Quantitative Risk Assessment Process of Fuel Assembly Retrieval from Spent Fuel Pool in Fukushima Daiichi Nuclear Power Plant Decommissioning

Transcription

1 PSA 2017 September 25-28, 2017 Quantitative Risk Assessment Process of Fuel Assembly Retrieval from Spent Fuel Pool in Fukushima Daiichi Nuclear Power Plant Decommissioning Presented by Akira Yamaguchi (University of Tokyo) Co-authors: Jang (UT), Narumiya (KEPCo), Yamanaka (TEPCo), and Hida (NDF)

2 Fukushima Daiichi Accident and Decommissioning Severe accident in Fukushima Daiichi Nuclear Power Station (FD-NPS) on March 11, 2011 Seismic-induced tsunami event Multi-unit event (core melt in three units) A few thousands of fuel assemblies were left in the spent fuel pools of four units Reactor buildings were seriously damaged and contaminated by release of radioactive materials and/or hydrogen explosion A risk management goal of the decommissioning project To control and reduce the risk of the FD-NPS so that the public and workers are not exposed to significant radiation and radioactive materials are adequately confined 2

3 Purpose of the Present Study To establish the risk management framework for the decommissioning of the FD-NPS on the basis of the best available knowledge and experiences. Prudent decision-making is required for every activity in the decommissioning project, especially given the public concerns on this activity. The Atomic Energy Society of Japan (AESJ) Has established the Risk Assessment and Management Working Group on the Fukushima Daiichi Decommissioning project 3

4 Risk Control and Management Strategy Hazard Potential / Potential Consequence Surveillance and control Risk with lower priority Risk with high priority Reduce hazard potential Likelihood of Loss of Confinement 4

5 Risk Management in Decommissioning Multiple risks are to be dealt with in parallel Fuel debris in containment vessels Fuel assemblies in the spent fuel pools (SFPs) Contaminated water Other solid wastes, etc. Risk characteristics are significantly different from those in an operating nuclear power plant Appropriate risk assessment strategy is needed Understanding of the risk characteristics Assigning priorities on individual tasks 5

6 Characterization of Risk Sources Risk with technical challenge Risk with high priority Risk with lower priority Source: NDF, Technical Strategic Plan 2016, July

7 Risk Significance of Hazard Source Severity of a risk/priority in risk management is determined by: Inventory of a hazard Mobility of a hazardous material Physical confinement performance Oversight and controllability, and Mitigation capability The risk can be controlled by Diminishing absolute amount of hazard Confining the hazard Mitigation of exposed hazard by improving abilities of anomaly detection and qualifying response to anomalies Using measurable metrics of the risk magnitude, priorities and resources are allocated to each of decommissioning activities in a rational and consistent way 7

8 Risk Assessment/Management Process Risk Triplet (Kaplan and Garrick) Risk is characterized and explained by answering three essential questions What can go wrong? How likely is it? and What are the consequences if it happens? Three risk surrogates have been proposed to be used in prioritization and decision making processes: Personnel risk Public risk, and Project risk 8

9 Four Principles of Risk Assessment Comprehensiveness All the possible scenarios are taken into consideration Efficiency Risk assessment is performed with reasonable cost and resources Measurability Risk is defined and evaluated as measurable quantities Effectiveness Effective and practicable risk management provisions can be proposed 9

10 Six Steps of Risk Management Step 1 Success Path Description Step 2 Threats Identification Step 3 Initiating Event and Likelihood Step 4 Failure Scenario and Likelihood Step 5 Consequences and Likelihood Step 6 Risk-Informed Decision Making Step 1: Describe the success path Step 2: Identify threats to each success path element Threat 1 Threat 3 Threat 2 A B C D E F End point Step 3: Determine Initiating Event Sequence 1 Sequence 2 Sequence 3 Sequence 4 Sequence 5 Sequence 6 Step 6: Risk insight and Decision Making Step 4: Quantify Failure Scenario Step 5: Evaluate Consequence and Likelihood 10

11 Steps 1 and 2: Combination of Success Path and Threats (Initiator) Threat Element of success path Sub-Endpoint Fuel Retrieval from Spent Fuel Pool Task Transport Fuel using Fuel Handling Machine System and Equipment Factor Societal Factor Management Factor Random failure Loss of power supply Natural hazard Earthquake (Small) Earthquake (Large) Typhoon / Strong wind Human error Public trust Malicious activity Miss operation Report minor incident Sabotage Project management Lack of workers 11

12 Step 3: Identification of Initiating Events Initiating Event Description For each combination of the task (element of success path) and threat, possible initiating event is described. Step 3: Initiating Event Countermeasure and Reasoning of Likelihood In the decommissioning, commonly applied countermeasures will be built in. They are considered in the evaluation of likelihood. Considering the initiating event and countermeasures, likelihood is evaluated. Reasoning of the countermeasures and the likelihood estimate is explained in detail. The reasoning will be used in the peer review of the risk assessment. Likelihood (H/M/L/VL) Likelihood is rated by H (high), M (medium), L (low) and VL (very low). Initiating events to which VL is assigned are excluded in the following evaluation. 12

13 Step 4: Failure Scenario Evaluation Failure Scenario With an initiating event, following scenario of system failure is identified. For this purpose, system analysis methods are used such as ET/FT method. Step 4: Failure Scenario (Likelihood and Consequence) Consequence, mitigation measure and Reasoning Analyzing the systems, whole scenario and consequences are shown. Considering mitigation measure, the consequence becomes mild. Commonly applied mitigation measures are considered. Reasoning of the consequence, likelihood and mitigation measures are explained as detail as possible. Likelihood (H/M/L) Likelihood is rated by H (high), M (medium) and L (low) based on the reasoning. Risk Importance Considering likelihood and consequences, risk importance is rated by H (high) and L (low). Failure scenario with L are excluded in the following evaluation. 13

14 Step 5: Prioritization of Failure Scenario Step 5: Scenario Prioritization Consequence Likelihood (H/M/L) Priority Consequence of failure scenario is evaluated in detail. If necessary, additional investigation is performed. Likelihood of failure scenario is evaluated in detail. If necessary, additional investigation is performed. With the consequence and likelihood, priority is determined for risk management 14

15 Three Step Screening and Prioritization Step 3 Initiating Event Frequency. Preventive Provisions are considered Step 4 Failure Scenario Frequency and Consequence. Mitigation measures are considered Step 5 Prioritization based on the Scenario Frequency and consequence If necessary, additional information is collected Three consequence type are considered: Personnel safety, public safety, and project schedule 15

16 First Application of Proposed Risk Management Approach Fuel Assembly Retrieval from Unit 3 SFP Operational Floor オペフロ全景 Reactor Building 原子炉建屋北面全景 N 撮影 :2011 年 3 月 24 日 撮影 :2012 年 2 月 21 日 N 撮影 :2013 年 10 月 11 日 撮影 :2013 年 10 月 11 日 16

17 Fuel Assembly Retrieval from Unit 3 SPF Risk management goal (three risk surrogates) No significant radiological material No Significant exposure and injury of workers No significant delay in the project Three phases of fuel assembly retrieval Preparation of Fuel Retrieval Fuel Retrieval Transport and Storage of Fuel 17

18 Underwater View of SFP of Unit 3, February, 2013 Source: TEPCo. Homepage 18

19 Phase I: Preparation Planning for Fuel retrieval Remove large rubbles in operational floor Remove large rubbles in the SF pool Decontamination and shielding Remove small rubbles in operational floor Training fuel handling Initiation of Fuel retrieval from the SFP Install cover for fuel retrieval Install fuel handling equipment Remove large/small rubbles (Environment) Install fuel handling equipment (Equipment) Training fuel handling (Training) 19

20 More Information on SFP October, 2015 Source: TEPCo. Homepage 20

21 Install Cover for Fuel Retrieval September, 2017 Source: TEPCo. Homepage 21

22 Phase 2: Fuel Assembly Retrieval Place On-site Transportation Cask Transport Fuel Close Primary Rid of Cask Carry Cask Out Close Secondary Rid of Cask To place fuel assemblies in a transport cask To transport fuel assemblies in the cask, and To carry the cask out safely Phase 3: Fuel Transport and Storage Dismantle Fuel Handling Equipment On-site Transport Cask Move Cask to Storage Pool Maintain Intact Storage in Pool Dry Cask Storage The retrieved fuel is firstly stored in the fuel storage pool and ultimately maintained in a dry cask 22

23 Quantification Method Selection of task Transport and Storage Fuel Selection of 10 Experts 2 Utility engineers 1 Regulator 1 Risk analyst 2 University professors 2 Vendor engineers 1 Decommissioning company engineer 118 Scenarios have been evaluated independently Ranking 1(least importance) to 10 (highest importance) 23

24 Conclusions Risk Assessment and Management Working Group on the Fukushima Daiichi Decommissioning project under AESJ Principles and framework of the decommissioning risk management has been proposed and developed. Screening processes are carefully employed so that the risk assessment is performed efficiently with limited resources without neglecting risk-dominant scenario. Whole the process is applied to fuel assembly retrieval work from the Unit 3 SFP. Risk assessment and management are required and the process needs to be updated in accordance with the most recent information and knowledge on the decommissioning works. 24