Theory of corrosion and backflow through feedwater lines to explain Fukushima radiactive water leaks

So how DID Reactor Vessel water get into the Pacific Ocean? at Dailykos by kbman

During normal operations steam flows out the top of the reactor in the main steam line which feeds the turbines. There are several devices involved and a number of turbine stages which use as much of the energy in the steam as it is practical to extract. The “dead” steam then passes through a condensing unit and heat exchange process to remove enough heat that it becomes water again. This is done by feeding the steam/condensate through a huge number of small tubes which are surrounded on their outside by cooling water drawn from the ocean. The ocean water absorbs the heat and carries it away and the condensate – water – is then pumped by the feedwater pumps back into the reactor.

When the plant(s) shutdown during the earthquake, their main steam line isolation valves would have shut immediately. (This theory applies to varying degrees to each plant.) The flow of water through the core would have switched to another cooling system which uses a heat exchanger to cool the reactor water. This and other similar systems were the ones which could not be used after the power failed.

As I’ve mentioned a number of times in diaries and comments, the most immediate threat I see from corrosion is on valves and seals. And in this case I believe it is backward flow through the feedwater line that is transporting reactor water to the turbine building and also to the ocean. The feedwater enters the reactor well above the level of the fuel rods. As such, that pathway has been constantly exposed to the reactor pressure and the steam that is present above the water line. This exerts backpressure on the feedwater line towards the pumps and heat exchanger. There may or may not be a cutoff valve on this line as well. If there was enough existing corrosion in the pump seals and cutoff valve to allow a little bit of flow, or if the cutoff valve did not properly close, it would allow backflow towards the heat exchanger.

Now here is one of those dirty little secrets that may come into play in this circumstance. It is almost a certainty that some number of heat exchanger tubes have leaks. This is almost inevitable over time in an operating plant, nuclear or otherwise, especially one which uses saltwater as the cooling medium. It would not be noted under normal operating conditions because the radioactivity of the water would not be that great. Remember when they said that the water the workers stepped in was 10,000 times the radioactivity of normal coolant? That high level of contamination is because of all the fuel that was busted open when the water level was lost in the first hours of this emergency. Some of it melted into clumps and fell to the bottom of the reactor vessel, other bits just came out of their cladding and the ceramic pellets disintegrated in the heat. The clumps are contained fairly well. It is the pellets that broke up that are coming out in the water. The water at unit 2 is much hotter than at unit 3 which may be the result of the unit 3 reactor water being diluted with less radioactive fuel pool water.

They have gotten the operations of these reactors to the point where there is not much radioactivity at all in the coolant/steam/condensate during normal mode operations. It is because of this that they could get away with many leaks in their heat exchanger. There would be no appreciable radiation detected in the water, no regulatory flags would be waved, and they could go on operating longer without going through the costly process of repairing corroded heat exchanger tubes. My belief is that the constant steam pressure on the feedwater inlet line caused enough backpressure to generate flow out the bad heat exchanger tubes and directly to the ocean. At first this would have just meant emptying the relatively low radioactivity water that had stalled there when the reactor was shut down. But as more and more volume drained down the piping acted as a condensing surface causing the exceptionally radioactive steam to become water and mix with the remaining stalled water.

And after days of blowing this stuff out to the ocean, there may have finally been enough corrosion on one of the local components in the turbine building to cause it to leak into the basement. Or that water may be coming through an entirely different pathway. Anyway, those are my thoughts on how this is happening. I welcome your take on things.

I just read another theory being advanced regarding the possibility that the ruptured torus at unit 2 could be the source of the much higher levels of radiation in that water. I discount this because of how long ago it was that they popped the torus. The very short-lived isotopes would be much lower already in that water.

Best-case scenario

I was asked in a comment earlier today to provide what I believed to be the best case scenario for how this thing ends. In the best case the heat at unit 1 continues to drop and they are able to get electrical pumps running in the next few days with the core cooling system soon to follow. Also in the best case scenario the heat exchanger for this system is fairly well intact and free of corrosion. The other reactors having their core cooling systems working would follow shortly after.

Meanwhile, they would be able to get enough water in each of the fuel pools to be able sufficiently cover the fuel to be able to safely work on the refuel floor. This would allow at least some cleanup of the explosion debris to make efforts at the fuel pools more efficient and effective. It would also make it possible to take whatever action is chosen to resolve the fuel pool issues at the various units. Again, in the best case they would identify some material or method for sealing the leaks in 3 and 4, at least temporarily – long enough to allow shoring up from below and more permanent or semi-permanent repairs.

Also in this scenario it will be possible in the near future to process the contaminated water stored on site, remove fuel from the reactor cores and fuel pools, and decommission the sites including scraping the top half foot of ground up from all over the site and dumping it as fill into the containments and then entombing the whole thing in concrete.

Likely-case scenario

Given the difficulties of the working environment and the problems with the leaking water I think it is optimistic to think they will have core cooling systems returned to service within the next week at any of the units. This will mean more feed and bleed to remove heat. Also, they will need to track down the exact pathway(s) water is taking to get to the turbine building and ocean. They may or may not be able to isolate the problem and stop the leaking. Next best would be to be able to contain the leak at its source and drain that container continuously. Obviously this doesn’t work if there are heat exchanger leaks but it may be possible to isolate this system using locally controlled manual cutoff valves.

I’m really not sure what is likely to happen with the leaking fuel pools. I fear that concrete entombment is likely in the future for at least one of them, possibly both units 3 and 4.

I expect the releases will need to continue for the next week at least, likely more until they get alternate means of heat removal back online. I also fear the ocean releases will continue for that same period of time, perhaps longer depending on the condition of the heat exchangers.

Cleanup and decommissioning may not be done as thoroughly as possible due to dose rates all around the site. They may well end up covering everything with sand, gravel, or other more suitable materials to contain the surface contaminants. I expect the site to eventually be abandoned and reactors 5 and 6 to also be decommissioned.

Worst-case scenario

You’ve heard this from any number of talking heads on TV. Noun-verb-Chernobyl. I seriously doubt the releases will be at the magnitude of Chernobyl, but if all three reactors fully melt down and break free of their containments and all 4 problem fuel pools go dry and burn then yes, it could get very ugly indeed. I don’t see this happening given current conditions, but if any one of the problems were to escalate beyond control then it could cause the others to become unmanageable due to lack of site habitability. I presume that each of the individuals responsible for managing the reactors and fuel pools is well aware of the stakes of failure.

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