TEPCO – Incorrect data for Fukushima Daiichi Reactor 1 PCV pressure

Incorrect data for pressure at Primary Containment Vessel of Unit 1 at Fukushima Daiichi Nuclear Power Station

(Brief Overview)November 29, 2011

Tokyo Electric Power Company

1. Event

We noticed discrepancy of data for pressure at Primary Containment Vessel of Unit 1 (Hereafter D/W pressure) with other supervisory instrumentation on November 21, 2011.

Thereafter we checked and on November 25, we realized possibility that the methods of compensating the data for D/W pressure may be inaccurate.

We checked the methods of compensating the data, and found out that the data for D/W pressure from at 11:00 am on October 28 when we changed data sampling device from primary device to digital recorder have not been compensated.

Also, we found out that the compensation formula itself used from at 17:00 pm on May 11 is incorrect.

 

2. Examination

We have examined how data for D/W pressure became incorrect and found out following points: (Reference document 1&2)

(1)Event 1: Neglect of Compensation Formula Usage

 

①After the earthquake, regarding data for D/W pressure of unit 1, our operators visited Central Operation Room(COR), read figures at D/W pressure recorder(PR1602-16) and recorded the gained figures at datasheet

②On May 11, as operation aiming converging reactor, we checked input-output characteristics of D/W pressure recorder of unit 1(PT-1601-69, hereafter PT) and we found out the out of tune at output signals at PT(hereafter gauge drift)

※1. Since D/W pressure recorder as primary device gains the pressure data from PT and display it, we calculated amount of compensation needed from gauge drift and decided D/W pressure

※1 as the amount of read figures at D/W pressure recorder at COR and the calculated amount of compensation needed.

※1 We press released regarding this issue on May 12,2011 (Reference document 3)

 

③As a result of ② above, we recorded both data of D/W pressure recorder read by our operator at COR and the compensated data at data sheet.

 

④As to reduce exposure to radiation, Equipment Maintenance Division at Fukushima Daiichi Stabilization Center started installation of digital recorders and remote monitoring device (hereafter CRT) on this summer to be able to read the data at Main Anti-Earthquake Building, and regarding D/W pressure, the installation was done on September 5.

We started to use the digital recorder from at 11:00 am on October 28 because the installing digital recorders and CRT for other data was done and we could gain the data at CRT in Main Anti-Earthquake Building.

 

⑤Before changing the data collection method, with recognition of documents released by the Equipment Maintenance Division and comparison of read data and compensated data, operator misunderstood that the data shown at digital recorder is already compensated value and decided to change the way of recording to data sheet, such as the operator wrote down only the read data at digital recorder(Before this change, both read data and compensated data were written down at data sheet).

Thus, from at 11:00 on October 28, data shown at the digital recorder was written down on data sheet, instead of compensated data that should be recorded.

⑥In daily meeting held at headquarters together with Fukushima Daiichi and Daini Nuclear Power Station and other related offices, we realized the differentials between data spoken from Power Station and daily published data of D/W pressure of Unit 1.

Thus we confirmed to Power Station regarding compensation of D/W pressure in the evening on October 21.

⑦It seemed that the figures of D/W pressure shown at digital recorder did not compensate gauge drift, so staffs at Power Station checked in detail.

As a result, we realized that the data of D/W pressure from 11:00 am on October 28 written down on the data sheet were not the compensated data but the figures shown at digital recorder.

 

(2)Event 2: Inaccurate Compensation Formula Usage

(Reference document 4) (Operation at site)

①We realized gauge drift at output signals on May 11 when we checked input-output characteristics of D/W pressure recorder of unit 1.

②In ordinal occasion, we calibrate the gauge (arrange the output signals), but the Airborne radiation of the working area(1st floor of reactor building) was extremely high(12mSv/h) and so to reduce exposure to radiation, we decided to monitor the plant by using compensated data that we calculate as D/W pressure.

(How the compensation formula was derived)

 

① Two TEPCO’s employee derived the compensation formula on May 11

 

② One of the employees (Employee A) calculated the range of error by comparing the calibrated figures and read data at pressure monitor device. The other employee (Employee B) calculated the process figure using the result attained from the calculation done by Employee A, and tried to derive the compensation formula.

 

③ Employee A provided Employee B with the range of error (Percentages of range of error at 600kPa range of device measuring) at two points such as “40kPa (6.67% output)”and”600kPa (100% output)”.

 

④ Then Employee B calculated the process figure at that two points using data provided by Employee A

 

⑤ Employee B supposed to calculate the process figure by multiplying the range of error and 600kPa range of device measuring, but calculated by multiplying the range of error and two calculated pressure provided by EmployeeA respectively and join the two points in linear line, and thought the linear formula is the compensation formula.

 

⑥ We did not realize the inaccuracy in the compensation formula and so we derived incorrect D/W pressure figures from May 11 to October 28.

 

3. Probable Cause

We researched the cause of event 1 and 2 and the facts we found are as follows:

(1)Event 1: Neglect of Compensation Formula Usage With change data sampling device, Equipment Maintenance Division which changed data sheet design found the fact as follows:

① Trend of data gained by adding read figures at primary data sampling device and calibrated figures and data shown at digital recorder proceeded in similar figures

② In the document released from Equipment Maintenance Division says “data shown at digital recorder is similar to primary device and data written down on the data sheet”, and so Operation Maintenance Division realized that the data of digital recorder and primary device is the same.

③ Operation Maintenance Division thought that it is easy to calculate D/W pressure compensation by digitalization.

④ As above ①to③, operator who designed the new data sheet realized that the digital recorder shows the calibrated data.

⑤ The realization above ④ was strong enough that the operator did not confirm whether the data at digital recorder is before calibration or after calibration.

⑥ Other operators also did not questioned the decision of “digital recorder shows the calibrated data” made by the operator who designed the new data sheet because the data from digital recorder is similar to primary device and data written down on the data sheet

 

As above, following causes are estimated:

・The operator who designed the new data sheet misunderstood that “D/W pressure at digital recorder shows the calibrated figure from PT” and so changed the design of data sheet without any consideration of the misunderstandings

・This misunderstood realization was taken over by the other operators

 

 

(2)Event 2: Inaccurate Compensation Formula Usage

①It was unusual operation environment at site and so we monitor the D/W pressure by using compensation formula instead of on site calibration.

②During derivation of the compensation formula, division of rolls, checking methods, and who checks were not defined in detail, and enough checks were not done for derived compensation formula.

 

4. Measurement for Prevention of Recurrence

We correct the data and do following measurement to prevent from recurrence (Reference document 5&6)

 

(1)Event 1: Neglect of Compensation Formula Usage

①We define the devices which are calibrated and the content of calibration in clear and precise manner and have them in common. We give direction document of Operation Maintenance Division to all the operator that says when change in the said device is made, ask Equipment Maintenance Division whether calibration is needed or not.

(2)Event 2: Inaccurate Compensation Formula Usage

①It depends on the operation environment on site, but when gauge drift is realized, we calibrate the gauge on site in principal

②We check twice for verification document of the calibration

 

5. Influence Caused by the Incorrect Data

(1)Re-calibration of D/W pressure using accurate compensation formula

As we recalculated the figures using accurate compensation formula for the period of inaccurate compensation formula was used (from at 17:00 pm on May 11 to at 5:00 am on October 28), we found out that the incorrect data showed higher by range from 0.00183MPa to 0.00202MPa.

Also, as for the period of compensation formula was not used and monitored D/W pressure (from at 11:00 am on October 28 to at 5:00 on November 29), we found out that the incorrect data showed lower by range from 0.00028MPa to 0.00062MPa.  (Reference document 5)

(2)Influence for Monitoring D/W Pressure

Caused by the Inaccurate Compensation Formula In this occasion, the maximum difference of the pressure is around 0.002MPa and the corrected data keeps over atmosphere pressure.

Thus, oxygen that may lead hydrogen explosion would not come into D/W, we see no problem in safety issue.

Also, as the difference between the corrected data and incorrect data are almost fixed, we think we could monitor the Primary Containment Vessel.

 

(3)Research on Similar Cases

We checked our published data on the view of adequacy of data sampling device, whether calibration is done and the content of the calibration (if calibrated), we confirmed that there is no incorrect data other than D/W pressure of Unit 1.

Also, we checked data that are compensated based on input-output characteristics regarding our published data and devices used to monitor, and we found out only D/W pressure of Unit 1 matches this case.

 

Attachment-(2)

 

Chronological Order

  • March 11 Since the earthquake disaster, the data of D/W pressure recorder (PR-1602-16) had been used as D/W pressure of Unit1.
 
  • May 11 As a result of calibration of local indicator of D/W pressure (PT-1601-68) and characteristic test of D/W pressure converter (PT-1601-69), we found gauge drift in PT.
  • May 11 We made compensation formula which takes gauge drift into account, using input-output characteristic test results. At that time, we made it using incorrect data.
  • May 11 We reported at the general meeting that as a result of calibration of D/W pressure gauge, gauge drift was found and we also reported how to treat D/W pressure data with compensation formula.
  • May 11 From the timing of usual data collection at 5 pm , we changed the operation to regarding the compensated data which are summation of data of D/W pressure gauge (PR-1602-16) and gauge drift as compensated D/W pressure.
 
  • October 28 From 11 am, we changed the way of data collection from collection at COR and local location to via digital recorder using remote monitoring device (CRT) of Main Anti-Earthquake Building.
 
  • November 21 At the general meeting, we noticed that there is a difference between Unit 1 D/W pressure data which were informed by the power station and the D/W pressure data which had been disclosed everyday. Thus, at the headquarters daily meeting in the evening, device compensation of D/W pressure was questioned and we started the following survey on Nov. 21.
  • Confirmation on whether D/W pressure data had been compensated and whether compensation had been necessary
  • Interview with related staff (which includes interview with operational off-duty staff through phones)
  • Confirmation of how compensation formula of D/W pressure was calculated
  • November 25 As a result of the survey, we confirmed the possibility of errors in the way of how D/W pressure data had been compensated (with or without compensation and compensation formula of D/W pressure).  Later survey revealed that indicated data by digital recorder had been recorded since 11 am of October 28, in stead of the compensated data and that some of the values used for compensation formula had been incorrect.
 

 

 

 

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