Technical Lessons Learned from the Fukushima-Daichii Accident and Possible Corrective Actions for the Nuclear Industry: An Initial Evaluation

Report Date: May 2011
Appendices: No
 
Premise

The accident at the Fukushima-Daichii nuclear plant has generated worldwide news and precipitated public concern about the safety of nuclear power in general. The accident has already caused some governments to re-think their nuclear energy policies, notably including the Japanese and German governments. There have been calls for cancellation of nuclear construction projects and reassessments of plant license extensions. This may lead to a global slow-down of the nuclear enterprise, based on the perception that nuclear energy is not safe enough. However, the lessons to be drawn from the Fukushima accident are different.

First, the accident was a result of the worst earthquake and tsunami in Japan’s modern history, an event which has caused the loss of over 20,000 lives and up to $300 billion in damages. Second, given the extraordinary magnitude of the initiating events (i.e. earthquake was 9.0 vs design 8.2, tsunami wave was 14 m vs design 5.7 m), the Fukushima-Daichii plant has performed relatively well in some respects and so far there is no evidence of major human errors in handling the crisis. It is noted that the containments at Units 1-3 have not massively failed, in spite of the exceptional loads they have been subject to, i.e. earthquake, tsunami, hydrogen explosions in the reactor buildings, steam discharges from the reactor pressure vessel, exposure to hot seawater, pressure above design limits for days. The release of radioactivity from the plant has been large (with contributions also from containment venting) and some workers have received significant radiation doses (>100 mSv whole-body equivalent), but health risks for them and the general population are expected to be negligible (see Appendix A). In fact, no loss of life has occurred or is expected as a result of the accident. Direct damage and casualties inflicted on Japan by the earthquake and tsunami far exceed any damage caused by the accident at the nuclear plant. The Fukushima accident has been rated at the maximum level (Level 7) on the IAEA nuclear event scale, indicating an accident with large release of radioactivity accompanied by “widespread health and environmental effects”, like Chernobyl. However, there are very significant differences between Fukushima and Chernobyl. Briefly, the amount of the release (~10% of Chernobyl), the presence of the containment structures, the radionuclides released (mostly iodine and cesium isotopes vs. the entire core inventory), the physical form of the releases (mostly aqueous vs. volatile), the favorable currents and winds at the site, and the timing of the release with respect to population evacuation resulted in vastly smaller overall consequences. Having said this, it is important to analyze the technical lessons that can be learned from Fukushima, so that the safety of nuclear plants in the U.S. and worldwide can be further enhanced and the attractiveness of nuclear energy sustained over the long term. An initial attempt to identify the key lessons from the Fukushima accident is presented here.

Objectives of the report

This report presents the reflections of members of the MIT Nuclear Science and Engineering faculty on the accident at Fukushima, and is offered as a contribution to the debate on the implications of the accident for the nuclear industry. Our purpose is twofold: we identify and discuss technical issues arising from the accident; and we begin a review of how the lessons learned can be used to improve the safety of current and future plants. The information is organized in six sections: “Emergency Power following Beyond-Design-Basis External Events”, “Emergency Response to Beyond-Design-Basis External Events”, “Containment”, “Hydrogen Management”, “Spent Fuel Pools”, “Plant Siting and Site Layout”. For each area, we present key issues observed at Fukushima and corrective actions that should be evaluated for implementation in current and future plants.

Note of Caution

    The technical feasibility and economic impact of the corrective actions discussed in this report have not yet been fully evaluated; they should therefore not be regarded as recommendations, but rather as ideas to be explored.
     
    Not all the information needed for a detailed reconstruction and analysis of the accident is yet available. The need for and merit of the corrective actions described in this document should be re-assessed as more accurate and complete information about the accident becomes available.
     
    The need for and merit of corrective actions should be evaluated on a plant- and site-specific basis. For example, it is noted that some U.S. plants already have water-proof rooms for flooding protection of the diesel generators and related equipment; therefore, the discussion in Section 1 below would not be very relevant to those plants.

Program:     NSP Nuclear Systems Enhanced Performance
Type:     TR 

RPT. No.: 25