If you’re feeling good because the Japanese government is reporting that the Steel Reactor Pressure Vessel is intact, understand they’re lying to you, or stupid (or both).
When a core melts down, the nuclear material and the control rods and the supporting structures all fall to the bottom of the containment vessel – if you have ever used a pressure canner, that’s a pretty good analogy. Molten uranium would be about 2,700 celsius – or abouy 4,500 degrees F… Steel melts around 2,700 F
so the mass of moltem uranium will fairly quickly melt the steel at the bottom of the reactor vessel.
So the containment building keeps the radioactivity safely inside the building as the mess eventually cools – except the containment building has blown up, so the radioactive material now is probably exposed right to the outside elements… The Japanese “Plan” is to flood the reactor vessel with sea water to two weeks to cool it down. The problem with that plan is the reactor vessel probably no longer has a bottom.
So what happens when you take sea water with salt and other ocean minerals and start pumping in onto a big pool of molten radioactive metal that is half the temperature of the surface of the sun? It won’t be pretty.
The NY Times has a very helpful interacive Graphic Here
It introduces yet another issue. They’ve been storing the spent nuclear fuel inside the containment building that blew up right next to the reactor containment vessel (at least the roof blew off). There is a big tank under the reactor high filled with water. The Times suggests the explosion could have been from steam generated in that area (which would happen if the fuel started falling into the water)
Well, they’re really gonna be steamed when they find out….
It all depends on whether what the core is maintained in what is called a “coolable geometry”. The core doesn’t sit at the bottom of the reactor vessel. It’s in the middle, and in a BWR there are also a bunch of instrument tubes that penetrate through the bottom of the vessel. Underneath the active core is a large plenum space that is normally filled with water, along with a massive steel support structure that holds up the core. The core doesn’t touch the sides of the reactor vessel. All around the outer edge of the vessel is a fairly large annulus that allows cooling water to travel down to the bottom of the vessel and then up through the core.
As long as the majority of the fuel assemblies have not collapsed into an uncoolable ball of molten uranium (and everything seems to indicate that hasn’t happened yet), they should be okay with the seawater plan. They can gradually introduce seawater into the outer annulus (downcomer) and it will flow through the core from the bottom up. Baby-steps are required. A BWR core has low power density and flow-through wet steam cooling can do the job as long as you keep the steam flowing. The worst thing they can do is instantly flood the hot core with cold seawater. Obviously the reactor will never operate again due to the corrosive action of seawater on stainless steel, but we all knew that when the containment roof blew off.
When robot cameras examined the extent of the TMI-2 meltdown engineers were hugely surprised at the extent of the core damage. Evidently the fuel cladding shattered like hot glass when operators finally got around to turning the ECCS pumps back on. But even so the meltdown never damaged the reactor vessel and the unit was eventually cleaned up and restarted years later.
Are you writing this from your knowledge of the subject or is this copy/paste material?
Good question, and thanks for asking. No, none of this is copy/paste material. I am a nuclear engineer with 32 years experience in safety analysis. In 1980-81 I worked on the team that performed analyses in defense of Babcock and Wilcox when they were sued by GPU, the owners of TMI-2. The GPU courtroom presentation went on for weeks. When B&W started making their presentation, GPU withdrew the case after the first day.
I obviously admit to bias in favor of nuclear power, but I’m as PO’d as anyone that the problems in Japan have gotten as far as they have without resolution.
McGuire is the plant that I toured while it was under construction – it is a Westinghouse PWR. That employee training was my frame of reference for how nuclear power plants work.
Since your post, I have read up on the difference between PWR and BWRs and talked some more with my cousin, so I now understand better how BWRs are controlled.
Your information was helpful, but I immediately doubted it based on the TMI information.
The positive outcome would be that this causes acceleration of replacing aging BWRs with ABWRs or develepment of reactors like are not capable of melting their fuel.
The GE-Hitatchi ABWR description I read suggests they are still vulnerable to losing both the external and onsite power at the same time… They just also add in a gas powered turbine to back up the backup power.
My working theory about root cause (with no evidence yet) is the diesel fuel either hasn’t been rotated out or was contaminated. They’re saying the tsunami hit about 8 minutes after the quake, but the plant didn’t go dark for an hour.
So either they were on backup power for 50 minutes which then failed, or the didn’t lose their primary power until then as the grid collapsed. Perhaps someone in Tokyo decided to cut power to the quake zone, knocking them offline.
Fukusuki #1 reactor #1 was built in the late 1960s, and went online in 1971. Ironically, it was to be shut down in about a week – I believe it was going to be decomissioned at that point, not just annual maintenance. The Fukushima #1 facilitity has 2 more reactors being built (#7 and #8) that are current safety designs – the advanced boiling water reactor has the ability to terminate the accident even if there is loss of all pumping.
The loss of the onsite generation capability is what makes this so bad – the “battery power” to last 8 hours was probably the motorgenerators in the control room – since there are 6 reactors, my guess is all the reactors are controllled from a single control room facility… but keeping power on for the computers, gauges and phone systems doesn’t mean they had power to make a water pump push with enough pressure to overcome the pressure inside the containment vessel.
The thing about the fire truck pumping water in isn’t very clear yet (nor how they are injecting boric acid via a fire truck). Variations on what they might be doing:
Pumping water through the power generator piping, – that would do next to nothing if the core is uncovered…
Pumping water into containment vessel… they’re talking it will take days to pump in enough water to fill the thing – if that much water has already evaporated, it is possible that the water will boil off faster than the fire truck can pump it in. The reactor vessel is going to fill from the bottom, and water underneath the fuel rods is not going to cool them. Cold water contacting superhot fuel rods will just cause them to shatter.
The third possibility (which does not sound like what they are doing – yet)… is pumping water into the base of the containment building – on the outside of the reactor vessel.
I think you have two pieces about TMI combined. Unit #1 was not part of the accident, and it is still operating. Unit #2 was the one that had the partial meltdown. It’s state is the core was removed, but the reactor vessel is still in place sitting empty. They are currently in the process of removing the power generation unit and moving it to a different power plant.
TMI was bad, but it was brought under control in time. They didn’t lose external power and had complete control of the plant – IF they had known what was going on and what to do. The containment building never was in serious danger – the main concern (by the politicians) was that the hydrogen would create an explosion that might rupture the containment vessel. The computer models suggested the hydrogen would resorb back into the water and there was no danger, which is what happened. They did vent a small amount of radioactive steam because the politicians overrode the scientists (if you'[re a conspiracy person – maybe they did that to scuttle nuclear power that they opposed…)
You are correct about TMI-2 not being in operation currently. There was a lot of industry discussion during the last 10 years about bringing it back it back into operation, and I thought they had. But a check of the NRC website shows that it is currently decommission. I knew that unit-1 has been back in operation for some time now.
We actually looked at the “sabotage” angle during the lawsuit between GPU and B&W over TMI. But the data didn’t support that. It showed that the plant operators (a) didn’t know what they were doing for the transient, and they were overwhelmed by too many nuisance alarms.
Latest thing on the news is they think the rods were exposed for a bit more than two hours.