Question about "Worst Case Scenario Meltdown"

the nukers keep saying everything is ok.... because the containment vessels "should" capture the melted fuel and contain it.

what exactly happens if that part fails? any predictions greatly appreciated.

Until hit hit ground watter where it would create a geyser of steam that would last until the fuel was exhausted.
but that is just my uneducated guess.

What also happens is that the ground water is contaminated and the contamination will spread: leaving a large are of uninhabitable useless land for millions of years.

Example of half life: http://en.wikipedia.org/wiki/Plutonium

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support nuclear energy on DU.

Fuel melting can cover a wide array of outcomes. At TMI, the fuel ended up looking like. Blob of molten steel, but at TMI, the vessel integrity was maintained, releases were low, and the sister Unit at he site is operational today. Worst case, with the core completely uncovered, the outcome would not be as good. Chernobyl is not a likely outcome... there, they had a prompt critical where the power of the reactor blew the vessel apart... The Japanese units have SCRAM'd and the nuclear reaction is shut down. If the core does melt, boron is being injected to maintain he shutdown condition throughout the core. The answer to your question about site dose rates... If the core is uncovered, dose rates would be very high and a risk to Operators of all site plants. If they couldn't maintain at least 2/3rd core coverage, then they would have to uncover it to prevent a high pressure excursion that could damage the vessel and containment. It sounds like they have restored power to the normal condensate/ feed water system to inject sea water. The reason to go ahead and do that (basically, at his point the Unit is destroyed, but that is probably the case anyway with a collapsed RX building and some failed fuel) is to keep the site dose rates down and maintain the integrity of the RX vessel. What they are doing has never been done, but has been in GE Mark I emergency plans since TMI. To be doing that, they must have gotten power to their normal feed water system... They have been busy... it isn't part of the usual design. My guess is that they are working small miracles. There are also reports of another Unit with a coolant leak. This is a normal kind of emergency we all trained for, but they lost most of their emergency systems when they lost their emergency generators. This is the Unit that has probably had to release radioactive steam, because their heat sink tank is over-heating. I don't believe this would pose a large risk to the Operators. They have systems and equipment to cope with it. This is why the populous was evacuated. The end game for them is to restore power to their emergency systems and shut the plant down to cold shutdown. Existing high pressure systems can cope, but they are gradually over- heating. They need to get suppression pool cooling or low pressure injection systems back to terminate the General Emergecy and prevent extensive daage he he reactors. It could easily take 3 or 4 days to do that... It isn't as simple as plugging in a portable generator. Their are a lot of complicated electrical circuits to pur through. Reminds me of Apollo 13. I am sure they have the very best people on it. I am more optimistic than I was initially that they will work through the problems. If they do with no major radIologocal release, then I would like to shake he crews hand. They should be treated as heroes.

in a nutshell, when fissile U-235 breaks into two pieces and releases energy, the two pieces are unstable - they have too many neutrons for their new size. So the neutrons decay, emitting beta and gamma rays.

The fission products are normally isolated from the environment by three barriers: the zirconium cladding around the fuel elements (which are uranium oxide pellets), then the reactor vessel itself, then the steel and reinforced concrete containment structure.

If water cooling inside the reactor is lost, the reactor will be shut down instantly by inserting the control rods, which shut down the chain reaction. The fission of fuel ceases instantly. The problem, however, is this: the existing fission products contained in the fuel continue to release a substantial amount of heat, and with no heat transfer medium to carry that heat away, the zirconium cladding will melt, releasing the uranium dioxide fuel pellets and the fission products (which are both solid and gaseous) into the reactor coolant system. If the reactor coolant system has been breached, then these products will be released into the reactor building itself.

From here: http://www.physicsdaily.com/physics/Nuclear_meltdown

This is the worst case scenario. So you see, if the core completely melts down, we will all know and we won't need any agency to tell us.

nice rundown

They may have the vessel full of water (BWR's are usually about 2/3rd full of water) and relief valves open to run the water into the primary containment. This might be necessary if the torus water has heated up over the last day and they can't cool it (can't w/o emergency power). The torus is the normal heat sink for pressure control. They have to fill the torus (a huge steel donut shaped tank below in vessel) before they can fill the metal containment around the RX vessel. Yes, they are probably injecting water into the RX to fill the torus which will fill and then flood the upside down light bulb shaped drywell containment.. To do this very quickly, they probably have power to the ordinary condensate system. They would have to have gotten power to the site. The emergency systems were probably damaged beyond quick repair when the roof fell in. In this scenario, you would not have to drain the RX vessel. Severe core damage averted... THAT is the problem! They can't cool the torus... that explains why the radiation levels have been fairly low. The core is still covered. If they don't flood the containment, the torus will overheat, over-pressurize, and you will eventually lose primary containment. Flooding the vessel and containment with sea water totals the plant, but probably avoids a serious radiological release. I think they can do it, if I am right about the circumstances in the plant. By the way, the other plants need containment cooling to, but they are different designs. The plant with the collapsed RX building is more urgent, because returning torus cooling to service any time soon was probably not possible.

as a last resort you fill the primary containment with water... We had river water, they have sea water... and you drain the RX vessel. Draining the core gives it air/steam cooling but the core would lose its geometry and you would have massive cladding failure of the fuel... but that is the only remaining way to depressurize the RX vessel. What you don't want is a high pressure rupture of the RX vessel to destroy primary containment and spread the radioactive material. The fuel may melt through the vessel, but it most likely wouldn't through the primary containment. If the reports of flooding the containment with sea water is true, I think this is what they are doing or going to do. This would be worse than TMI, which melted fuel without any loss of vessel integrity... And they were able to reflood their core once they realized that it had been uncovered. Expect radiation levels to rise dramatically if the core is uncovered. The question I don't know the answer to is the status of the other Units... When will they be in cold shutdown? They need to get them to cold shutdown in case the control rooms becomes uninhabitable.

We really don't know for sure. Too much depends on the details of the internal state of the reactor pressure vessels, which is obviously unobtainable.

Worst case scenario ... if the core slags, it eventually melts a hole through the floor of the pressure vessel and drips onto a area of the containment building floor intended to catch molten core.

The core material should spread out, which is necessary to prevent enough material from accumulating in a small enough volume to start a low rate chain reaction. Really, there should be no possibility of THAT occurring.

What happens next ... this stuff is obviously hot. Very hot. Hot enough to melt through some of the toughest steel ever manufactured. The floor has been designed to dissipate that heat and contain it until the decay products inside the core material stop decaying and producing heat. But it will erode slowly under the thermal load.

My concern is that we have no certainty that the containment floor structure is still up to spec. It has been through an 8.9 earthquake and many strong after shocks. If its structural integrity has been compromised ... the core material could penetrate the previously impenetrable containment floor.

What happens next would then be determined by local geology, which I know nothing about. If the molten core hits the water table, a major release of radioactive material could and probably would occur.

Now, that is really, really bad. Horrible. But Chernobyl was much, much worse than we are talking about here. That does not mean I think the worst case scenario is acceptable. "Not as bad as Chernobyl" does not automatically produce a judgment that "so nuclear power is OK". That's sort of like saying "Hurricane Katrina wasn't so bad because Haiti was so much worse."

This is a bad situation. It is far from "ok".

I'm what you condescendingly call a "nuker". (You can ask Bananas.)

So there you have it.

By the way, please don't use the word "nuker" anymore. It isn't just a minced call-out, it sounds a lot like the word racists call the black folks.

--d!

fucking had it with you guys, plz seek help

And I know there are corporate pro nuclear shills etc. but I accept that there are also some intelligent sincere people who support nuclear power (which inlcudes some but not all of those writing with that positgion here).

I just think they are wrong :) Seriously, people have always had a tendency to underestimate the unpredictability and likelyhood of adverse circumstances and the potential for "previously unforseen events". Humans also seem to have an instinctive need to believe that they we can tassert control over nature and our own fates. We notoriously overestimate our power and abilities to control to our satisfaction scenarios whose outcomes are important to us.

In doing risk assessment, humans tend to subjectively bias against unlikely variables that could complicate matters, to the extent that in some cases they are simply dismissed and not taken into account. For example here in the USA the Nuclear Regulatory Commission simply refused to consider whether nuclear plant security could withstand an attack by a large group of terrorists with state of the art weapons - choosing instead to assume smaller less heavily armed forces would conduct a potential assault. To may observers it seems like they decided on a level of threat to assess that would be managable by utilites employing the level of security that utilities claim they can afford to employ.

Acts of intentional sabatague are rarely factored in fully, especially those by deranged individuals who might suddenly snap as opposed to those with an ideological agenda. Human greed is always insufficiently accounted for. What good are inspections when inspectors can sometimes be bought off?

It is the awful severity of the worst case scenarios involving nuclear energy that should make it beyond the pale. We show fate tempting arrogance when we presume they can always be headed off with good planning.

http://www.tepco.co.jp/en/press/corp-com/release/11031312-e.html

Press Release (Mar 13,2011)
Plant Status of Fukushima Daiichi Nuclear Power Station (as of 9pm March 13th)


All 6 units of Fukushima Daiichi Nuclear Power Station have been shut down.



Unit 1(Shut down)
- Reactor has been shut down. However, the unit is under inspection due
to the explosive sound and white smoke that was confirmed after the big
quake occurred at 3:36PM.
- We have been injecting sea water and boric acid which absorbs neutron
into the reactor pressure vessel.

Unit 2(Shut down)
- Reactor has been shut down and Reactor Core Isolation Cooling System
has been injecting water to the reactor. Current reactor water level is
lower than normal level, but the water level is steady. After fully
securing safety, measures to lowering the pressure of reactor
containment vessel has been taken, under the instruction of
the national government.

Unit 3(Shut down)
- Reactor has been shut down. However, as High Pressure Core Injection
System has been automatically shut down and water injection to
the reactor was interrupted, following the instruction by
the government and with fully securing safety, steps to lowering
the pressure of reactor containment vessel has been taken. Spraying
in order to lower pressure level within the reactor containment vessel
has been cancelled.
- After that, safety relief valve has been opened manually, lowering
the pressure level of the reactor, which was immediately followed
by injection of boric acid water which absorbs neutron, into
the reactor pressure vessel.

Unit 4 (shut down due to regular inspection)
- Reactor has been shut down and sufficient level of reactor coolant
to ensure safety is maintained.
- Currently, we do not believe there is any reactor coolant leakage
inside the reactor containment vessel.

Unit 5 (outage due to regular inspection)
- Reactor has been shut down and sufficient level of reactor coolant
to ensure safety is maintained.
- Currently, we do not believe there is any reactor coolant leakage
inside the reactor containment vessel.

Unit 6 (outage due to regular inspection)
- Reactor has been shut down and sufficient level of reactor coolant
to ensure safety is maintained.
- Currently, we do not believe there is any reactor coolant leakage
inside the reactor containment vessel.

Casualty
- 2 workers of cooperative firm were injured at the occurrence of
the earthquake, and were transported to the hospital.
- 1 TEPCO employee who was not able to stand by his own with his hand
holding left chest was transported to the hospital by an ambulance.
- 1 subcontract worker at important earthquake-proof building was
unconscious and transported to the hospital by an ambulance.
- The radiation exposure of 1 TEPCO employee, who was working inside
the reactor building, exceeded 100mSv and was transported to
the hospital.
- 2 TEPCO employees felt bad during their operation in the central
control rooms of Unit 1 and 2 while wearing full masks, and were
transferred to Fukushima Daini Power Station for consultation with
a medical advisor.
- 4 workers were injured and transported to the hospital after explosive
sound and white smoke were confirmed around the Unit 1.
- Presence of 2 TEPCO employees at the site are not confirmed

Others
- We are currently coordinating with the relevant authorities and
departments as to how to secure the cooling water to cool down
the water in the spent nuclear fuel pool.
- We measured radioactive materials inside of the nuclear power station
area (outdoor) by monitoring car and confirmed that radioactive
materials level is higher than ordinary level. Also, the level
at monitoring post is higher than ordinary level. We will continue
to monitor in detail the possibility of radioactive material being
discharged from exhaust stack or discharge canal. The national
government has instructed evacuation for those local residents within
20km radius of the periphery because it's possible that radioactive
materials are discharged.

- We will continue to take all measures to restore the security of the
site and to monitor the environment of the site periphery.

http://www.tepco.co.jp/en/press/corp-com/release/11031312-e.html

I would suggest that many parts of Japan could simply be "Chernobilized", that is uninhabitable. Major population shifts to other parts of Japan or other countries.

is pretty bad indeed, but probably would not be nearly so bad as Chernobyl in that the contamination area would be much more localized.

I think the worst case scenario involves a cascade. We now have least 5, probably 6 unstable reactors within 10 miles of each other. Three are right next to each other. It seems as if they are sure that two of those have had exposed fuel rods. Water levels in at least one (reactor number 2) aren't responding to pumping efforts like they should. I haven't seen an update on 3 at Daiichi that said the water level was rising. They've put boric acid in reactors 1 and 3, but I don't know if it is enough to stop the reaction yet, and they still have to maintain water levels for a while. Plus there's the spent fuel, etc.

So if one real failure occurs here, and makes it dangerous to work close to the source, it seems likely that a waterfall of failures might result. Yes, if a reactor shell breaches the fuel probably will fall through and be contained in the containment shell, but they would have to keep venting highly radioactive steam to maintain that containment, and pumping water in there, and pressures would be very high. I'm guessing that there would a valve failure or something like that. Hydrogen definitely, and you'd be venting that outside....

Basically what I'm saying is that I would have gone the boron route for 1 on Friday, and at this point I would be doing it for all six as quickly as I could. Decommission the whole facility, because you've got too much uncontrollable danger here. They seem to have been trying to wait to accomplish the evacuation before venting and so forth.

Because of the reactor types at Daiichi, total radiation exposures ought to be somewhat limited. Or so I'm told by an an engineer who should know.

But still, if you get a cascade of failures you're really dealing with all the fuel, spent or active, as radiation sources, and though a lot of it should pretty cold by now, it's still going to emit a lot of radiation with a breach. And if you can't get in there to maintain the cooling and water levels.... The different reactors are in different buildings. Maybe you could work there, maybe you couldn't.

I think you would have to entomb chunks of the facility and then remove the chunks onto barges and dump them in deep water. It would be very difficult and dangerous. You'd have a lot of contamination. I don't think you'd be able to let people back close to Daiichi for some time.

But unlike Chernobyl, it shouldn't produce radiation spread over such a wide distance. In Chernobyl, the reactor blew twice. There was a steam explosion (this is reconstructed) which destroyed the reactor shell and then there was a nuclear explosion just a few seconds later. At that point fragments of the nuclear fuel ended up outside the structure, and fires were started on the roof of a neighboring reactor.

The explosions happened on the 26th of April. There was some sort of cloud witnessed by locals as a result of the explosions. The internal fire, which was burning up the graphite (part of the reactor core) continued to burn until May 10th. The initial explosions threw a column of extremely radioactive debris high into the air. This was probably complicated by additional fragments and tiny detritus particles from the fire that kept burning in the reactor core for two weeks. So the total emissions shot up into the air were massive. The temperature inside the core was estimated at 1200 Celsius.