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25th Anniversary Of Three Mile Island
fbform writes "March 28, 2004 is the 25th anniversary of the Loss Of Coolant Accident (LOCA) at the nuclear power plant on Three Mile Island, Pennsylvania. It's a good time to reflect on the impact it has had on our nuclear safety policy and interface design in general."
...is seeing how the fuck these accidents actually happen. Both Chernobyl and TMI seem to be based on a ridiculous chain of events fuelled by unfortunate coincidence, fallible mensuration equipment and human idiocy.
For instance, at TMI, there was a massive chain of events going like this (I'm taking this from the Wikipedia article). If any of these steps were omitted an accident never would've happened:
1. "The plant's main feedwater pumps in the secondary non-nuclear cooling system failed at about 4:00 a.m. on March 28, 1979"
2. "This failure was due to either a mechanical or electrical failure and prevented the steam generators from removing heat."
3. "First the turbine, then the nuclear reactor automatically shut down. Immediately, the pressure in the primary system (the nuclear portion of the plant) began to increase."
4. "to prevent that pressure from becoming excessive, the pressurizer relief valve (a valve located at the top of the pressurizer) opened."
5. "The valve should have closed when the pressure decreased by a certain amount, but it did not. Signals available to the operator failed to show that the valve was still open. As a result, the stuck-open valve caused the pressure to continue to decrease in the system."
6. "Meanwhile, another problem appeared elsewhere in the plant. The emergency feedwater system (backup to main feedwater) was tested 42 hours prior to the accident. As part of the test, a valve is closed and then reopened at the end of the test. But this time, through either an administrative or human error, the valve was not reopened -- preventing the emergency feedwater system from functioning."
7. "As the system pressure in the primary system continued to decrease, voids (areas where no water is present) began to form in portions of the system other than the pressurizer."
8. "Because of these voids, the water in the system was redistributed and the pressurizer became full of water."
9. "The level indicator, which tells the operator the amount of coolant capable of heat removal, incorrectly indicated the system was full of water."
10. "Thus, the operator stopped adding water. He was unaware that, because of the stuck valve, the indicator could, and in this instance did, provide false readings."
And so on and so forth. This is terrific shit. Seeing how many stages the thing went through just makes me glad this happened somewhere other than the decomposing USSR. With better engineering of measurement tools the whole thing would never have happened.
The thing which I can not fathom about the American nuclear power policy is that they are encouraged to make HUGE reactors. (Had to look this up for nuclear physics class at one point) The US Navy has an almost perfect record with identical, small reactors. I conject that the safety part of the equation has been figured out. I persistantly wonder why it's a bad thing not to just use the design from a submarine and just put 12 of them in a row, all of the same design, and man them with ex-Navy personnel.
;P
At this point, I'd put a dog on a treadmill generator to not have coal power though...or an ignorance-rutting politician.
--degs at 68k dot org
In all seriousness, if anybody has any questions they'd like me to pass on I'd be more than willing to. I'll post the answers here or in a JE or somewhere.
Triv
What do you mean "could"?
In terms of lives lost, damage done, or just about any other measure you care to name, provided you restrict yourself to a competent design, nuclear fission is ALREADY the safest power generation technology known to man. Read "The Health Hazards of NOT Going Nuclear" by Dr. Petr Beckmann.
The key phrase in that sentence is "competent design." One of the key parameters in any nuclear reactor design is the void coefficient, and, most particularly, the sign of the void coefficient.
From http://www.nrc.gov/reading-rm/basic-ref/glossary/
From http://www.disenchanted.com/dis/lookup.html?node=
Briefly, if a reactor is designed with a positive void coefficient, it will inherently have a risk of a Chernobyl-style thermal runaway. If a reactor is designed with a negative void coefficient, it will not have that particular hazard. This fact was known to the Soviet reactor designers, who designed the RBMK reactor at Chernobyl (among other places), and was also known the US designers who wrote the US standards for reactor design. Positive void coefficient designs are flat-out illegal in the United States.
To do the safety analysis, you have to take, for example, black lung deaths of coal miners into account, and supertanker oil spill environmental damage. You also have to take into account the number of people who will, while attempting to install solar water heating panels on their roofs, will slip, fall, and break their necks.
If you want to prattle about radiation hazards, bear in mind that every lump of coal you burn, every drop of oil, every cubic foot of natural gas, contains some amount of radioactive carbon-14, and the ash (and emitted CO2) is thus radioactive waste. Ditto for wood. (Wood smoke contains other nasty things.)
I used to work in the radiation safety field and went on a technical tour of TMI just before the change in owners (current owner is AmerGen).
We were able to visit some aspects of the non-functioning side - the cooling towers (I have photos I took while standing inside one, and here's another), the empty turbine room, and the control room.
Surprisingly standing around the skeletons of the non-functioning cooling towers wasn't nearly as strange as comparing the turbine rooms between the functioning and non-functioning sides of the plant.
Anyone who has seen a turbine room in any kind of large power plant knows how huge they are. The turbine room used for the functioning reactor was hot, noisy, and full of intimidatingly large equipment. The huge emptiness of the unused turbine room was just plain strange in comparison.
IMNSHO, the worst thing about the TMI accident was the lack of communication both inside and outside of the plant. We can only hope that we've learned from our mistakes.
Chernobyl was a graphite-moderated reactor, which means that the fast neutrons were slowed by bouncing off the carbon atoms.
An interesting thing about water is that it has two effects in fission reactors:
1. It acts as a moderator (bouncing neutrons off the hydrogen atoms of water molecules is one of the best ways to slow a neutron down).
2. Water also acts as a poison to the chain reaction. The hydrogen atoms do have an affinity to sucking up neutrons and turning themselves into deuterium and tritium. This effect causes the fission chain reaction to peter out.
Which effect predominates depends on the physical geometry of the core and the layout of fuel, water, control rods, graphite, whatever else is in the core.
At TMI the moderation effect of water predominated, at Chernobyl the poison effect.
This means that at Chernobyl the primary coolant acted as a poison to the chain reaction - so remove the coolant and the nuclear reactions run amok - not an explosion, but all kinds of bad stuff. And that "bad stuff" includes, IIRC, a phase transformation of the graphite at a really high temperature that releases a lot of energy.
Conversely, at TMI when the core lost its coolant fission stopped and only decay heat from the radioactive decay of fission products remained - more than an order of magnitude less than rated reactor peak power depending on power history of the reactor (i.e., if the reactor has been running at 100% power for a few weeks, decay heat production is maxed at about 7% of full power, and decays rapidly)
But the loss of coolant at Chernobyl and resultant runaway nuclear reactions caused a steam explosion of the remaining coolant in the core that severed all emergency coolant connections into the core (and kill everyone in the reactor building itself, IIRC). This steam explosion probably would not have breached any containment vessel, but the later energy release from the graphite and the fires almost certainly would have anyway.
And Chernobyl was all caused by dumbasses shutting down the reactor protective systems designed to prevent them from running the reactor in such a condition. Chernobyl had safety features to prevent operation in the range where the disaster that happened would be possible (which was actually highly dependent on power history since the radioactive fission products also have a huge effect on fission in the core [ iodine-136, IIRC]), but since the engineers had a test they just had to perform even though the reactor hadn't been shutdown for a few days like it was supposed to be, they simply shut down the system that was designed to prevent the reactor from going kaboom.