Information obtained through past PCV internal survey

Here is a summary of the examinations conducted so far on the PCV internal survey.

(1) Unit 1 PCV internal survey

1. Objective: Inserting inspection device from PCV penetration of Unit 1 (X-100B penetration), conduct survey for the "information on the first floor grating in the PCV."
2. Method: In October 2012, the inside of the PCV was examined using a CCD camera and accumulated water was sampled. In April 2015, the outside of the pedestal was examined using a shape deformation robot (B1 examination).
3. Information obtained: The following information was obtained. Figure 1. shows part of the examination results.
1. ① No large damage on the existing facility (e.g. PLR pump, PCV inner wall and HVH)
2. ② Dose rate was approximately 10 Sv/h.
3. ③ PRL piping shielding unit confirmed fallen.
4. ④ The access route to the bottom of the D/W was confirmed but the deposits are scattered over a wide range.
Since Information ④ above had revealed that measures against deposits would be required when subsequent examinations are made or fuel debris is taken out, the procedure for examinations on the outside of the pedestal at the bottom of the PCV (B2 inspection) was reviewed and as a result, the examinations were postponed to fiscal 2016. No fuel debris was found in the survey this time.





Figure 1　　Results of Examinations on the outside of the Pedestal at the bottom of the PCV (Unit 1 B1 inspection)





4. Considerations: It can be estimated that the temperature at the periphery of the grating on the 1st floor might exceeded 328℃, which is the melting point of lead since PLR piping shielding units (lead wool mattress) have fallen.
5. Challenge: Back and forth motion should be used while checking the crawler portions. During the counterclockwise inspections, inspection crawler robot was stuck in the gaps between grating bars in the area between PLR pump and air-conditioning unit.

(2) Unit 2 PCV internal survey

1. Objective: To verify fallen objects on the platform, damage states, and access route to the periphery of the bottom of the PCV using the internal survey robot.
2. Method: In March 2012 and August 2013, dose rate measurements were made, the inside of the PCV was examined using a CCD camera, and accumulated water was sampled through the PCV penetration (X-53 penetration). An Inspection was scheduled in August 2015 based on an internal survey robot through the X-6 penetration.
3. Information obtained: The following information was obtained. Figure 2 shows some of the preparation processes of the examination.
1. ① The dose rates on March, 2012 and August 2013 were 31 and 73 Sv/h respectively. The does rate varies depends on the area and.
2. ② Although internal survey for the pedestal inside the PCV (A2 inspection) was planned, eluted materials were confirmed near the CRD hatch (X-6 penetration) and peripheral dose rate exceeded the assumption significantly. Consequently the scheduled examination was postponed to fiscal 2016 in order to take countermeasures to decrease the dose before conducting the examination.
4. Challenge: Timing of the inspection was postponed to FY2016 since the measures are required to reduce the radiation dose around the X-6. The future scope of the PCV repair including some peripheral areas will be required since low temperature history for the X-6 during the progress of the event is assumed, instead of leaching from X-6.

Figure 2　　Conditions Revealed during Preparations for the PCV internal survey (Unit 2)

(3) Unit 3 PCV internal survey

1. Objective: The inspection were aimed to check, in particular, that the inside of the PCV is properly kept cooled inserting inspection instruments (a camera, thermometer, and dosimeter) from the PCV penetration of Unit 3 as well as to obtain information that would be useful in reviewing the examination method to be used in the future.
2. Method: In October 2015, dose rate measurements were made, the PCV internal survey using CCD camera, and accumulated water was sampled.
3. Information obtained: The following information was obtained. Figure 3 shows part of the examination results.
1. ① No damage was found on the examined sections of structures or walls.
2. ② No damage was found on the examined sections of the X-6 penetration and CRD rail.
3. ③ Deposits were found on the CRD rail and the grating on the first floor.
4. ④ Sediments were observed on the CRD rail and gratings on the 1st floor. (transparency under the water inside the PCV was fine). .
5. ⑤ Water level inside the PCV was OP: approx. 11,800mm. Almost consistent with the estimated value.
6. ⑥ The maximum radiation dose detected in the gas phase inside the PCV was approximately 1Sv/h.
7. ⑦ The quality of the accumulated water inside the PCV indicated that the inside of the PCV is not in a severely corrosive environment; rather, it is in a less corrosive environment.
At present, an examination of the inside of the pedestal of the PCV is being considered that is based on such as underwater swimming device put into the PCV through the X-53 penetration.
4. Consideration: The radiation dose inside PCV is the lowest among the Units 1-3. This is considered to be because of shielding due to high stagnant water level.
5. Lesson Learned: Water level coordination or waterproof equipment will be required for PCV internal survey since stagnant water level is high.

Figure 3　Results of (Preliminary) PCV internal survey (Unit 3)

(4) Results of the Sampling of the Accumulated Water

Figure 4 shows the analysis results of the concentration of Cs-137 in the accumulated water sampled during the PCV internal survey. Since the samples were taken from different units at different timings, it is difficult to determine the trend for a specific unit. However, as a whole, the concentration of Cs-137 is on the decline because water treated with an adsorption system for Cs and Sr multi-nuclide removal system has been injected into the PCV as cooling water. Figure 5 shows the analysis results of the concentrations of Cs-137 in the accumulated water sampled in the T/B, building for processes, and high-temperature incinerator building. Although the T/Bs have penetration and affected by the connections of drainage from neighboring facilities, the concentrations in the accumulated water in the T/Bs for Units 1, 2, and 4 are gradually decreasing. For all of Units 1, 2, and 3, the concentrations of Cs-137 in the accumulated water are lower inside the PCVs than in the torus rooms. For each unit, at the time of the accident, the SR valve was in action and the volatile FPs inside the reactor were introduced to the cooling water inside the S/C. In addition, for Units 1 and 3, the volatile FPs were introduced to the cooling water inside the S/C even before ventilation. With these facts, it is assumed that Cs-137 has been released from the S/Cs although no measurement has been made of the accumulated water inside the S/Cs.

Figure 4 Concentrations of Cs-137 in the Sampled Water

Figure 5　Concentrations of Cs-137 in the Water Sampled in the Turbine Buildings and Water Injection System Diagram