Information Portal for the Fukushima Daiichi Accident Analysis and Decommissioning Activities

Estimation by actual unit investigation

Purpose

  1. To investigate the distribution of the fuel debris and the conditions inside the reactor vessels to obtain basic information for studying the fuel debris retrieval methods and condition of the radiation dose inside the reactors.

Current status

As investigation of conditions inside the actual reactor, inspections for specific locations, such as the inside of the PCV, RPV, S/C and torus room, and measurements of the fuel debris distribution using muon detection system are to be conducted. Muon detection technology is a measurement technology utilizing the characteristics of cosmic ray muon that scatters exclusively in high density materials and travels in a straight line if there is none. It is categorized to transmission method, scattering method and transmission method using nuclear emulsion plate depending on the detection method. Also, PCV internal survey can be categorized to the observation using a fiber scope camera, sampling and analysis of stagnant water, CCD camera, dosimeter and unmanned robot equipped with a thermometer.

The following is the status of the visual inspection using remote devices.

PCV/RPV internal survey

The internal PCV/RPV survey is an effective method to study the fuel debris retrieval policy by getting the information on the current status of the plant including the photographs, damage state of the equipment,radiation dose and temperature. This section describes the status of the study on the PCV internal survey for Units 1-3 performed in FY 2015 and study status of RPV internal survey.

(1) Unit 1 PCV internal survey

  1. Purpose: Collection of information of the 窶廨rating on the 1st floor inside the PCV.窶
  2. Method: Inspection device was inserted from the PCV penetration (X-100B penetration) and inspection of the outside of the pedestal (B1 inspection) was conducted using shape-changing robot on April 2015.
  3. Information obtained:

    1. No large scale damage on the existing facilities (e.g. PLR pump, wall inside the PCV, HVH) was observed. (No fuel debris was found.)
    2. Dose rate was approx. 10 Sv/h.
    3. PLR piping shielding units were confirmed fallen.
    4. The access route to the bottom of the D/W was confirmed but the deposits are scattered over a wide range.

  4. Considerations: It can be estimated that the temperature at the periphery of the grating on the 1st floor might exceeded 328 deg. C, which is the melting point of lead since PLR piping shielding units (lead wool mattress) have fallen.
  5. Issues: 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. Also, when installing thermometer after the B1 inspection, low visibility was occurred due to the sediments stirred up in the stagnant water.Therefore, the internal survey of the outside of the pedestal in the PCV (B2 inspection) was postponed to FY2016.

(2) Unit 2 PCV internal survey

  1. Purpose: 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: Inspection using internal survey robot inserted from X-6 penetration
  3. Information obtained:

    1. 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.

  4. Issues: 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 penetration during the progress of the event is assumed, instead of leaching from X-6. Also, multiple methods have been applied to provide decontamination to the areas close to the X-6 penetration but it takes time more than expected.

(3) Unit 3 PCV internal survey

  1. Purpose: Information collection to contribute to the verification of cooling state in the PCV and studies on the future investigation method.
  2. Method: The inspection device (camera, thermometer and dosimeter) was inserted via PCV penetration (X-53 penetration) in October 2015. The dose rate measurement, PCV internal survey using CCD camera, and stagnant water sampling were performed.
  3. Information obtained:

    1. Sediments were observed on the CRD rail and gratings on the 1st floor. (transparency under the water inside the PCV was fine)
    2. Water level inside the PCV was OP: approx. 11,800mm. Almost consistent with the estimated value.
    3. The maximum radiation dose detected in the gas phase inside the PCV was approximately 1Sv/h.

  4. Consideration: The radiation dose inside PCV is the lowest among Units 1-3. This is considered to be because of shielding due to high stagnant water level.
  5. Issues: Water level coordination or waterproof equipment will be required for PCV internal survey since stagnant water level is high.

(4) RPV internal survey

It is very effective to confirm the conditions of the fuel debris inside the RPV, structures and environment directly before the commencement of the retrieval work in order to carry out the retrieval work in a reasonable manner. As a result of the studies on the RPV internal survey method, the investigation accessing from the piping connected to the nozzle located on lateral side to the inside of the RPV, it was found difficult to reach the appropriate locations and conduct inspections from technical perspective. Accordingly, internal survey method which establishes holes on the shield plug and upper part of the PCV from the operating floor to access the inside the RPV was selected as a development object. The development to confirm the feasibility is currently underway. With regards to creating an opening in the upper part of the PCV as major technical issues, the possibility of sealing technology to control radioactive materials released from the inside of the PCV is confirmed through the element test. Also, the element tests were conducted for the technology to confirm feasibility of creating an access hole through the complicated internal structures from reactor core region from the operating floor level. In addition, the concepts of the system required for the RPV internal survey on the site was studied and it indicated the necessity of considerable preparation for the inspection-related systems including the measures to control the release of radioactive materials.

The requirements on the entire system during the boring and inspection are to be summarized as future issues. The detailed studies are to be conducted for technologies for the internal survey to be applied as well as the verification of the possibility. On the basis of the site conditions, the detailed studies are conducted for the improvement of reasonable plan for the RPV internal survey and timing of implementation for each Unit. Also, since the scales of the system may become larger, survey items and its degree of importance in the survey needs are to be studied combined with the technical FS. The evaluation will need to be performed from the perspective of cost effectiveness in consideration of the risk involved in the survey. It is important to conduct these evaluations systematically while making decision in appropriate timing.

Also, before the commencement of the development, it is desirable to perform the researches on the details of other technical development and technical information in Japan and abroad with incorporating the reasonable method flexibly.

Muon detection

Measurement of the fuel debris distribution using muon detection technology is as follows:

(1) Unit 1

The fuel debris distribution measurements were performed by muon detection of transmission method twice from February to May and from May to September, 2015. The results of these measurements indicated that in the original reactor core region indicated that there was neither water nor fuels larger than 1 m, which can be identified by muon detection using transmission method.

(2) Units 2 and 3

The distribution measurement of the fuel debris was performed by muon detection of transmission method using nuclear emulsion plate for Unit 2 and it implied there are no high density materials (fuels) in the reactor core regions.

Also, the measurement for the core region and RPV lower plenum by the transmission method has been started from May 2016. The evaluation will be performed after the data measurement for more than three months.

The measurement plan is also required for Unit 3.