An in-depth article in this week's Rolling Stone - America's Nuclear Nightmare - author Jeff Goodell offered a prophetic warning about the actual dangers those 31 antiquated GE Mark I and II reactors still operating in the United States represent:
The Japanese disaster should have been a wake-up call for boosters of nuclear power. America has 31 aging reactors just like Fukushima, and it wouldn't take an earthquake or tsunami to push many of them to the brink of meltdown. A natural disaster may have triggered the crisis in Japan, but the real problem was that the plant lost power and was unable to keep its cooling systems running — a condition known as "station blackout."
Yesterday's record-breaking tornados in Alabama, in addition to causing tremendous death and destruction, also caused a 'station blackout' for all three reactors at TVA's Browns Ferry station near Athens, 84 miles north of Birmingham. No, the nukes weren't actually hit by the monster twisters that ravaged Birmingham and Tuscaloosa and killed nearly 300 people in several states. The blackout occurred when the storms knocked out transmission lines providing power to the reactors. TVA now estimates offsite power will not be restored for at least three days, possibly a week.
UPDATE: World Nuclear News reports that units 2 and 3 were brought to cold shutdown at 2:43 a.m. this morning, just over ten hours after shutdown. Unit-1, however, is still hot and TVA 'hopes' to get it to cold shutdown as well, though it doesn't say when. The event notification to NRC fell under the header of...
"when the normal and alternate power supplies for essential equipment were unavailable for more than 15 minutes." TVA stressed that "safety systems performed well."
What happens when a natural disaster or something so mundane as squirrels playing around on transmission lines cuts the off-site power to these venerable antiques? Goodell explains:
At U.S. reactors, power failures have been caused by culprits as mundane as squirrels playing on power lines. In the event of a blackout, operators have only a few hours to restore power before a meltdown begins. All nukes are equipped with backup diesel generators, as well as batteries. But at Fukushima, the diesel generators were swamped by floodwaters, and the batteries lasted a mere eight hours — not nearly long enough to get power restored and avert catastrophe. NRC standards do virtually nothing to prevent such a crisis here at home. Only 11 of America's nuclear reactors have batteries designed to supply power for up to eight hours, while the other 93 have batteries that last half that long.
When the lights went out yesterday at Browns Ferry, one of the backup diesel generators was down for maintenance, though power enough to scram the reactors was available via the other generators and batteries. Offsite power was partially restored later in the evening, hopefully to replace continuing coolant circulation power to the unit forced to rely on battery backup, but it may be some time before full power can be restored to spell the diesels.
TVA's spokesperson in Knoxville, Barbara Martocci, has done an admirable job of offering the usual couched doublespeak about the true situation at the plants, which carefully skirts the issue of time-delay between 'station blackout' and startup-to-power of the backup power generators and battery engagement to enable shut-down of the reactors:
Ms. Martocci said the TVA didn't know how long it would take to restore full power to the Browns Ferry plant. She said the plant needs "redundant" sources of power before they can turn it back on.
What Martocci is speaking of turning "back on" is NOT the generation capacities of the three reactors, but offsite power to the entire reservation. Which sort of makes me wonder how long that battery they're using is supposed to last. Still, we may hope that TVA workers won't do the same thing workers at Fukushima did when they 'forgot' to fuel the portable backup generator brought in when their battery power ran out.
At any rate, this provides a very excellent example of how U.S. nuclear power stations can be as vulnerable to blackouts caused by natural (and, conceivably, unnatural) disasters as are Japanese stations stupidly sited on tsunami-prone coastlines right next to the most active tectonic subduction zone on the planet. It doesn't take a super-quake or super-tsunami to knock out nukes. Some predictable springtime wind will do the trick, and the U.S. is famous for that sort of thing.
TVA did declare an "Unusual Event" to the NRC 30 minutes after the loss of power, the lowest of 4 emergency event classifications. And TVA reports that it was able to scram all three reactors once the battery and diesels took over to supply the necessary power. Yet there are probably people here who may wonder why the reactors weren't designed to automatically scram in the event of station blackout, given that loss of power to the systems means a total loss of cooling capacity when the pumps shut down and in such a situation continuing fission in the reactors means exponential heating that can start a meltdown in less than two minutes - as has actually occurred on occasion.
A PWR [Pressurized Water Reactor] has control rods that were designed to fall via the simple force of gravity when things go wrong (retrofit with drivers when it was learned that gravity doesn't always work). The three Browns Ferry reactors are identical to those melting at Fukushima right now. Their rods were not designed to 'drop' in the event of a loss of power. In fact, the control rods on these early GE models MUST have power to do their jobs - they feed upwards from the bottom of the reactor rather than fall from above. This makes the time lag between loss of power and availability of emergency power very important. I looked around for information sources and found quite a few, but it isn't clear whether or not the Browns Ferry units use a residual mechanical (turbine-based) means to insert the control rods in a LOOP [Loss Of Offsite Power] transient or must await the power-up of EDGs [Emergency Diesel Generators].
TVA assures us that all three reactors were able to scram and are now in "safe shutdown" mode, meaning there is power enough from the generators and partial offsite power restoration (the battery wouldn't last longer than 4 hours, far as I can tell from NRC requirements) to power ECCSs [Emergency Core Cooling Systems] and circulatory pumps. Presumably, TVA will be able to get diesel fuel to the site without trouble if the outage goes on for days, unlike poor Fukushima where the larger natural disaster made that impossible even if the diesels had not been washed away.
What they are carefully NOT talking about is whether any of the units suffered any damage during the ~30-40 seconds' worth of exponential heating between the total loss of power and availability of sufficient backup power to insert the control rods, if residual turbine power isn't used for that purpose. The temperature transient of an emergency scram from power is considerable in the 'best' of situations, after all. A still-fissioning core [see BWR Safety Systems, per the limiting case of ATWS - Anticipated Transient Without Scram] can reach 1000 ºF [540 ºC] in 18 seconds, 1200 ºF in 40 seconds [650 ºC]. Peak temperature by the time all the ECCS systems are full-on can go to 1200 ºC [2200 ºF] per NRC in 'hot spots' in the lower core before full cooling flow is restored.
Situations like this can release radioactive nobles and iodine, fission product gases that escape through even minute imperfections in the cladding welds if fuel overheats and swells. Certainly not much compared to Fukushima, but enough to elevate local levels. Hope someone's keeping independent track. This country's nuclear establishment is far more reluctant than the Japanese to let people know when it's good to take some kelp and/or stay indoors.
When factoring the chances of core damage for various kinds of events at nuclear plants, the NRC's evaluation looks like this -
In other words, for all causes of core damage events, station blackout accounts for 70% of the probabilities. Station blackout is the primary cause of the disaster ongoing at Fukushima. In August of 2003, a widespread grid failure here in the United States caused station blackouts at nine nuclear power plants at the same time. It didn't last as long as TVA expects the outage at Browns Ferry to be.