On the one-year anniversary (I know you're getting sick of the phrase..) of the Fukushima I Nuclear Power Plant accident, the US Nuclear Regulatory Commission has issued orders to improve safety of the commercial nuclear power plants in the US.
From the NRC's press release on March 9, 2012, they are:
The first Order requires the plants to better protect safety equipment installed after the 9/11 terrorist attacks and to obtain sufficient equipment to support all reactors at a given site simultaneously.
The second Order requires the plants to install enhanced equipment for monitoring water levels in each plant’s spent fuel pool.
The third Order applies only to U.S. boiling-water reactors that have “Mark I” or “Mark II” containment structures. These reactors must improve venting systems (or for the Mark II plants, install new systems) that help prevent or mitigate core damage in the event of a serious accident.
Plants have until Dec. 31, 2016, to complete modifications and requirements of all three Orders.
There are evidences that the venting may have caused the hydrogen explosions of the reactor buildings because the vent pipes were connected to the pipes for the gas management systems BEFORE they connected to the exhaust stacks. At least part of hydrogen and other gasses, instead of dispersing into the atmosphere through the exhaust stacks, may have come back into the reactor buildings via the gas management system pipes, the Nuclear and Industrial Safety Agency admitted in December last year, 9 months after the explosions.
About the "enhanced equipment" to monitor water levels in the SFPs, I wonder what happens if station blackout happens and the operator cannot read any instrument, as it happened at TEPCO's plant.
Order No.3 requires the installation of new venting systems for Mark II containment structures. Fukushima Reactor 6 is Mark II. I actually do not know if Reactor 6 has the venting system.
If anyone knows the link to the details of these NRC orders, please post on the comment section. I want to know the details. I haven't found it, but I haven't looked carefully enough on the NRC site.
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17 comments:
laprimavera: maybe this is what you're looking for?
http://www.nrc.gov/reading-rm/doc-collections/commission/srm/2012/2012-0025srm.pdf
Thank you! That looks like it.
Can anyone explain what "hardened vent" involves?
The venting is to reduce pressure in the containments in an emergency. At Fukushima, the pressure in the containments was too high for the fire-truck pressure pumps to overcome. It was at the same time too low to break the rupture discs in the vent outlets.
To vent the containments, operators had to open electric-operated valves without electric power. The various batteries they had on hand would not work. Valves were located in areas with lethal radiation or floods of steam. Consequently, containments could not be flooded with (sea)water as necessary to prevent melt outs.
The media, Tepco and Japgov insist on hydrogen explosions. However, it is likely that hydrogen was a single component of steam explosions. First of all, the cores were without cooling for hours. The Mark 1 cores begin to melt in 40 minutes or so after loss of coolant circulation. Steam pressure within the pressure vessels in Mk 1 units forces the water out of the PV into the suppression pool which has become an inadvertent condenser. Remember, the service water condensers were destroyed by the tsunami and the primary steam circuit valved off from the main condenser. The only form of condenser was the suppression pools. These pools would have drawn water from the PVs while steam in the vessels would have provided a push.
Molten fuel flowed into water in suppression pools to cause steam explosions: #3 was particularly violent and ejected core material through dryer pool area. All of the explosions originated in suppression pools except #4 which took place in SFP (operators simply forgot to check #4 pool and allowed the water level to drop below the uppermost fuel assemblies).
Operators/managers continue to lie about the reactors and what happened, makes it harder for NRC to determine fixes. Best to have 'normally open' vents with active systems to keep the valves closed.
Another big problem is the 20yr extensions and 'up-power' licenses. Next meltdown will have PV's simply blow up or be blown out of the top of the reactor. Surprised this didn't happen due to thermal shock and water hammers during scrams (this may have happened in unit #1).
'Hardened vent' refers to an improved system of pipes etc for venting thats been around quite a long time. It was recommended by the late 1980's as a result of greater understanding of mark 1 containment & venting deficiencies. The 'hardened' bit refers to the fact that the new venting system would actually be designed to withstand the sorts of pressure that could come into play during a vent during an emergency, a vent designed to reduce pressure in the containment. Because before these hardened venting systems were added to many plants, the exiting venting system wasn't designed to handle high pressure, and as a result they were worried about the non-hardened vent system leaking substances into the reactor building during a vent, and other problems.
Fukushima had the hardened vents fitted at some point in the past. However as your article mentions, the hardened vents failed to solve all of these issues at Fukushima, the gas etc found a different way to get back into the building. So now the hardened vents need improving.
@elbow, thank you for info on "hardened vent". I think TEPCO had said Fukushima had those vents, but I heard from someone who worked there in the past who said vent pipes were the same as those used for air ducts. Hard to believe, though.
@steve, can you elaborate on how molten fuel found its way into suppression chamber?
There was a Japanese researcher who said both Reactor 1 and Reactor 3 explosions involved suppression chambers. He said suppression chambers were damaged and a large amount of steam was released in both cases. No one paid attention because he is not a nuclear expert. His expertise is explosive chemicals, and he knows explosion.
The downcomer vents lead straight from the pedestal area under the pressure vessel to the suppression pools. Since there was no water being pumped into the containments (pressure too high) there would be little or no water in the dry wells but high water levels in the suppression pools. The vents would be a highway straight to the water.
http://2.bp.blogspot.com/-KOJNa5L32hY/TZPojz5VjLI/AAAAAAAAArI/ZPFkeihFav8/s1600/BWRMarkIcontainment.png
I have seen other suggestions of explosions starting in the suppression pools as well. These seem to be an obvious weak link.
"...but I heard from someone who worked there in the past who said vent pipes were the same as those used for air ducts. Hard to believe, though."
I think the guy could be right. The normal air duct sturdiness due to mechanical resistance requirements probably will make them withstand four atmospheres of pressure (which was about the intended blowout pressure) without blowing apart, even though they were not primarily designed for such purposes.
But as unit 2 drywell pressure reached almost 10 atmospheres, it is probably no longer safe to assume venting pressures will not blow apart normal air ducts.
Regarding the comment of Virginia Steve, I have to add another possibly important point regarding the suppression pools.
If a mix of steam and hydrogen gas passes through the downcomers, only the steam part will condense to water again. The hydrogen part will collect and concentrate in the torus' air. Everybody reading this blog knows what consequence this potentially can have.
Anyway, I find interesting that there seems to be no mention of the sump clogging problem that quite definitely occurred at unit 2.
I suspect this is because there has not been found any solution for this problem, so it just gets ignored.
2 possibilities: hydrogen vs steam explosion. There is very little evidence to corroborate primarily steam explosions. The preponderance of the evidence to date favors the theory of primarily hydrogen explosions. Buoyancy calculations of the fireball of unit3 imply a temperature AFTER expansion of about 2000°C - consistent with a hydrogen explosion and inconsistent with a steam explosion (again AFTER expansion). There is no evidence to corroborate the theory that the unit 4 explosion originated in sfp4. There is clear evidence that hydrogen from unit3 made its way into unit4. There is also the possibility that sfp4 also released hydrogen due to hydrolysis caused by boiling in the presence of high radioactivity, but there is no evidence that the water level dropped low enough to uncover the fuel rods, although the level may have dropped substantially. Steam explosions can be very powerful because they are BLEVEs - boiling liquid expanding vapor explosions. As pressure is reduced due to a rupture (usually of a tank), there is flash evaporation of the remaining liquid which feeds the expansion of the explosion. At Fukushima, the liquid levels in the RPVs dropped rapidly after core cooling stopped. In fact, meltdown had to happen AFTER the liquid levels dropped due to water being turned into steam. As a result, there would have been very little water left in the RPVs to provide the expansion normally seen in BLEVEs - an argument against the primarily steam explosions. In short, hydrogen explosion are more likely, but certainty will take many more years of investigation.
Are there any plans to address the problems of the RPV cap or is it unnecessary as the comments made regarding the leeky cap just malicious rumors?
Malicious rumors or a tool to sell some books..
In my opinion the explosions were overanalysed because they were one of the few dramatic events at Fukushima that people could see with their own eyes.
Given the discussion we had about reactor 2 recently, I think new meaning comes to the phrase that 'its the quiet ones you have to watch'.
And reactor 2 is certainly the one we can look at most closely at this point when it comes to the suppression chamber, Even though the company has now distanced itself from the idea that the explosive sound on the 15th came from the suppression chamber, its been one of the only forms of containment damage that they;ve been prepared to mention. And not only did the reactor 2 suppression chamber meter did stance things, not matching the drywell pressure and then falling to 0. And they failed to vent the suppression chamber at reactor 2. And they had switched the cooling system of reactor 2 to use the suppression chamber water days earlier, and then failed to monitor its pressure or temperature for several days. By the time they did start monitoring it things were not good, and they were very worried that if they let pressure out of the reactor then the suppression chamber would struggle to cope unless they depressurised it by venting first. But then they couldn't vent it, which is why they tried the dry vent out of desperation,which would not have helped the suppression chamber directly but was better than letting containment fail in a potentially dramatic way.
So, the suppression chamber at reactor 2 had lots of reasons to go wrong that did not involve corium falling into it. But I have certainly seen an old paper which looks at what could happen in these sorts of reactors with mk1 containment if the core melts, and the possibility of fuel entering the suppression chamber via the downcomers is there. I will try to find a link to the document in question.
When looking at suppression chambers in general, Fukushima Daini is also one to study because if memory serves me correctly it was loss of suppression chamber function due to their temperatures getting too high which triggered the emergency legislation at those plants in the days before they brought things under control at that plant.
I found the study that I said mentions the core-> suppression chamber via downcomer path that someone else mentioned in their comments.
I was mistaken. The study actually talks about core ending up in the torus room, not the torus (Suppression chamber) itself.
Its on pages 5 & 6 of this document, and its talking about the molten core eating away at the drywell liner, and what pathways this opens up.
http://www.osti.gov/bridge/purl.cover.jsp?purl=/5835351-nR29Hq/5835351.pdf
@ 3.33pm Hadn't they been pumping seawater into unit 3 for a good few hours before the explosion? So there couldn't there have been enough water for a steam explosion to happen?
Supposedly the vent systems in Japan were "hardened" but not mandatory systems so they didn't get detailed scrutiny. Operators voluntarily installed them. In the US at first much was made about Japanese venting systems but then it came out that venting systems in the US while mandatory were generally not required to prove they would work. A couple of US BWR Mark 1 units don't HAVE venting systems. The whined they would be too hard to install but the NRC has not been forthcoming about what plants don't have vents. The Mark II systems are supposedly superior so seeing them included indicates the NRC knows something they are not telling and it worried them greatly. BTW Daini's units were in need of venting but so far what I have found it sounds like they didn't vent? Not sure if they feared explosions like at Fukushima or if they were able to get the units cooled down enough to put it off? The NRC knows the Mark 1 units in the US pose a major threat but won't shut them down. They just keep quietly issuing fixes.
They had a lot more luck at Daini. The tsunami height wasn't as bad there, and they didn't lose all off-site power, one transmission line was still intact. They still got fairly close to a meltdown at 3 of the reactors, with the suppression chamber temperatures going over 100 degrees C, but by restoring cooling systems they were able to bring things under control within days.
Just remember that they also had contaminated water in some basement or other at Daini with no plausible explanation how it got there ... Am still not convinced that all was "well" at Daini.
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