Fukushima Unit 2 - Xenon gas detected

Fukushima Update #80, February 13 & 14, 2012

by Nelle Maxey

Xenon gas has been detected in Unit 2.
TEPCO's press handout from this morning has the details.
An ENEnews story translates the amounts in the press handout:
Sampling time: February 13, 2012, 16:24-16:54 (charcoal filter)
• Xe-133 @ 0.016 Bq/cm3 (5 day half-life) or 16,000 Bq/m3
• Xe-135 @ 0.023 Bq/cm3 (9 hour half-life) or 23,000 Bq/m3
This is bad news as the detection of this gas points to re-criticality occurring in Unit 2.
We will just have wait to see how NISA and TEPCO handle this now.
I can only assume more boric acid will be injected into the reactor.

Here is a discussion from Wikipedia on the last incident of Xenon detection at Unit 2 last November. For comparison the levels are higher this time with 16 and 23 parts per thousand per cubic centimeter.

"Concerns over re-criticality
On 1 November 2011 TEPCO said that Xenon-133 and Xenon-135 was detected in gas-samples taken from the containment vessel of reactor 2, in a concentration of 6 to 10 (or more) parts per million becquerels per cubic centimeter. Xenon-135 was also detected in gas samples collected on 2 November. These isotopes are the result of nuclear fission-reaction of uranium. Because the short half-lifes of these gases: (Xe-133: 5 days Xe-135: 9 hours), the presence could only mean that nuclear fissions were occurring at some places in the reactor. Boric-acid was poured into the reactor in an attempt to stop the fission-reactions. No significant change in temperature or pressure was found by TEPCO, so there was no sign of large-scale criticallity. The reactor-cooling was continued, but TEPCO would examine the situation at reactor 1 and 3 also.[213][214][215][216] Professor Koji Okamoto of the University of Tokyo Graduate School made the comment that localized and temporary fission might still happen, and that the melted fuel could undergo fission, but the fuel was probably scattered around. However, neutrons from radioactive materials could react with the uranium fuel and other substances. Self-sustaining chain reactions were unlikely, thanks to the huge amounts of boric acid that have been poured into the reactor…."


February 13, 2012

The mystery deepens. Here are some reports from the Japanese press on the situation.

Hot reading at reactor blamed on faulty gear
By Kazuaki Nagata and Minoru Matsutani, Staff writers
"The temperature in the pressure vessel of reactor 2 at the Fukushima No. 1 nuclear power plant exceeded 90 degrees Monday, Tokyo Electric Power Co. said, but discounted the reading as the result of a faulty thermometer.
"Two other thermometers at the same height – about 3 meters from the bottom of the pressure vessel – read 33.1 and 32.9 degrees at 10 a.m., while one read 91.2 degrees.
"After more water was injected into the reactor at the troubled Fukushima Prefecture plant Sunday afternoon, the two thermometers in accord showed a drop in temperature while "the problematic one has stayed high, so we think it's more likely now that the thermometer is broken rather than the temperature is actually rising," Junichi Matsumoto, a Tepco spokesman, said at a morning news conference…."
Source: http://www.japantimes.co.jp/text/nn20120213x1.html

NHK reports briefly as follows:
TEPCO: broken thermometer may show high temperature
"Tokyo Electric Power Company says a malfunctioning thermometer at the disabled Fukushima Daiichi nuclear power plant is likely to blame for the high temperature reading in one of the reactors.
"The reading of one of the thermometers at the bottom of the No. 2 reactor began to rise late last month. On Sunday the temperature exceeded the critical safety threshold of 80 degrees. On Monday the reading rose to 94.9 degree Celsius at noon.
"The utility firm says it thinks the thermometer is broken since the readings of 2 other thermometers set at the same height dropped to about 33 degrees.
"The company says an inspection showed that a cable inside the thermometer is probably cut, resulting in a false reading."
Source: http://www3.nhk.or.jp/daily/english/20120213_28.html

Just to clarify the last statement in the NHK report, there was no "inspection" since the sensor is located on the outside of the RPV behind a concrete shroud inside the primary containment vessel which no one can enter. What TEPCO did was an electrical test from the control room of Unit 2.
SKF as usual has the most complete report on the press conference this evening (Japan time). You will see there is much questioning regarding what is going on.
TEPCO Evening Press Conference 2/13/2012: Reactor 2 RPV Thermocouple Is Broken, Matsumoto Says
Press conference live link is here.[in Japanese]

Reactor 2 RPV's thermocouple that's been going up seems to have finally broken. (Or else…)

15:00 285.4 degrees Celsius
16:00 260.9
17:00 275.9

The last time this particular thermocouple went to that level, it was March 2011.
TEPCO was measuring the electrical resistance of the thermocouple before the temperature suddenly shot up to 285 degrees at 3PM.
Resistance 500 to 530 ohm.
The instrument has totally failed, says TEPCO's Matsumoto.
Interesting question from a reporter from Nico Nico:
Q: Are you going to test the other two thermocouples at the bottom of the RPV?
A: No.

Yomiuri: Did measuring the resistance break the thermocouple?
A: We don't know why the temperature shot up after we finished the measurement.

Yomiuri: Do you know when this thermocouple broke?
A: At least, until the end of January it was showing the same trend as the other two. We want to carefully compare with other thermocouples.

Jiji Tsushin says right before 3PM, the temperature was 342.2 degrees Celsius.


Q: Who decided that the thermocouple was broken? Was the manufacturer consulted?
A: TEPCO decided. We have experience in maintenance of the thermocouple. (Matsumoto sounds very testy.)

Q: Is there a possibility of a thermocouple showing temperature lower than what really is and therefore it is broken?
A: If this thermocouple were correct, there would be other thermocouples that would show higher temperature. The other temperatures are trending down. So we think this particular thermocouple is broken.

Q: The temperature rise in early February – was it related to the instrument failure now?
A: We think so. But we didn't know at that time whether it was actually a rise in temperature or the instrument failure. The temperature did go down after increasing the water injection.

Q: How reliable is it to judge "recriticality" by xenon-135?
A: We think it is a reliable indicator.

Q: When was 342.2 degrees Celsius recorded?
A: We finished the testing at 2:54PM. So it must be between that time and 3PM. We'll have to check.

Q: How high did the temperature go? (looking at the graph that was provided)
A: It went overscale, so the graph shows temperatures like that [over 400 degrees Celsius]."

Here is the TEPCO one-second instrument recorded temperature chart they are looking at in the final question above:

The original is larger and you can see that the sensor went off scale right after TEPCO took the resistance reading at 2:56pm. The wide blue band is high temperatures in the 250 degree range prior to the testing. Source: http://www.tepco.co.jp/en/nu/fukushima-np/images/handouts_120213_10-e.pdf

This is the second sensor which has failed in Unit 2 in the same general location at the bottom of the RPV. If you remember from previous postings, the CRD Housing temperature sensor slowly climbed in mid-January and then soared off scale, then plunged into the negative off scale. For a few days there were no recordings of its temperature in the daily parameter reports. Then the temperatures began to be recorded again (with no explanation from TEPCO).
Nothing is settled yet.

5 Issues/yr — $25 print; $15 digital