The "3.11" Japan Fukushima nuclear accident was almost one year old. Although the three damaged reactors have achieved the goals set by the government to enter the phase of nuclear fuel removal and reactor decommissioning, the impact and aftermath of the accident are still evolving. The Japanese government wants to stabilize the situation and restart the safety review of qualified nuclear power units. The safety concerns of the public and local governments are major obstacles. Other countries in the world that are nuclear or prepared to develop nuclear power and the international community are conscientiously analyzing and drawing lessons from the Fukushima accident, formulating various measures to ensure nuclear safety, and restoring the normal development of nuclear power. There are still widespread differences and disputes in understanding. The “nuclear recovery” to cope with the growth of energy demand and the pressure of emission reduction has been suppressed, and the emission of greenhouse gases is increasing... How to treat the Fukushima nuclear accident and expand the future of nuclear power will require more work. Among them, the key is the "cognition" problem of the government decision-making level and the public.

The milestone significance of the Fukushima accident should be said that through extensive investigations and analysis, the cause of the Fukushima nuclear accident has become clear and the lessons learned should be gradually clear. Simply put, it is the catastrophic impact of an extreme external natural event combination on a fortified/prepared nuclear power plant. Fortunately, the consequences are not as severe. Relative to the deaths and other losses in the coastal areas directly caused by natural disasters, the impact of the Fukushima accident on the health of the surrounding residents is still "psychological" [1-3].

However, the understanding of the Fukushima accident must be further deepened and transformed by the entire society, especially the nuclear industry. The representative view is that the US Nuclear Regulatory Commission puts forward the investigation conclusion: “All incidents involving nuclear reactor core damage and uncontrolled leakage of radioactive material, even if they do not have a significant impact on public health, are essentially unacceptable”[4] ]. Taking all possible precautions and mitigation measures to ensure that “the accident of radioactive material leakage out of control” does not occur, it should become the safety goal pursued by the world’s nuclear industry.

As early as the beginning of the peaceful use of nuclear energy, the pioneers of nuclear science realized the characteristics of nuclear energy and proposed the fundamental principles for ensuring nuclear safety. This is reflected in the operation of nuclear reactors:

- Stop the reactor immediately, stop the chain nuclear reaction and allow the core to enter and maintain a deep subcritical state;

- The main circuit is depressurized, cooled, and discharges nuclear core decay heat;

- Maintain the integrity of the containment vessel and prevent radioactive material from entering the external environment uncontrollably.

From the initial stage of reactor development to the eve of the Fukushima accident, nuclear science and nuclear industry have been working under the guidance of the “Defense in Depth” philosophies, combined with operational experience feedback and forward-looking safety research to recognize and implement these three principles, so that the safety of operational nuclear reactors is constantly improved. Promote. The "milestone" significance of the Fukushima accident is to supplement, perfect, and correct the understanding of the nuclear science and nuclear industry: the impact of external events on reactor safety is underestimated, and the impact of extreme external event combinations on multiple piles of sites may have a negative effect. The "defense in depth" philosophy is flawed. Faced with a wide range of possible challenging scenarios, multiple barriers may be disintegrated. A simple defense may be "preventive" and requires a "transition" concept; the key to ensuring reactor safety is to ensure nuclear fuel cladding. Sealing must be based on “digging” to reliably discharge decay heat; the integrity of containment is more important than sealability and is the ultimate means of ensuring controlled release of radioactivity.

For the three principles of ensuring reactor safety, there is no disagreement between nuclear science and nuclear industry, and the general public can understand it. However, there are different views and understandings on the implementation and implementation of the three principles. The problem lies not in the methods and means of “stopping the reactor immediately, stopping the chain reaction, making the core enter and maintaining a deep subcritical state”, but rather the goal of “extracting heat from the reactor core” and the understanding of “safety shell integrity”.

Regardless of the cause of reactor accidents for any reason, the first task after an emergency shutdown is to “discharge” the remaining heat from the core, the goal being to maintain the tightness of the nuclear fuel cladding. Various measures have been taken to make the main system cool down and depressurize, and it is of primary importance to make full use of the existing cooling agent in the reactor system to carry the waste heat out of the core. The most extreme and most difficult condition is that the reactor loses "all" power (external power, internal AC, DC power, compressed air and control meter display), extensive infrastructure damage and even lose the main control room function, and "exhaust heat" There is no change in the "requirements".

For modern light water reactors, the time range in which the system itself maintains nuclear fuel cladding integrity under such extreme external conditions is related to stack type and design. As long as a portable emergency power system is used to start a usable emergency cooling system to fill the core within this time range, water vaporization in the core will discharge excess heat, which can resolve the crisis and earn further emergency preparation time (12 hours). In short, the “core creed” of nuclear safety is that the flow of water in the core can be used to win time, thereby avoiding the core melting that is so serious as to cause the Fukushima accident.[5]

Maintaining the integrity of nuclear fuel cladding is a prerequisite for maintaining the integrity of the containment vessel. Nuclear fuel cladding is complete, no zirconium oxide oxidation occurs, and the heat generation is much smaller. Even in the event of main system pipeline leakage and/or nuclear fuel cladding rupture, ensuring the integrity of the containment shell is still more important than tightness. The risk of radioactive release from the containment exhaust and even the exhaust gas in a timely manner is very small. The use of containment to filter exhaust gas (gas) measures is a controlled, consequence-predictive risk, which can at least reduce the amount of released radioactive total by 4-5. Order of magnitude [6].

The above-mentioned extremely difficult external conditions are not impossible or “unthinkable”. The accident at Fukushima corresponds roughly to the above, but it is not the worst. If the epicentre of the earthquake is closer to the power plant or the geological conditions are more unfavorable, there is a certain defect in the quality of the manufacturing of the reactor's important systems and equipment and the quality of the civil works, and damage or leakage may occur, and the situation will be more complicated. In this regard, even the most advanced light water reactor design cannot completely eliminate the possibility of core damage and radioactive release.

The concept of "filling water-exhausting steam" of the core "filling water-exhausting steam" is to make full use of the existing water source of the main loop system to keep the water in the core to keep flowing; to keep the integrity of the nuclear fuel cladding as the goal, to make the heap The water vaporization/evaporation inside the core exhausts the core waste heat, which will win further preparations for long-term reliable discharge of waste heat.

The PWR's main circuit system remains intact, and the steam generator (SG) can be used to reliably remove waste heat. The outdoor atmosphere is a reliable heat sink and there are many available water sources. Fire water is the most convenient choice. The discharged steam does not have any radioactivity and does not have any adverse effects on the environment.

Relative to boiling water reactors, there is a leakage in the pressure boundary of the main circuit of the PWR. The situation is more complicated because there are not so many passive residual heat removal systems. In order to ensure the integrity of nuclear fuel cladding, the only way is to reduce the temperature and pressure in the maximum allowable limit of the cladding material [7], to “convert” the PWR into a boiling water reactor, and to use “water filling-exhaust” The mode discharges thermal energy.

In the “post-Fukushima” era, modern nuclear reactors must be viewed as an extreme scenario that they may face and seriously analyze and study. It is the mission of the owners of nuclear power plants to analyze the best strategic actions that each reactor can actually take in this state. But regardless of the specific circumstances, the universally applicable, flexible, and simple means is that there are mobile power and water sources on site. Specific, simple, and convenient are battery cars, fire engines, and hoses. . . . . . According to recent reports from the United States Nuclear Research Institute, the emergency facilities that the nuclear industry voluntarily purchased are such devices [8]. Advanced reactors, some using "innovative" design to derive the reactor core heat (such as AP1000), and some to strengthen the various barrier capabilities (such as EPR), but now realize that sooner or later have the above-mentioned mobile power and water.

If so, with such simple measures, the safety of modern reactors can be at least a “step”. Because of the total reactor core damage frequency (CDF) in modern reactors, “whole plant blackouts” are the main cause (25-70% or even higher) [9].

This kind of thinking, Americans call it a "mitigation" strategy and include the category of "defense in depth" philosophy [4]. From the point of view of proactively resolving contradictions, the concept of "drowning" is more precise and the concept is clearer and more reasonable.

Initial Response - "Grooming" Concept Implementation Points To implement the "distraction" concept, we must pay attention to and give full play to people's subjective initiative. In past nuclear safety analysis, the negative aspects of “human factors” were “magnified” and lacked the “dialectical” view. Nuclear safety depends mainly on "people." In an extremely complex, difficult, or “rule-free” situation, people’s subjective initiative may even “play on the spot” and may win a full victory at a very small price. The “heroic behavior” and “reckless behavior” are not encouraged, but the professional ethics and sacrifice spirit of the nuclear industry must still be upheld in the most difficult, critical, and critical moments.

The implementation of the "drainage" concept mainly involves "software" improvements and minimum procurement. Changes in safety-related systems (equipment) are rare, and they are very subtle improvements. In the case of pressurized water reactors, the recommended “hardware” improvements for some designs of advanced reactors are:

- The valve control mode of the steam/diesel driven auxiliary feedwater system adds a "fault-input" mode.

- Regulator pressure relief valve control loop Add "continuous buck" control function. In the case of a leak at the main circuit pressure boundary, the water in the reactor core is maintained and turned into a “water-exhaust” mode, draining the core waste heat.

Implementation points:

- Conduct systematic research, analysis and preparation of the unit and the site to determine the time limits for action, "entry conditions" and action steps for different response strategies.

- The means of implementation must be strong, simple, flexible and easy to implement. With the simplest training, even on-site operations and even support staff (such as fire and security personnel) will perform.

- Power plant leaders and management must first receive training and “in-person” to participate in actual drills.

- The adaptability changes of power plant organization and staffing are made to clearly authorize the first responsible person and backup candidate at the site. The crew value is the "first" responsible person, and the on-site duty "safety engineer" (also called "on-duty technical consultant") is the most suitable "reserve" candidate. The safety engineer is affiliated with the nuclear safety department of the power plant and has separate duty positions and on-site inspection and inspection duties [10].

- When a serious incident occurs, immediately notify the emergency response command of the power plant and perform analysis and judgment at the same time. The initial response action can be taken within 30 minutes without a request or approval:

â–  PWR main loop system is basically complete:

● Manually open the main steam system atmospheric release valve within 30 minutes to ensure that the auxiliary water supply system starts normally to supply SG water, monitor and maintain the S** position (less than water), and simultaneously replenish the condensate tank;

● If the auxiliary feed pump fails, a mobile pump must be used to supply SG with water at a rate of 40m3/hour within 90 minutes.

â–  Leakage of the main system pressure boundary:

● Depending on the trend of the main system's step-down rate and core temperature, it is decided whether or not to implement continuous depressurization of the regulator pressure relief valve within 30 minutes. At the same time, determine the position of “water filling”, and continuously fill the core with 70m3/hour of flow within 90 hours to control the core temperature and pressure, and realize the “water filling-exhaust” operation mode.

●Depending on the evolution of the reactor core status, timely use of “additional” backup mobile water sources to achieve a “safety containment flooding” through 70 m3/h flow through the containment spray or low-pressure injection pipe.

● Continuously monitor the temperature and pressure in the containment vessel. Manually insert the containment vessel to filter the exhaust gas (gas) system at the appropriate time to ensure the integrity of the containment vessel.

- The most basic tools are illuminated "miner" helmets, means of communication and necessary electrical tools.

- The main information relied on is the core/SG temperature and pressure, and the water level indicator is not reliable. In order to ensure that the core nuclear fuel assembly is not exposed, the SG is not full of water, and the “correlation curve of core/S** position with temperature and pressure” should be plotted through simulation or accident analysis.

Initial Response – The “distraction” concept is implemented as a “bridge” between the normal operation of a power plant and the emergency response of a serious plant event and the “transition process” of emergency response to the plant in the event of a serious accident in a power plant. A good transition can create the best conditions for the follow-up response, restore normal cooling functions as soon as possible, and enter the “cold shutdown” state.

The accuracy of the time-frame for action and water flow quoted in the text must be combined with the specific design of the unit and validated using models/procedures (eg, commonly used MELCOR, MAAP) [11].

Simple Conclusion The milestone significance of the Fukushima accident lies in revealing that nuclear science and nuclear industry safety concepts need to be “transformed”: the key to ensuring nuclear safety is to maintain the integrity of nuclear fuel cladding; the philosophy of “defense in depth” needs the assistance of the “distraction” concept. “Water-exhaust” (not exhaust) is the ideal mode; the integrity of the containment is more important than the tightness, and the containment filter exhaust/gas is the last resort to reduce the effects of radioactive environment.

Serious accidents are small-probability events after all. Some of the exposed design defects need to be eliminated (such as system isolation and sealing), but the necessity and effectiveness of large-scale fixed defense methods (such as wave-breaking dams and fences) cannot be determined. Portable power and mobile water sources are more flexible and effective.

The safety performance of LWRs is constantly improving. Equipment manufacturing and construction quality are the material basis for ensuring nuclear safety, but the system and culture are serious challenges facing the nuclear industry. Human beings can absorb past experiences and lessons and be prepared to respond to design benchmarks and hyperdesign benchmark events at any time without repeating the "past stories."

It is entirely personal and has nothing to do with any organization or individual;

Note: (Citation data can be found on the Internet)

1. WNN, Low risk from major accident consequences, 02 February 2012

2. US NRC,MODELING POTENTIAL REACTOR ACCIDENT CONSEQUENCES,NUREG/BR-0359,January 2012

3. MIT, Technical Lessons Learned from the Fukushima-Daichii Accident and Possible Corrective Actions for the Nuclear Industry: An Initial Evaluation, MIT-NSP-TR-025, May 2011.

4. US NRC, Recommendations for Enhancing Reactor Safety in the 21ST Century, July 12,201

5. Declan Butler, France'imagines the unimaginable', Nature news 11 January 2012

6. The M310 reactor is designed with a containment filter exhaust system; the vaporization purification effect is ~100; the filter exhaust system purification effect is greater than 500, so the total efficiency is ~2x10-5.

7. David Lochbaum, director of UCS nuclear safety engineering, once pointed out that after the emergency shutdown of Unit 1 in Fukushima, the reactor core cooled and the heating rate reached 164°C/hour and 138°C/hour. Exceeding the specification (cooling rate of 55°C/hr), no problems were found, proving that the zirconium alloy cladding of boiling water reactor fuel can withstand high cooling rates without damage. For details, see David Lochbaum, Fukushima Dai-Ichi Unit 1: The First 30 Minutes, UCS, May 24, 2011

8. USNEI, U. S. Nuclear Industry Adopts Initiative to Acquire More Emergency Equipment,February 21,2012

9. INL, Analysis of Station Blackout Risk, NUREG/CR-6890, Vol. 2, December 2005

10. After the Sanli Island accident, many nuclear power plants in the world had added “on-duty technical consultants” or “safety engineer” positions to handle abnormal operations and major accident handling functions. Although this type of setting for many nuclear power plants remains, most of them are no longer “walking down”. It now appears that restoring "shifting" is an effective measure.

11. The citations for time frame for action and water flow are based on the following three data: 1) NE1, B. S. b Phase 2&3 Submittal Guideline, NE106-12 Revision 2, December 2006; 2) Shih-jen wang, Chun-sheng Chien, and Te-chuan Wang, Simulation of Maanshan TM LB`sequence with Melcor 1.8.3, Nuclear Technology, Vol. , 126 APR. 1999; 3) NUREG-1150-vol. 1 Severe Accident Risks: An Assessment for Five U. S. Nuclear Power Plants, Final Summary Report part-2,Summary of Plant Results,December 1990