News Japan Earthquake, Tsunami, & Nuclear Disaster

[Rtyh-12]

It is a BWR so all steam in the plant is contaminated. This contamination is mostly nitrogen with a half-life of 16 minutes, if the reactor fuel remains intact.

You mean that that vapour is harmless?

 

[Urwumpe]

You mean that that vapour is harmless?

No, it just means that the biggest danger is directly to the plant. It also contains a lot of nasty gases, which last longer. A turbine of a BWR is always a pretty radioactive place, but the first hour after shutdown is the really dangerous phase then.

How bad the meldown can be?

What's worse? A pressure vessel failure would pulverise the already present content and leave the open gap for the worse stuff that would follow the imminent meltdown.
Preventing the breach could cause a meltdown, which would, as far as i understand, merely destroy the plant. Or is there a chance of america syndrome there?

A meltdown would be essentially an unstoppable heating of the fuel elements until the fuel leaves the fuel cladding and turns into a blob of molten metal and slag. Once the melt down starts, it is very hard to stop it. A small local damage to the fuel elements is easily contained, but if your cooling system failed completely, the heating by the residual nuclear reactions will take much longer to cool down, especially if the fuel left the controlled fuel elements and becomes a hot mass. It won't become supercritical and explode, but simply just slowly melt through everything until the radioactivity dropped low enough.

There are some phases that you have to expect in a meltdown, according to a properly sourced article on wikipedia:

1. Core uncovery. In the event of a transient, upset, emergency, or limiting fault, LWRs are designed to automatically SCRAM (a SCRAM being the immediate and full insertion of all control rods) and spin up the ECCS. This greatly reduces reactor thermal power (but does not remove it completely); this delays core "uncovery", which is defined as the point when the fuel rods are no longer covered by coolant and can begin to heat up. As Kuan states: "In a small-break LOCA with no emergency core coolant injection, core uncovery generally begins approximately an hour after the initiation of the break. If the reactor coolant pumps are not running, the upper part of the core will be exposed to a steam environment and heatup of the core will begin. However, if the coolant pumps are running, the core will be cooled by a two-phase mixture of steam and water, and heatup of the fuel rods will be delayed until almost all of the water in the two-phase mixture is vaporized. The TMI-2 accident showed that operation of reactor coolant pumps may be sustained for up to approximately two hours to deliver a two phase mixture that can prevent core heatup."[6]
2. Pre-damage heat up. "In the absence of a two-phase mixture going through the core or of water addition to the core to compensate water boiloff, the fuel rods in a steam environment will heatup at a rate between 0.3 K/s and 1 K/s (3)."[6]
3. Fuel ballooning and bursting. "In less than half an hour, the peak core temperature would reach 1100 K. At this temperature, the zircaloy cladding of the fuel rods may balloon and burst. This is the first stage of core damage. Cladding ballooning may block a substantial portion of the flow area of the core and restrict the flow of coolant. However complete blockage of the core is unlikely because not all fuel rods balloon at the same axial location. In this case, sufficient water addition can cool the core and stop core damage progression."[6]
4. Rapid oxidation. "The next stage of core damage, beginning at approximately 1500 K, is the rapid oxidation of the Zircaloy by steam. In the oxidation process, hydrogen is produced and a large amount of heat is released. Above 1500 K, the power from oxidation exceeds that from decay heat (4,5) unless the oxidation rate is limited by the supply of either zircaloy or steam."[6]
5. Debris bed formation. "When the temperature in the core reaches about 1700 K, molten control materials [1,6] will flow to and solidify in the space between the lower parts of the fuel rods where the temperature is comparatively low. Above 1700 K, the core temperature may escalate in a few minutes to the melting point of zircaloy (2150 K) due to increased oxidation rate. When the oxidized cladding breaks, the molten zircaloy, along with dissolved UO2 [1,7] would flow downward and freeze in the cooler, lower region of the core. Together with solidified control materials from earlier down-flows, the relocated zircaloy and UO2 would form the lower crust of a developing cohesive debris bed."[6]
6. (Corium) Relocation to the lower plenum. "In scenarios of small-break LOCAs, there is generally. a pool of water in the lower plenum of the vessel at the time of core relocation. Release of molten core materials into water always generates large amounts of steam. If the molten stream of core materials breaks up rapidly in water, there is also a possibility of a steam explosion. During relocation, any unoxidized zirconium in the molten material may also be oxidized by steam, and in the process hydrogen is produced. Recriticality also may be a concern if the control materials are left behind in the core and the relocated material breaks up in unborated water in the lower plenum."[6]

No america syndrome as the image paints, but rather that the pressure vessel will be destroyed and the containment building becoming a nuclear ruin by structural integrity of the pressure vessel being reduced. Usually, there are special catch basins below the pressure vessel for that case, that are meant to permit the molten mass to spread out over a larger surface and cool down faster without leaving the building.

In the best case of a full meltdown, you get something like that:

The nuclear core is completely destroyed, but the pressure vessel intact.

During Chernobyl, stalagmites of corium formed at damaged pipes below the reactor.

[ame="http://en.wikipedia.org/wiki/Nuclear_meltdown"]Nuclear meltdown - Wikipedia, the free encyclopedia[/ame]
http://en.wikipedia.org/wiki/Corium_(nuclear_reactor)

Kind of makes me happy that I work at a PWR.
I wonder if they have that goofy rod insertion/extration method that our BWR at Browns Ferry use. Their rods are beneith the reactor, whereas ours are above (gravity is our friend in a worst-case).

Yes, exactly that one. Also it is one of those old reactors without fine control over the rods.

As far as I can tell, a BWR can't have the rods insert from above at all.

 

[PhantomCruiser]

...a BWR can't have the rods insert from above at all.

Yeah, we were just talking about that here a few minutes ago. I'm still a nuclear noob (only been here about 5 years). I came from fossil power and combustion turbines, to the PWR here, so there's plenty about it that is still new to me.

As near as I/we can tell, it may be to protect the internals from the heat exposer associated with the steam bubble? (then again, none of use are engineers). Our reactor is completely submerged/encased in water, the steam bubble is in the pressurizer, another vessel entirely.

The catch phrase is that "nuclear power is special and unique". That it is...

 

[Topper]

Is it true that there are vents open and the powerplant and also the air arround is conterminated? I've heared that here in the news, but i have no other confirmation..
Or is the contermination "only" inside the building?

 

[Urwumpe]

The reason is likely because of the steam dryer and steam separator being inside the pressure vessel and above the core.

http://www.nucleartourist.com/type/bwr.htm

---------- Post added at 09:42 PM ---------- Previous post was at 09:39 PM ----------

Is it true that there are vents open and the powerplant and also the air arround is conterminated? I've heared that here in the news, but i have no other confirmation..
Or is the contermination "only" inside the building?

It will be vented over the exhaust stack.

See this image as reference how a fairly "modern" BWR has its emergency system:

(This one lacks modern features like a Torus)

 

[Belisarius]

The latest from NHK - they are going to vent the steam soon:

Nuclear plant to vent air
Air that may contain radioactive materials will be vented from a nuclear power plant in quake-stricken Fukushima Prefecture. The Tokyo Electric Power Company has decided to release air from the reactor's containment vessels, aiming to avoid their breakdown.

The company issued a warning about its Fukushima Number One Plant early on Saturday morning. It said the pressure value for the reactor's containment vessels had risen, and that if the value was correct, the vessels could break down.

Tokyo Electric says the amount of air to be released will be small and that it will notify residents near the plant before it starts releasing the air. The company says it will check the amount of radioactive materials in the air before the release.

Earlier, the government issued an evacuation advisory to people living within a 3-kilometer radius of the Fukushima Number One plant.
Saturday, March 12, 2011 03:56 +0900 (JST)

http://www3.nhk.or.jp/daily/english/12_15.html

 

[MeDiCS]

Newest reports say about 90,000 missing, and the quake reevaluated as having a magnitude of ~9.0 (some even say 9.1). Horrible event.

 

[Topper]

I've just heared that they can't open the vents because they have now electrical power, so the pressure inside the reactor is increasing further.
The evacuation area has been increased from 3km up to 10km. God bless them...

 

[Dambuster]

From Al Jazeera live blog:

6:46am
AFP is quoting the Kyodo News Agency as reporting the Japan safety panel saying radiation at the Fukushima nuclear power plant has reached levels 1,000 times of normal levels.

You can be sure we'll bring you more on those reports.

Pressure has been building at the nuclear plant after a reactor's cooling system was damaged in the earthquake.

Meanwhile, contrary to earlier reports, officials say that the US did not deliver nuclear coolant material, and that Japanese authorities handled the situation themselves.

---------- Post added at 10:01 PM ---------- Previous post was at 09:53 PM ----------

And another:

6:56am
There may be some accuracy to these reports of increased danger at Fukushima nuclear power plant. AP is now reporting radiation levels have surged outside the facility, and Japanese officials have expanded the area subject to evacuation.

---------- Post added at 10:03 PM ---------- Previous post was at 10:01 PM ----------

This seems to be a good place for updates:
http://live.reuters.com/uk/Event/Japan_earthquake2

 

[Urwumpe]

That is the problem with all nuclear power plants - despite all the electricity that they produce, their control systems rely on electricity from the external power network, and take failures not lightly. Even if you have emergency generators, the black out leaves you in a bad situation until everything is back.

 

[Sky Captain]

They have 4 other reactors on site and there are 2 diesel generators per reactor, one diesel should be enough to run coolant pumps. Why they can't just take some heavy duty extension cord and connect a working diesel generator from another reactor to coolant pumps?

 

[Urwumpe]

They have 4 other reactors on site and there are 2 diesel generators per reactor, one diesel should be enough to run coolant pumps. Why they can't just take some heavy duty extension cord and connect a working diesel generator from another reactor to coolant pumps?

No such cord. The cord is meant to be delivered from the USA.

 

[PhantomCruiser]

What I heard therough the 'network/grapevine' is that one set of diesels had been take off-line for maintenance, while others dropped out due to damage (from the quake?).

We also have 4 sets of diesels for safe shut-down. And they can cross-connect from unit 1 to 2. We HAVE TO maintain 2 independant methods of offsite power as well. An advantage we have is our hydroelectric plant next door (also TVA). We no longer have Watts Bar fossil available, but have a separate transmission line from the grid providing our 2nd source of offsite power.

From what we've gathered so far is, that everything crapped out at basically the same time (for all practical purposes).

 

[Urwumpe]

From what we've gathered so far is, that everything crapped out at basically the same time (for all practical purposes).

What I heard is that a lot of equipment was still damaged or under repairs since the last strong Earthquake there, this one was essentially a worst case scenarios big brother.

 

[Artlav]

radiation at the Fukushima nuclear power plant has reached levels 1,000 times of normal levels.

Isn't that still just a few dozen mSv?
"Normal" should be slightly over background, no?

No such cord. The cord is meant to be delivered from the USA.

Well, that's embarrassing... But i guess you can only plan for that big a problem.

 

[Sky Captain]

So they are left with only 3 working diesels and they have none to spare for fourth reactor?

 

[Zeehond]

Fukushima reactor II is now also in trouble, according to NKH TV.

 

[Topper]

I don't think that they can prevent the meltdown, but of course, i hope it!

 

[Sky Captain]

How much power is recquired for those cooling pumps? Powerful diesel generators are also used in other places like construction sites. It just seems weird that no suitable generator can be found in nearby area and transported to site even by heavy lift helicopter if roads are not usable

 

[Urwumpe]

Isn't that still just a few dozen mSv?
"Normal" should be slightly over background, no?

Yes, and such values could also be produced by releasing steam from the reactor vessel in a controlled way. Already small releases of radioactivity cause such increases at the measurement stations.

But that is also not small. normal activity at a nuclear power station is 0.15 µSv/h so we are talking about around 0.15 mSv/h. That means after 24 hours at such intensity, you have already received your annual dose.

---------- Post added at 11:41 PM ---------- Previous post was at 11:33 PM ----------

How much power is recquired for those cooling pumps? Powerful diesel generators are also used in other places like construction sites. It just seems weird that no suitable generator can be found in nearby area and transported to site even by heavy lift helicopter if roads are not usable

Hard to tell, but we are talking of 50 MW of thermal energy that needs to be moved away. If you use water for cooling, you need to bring 22 tons of water every second from 50°C to boiling (at normal pressure). Not really realistic for a nuclear reactor.

Typical flow is 45,000,000 kg/h water by the pumps at about 100% power -> 12.5 tons every second . Using this data you need 875 kg water every second for cooling now. The higher the pressure at the reactor vessel, the more power the pumps need. Depends on the pressure drop.