so unit 3 is about 40 years old.
Unit 3 had only just gotten back from the shop
I’d guess the timing is what bugs ČEZ the most. Unit 3 was offline from July 17 to September 20 for its scheduled outage, when part of its fuel was swapped out and its equipment was inspected and serviced.
That was a longer outage than usual. Dukovany’s reactors normally run for about 15 months between outages and then come down for roughly a month, according to ČTK. This summer, though, unit 4 shut down alongside unit 3 in August so the plant could work on equipment the two units share.
Unit 3 went back on the grid on September 20. It lasted 41 days.
It’s basically the nuclear version of picking your car up after a big service and spotting a puddle under it in the driveway a few weeks later. Annoying, sure. Still better than not noticing the puddle.
Here’s what we don’t know. I can’t find anything from ČEZ or SÚJB connecting the leak to the summer work, and nobody has suggested a link. The timing may just be bad luck.
Sources: SÚJB, ČEZ, ČTK.
Dukovany does a lot of the Czech grid’s heavy lifting
The plant sits about 19 miles southeast of the town of Třebíč, and ČEZ says it covers roughly a fifth of the country’s electricity use. It produced 14.68 terawatt-hours last year, according to ČTK. That makes it pretty much the workhorse of the Czech grid, which probably explains why one unit going down for a week makes the national news there.
ČEZ expects to run the four units for up to 80 years from startup. Korea’s KHNP is set to build two new reactors next to them, and the first one is due to be finished in 2036.
If you’re keeping score, that’s a Soviet-designed reactor that’s eventually supposed to run on fuel from an American company, with Korean reactors going up next door. Westinghouse has already delivered the first fuel assemblies meant for Dukovany, ČTK reported on Friday. I’d argue that’s the most interesting thing going on at the site right now, even if it isn’t the one making headlines this week.
So when does unit 3 come back?
SÚJB expects unit 3 to return to service and reach full output on November 6. Doležal didn’t commit to that date, though. He said the length of the shutdown will depend on how long the repair takes and on the checks that follow it.
I wouldn’t treat November 6 as a hard date, and it doesn’t sound like ČEZ does either. Bringing a unit back from an outage to full power normally takes several days, with tests at each power level along the way, according to ČEZ. So if the valve job runs long, the restart probably slides with it.
The regulator, for its part, says it’s watching. “We are carefully supervising all activities,” SÚJB wrote in its notice.
SÚJB’s notice, posted October 30, still has Dukovany’s unit 3 back in operation and at full output on Friday, November 6.
