1. Strategic Plan Basic Concept
1.1. The Strategic Plan is designed as a mid- and long-term risk reduction strategy
1.2. Five Guiding Principles for risk reduction: ”Safe, Proven, Efficient, Timely and Field-oriented”
1.3. Classification of major risk sources
1.4. How to reduce risks (Fuel debris and waste management)
1.2. Five Guiding Principles for risk reduction: ”Safe, Proven, Efficient, Timely and Field-oriented”
1.3. Classification of major risk sources
1.4. How to reduce risks (Fuel debris and waste management)
1.1. The Strategic Plan is designed as a mid- and long-term risk reduction strategy design
Despite currently being maintained at a certain level of stability, the Fukushima Daiichi NPS is in a state that differs from other nuclear power stations in that there is building damages, the presence of fuel debris*1 and spent fuel, water contaminated with radioactive materials and various types of radioactive waste. The possibility of risks associated with radioactive materials arising during the course of future decommissioning work cannot be denied.
The risks posed by radioactive materials in the decommissioning of the Fukushima Daiichi NPS are recognized as being greater than those encountered during the decommissioning of other nuclear power plants.
For this reason, the basic policy behind the decommissioning of the Fukushima Daiichi NPS is to continuously and promptly reduce the risks associated with radioactive materials generated by the accident that do not normally exist in nuclear power plants. Therefore, the Strategic Plan can be considered as a mid- and long-term "risk reduction strategy design.”
*1 Fuel debris: Molten and solidified fuels
For details, please refer to 3. Basic Concept of the Strategic Plan in the Strategic Plan 2015.
The risks posed by radioactive materials in the decommissioning of the Fukushima Daiichi NPS are recognized as being greater than those encountered during the decommissioning of other nuclear power plants.
For this reason, the basic policy behind the decommissioning of the Fukushima Daiichi NPS is to continuously and promptly reduce the risks associated with radioactive materials generated by the accident that do not normally exist in nuclear power plants. Therefore, the Strategic Plan can be considered as a mid- and long-term "risk reduction strategy design.”
*1 Fuel debris: Molten and solidified fuels
For details, please refer to 3. Basic Concept of the Strategic Plan in the Strategic Plan 2015.
1.2. Five Guiding Principles for risk reduction: “Safe, Proven, Efficient, Timely and Field-oriented”
The following have been established as the Five Guiding Principles for risk reduction in the decommissioning of the Fukushima Daiichi NPS. Technical options are narrowed down with these key words in mind.
For details, please refer to 3. Basic Concept of the Strategic Plan in the Strategic Plan 2015.
(1) Principle 1: Safety Reduction of risks*2 posed by radioactive materials and ensuring workers' safety
Recognizing the high level of risk posed by the damaged reactors, it is important to make efforts to effectively secure safety and reduce risks, by putting priority on "immediate risk reduction to achieve safe and stable conditions."
With regards to workers' safety, sufficient attention must be given to preventing accidents and injuries. As work is performed in a highly radioactive environment, management of work hours, installation of radiation shielding, and use of protective equipment must be ensured in an effort to reduce exposure.
*2 Reduction of risks posed by radioactive materials: Impact on the environment and exposure to workers.
With regards to workers' safety, sufficient attention must be given to preventing accidents and injuries. As work is performed in a highly radioactive environment, management of work hours, installation of radiation shielding, and use of protective equipment must be ensured in an effort to reduce exposure.
*2 Reduction of risks posed by radioactive materials: Impact on the environment and exposure to workers.
(2) Principle 2 : Proven Highly reliable and flexible technologies
As a way to carry out measures that are required to be taken in a relatively short term, in order to minimize the risk of developmental failure, feasible technologies and knowledge available in Japan and abroad with a high TRL (Technology Readiness Level) should be adopted and applied by improving them so that they meet site conditions at the Fukushima Daiichi NPS.
The technologies should be selected, which enables flexible application to unexpected situations or changes in conditions. Work should be carried out in an phased manner so that the course of action be adjusted as necessary. It is also important to put alternative plans in place.
The technologies should be selected, which enables flexible application to unexpected situations or changes in conditions. Work should be carried out in an phased manner so that the course of action be adjusted as necessary. It is also important to put alternative plans in place.
(3) Principle 3: Efficient Effective utilization of resources (human, physical, financial, space, etc.)
Human:
It is necessary to plan and manage the total exposure for each worker during the period of work in order to secure manpower over a long period of time in the highly radioactive environment. Human resource development and the next generation technology transfer are also important.
Physical:
It is likely that any equipment or item brought into the Fukushima Daiichi NPS will be treated as radioactive waste. In order to reduce the amount of waste, unnecessary objects should not be carried in and anything brought onto the site must be effectively utilized to the fullest.
Financial:
All activities must always be carried out considering cost effectiveness of the work and the effect of the investment on technical development and facilities.
Space:
Available space must be used in an effective way, including the preparation and securing of work areas and transport routes, since a vast area of land is necessary for contaminated water tanks and temporary waste storage facilities.
(4) Principle 4: Timely Awareness of time axis
It is important to set a specific target time for “actions to be taken promptly,” “actions that require steady implementation” and “actions to be carried out over a long time.” In addition, multilayered preventive measures against project risks are also important to avoid time loss and additional rework.
On the other hand, “timeliness” is not the immediate focus of long-term issues such as waste management and decommissioning. However, since new regulatory systems and standards may need to be established for the treatment of the damaged nuclear power station and the generated waste, sufficient lead time should be taken into account.
On the other hand, “timeliness” is not the immediate focus of long-term issues such as waste management and decommissioning. However, since new regulatory systems and standards may need to be established for the treatment of the damaged nuclear power station and the generated waste, sufficient lead time should be taken into account.
(5) Principle 5: Field-oriented Emphasize the Three Actuals (“the actual place, the actual parts and the actual situation”)
Through understanding of precise need based on actual site conditions, actual structures, systems and components, and what is actually happening at the site, technologies needs to be chosen focusing on the applicability to the site. The site applicability process assesses whether the technologies undergoing feasibility studies are actually applicable to site conditions and the environment at the Fukushima Daiichi NPS. (Environmental resistance (radiation, temperature and humidity, light intensity, etc.) accessibility, transportability, and work space.)
For details, please refer to 3. Basic Concept of the Strategic Plan in the Strategic Plan 2015.
1.3. Classification of major risk sources
Since the containment function of the Fukushima Daiichi NPS was lost due to the accident, it is important to identify the risk sources that require countermeasures to be taken and determine the order of priority and approaches for the risk reduction for each of the identified risk groups.
The concept of the risks posed by radioactive materials is shown below.
The level of risk is shown through the relationship between the “hazard potential*3”on the vertical axis and “likelihood of loss of containment function” on the horizontal axis, and it is higher in the upper right side and lower in the lower left side.
*3 Hazard potential: degree of impact posed by harmful materials
The level of risk for major risk sources in the Fukushima Daiichi NPS is shown below.
In order to ensure the reduction of risks posed by radioactive materials, it is necessary to identify risk sources, analyze these risks, and determine the order of priority of measures for risk reduction. Major risks existing in the Fukushima Daiichi NPS are classified into the following three types:
For details, please refer to 3. Basic Concept of the Strategic Plan in the Strategic Plan 2015.
The concept of the risks posed by radioactive materials is shown below.
Fig. 3-1 Level of risk
The level of risk is shown through the relationship between the “hazard potential*3”on the vertical axis and “likelihood of loss of containment function” on the horizontal axis, and it is higher in the upper right side and lower in the lower left side.
*3 Hazard potential: degree of impact posed by harmful materials
Hazard potential:
Defined as material with properties considered in terms of ease of leakage or migration contained within the total amount of radioactive materials in the risk source. Typically, radioactive materials are in the form of gases, liquids and solids. The likelihood of leakage or migration and dispersion differs in the event of loss of containment function decreases in the order of gases, liquids and solids. Other properties include powders and radioactive sludge*4.
＊4 sludge： Substance like mud including radioactive materials
＊4 sludge： Substance like mud including radioactive materials
Likelihood of loss of containment：
Likelihood of the frequency of initiating events (loss of power supply, internal fire, inundation, hydrogen explosion, malfunction, erroneous operation, earthquake, tsunami, volcanic activity, tornado, external fire, typhoon, etc.) and the integrity of the facility (building, facility etc.) that contains the risk source.
Major risk sources:
- Fuel-related (fuel debris and fuel assemblies in the pools)
- Contaminated water-related (contaminated water in the buildings, trenches, and tanks)
- Waste-related (waste adsorption column of water treatment system, waste sludge from water treatment system and solid radioactive waste.)
- Contaminated water-related (contaminated water in the buildings, trenches, and tanks)
- Waste-related (waste adsorption column of water treatment system, waste sludge from water treatment system and solid radioactive waste.)
The level of risk for major risk sources in the Fukushima Daiichi NPS is shown below.
Fig. 3-5 Risks at the Fukushima Daiichi NPS
In order to ensure the reduction of risks posed by radioactive materials, it is necessary to identify risk sources, analyze these risks, and determine the order of priority of measures for risk reduction. Major risks existing in the Fukushima Daiichi NPS are classified into the following three types:
Classification of major risk sources existing at Fukushima Daiichi
(1) Risks to be addressed as soon as practicable：
(2) Risks to be addressed safely, effectively and carefully with thorough preparation and technologies to realize more stable conditions：
(3) Risks to be addressed on a long-term basis：
Contaminated water in trenches, buildings and tanks, and fuel assemblies in the pools.
(2) Risks to be addressed safely, effectively and carefully with thorough preparation and technologies to realize more stable conditions：
Fuel debris.
(3) Risks to be addressed on a long-term basis：
Waste adsorption column of water treatment system, waste sludge from water treatment system and solid radioactive waste.
For details, please refer to 3. Basic Concept of the Strategic Plan in the Strategic Plan 2015.
1.4. How to reduce risks (Fuel debris and waste management)
Risk reduction strategies for the Fukushima Daiichi NPS are summarized in the following figure (logic tree).
Fig. 3-7 Logic tree for reduction of risks posed by radioactive materials
For safe and steady retrieval of fuel debris, thorough preparations will be required and there are a number of challenging issues that lie ahead. The Strategic Plan (Chapter 4) therefore mainly studies fuel debris retrieval.
Fig. 3-8 Logic tree on risk reduction for fuel debris
Fig.3-9 Logic tree on risk reduction for waste
For details, please refer to 3. Basic Concept of the Strategic Plan in the Strategic Plan 2015.