{"id":19628,"date":"2026-09-09T09:30:20","date_gmt":"2026-09-09T13:30:20","guid":{"rendered":"https:\/\/www.autonocion.com\/us\/?p=19628"},"modified":"2026-09-09T06:16:18","modified_gmt":"2026-09-09T10:16:18","slug":"illinois-reactor-rods-that-never-moved","status":"publish","type":"post","link":"https:\/\/www.autonocion.com\/us\/illinois-reactor-rods-that-never-moved\/","title":{"rendered":"An Illinois reactor that regulators just cleared to run until 2049 was shut off by hand at 4:31 in the morning because an instrument said its control rods were moving when they weren’t, half its protection system fired on its own, and it was still at zero five days later"},"content":{"rendered":"

American nuclear reactors have spent the last couple of years collecting extra decades. Utilities keep filing for 20-year extensions on plants that were supposed to be retired by now, regulators keep granting them, and the data center boom has made every megawatt that’s already built look like a bargain. Dresden, out in the farmland southwest of Chicago, got its turn last December, and Constellation<\/a> says the renewed paperwork runs Unit 2 to December 22, 2049 and Unit 3 to January 12, 2051.<\/p>\n

Then at 4:31 on Thursday morning, somebody in the Dresden control room shut Unit 2 off by hand.<\/p>\n

The reactor was fine. What broke, according to what Constellation told the Nuclear Regulatory Commission<\/a> a few hours later, was the gear watching it.<\/p>\n

Here’s the filing. Unit 2 was in Mode 1 at 100 percent power. The control room got an indication that control rods were moving, and the reactor protection system’s “A” channel went into a half scram. Both of those came from a nuclear instrumentation failure, the licensee said, and the crew answered by tripping the reactor manually. The trip wasn’t complex, and everything responded normally afterward.<\/p>\n

The plant settled into Mode 3, hot shutdown, with decay heat coming off through the main turbine bypass valves. Unit 3 never noticed. The NRC logged it as Event Number 58436 under the four-hour rule covering a protection system actuation with the reactor critical.<\/p>\n

So what’s a half scram?<\/h2>\n

A scram is the emergency shutdown, the one where every control rod goes into the core at once and the chain reaction stops. You can’t get one out of a General Electric boiling water reactor without agreement from two independent trip systems. NRC training material on the GE design says it flatly: both systems, A and B, have to be in the trip condition to produce a full scram, and one of them tripping alone gives you what everybody calls a half scram.<\/p>\n

A half scram doesn’t shut anything down. The reactor’s still making power at 100 percent, and the control room is looking at a board telling them half the shutdown logic has already fired with nothing real behind it. That’s an uncomfortable board to stand in front of at 4:31 in the morning, and I’d have pushed the button too.<\/p>\n

So what counts as a nuclear instrument?<\/h2>\n

You’d assume a reactor gets measured from the outside, the way you’d check an engine with a gauge on the dash. It doesn’t. A boiling water reactor is full of small neutron detectors sitting down among the fuel, and NRC training material for the GE design describes how each averaging channel pulls in as many as 17 of them to work out how hard the core is running. Those channels feed the protection system directly, and any single one of them calling for a scram gives you a half scram by itself. So a detector or an amplifier going bad is enough to put half the shutdown system into the trip condition while the core underneath it behaves perfectly.<\/p>\n

Constellation’s notification doesn’t name the instrument that failed, and it doesn’t explain how a nuclear instrumentation failure ends up presenting as control rod movement. I can’t find a follow-up filing that spells it out, and I wouldn’t expect one this week. The NRC’s resident inspector was notified, which is routine, and inspection work like that generally surfaces in a quarterly report months down the line.<\/p>\n

\n
\n
Began operating<\/div>\n
1970<\/div>\n
General Electric Type 3 boiling water reactor, wet Mark I containment.<\/div>\n<\/div>\n
\n
Licensed thermal power<\/div>\n
2,957 MWt<\/div>\n
Heat the core is licensed to make, per the NRC operating license.<\/div>\n<\/div>\n
\n
TARGET<\/div>\n
Licensed through<\/div>\n
Dec 22, 2049<\/div>\n
Second license renewal, approved by the NRC on December 16, 2025.<\/div>\n<\/div>\n
\n
Power on September 8<\/div>\n
0%<\/div>\n
NRC daily status report, five days after the trip. Unit 3 was at 100.<\/div>\n<\/div>\n<\/div>\n

Dresden 2 is a 1970 machine with a Mark I containment<\/h2>\n

The NRC’s page for the unit<\/a> lists a General Electric Type 3 boiling water reactor licensed for 2,957 megawatts of thermal power, with a wet Mark I containment, at Morris, Illinois, 23 miles southwest of Joliet. The station sits on the south bank of the Illinois River where the Des Plaines and the Kankakee run together. Unit 2 began operating in 1970, which puts it among the oldest reactors still turning in this country. The Energy Information Administration pegged the average age of the American fleet at about 44 years as of March 2026.<\/p>\n

The Mark I is the containment design used at Fukushima Daiichi, and Unit 1 over there was a BWR\/3 with a Mark I, so it’s the same reactor generation and the same containment as Dresden 2. That’s a fact about design lineage, and it isn’t a claim about this event. It’s frankly the first thing anybody asks about a 1970 BWR, so it’s better said out loud than left hanging. Constellation told the NRC there was no impact on the health and safety of the public or plant personnel, the trip was uncomplicated, and the unit’s been sitting quietly in hot shutdown since.<\/p>\n

Dresden wears its age pretty well, honestly. The retired unit next door was the first full-scale privately financed nuclear plant in America, it started up in 1960, it shut down in 1978, and it’s a designated Nuclear Historic Landmark. You’ll see all three of them in photos of the site.<\/p>\n

Unit 3 has been at full power the whole time<\/h2>\n

Dresden runs its two reactors about as close together as two reactors get, and only one of them tripped. The NRC’s daily status report<\/a> for Tuesday, September 8 puts Unit 3 at 100 percent and Unit 2 at zero, five days on. Five of the 95 units on that report were at zero, and Dresden 2 was one of them.<\/p>\n

There’s a useful comparison a few weeks back. When three control rods dropped into Saint Lucie Unit 1 in Florida<\/a> on August 13 and the crew tripped that reactor by hand at 9:47 in the morning, the Florida plant came back fast enough that both its units are at 100 percent on this same September 8 report. Constellation doesn’t publish restart schedules unit by unit, so that daily report is what you’ve got.<\/p>\n

One thing to keep separate, because the two sit on the same NRC event page and that’s an easy conflation to make. On September 2, Westinghouse told the agency that springs inside a batch of nuclear plant breakers got acrylic paint instead of dye<\/a>, and 13 plants have to go check their switchgear over it. Dresden isn’t on that list, and the paint has nothing to do with the instrument that failed in Illinois.<\/p>\n

Constellation’s page for the plant says the second renewal came through on December 16, 2025, and that Unit 2 is licensed until December 22, 2049. On the NRC’s status report for September 8, 2026, that reactor was making zero percent power.<\/p>\n","protected":false},"excerpt":{"rendered":"

American nuclear reactors have spent the last couple of years collecting extra decades. Utilities keep filing for 20-year extensions on … <\/p>\n

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