Unit 2: What May Be Going On Inside

Water flow changes at unit 2 likely contributed to the recent heat up. Why changes back and forth between the feedwater system and the core spray system caused a temp increase gives some hints at the location of at least part of the melted fuel.

Location of the sensor:
There was one sensor with an increasing temperature. Other sensors around the same level inside the reactor were not increasing as this one sensor was.
Sensor location signified by the green line. It is roughly 30 degrees off of the zero mark used on TEPCO’s diagram.

This puts the sensor on a plane as shown in these images. The sensor is in the RPV, lines drawn longer in some cases to clearly illustrate location.


The side of the RPV where the sensor resides also happens to be the side where the scope investigation was done

The combination of changes to the watering with the feed water system and the core spray system caused the one area to heat up inside the RPV. A paper on the TMI accident shows the melt of the fuel inside the reactor there slumped to one side of the RPV, burned through the side of the baffle layer and began to damage the core barrel steel.

This shows that a fuel melt doesn’t always happen in a uniform manner. Fuel inside unit 2 at Daiichi could have burned a hole through the side of the shroud, allowing some fuel to reside inside
the shroud and some outside the shroud. The raising, lowering and rebalancing of water flow between the 2 systems could have removed a combination of watering that fed water to both sides of a section of corium that had burned through the shroud.  Now with both water systems running at higher volumes the temps appear to be going down.

The orginal work that started the heat up problem and water flow changes:
To improve reliability of water injection to a reactor, line connecting to injection piping in water injection line of reactor injection pump on the hill had replaced with polyethylene piping.. At 10:55 am in February 2, we adjusted water injection volume to the reactor of Unit 2 through the feed water system from approx. 3.9 m3/h to approx. 3.0 m3/h, and the volume from the reactor core spray system from approx. 5.1 3/h to approx. 6.0m3/h. We confirmed the decrease in volume of injecting water to reactors, so at 3:15 pm on February 2, we adjusted volume of water injection in each reactor as follows: Feed water system from approx. 2.5 m3/h to approx. 3.0 m3/h, Reactor core spray system from approx. 5.2 m3/h to approx. 5.5 m3/h.

Feb 1st statement:
After we finished replacing the water injection line from the reactor water injection pump on the hill to polyethylene pipe in order to improve the reliability of water injection to the reactor, we have been changing the water injection volume to the reactor gradually. At 11:50 pm on February 1, we adjusted the water injection volume to the reactor through the feed water system from approx. 5 m3/h to approx. 4 m3/h, and the volume from the reactor core spray system from approx. 4 m3/h to approx.5 m3/h.

Boric Acid Injection
“We injected boric acid into the reactor as a safety countermeasures against the re-criticality from 0:19 am to 3:20 am on February 7,”

TEPCO temps at TE-2-3-69H1 in unit 2
2/4 23:00      66.1
2/5 5:00        67.4
2/5 11:00      68.6
2/5 17:00      69.4
2/5 23:00     70.3
2/6 5:00       70.6
2/6 11:00     71.0
2/6 17:00     69.2
2/6 23:00     69.9
2/7 5:00       72.2
2/7 11:00     69.6
2/7 17:00     68.5
2/7 23:00     67.4
2/8 5:00       66.7

Feedwater data

Feb 8 – 3pm (JST)
Feed Water System: Approx. 6.5 m/h
Core Spray System: Approx. 6.8 m3/h.

Feb 8 – 11am (JST)
Feed Water System: Approx.6.4 m3/h
Core Spray System: Approx.6.8 m3/h

Feb 8 – 5am (JST)
Feed Water System:Approx 6.5 m3/h
Core Spray System:Approx 6.8 m3/h

Feb 7 – 3pm (JST)
Feed Water System:Approx 6.7 m3/h
Core Spray System:Approx 6.7 m3/h

Feb 6 – 11am (JST)
Feed Water System:Approx 6.8 m3/h
Core Spray System:Approx 3.8 m3/h

Feb 5 – 12:52am (JST)
Feed Water System:Approx 5.8 m3/h
Core Spray System:Approx 3.8 m3/h

Feb 4 – 11:00am (JST)
Feed Water System:Approx 4.8 m3/h
Core Spray System:Approx 3.8 m3/h

Feb 3 – 7:20pm (JST)
Feed Water System:Approx 4.9 m3/h
Core Spray System:Approx 3.8 m3/h

Feb 3 – 3:00pm (JST)
Feed Water System:Approx 2.9 m3/h
Core Spray System:Approx 5.7 m3/h

Feb 2 – 10:55am (JST)
Feed Water System:Approx 3.0 m3/h
Core Spray System:Approx 6.0 m3/h

Feb 1 – 11:50am (JST)
Feed Water System:Approx 4.0 m3/h
Core Spray System:Approx 5.0 m3/h

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