feedmegin posted:

Biscuit, old chap. Cookies are a subset thereof but I doubt you meant Marylands. Tell me more

Lady Radia posted:

wait, just to clarify: the other two guys in the room also got slammed?

quote:

The Three Mile Island accident (TMI) is notable, not because of the immediate impact on human lives, but because of the profound effect it had on the field of safety science.

Before TMI, the prevailing theories of accidents was that they were because of issues like mechanical failures (e.g., bridge collapse, boiler explosion), unsafe operator practices, and mixing up physical controls (e.g., switch that lowers the landing gear looks similar to switch that lowers the flaps).

But TMI was different. It’s not that the operators were doing the wrong things, but rather that they did the right things based on their understanding of what was happening, but their understanding of what was happening, which was based on the information that they were getting from their instruments, didn’t match reality. As a result, the actions that they took contributed to the incident, even though they did what they were supposed to do. (For more on this, I recommend watching Richard Cook’s excellent lecture: It all started at TMI, 1979).

TMI led to a kind of Cambrian explosion of research into human error and its role in accidents. This is the beginning of the era where you see work from researchers such as Charles Perrow, Jens Rasmussen, James Reason, Don Norman, David Woods, and Erik Hollnagel.

TMI was significant because it was an event that could not be explained using existing theories.

quote:

The accident started when two secondary cooling system pumps stopped operating (probably because of a false control signal that was caused by leakage of perhaps a cup of water from seals in the condensate polisher system). With no secondary cooling system flow, the turbines shut down, and heat stopped being removed from the secondary system by the turbines and cooling tower, and therefore, heat stopped being removed from the primary system through the heat exchanger.

The emergency feedwater pumps activated, to remove the heat that was building up in the reactor, now that the secondary cooling system was no longer removing it through the turbines, cooling tower, and heat exchanger. These emergency pumps circulate water in the secondary cooling system, which boils off because the energy is not removed by the turbine, and draw in replacement water from the emergency water storage tank.

Both emergency feedwater pumps were operating against closed valves: they had been closed during maintenance two days earlier. The operator did verify that the pumps were operating, but did not know that they were accomplishing nothing because of the closed valves. One of the two indicator lights on the control panel that might have alerted them to the valves being closed was obscured by a repair tag hanging on the switch above it. It was only eight minutes later that this problem was discovered.

With no secondary circulation, no more heat was being removed from the reactor core, its temperature started to rise, and the automatic emergency shutdown procedure, known as a "scram," was started. This involves the rapid insertion of control rods whose composition includes a large percentage of neutron-absorbing materials. This absorbs most of the fission neutrons before they have a chance to initiate a new fission event, stopping the chain reaction. It does not immediately stop the release of heat in the reactor core. Because many of the fission products are unstable nuclei, with half-lives ranging from fractions of a second to days, heat continues to be released in a nuclear reactor core for quite some time after the chain reaction itself is stopped by the scram. Because no heat was being removed through the secondary coolant system, temperatures and therefore also pressures rose within the core and primary coolant system.

The Pilot Operated Relief Valve (PORV) is designed to valve off enough coolant from the primary system to keep pressures at safe levels. It initially opened because of the pressure rise resulting from the cooling failure. It was instructed to close, to keep the bubbles squeezed small. It did not close, and therefore the radioactive primary coolant continued to be drained into the sump and the bubbles in the core grew larger and larger as coolant turned to steam at the reduced pressures. Steam is much less effective at conducting heat away from the reactor fuel rods, so their temperatures rose even faster, reaching values that permitted them to resume fissioning.

As soon as the pressure had been adequately reduced, a signal was sent to the PORV to close again. The control panel included an indicator light that showed that this signal had been sent. Unfortunately, despite the indicator light showing that the valve was being told to close, it did not in fact close. The primary cooling system stayed open for 140 minutes, venting 32,000 gallons, one third of the core capacity, and keeping the pressure in the core at a much lower level than it would have been with the PORV properly seated.

The reduced pressure caused steam bubbles to form, with four results:

1. It reduced the effectiveness of the cooling wherever those bubbles were in direct contact with the fuel rods.
2. It impeded the flow of coolant through the core and the pipes.
3. The fuel rod temperatures rose.
4. The chain reaction resumed, releasing even more heat.

The High Pressure Injection (HPI) pumps activated (one automatically, one by operator intervention) to flood the core with cold water. Reactor containment vessels are made of steel, and in operation are exposed to large amount of radiation, especially neutrons. The steel becomes brittle with age, prone to shattering. The shock of HPI operation with its cold water is a risk to be traded off against the risk of letting the core heat up.

To reduce the risks of high pressure operation, and in particular, the risks of hydraulic shock waves traveling through the plumbing, the reactor is designed with a large surge tank. This tank, known as the "pressurizer," normally has its bottom half filled with water and its top half with steam. Heating or cooling the steam permits the pressure in the reactor core and primary cooling system to be controlled. Compression of the steam absorbs any hydraulic shocks that reach the pressurizer. If the pressurizer fills up with water, as it will if the steam all condenses, then hydraulic shock waves moving through the plumbing (caused, for example, by opening or closing of valves, or the starting or stopping of pumps) will not be absorbed, and may therefore cause pipes or pipe-joints to break.

In order to prevent this well-recognized risk to a safety system, the operators followed standard procedures and reduced High Pressure Injection when the pressurizer pressure indicator rose toward the point that would indicate that it was about to fill.

The reduced pressure also caused cavitation in the reactor coolant pumps, which could erode the moving metal parts of the pump, distributing fragments throughout the coolant (where they would destroy other pumps and valves in the system), so the reactor coolant pumps had to be shut down, further reducing coolant flow.

All four of these failures took place within the first thirteen seconds, and none of them are things the operators could have been reasonably expected to be aware of.

With the loss of one-third of the coolant and the sustained operation at reduced pressure but high temperature, the zirconium cladding of some of the fuel rods reacted with the water, oxidizing the zirconium and leaving free hydrogen behind. The hydrogen accumulated in the reactor, forming pockets that prevented water from reaching parts of the core to cool it.

In the reactor core, this hydrogen is inert, because there is no oxygen available to burn with it. During a loss of coolant incident, however, it is likely that some of the hydrogen will be carried out along with the coolant. It will then collect in the containment building, where it will have oxygen available. […] At TMI, the hydrogen-air explosion took place 33 hours into the accident and produced an over-pressure fully half of the design strength of the building.

Captain Foo posted:

China syndrome is a good movie

Captain Foo posted:

China syndrome is a good movie

rotor posted:

hollywoods demonization of nuclear power is largely why we're still stuck with coal.

Doom Mathematic posted:

I also partly blame The Simpsons.

Sagebrush posted:

MononcQc posted:

to be clear, the building (which was stronger than usual because it was near an airport and wanted to withstand a plane crashing into it) and the amount of redundant failsafes did their jobs, but the nature of the failure itself and its mechanisms are what baffled people and eroded trust in the existing safety models. The link above is an overview of Perrow’s book as written by a reviewer.

Perrow’s conclusions were that some distributed systems require tight coupling and complex interactions to work and control highly dynamic and complex situations. unfortunately these properties put together result in accidents being something we need to consider as “normal” — as an inevitable consequence of these systems existing and operating.

This does not mean in practice that nuclear power is more or less dangerous than other systems, but had a pivotal shift in the way safety sciences framed themselves and the type of activities they prioritized.

namlosh posted:

I know I found this video from these forums but didn’t see it posted in this thread yet. it’s one of the more viscerally disturbing and enlightening radiation vids I’ve ever seen. the lady’s probably an idiot, but glad she filmed finding a piece of the Chernobyl reactor fuel

https://youtu.be/ejZyDvtX85Y
second video confirms it via spectroscopy iirc

Eeyo posted:

idk, maybe i've just been exposed to that school of thought in the background during my education but the idea that accidents are kind of inevitable just as emergent properties of systems seems pretty natural/obvious. like sure i can believe that people believed you could make systems with inherent stability (and therefore accident proof?), but like that's got to be a very small class of systems.

quote:

It appears that there are enormous differences of opinion as to the probability of a failure with loss of vehicle and of human life. The estimates range from roughly 1 in 100 to 1 in 100,000. The higher figures come from the working engineers, and the very low figures from management.

distortion park posted:

Agreed, making what is visually an industrial accident + fire into a gripping 5 part series without completely walking over the facts is pretty impressive.

Beeftweeter posted:

the series is also super good at ratcheting up a sort of existential horror

namlosh posted:

glad she filmed finding a piece of the Chernobyl reactor fuel

quote:

I can count how many times I've visited Chernobyl on one hand, 7 times now.

distortion park posted:

I was browsing Wikipedia reading about shipbourne nuclear power and it's wild. USS enterprise had 8 nuclear reactors! Soviet subs had two each! Reading the stats they are lower power than plants for power generation but still around 100MW electrical output. How come the military can put reactors in a sub when it costs mega billions to build one on land? Are they just a lot less safe or something?

distortion park posted:

It's the naval vs land based cost difference I'm interested in. Here's some numbers from wikipedia

About $3 billion for a S9G with a submarine thrown in for free, 210MW but that's total power out, electric equivalent would be less.

About $30 billion for an expansion to an existing land based plant, producing just over 2000MW electricity.

Like I'd have assumed that a smaller powerplant that has to operate with minimal maintenance for long periods at sea would be dramatically more expensive than a large scale extension to an existing plant per MW, but that doesn't seem to be the case (at least it isn't obvious from the rough numbers here). The point about subs not normally operating anywhere near their full capacity perhaps explains some of it though.

E: Interesting article here with better researched numbers: https://constructionphysics.substack.com/p/why-are-nuclear-power-construction-c3c

fart simpson posted:

how’s that thorium reactor going in china? any news?