Fukushima Update: Unit 2 Temperature Rise
by Nelle Maxey
TEPCO increased the volume of water being injected into Unit 2 by
35% in the early hours of February 7.
This attempt to lower the temperature has stabilized it around 70 degrees
C.
But it has not lowered it significantly.
The best article on the net explaining what is going on at Unit 2 is the
one below from Asahi news. It has the fewest errors and omissions and also has
an excellent drawing and an excellent chart which you can see at the link to the
article.
Tokyo Electric Power Co. is
taking steps to prevent a possible self-sustaining nuclear chain reaction at
the crippled Fukushima No. 1 nuclear power plant.
Readings from a thermometer at
the bottom of the No. 2 reactor's pressure vessel rose from 50.8 degrees at 5
a.m. on Feb. 1 to 73.3 degrees at 7 a.m. on Feb. 6.
Melted fuel is believed to have
accumulated at the bottom of the reactor, but high radiation levels have
prevented workers from checking the exact situation within the reactor.
After the flow of cooling water
was increased to 10.6 tons per hour on Feb. 6, up from 8.6 tons two days
earlier, the temperature fell to 69.2 degrees at 5 p.m. on Feb. 6. That night,
TEPCO injected boric acid into the reactor to prevent criticality, the point
at which a nuclear fission reaction becomes self-sustaining. Boric acid
absorbs neutrons, which induce nuclear fission.
The Nuclear and Industrial
Safety Agency (NISA) instructed the company to consider injecting boric acid
earlier in the day.
TEPCO also plans to increase
the amount of cooling water by 3 tons per hour.
At a news conference on Feb. 6,
Haruki Madarame, chairman of the Nuclear Safety Commission of Japan, told
TEPCO and NISA to keep the public informed.
“We expect them to address the
public’s concerns by methodically explaining what could happen and how they
plan to deal with it,†Madarame said.
A TEPCO official said there
were no signs that the melted fuel had reached criticality. The official said
the level of radioactive xenon, an element with a short half-life, remained
below a measurable detection limit, and that monitoring devices around the
nuclear power plant have not detected a rise in radiation levels.
TEPCO officials said the rise
in temperatures was not steep enough to indicate that criticality had been
reached.
However, a temperature of 80
degrees or more at the bottom of the pressure vessel would ring alarm bells.
TEPCO has assumed a margin of error of up to 20 degrees for the thermometers
in the reactor because it is not clear what damage was done to them by the
Great East Japan Earthquake.
Therefore, a reading exceeding
80 degrees could mean an actual temperature of more than 100 degrees,
compromising the reactor’s status as being in cold shutdown.
The rise in temperatures
appears to coincide with changes in the flow of water through two separate
systems for cooling the No. 2 reactor: the feed water system and the core
spray system. TEPCO temporarily increased the amount of water being pumped
through the feed water system and reduced the amount of water going through
the core spray system as it strengthened outdoor piping in late January.
After that work was completed,
it gradually decreased the amount of water flowing through the feed water
system and increased the flow through the core spray system in an effort to
restore flows to the setup before the strengthening.
The thermometer that has
produced the high readings is located just under the feed water system. Its
temperature readings rose when the water passing through the feed water system
was reduced and water going through the core spray system was increased.
Readings from two other thermometers at the same height in the reactor have
been stable at 44-45 degrees.
“The temperature may have risen
because water has not reached part of the fuel since the amount of water
through the feed water system decreased and the flow of water changed,†said
an official at TEPCO’s Nuclear Power and Plant Siting Division.
I updated the temperature and
water injection chart to show the data as released in the this morning
(link above):
source: http://ajw.asahi.com/article/0311disaster/fukushima/AJ201202070027
TEPCO also released a
temperature chart in a press
handout today.
It clearly shows the
stabilization, but no reduction, in the temperature. (Note the 0, 135, and 270
are NOT degrees of temperature, but refer to the location of the 3 thermometers
on the circular RPV in the accompanying picture.)
source: http://www.tepco.co.jp/en/nu/fukushima-np/images/handouts_120207_06-e.pdf
NHK also carries a story today
which makes clear TEPCO is not sure what is causing the temperature rise in Unit
2. Many press stories comment on the “unknown causeâ€.
Temperature remains
high at damaged reactor
An unknown rise in temperature at
one of the reactors at the damaged Fukushima nuclear plant is troubling its
operator. Tokyo Electric says the temperature hasn't gone down even after it
increased the volume of cooling water on Tuesday.
One of the thermometers
at the bottom of reactor No. 2 at the Fukushima Daiichi plant gradually rose to
about 70 degrees Celsius since January 27th. It had stayed around 45 degrees
before.
In an effort to lower the temperature, the operator increased the
amount of water sprayed on the nuclear fuel by 3 tons to 13.5 tons per hour
Tuesday morning.
But Tokyo Electric said readings were down only about 3
degrees after some 5 hours of operation, hardly showing signs of
improvement.
The utility said the flow of water in the reactor may have
changed after plumbing work in late January, causing difficulties in cooling
part of the melted nuclear fuel.
It added that no temperature rise has
been observed at 2 other thermometers in the same reactor and that it will
continue to carefully monitor the reactor.
TEPCO has been unable to
visually confirm conditions inside the reactors since the nuclear disaster last
March because of high radiation.
Tuesday, February 07, 2012 13:05
+0900 (JST) [video at link] http://www3.nhk.or.jp/daily/english/20120207_21.html
