Japan Earthquake: nuclear plants

Dmytry

afaik this system may have rather low throughput and could not be used in the event of serious emergency. But that may depend to particular reactor.

tsutsuji

http://www3.nhk.or.jp/news/genpatsu-...807/index.html The Areva system chemical pump was restarted after 3 and a half hours. Tepco says the pump tripped because a high viscosity increased the load, but is still clueless about why the backup pump did not start. Concerning the pump that stopped at the Kurion system, the recovery is nowhere in sight. The NISA has requested Tepco to write a report on the causes of the water treatment facility troubles.

http://mainichi.jp/select/jiken/news...40141000c.html Tepco decided to reduce the chemical pump's flow rate, while increasing the frequency of chemical injections. Without evaporation systems, the desalinating facility produces 1.5 times more high concentration salty water than freshwater. The two evaporation systems that were launched yesterday can bring this rate down to 30%.

http://www.tepco.co.jp/en/nu/fukushi...10808_01-e.pdf "Diagram of Desalination System"

http://www.tepco.co.jp/en/nu/fukushi...10808_02-e.pdf "Leakage detected between cable duct for starting transformer and control building Unit 3, Fukushima Daiichi Nuclear Power Station"

http://www.tepco.co.jp/en/nu/fukushi...10808_03-e.pdf (page 2) From 10:03 to 10:43 AM on 8 August, unit 5 reactor cooling was stopped to enable the switching of power supply for RHRS pump (C).

MadderDoc

Yes, in all likelihood the venting systems used by the operators were not the Emergency Gas Treatment Systems, but rather the retrofitted hardened vents we have been talking about. Here's the unit 1/2/3 diagrams of the hardened vent lines cropped from the report of the Japanese governement to the IAEA (see attachments for better rendering):

Attached Thumbnails

   

tonio

Concerning the source of the high radiation readings, maybe the following information from http://www.world-nuclear-news.org/RS...ng_308112.html is of interest:


Second high radiation reading
03 August 2011
Another radiation hotspot has been found at the Fukushima Daiichi site - again in part of the emergency venting and filtration system.

Yesterday an extremely high reading of 10,000 millisieverts per hour was announced as having been found in pipework leading to an exhaust stack. Today that was followed by a reading of over 5000 millisieverts per hour inside one of the buildings.

Both of the readings were taken from parts of the Standby Gas Treatment System (SGTS), through which steam was vented to relieve reactor pressure during the accident in March. In that sense, it is highly likely that Tokyo Electric Power Company will make several more findings of radiation hotspots as it conducts stabilisation work inside the power plant buildings.

The first hotspot was detected in a part of the SGTS pipework immediately before the exhaust stack shared by units 1 and 2. The latest one is on the second floor of unit 1's turbine building, near the entrance to a room related to the SGTS.

Each reactor building has an SGTS which maintains slightly lower air pressure during normal operation to reduce the possiblity that potential contamination could exit the plant building through any tiny holes. It has air intakes and components on several floors. Should contamination be present in the buildings for any reason, the SGTS is there to filter the air before release through the stack. It also provides the filtered route for venting from the reactor system, as used during the accident.

The radiation levels indicated by these off-the-scale readings mean that no worker can approach to within a few metres of the areas to take detailed manual measurements. Instead, levels were estimated using gamma cameras mounted on robots.

joewein

The SGTS in unit 1 has two units capable of filtering 1800 m3/h each, capturing > 97% of iodine. In unit 2 and 3 there are also two units, capable of filtering 2700 m3/h each, capturing > 99.9% of iodine (source: http://fukushima.grs.de/sites/defaul...2011-06-08.pdf -- page IV-15).

It's a good system except you can't use it when you really need it, because it needs power for its electric blowers and dehumidifiers and can't cope with high temperatures or pressures or very rapid releases.

The weight of 1800 m3 of steam at 100 deg C and 1 bar is just about a single ton of water. You can see how this was not designed to clean up vent gases from a boiling suppression chamber holding thousands of tons of water.

zapperzero

http://channel6newsonline.com/2011/0...-melted-twice/

etudiant

So if this insight is put together with the earlier recognition by the NRC that a 24 hr SBO meant a meltdown,
the basis for the US governments recommendation for a 50 mile evacuation zone becomes quite clear.
It also does suggest very strongly that nuclear facilities should have belt, suspenders and a girdle, just in case.
Certainly the current designs do not appear to fail at all gracefully if the emergency venting is just flushed through the stack. That just maximizes the problem. What is the rationale for such a design?

joewein

I think the rationale is the mistaken assumption that extended station blackouts do not happen in the first place. It's a "beyond design base" condition.

However, venting from the wet well air space as in unit 1 does still seem the lesser evil compared to an uncontrolled containment failure from over-pressure as may have happened in unit 2, which is assumed to have released a lot more contamination than unit 1 (think Iitate-mura). At least the vented gas has been scrubbed to some extent by first bubbling it through the pool water in the torus. With a cracked containment all bets are off what gets released.


The hardened vent path was mostly designed with hydrogen releases in mind. The Areva report by Dr. Braun estimate H2 production from the Zirconium-steam reaction in unit 1 as 300-600 kg and 300-1000 kg in units 2 and 3.

At 100 deg C the density of H2 is about 0.065 g/L, or 15 m3 per kg. Therefore to release the non-condensable hydrogen from the air space of the torus would have involved a release of 4500-9000 m3 in unit 1 and 4500-15000 m3 in the other units, not counting the steam also accumulated there or allowing for temperatures beyond 100 deg C.

SpunkyMonkey

Perhaps, but to me it looks like a water-carried distribution of the similar-colored dark stuff splattered around and heaped at the base of the same pipe. And because that heap of red-brown gunk is extremely radioactive and associated with a ventilation/filtration system, it may well be (largely) cesium-vapor residue, which is also dark red-brown.

What we can say for sure is that the stain lines are consistent with some red-brown substance washed down with rain. But that does not entail that it's rust.

Yes I see the tape, it's obvious, but what kind of tape allows rust to fester as you suggest? I don't see why the tape increases the likelihood of the staining being rust. Maybe it's to stop corrosion as you suggest, or then maybe to reduce the escape of cesium-vapor residue in the event of a filtration-system rupture. It might be there for a number of reasons. I doubt we can infer much from the tape being there.

swl

When it rains hard, as it has recently in the Fukushima area, what happens to all the water that falls down the ventilation stack? After rinsing down the inner walls of the stack, it then hits the bottom, and what? Does anyone know if there is an automatic drain mechanism? Knowing that heavy rain is common here, I'm certain that something has been done to address this issue, but I don't remember reading about it. I'm curious where rain water and condensate goes after reaching the bottom of the stack.

AtomicWombat

I agree with joewein.

As I take it, the rationale behind the hardened wet well vents is as a last ditch attempt to prevent a hydrogen explosion or primary containment over-pressurisation, on the basis that either event would release more radioactivity (due to catastrophic failure of the containment) than direct venting of the torus to the environment.

See:
http://www.gereports.com/venting-sys...rk-i-reactors/
http://www.nrc.gov/reading-rm/doc-co...9/gl89016.html

Ironically all 3 operating units experienced hydrogen explosions and the hardened vents did nothing to prevent these. It appears that in at least one case (unit 1) the hardened vent was operated as well, created the worst of both worlds.

AtomicWombat

I have a nasty suspicion that the rain water simply is passed to the normal storm water system and then to the ocean and/or ground water. The rationale being that on the "design basis" only filtered gases are exhausted through the stack. Remember that the hardened vent was a retrofit.

westfield

This document appears to indicate the operators aligned the venting from both drywell AND S\C. No scrubbing from the drywell venting.

The document also indicates very high dose rates in the buildings and onsite well before the venting even took place. To my laybrain that seems odd - was there containment failure before they even got to vent?

source : TEPCO


On the SGTS\HVS contamination - why the high dose rates in the Unit 1 Turbine building early on? And why does SGTS even go into the turbine building? Why does the SGTS appear to be HEAVILY contaminated, it shouldn't have even been possible for it to be working after loss of power. So SGTS just opens itself up on loss of power? WTF. The more I read about the design of these systems the less I want to know, kind of.

SpunkyMonkey

By stalagmatic accumulation I refer to the process slow water-carrier driven stalagmite formation. The rust-color residue has accumulated to a few inches in height at the base of the feed-in pipe's interface with the main vent, and that heap of gunk is the highest-dose spot.

Moreover, the same rust-colored stain (that I propose is cesium-vapor residue) is also seen emanating from the interface seam. This constitutes a second unique source of rust-color staining and flow pattern versus that coming down the pipe, and both pathways would logically converge at the accumulation point. The rust-colored staining clearly appears to be associated with the highest-dose accumulation of dark-red gunk.

joewein

If the RPV steam from the pressure release valves is always released into the wetwell, the air space there should be much more contaminated than the nitrogen in the drywell. The drywell would get more severely contaminated later on, once the RPV melts through or the seals around it start leaking, but initially all the good stuff ends up in the torus.

There may be some exchange of pressure via the downcomer tubes between the two if there is a large pressure differential (will water get pushed up the tubes if pressure gets too high in the torus, opening a path for contaminated gas to leak from the torus into the drywell?), but my understanding is that until the RPV gets damaged by excessive temperatures the wetwell would be the more contaminated of the two spaces.

It says they prohibited entry to R/B 1 at 21:51 on 2011-03-11 because of radiation (about seven hours after the quake). Tepco estimates that fuel was exposed five hours after the quake because the water level had fallen too far. So by the time they prohibited entrance, the meltdown had been in progress for about two hours. I don't know what the containment pressure was like by then, but 4 hours later, at 02:00 JST it was 0.6 MPa and at 05:30 it was at 0.82 MPa. The containment may already have been near or at design pressure (0.42 MPa = 4.2 bar) when the radiation went up.

For radiation levels inside the building to go up without venting there must have been some leaks. But in a way it is not surprising that the unit 1 containment was leaking in 2011 (when it was 40 years old and stressed to the max by a melting core) when it had already leaked unacceptably in 1992 during routine inspections when tested at 3 bar:

2.5% of several thousand cubic meters of nitrogen at 3 bar is several hundred cubic meters that would have leaked per day.

Probably because most of the pipes coming out or going into the containment go next door to the turbine building. It's like the belly button of the reactor. The turbine hall is also more spacious.

joewein

That is a very interesting question. Note that the guy who measured the radiation with a 3 meter long pole (and took a 40 mSv hit) is standing next to what looks like a sewer lid inside the structural frame of the stack pipe.

tsutsuji

The following NISA report, written in December 2002, contains a time-line. Here are a few translated excerpts :
The 28 May 2004 Tepco press release announces the following :
The 15 December 2010 press release about regular inspection No. 26 (March 2010 -December 2010) says :
http://www.tepco.co.jp/nu/f1-np/pres...a/bi9714-j.pdf (page 5) 17 February 2009 : 0.176%
http://www.tepco.co.jp/nu/f1-np/pres...a/bi8116-j.pdf (page 7) 12-13 September 2007 (24 hour test) : 0.101%