Estimation based on knowledge and experiments


Current status

Estimation from the knowledge and experiments can be categorized into three cases, which are the estimation based on the past accidents and researches, engineering estimation on plant data and experiments conducted using simulated fuel debris.

The examples of core meltdown accident include the accident at TMI-2 and Chernobyl Nuclear Power Plant Unit 4. The findings to be obtained will be utilized in the estimation of the behavior inside the RPV and estimation by the MCCI. The researches performed in the past include the FP tests performed at the Phebus reactor in France and MCCI test at the U.S. Argonne National Laboratory. Those results were reflected to the models of the severe accident analysis code.

The engineering estimation based on the plant data is considered in line with the plant parameter, such as the fuel debris distribution evaluation by the heat balance method.

The simulated debris for testing was fabricated considering the severe accident phenomenon progress at the Fukushima Daiichi NPS in reference to the TMI-2. Using the simulated debris, the data on the mechanical,chemical and physical properties are collected.

In this section, the status of estimations for fuel debris distribution based on the plant data and for the fuel debris properties based on the experiment using simulated debris is summarized.

Engineering estimation based on the plant data

(1) Estimation by heat balance method

The ratio of the fuel debris in the RPV and PCV was estimated based on the heat balance assuming that the temperature of the cooling water injected to the RPV is elevated to temperature of the accumulated water by the heat source (fuel debris) inside the RPV/PCV. That is the balance of the heat input (heat capacity of injected water and decay heat) and heat emittance (heat emittance from the PCV wall surface to the building or to the atmosphere and temperature elevation of cooling water by the fuel debris). The estimation results using heat balance method and the overview and estimation results of heat balance method are shown in this page (NDF Technical Strategic Plan 2016,page A-29,A-30,A- 31)

(2) Estimation based on the trend of plant parameter

The heat source (fuel debris) in the RPV was estimated based on the trend of the temperature around the RPV in the post-accident condition, water temperature of S/C, amount of injected water via feedwater (FDW) system and reactor core spray (CS) system. The FDW system is a system that introduces the water cooled by condenser into the RPV during the normal operation of BWR. If the integrity of the bottom of the RPV is maintained, the cooling water will accumulated inside the RPV and water level will be raised. However, since increase in the water level was not observed, it is assumed that the bottom of the RPV has been damaged and the cooling water was dropping from the damaged portion to the inside of the pedestal.This indicates that the water injection using the FDW system was not able to cool down the reactor core even it can cool down the bottom part of the RPV in the post-accident conditions. On the other hand, the CS system is a core spray system for the coolant loss accident and is installed along the walls of the core shroud immediately above the reactor core. In the CS system water injection, the cooling water is flowing through the space from the reactor core to the bottom of the RPV and the scape can be cooled down. Table 2 shows the estimation results of the fuel debris locations for each Unit.

Also,this page shows the estimation method described above and its results.

Table 2縲縲Results of estimation of fuel debris locations based on the trend of plant parameter

Estimation of fuel debris properties based simulated fuel debris experiments

The studies, such as on the retrieval, collection and storage of fuel debris will require the data on the features of the fuel debris located inside the reactor. Therefore, in addition to the knowledge obtained to date (e.g. TMI-2 accident and severe accident research), analyses and tests using simulated debris are performed and fuel debris properties are estimated based on these data.

Also, the developments are being carried out for the technologies required to analyze and measure the fuel debris which will be retrieved from the reactors.

(1) Understanding of features using simulated debris

  1. 1)  Characterization of metal debris
  2. Mechanical of metal oxides properties, such as of Zr (O), which is zirconium that oxygen is dissolved in, estimated to be contained in the metallic layer of the fuel debris, has been measured.

  3. 2) Characterization of the products caused by the reaction specific to the accident at the Fukushima Daiichi NPS.
  4. The following data is being collected: mechanical properties and the formation phase of simulated fuel debris forms a solid solution of fuel, oxides of stainless steels, FP elements, and sea salt compositions.

  5. 3) Characterization for the heterogeneity
  6. Since the evaluation of the heterogeneous properties as a large lump was difficult to be performed in Japan, the evaluation tests for the mechanical properties are being conducted for the large scale products of MCCI at CEA, France.

Also, at the National Nuclear Center, Kazakhstan, the data of the fuel debris in a powder form, which is the molten material that solidified by water cooling and physical property such as the particle size,density, phase of the condensed solidified materials has been obtained in the characterization test for the large scale molten and solidified metal ceramics.

In addition to the results of characterization using simulated debris described above, the fuel debris properties are estimated based on the knowledge obtained to date (e.g. experience from TMI-2 accident, severe accident research) and summarized in the list. In particular, the macro properties, e.g. compressive strength and uranium content, and micro properties e.g. the mechanical properties and thermal properties such as thermal conductivity are summarized based on the literature survey and experimental results of each location of the fuel debris estimated by the severe accident progression analysis.

Also, the estimation of the properties, such as external appearance and shape of the fuel debris are being carried out temporarily for the fuel debris which has been estimated by the severe accident progression analysis based on the TMI-2 accident and tests, in the RPV/PCV. The estimation results are shown in this page.(NDF Technical Strategic Plan 2016,page A-35,A-36,A-37)

Based on the needs of the information on the fuel debris properties which are required for the studies on the fuel debris retrieval, the list of the above properties will be updated in collaboration with the comprehensive analysis and evaluation of internal PCV condition.

(2) Analysis of actual debris properties

The properties of actual debris (e.g. mechanical properties and chemical composition) are required information for safe fuel debris retrieval. Therefore, the needs of analysis from the related projects are summarized as an analysis plan so as to collect the required information on the fuel debris properties reasonably and reliably.

Also, since the fuel debris formed in the state which has never been experienced before will be handled, the analysis flow is being studied comprehensively to identify the required development items, and the analytical techniques including the dissolution method of actual fuel debris and analysis method for chemical form are currently developed. In this regard, the studies are conducted for the cask required for transportation specimen with high radioactivity. These studies are to be conducted steadily according to the processes for fuel removal from SFPs and establishment of analysis and research facilities.

As a facility for analysis of the actual debris, testing facilities where specimen with high radioactivity can be handled will be required. Although the existing facility of JAEA located in Ibaraki district can be utilized at this moment, the facilities are not considered to meet a broad range of demands. Radioactive Material Analysis and Research Facility No.2 is, therefore, planned to be established in Okuma machi.

The studies on the plan for the analysis of actual fuel debris properties are to be revised including the priorities, frequency and timing of the analysis. Also it is important to reflect the studies to the specifications and operation method of the Radioactive Material Analysis and Research Facility No.2 appropriately. Also, the analysis needs in the stages of fuel debris retrieval, stable fuel debris storage, waste conditioning processing and disposal, and in addition, analysis needs over the mid and long term, such as the safe research needs of damaged reactors are to be satisfied in the establishment of analysis plan. With regard to the facility for analysis, it is appropriate to study the use of the existing facilities of JAEA in Ibaraki district, as needed.

To analyze and evaluate the obtained data, it is also require establishing a structure that incorporates the opinions from the experts in Japan and abroad.

The storage method, canisters and transportation casks of the specimen for analysis are required to be studied separately from the collection and storage for the full-scale retrieval. Also, the technical issues related to the exportation are required to be studied as needed.

Also, in parallel, the issues related to the analytical techniques using a small quantity of sample which can be obtained by the PCV internal survey are required to be carried out.