Friday, August 3, 2012

Ooi Nuke Plant Reactor 4 Had 137 Alarms Set Off in 10 Days, But "Not A Problem", Says KEPCO


KEPCO seems to have much thicker skin than TEPCO, if that's even possible.

KEPCO says the alarm at one of the thermocouples inside the Reactor 4 Pressure Vessel has been triggered 137 times since the reactor started the full operation on July 25, but it's not a problem, things will sort themselves out soon.

From Mainichi Shinbun Fukui local version (8/3/2012):

関西電力は2日、大飯原発4号機(118万キロワット)の原子炉内の1次冷却水の温度が部分的に上昇したことを示す警報が同日午後3時現在で計137回作動したと発表した。警報は4号機がフル稼働に入った7月25日早朝に初めて作動。燃料集合体(193体)のうち1体について、周辺の冷却水の温度が警報基準に達した。

KEPCO announced on August 2 that an alarm was sounded 137 times as of 3PM that day which indicated the rise in local temperature of the primary coolant [water] inside the Reactor Pressure Vessel in Reactor 4 (1.18 million kilowatts) at Ooi Nuclear Power Plant. The alarm was first sounded in the early morning on July 25, when the reactor started the full operation. The temperature of the coolant around one of the nuclear fuel assemblies (193 assemblies total) rose to the level that would trigger an alarm.

 関電によると、過去に1回使用した比較的燃焼度の高い燃料だったため、流量の変化で一時的に冷却水の温度が上がった。燃焼度に応じた燃料全体の配置に問題はなく、今月中旬ごろには安定した状態になるとみている。

According to KEPCO, the particular fuel assembly had been used once before and had relatively high fuel burnup. The company says that a change in the amount of flowing water temporarily raised the temperature of the coolant. KEPCO doesn't think there is any problem with the fuel assembly arrangement based on the fuel burnup, and says the temperature will stabilize by the middle of this month.

 また、7月30日には取水口付近に漂着したクラゲの影響で3、4号機が共に取水量を抑えたため発電出力が最大約1・8%落ちた。

On July 30, the power output was decreased by 1.8% as both Reactors 3 and 4 reduced the amount of seawater intake because of jellyfish.


(So jellyfish are still protesting...)

KEPCO's press release on 8/2/2012 includes their explanation (in the attachment No.3) of why the alarm is being triggered and why it is no problem, in a very technical term (again, they outdo TEPCO).

From what I could figure, the thermocouple at this particular fuel assembly has been registering temperature higher than the standard (between 305 and 336 degrees Celsius), but it is because this assembly has high fuel burnup having been used once already, and as the time goes the relative output of the fuel assembly will drop, and the temperature will drop. The current cycle is the cycle No.15, and KEPCO uses more fuel assemblies that have been used once than in the previous two cycles, so the higher temperature is to be expected. As long as the temperature is below the saturation temperature (345.3 degrees Celsius), there is no problem with cooling.

(I put the English labels. KEPCO's press releases in English are almost all about financial results for the investors. They do have an English press release on the restart of Ooi Nuke Plant, dated June 16, 2012, if you're interested, here.)



Ooi Nuclear Power Plant's two reactors are both pressurized-water reactors from Mitsubishi Heavy Industries.

The temperatures of the coolant inside the PWR, according to wiki, is 275 degrees Celsius as the water enters the RPV at the bottom, and 315 degrees Celsius as it flows upward to the reactor core.

11 comments:

Anonymous said...

Okay, maybe I'm splitting hairs here, but if an alarm goes off - and does so 137 times - there IS a problem. That's the purpose of the alarm, to alert to a problem. It may be one that can easily be controlled (am not a technical person and have no idea), but it IS a problem nonetheless.

If the higher temperature "is to be expected," then why would alarms be calibrated to sound in a situation that is supposedly normal? This doesn't make sense to me.

Maybe someone with a better understanding of the technical aspects can explain further?

Thanks.
*mscharisma*

Anonymous said...

The "CHECK ENGINE" light is on folks. Should we ignore it until the next shakken?

Anonymous said...

"On July 30, the power output was decreased by 1.8% as both Reactors 3 and 4 reduced the amount of seawater intake because of jellyfish."
translation:
we, the jellyfish, are demonstration for cooler ocean waters!

Anonymous said...

In IT, following the conventions of the well-known recommandations called ITIL, a repeating incident is known as a problem... I wonder why the same names are not given in the Nuclear Industry?

Anonymous said...

Lets just ignore everything and make money, to hell with it ... even if it melts down they will go into "Fukushima mode" again ...cover up, lie ,distort information, victimize, anything but treat citizens with compassion.

pat said...

different types of alarm, it cn give a warning if you have a local hot spot,

The problem i see isn't the local hot spot, it's the fault line under the rac.

Anonymous said...

With nuclear power nothing is ever a problem until it is. Chernobyl was so "safe" they didn't use containment it wasn't a problem until oopsy. The THTR 300 pebble bed reactor in Germany had the same level safety when it had a failure that couldn't happen in the designers mildest dreams.

The most distressing thing about Ooi is when there really is a problem they'll just say "how could we know, the alarm system was faulty" and the worst part is the media will agree. Just like they agreed who could know a tsunami would strike Fukushima.

Atomfritz said...

This report I had difficulties to really understand.

Kepco said "According to KEPCO, the particular fuel assembly had been used once before and had relatively high fuel burnup. The company says that a change in the amount of flowing water temporarily raised the temperature of the coolant. KEPCO doesn't think there is any problem with the fuel assembly arrangement based on the fuel burnup, and says the temperature will stabilize by the middle of this month."

First, a fuel assembly with high burnup should produce less energy, not more.

Second, Kepco doesn't tell why the change of the amount of water flow. Localized boiling can be on a location different from the thermocouple placement.

Before a massive development of steam at the saturation temperature, which would clog the fuel assembly effectively, resulting in fuel assembly damage or even a partial meltdown, there is already the phenomenon of steam films at the surface of fuel assemblies at temperatures significantly below the saturation temperature.
Such steam films should be avoided at all costs because they reduce cooling and are accident precursors.

Operating parts of the reactor at almost the saturation temperature (instead with the common 30 degrees safety margin) demonstrates that Kepco safety culture is no better than Tepco's.
A short cooling disturbance could result in a catastrophe.
To avoid the risk, Kepco could operate the reactor at, say, 90% capacity for one or two months to allow for settling of the unequally distributed power of the new core, instead of riding the tiger.

With alarms blaring daily and being ignored, what will happen if some time the alarm doesn't indicate only a close call, but a real mishap?

Anonymous said...

I am no nuclear engineer but my understanding is that as the nuclear fuel assembly becomes too hot the water around it turns entirely into steam and its ability to remove heat from the assembly decreases, hence the assembly becomes even hotter and you enter a vicious circle.
At this point I suppose you really *have* to insert the control rods but is it enough? there might be something else in the picture, maybe related to the role the water has as a moderator of the nuclear reaction; I imagine steam is a worse moderator than water because it is less dense.
A comment from a technical reader would be welcome.
Beppe
Beppe

Atomfritz said...

@ Beppe

I am no nuclearist, but some sort of an interested "enthusiast", too.

To my understanding, the water flows at very high speed, so localized overheating starts with a bubble stream, turning into a steam film, providing thermal insulation via reduced cooling efficiency.
The negative void coefficient doesn't factor in much as long as it's only small bubbling and film, so that the chain reaction intensity doesn't go down significantly. So the fuel assembly won't cool down instantly, even if we keep the decay heat out of view.

But this changes at the "saturation temperature" at about 345C, when the water cannot take any more heat energy without heavily boiling at operating pressures.
Then bubbles develop, forming an obstacle for water flow.
In consequence water takes another path than the steam-clogged overheated region.
There the overheating continues because of insufficient local cooling, leading to potential localized melting.
Remember, the heat inside the fuel assemblies is above 2000 degrees.
Most cases leave only single fuel assemblies damaged because the resulting interruption of cooling has been short only, but sometimes there is less luck (like at Three Mile Island).

The nearer you approach the critical temperature at around 345 degrees C, the lower the safety margin becomes.

Depending on your safety attitude, you either could operate the reactor at about, say 90% power for a few weeks or months to level out the reactivity of the reactor after reloading (often done in Germany) or to wittingly/intentionally reduce the safety margin (increasing profit and risk, *epco style), like at Ooi.

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