If, is a decision for the public. Nuclear agencies are tasked with safety standards that make the nuclear industry designs and procedures predict a huge variety of combinations of potential events and responses to those events. Public perception of the competence of these agencies to do that job is very important when making that decision. In our democracy, the opinion of all individuals is considered and open for the public to review.
A number of opinions on the safety of design aspects of nuclear plant design, spent fuel storage and other elements have been raised that the NRC has addressed which are now open to more rigorous review. I want to touch on a few of those before discussing my view on plant design.
First is partial melt down. Melt down is not a very good term in my opinion. Fuel damage is more accurate. The first and second levels of containment are fuel (pellets in this case) and the cladding that houses the fuel pellets. The cladding is made of a material call zircaloy. It is a zirconium metal alloy that has different material properties than the fuel pellets. Zircaloy has a melting point of ~1820 degrees C that is lower than the melting point of the fuel, oxides of uranium and in one case Plutonium in Mixed Oxide (MOX), which is ~2700 degrees C (http://www.ornl.gov/~webworks/cpr/v823/rpt/109264.pdf). Zircaloy can also burn at temperatures between (Correction)
From the NTC Design requirements:
"Criterion 50--Containment design basis. The reactor containment structure, including access openings, penetrations, and the containment heat removal system shall be designed so that the containment structure and its internal compartments can accommodate, without exceeding the design leakage rate and with sufficient margin, the calculated pressure and temperature conditions resulting from any loss-of-coolant accident. This margin shall reflect consideration of (1) the effects of potential energy sources which have not been included in the determination of the peak conditions, such as energy in steam generators and as required by § 50.44 energy from metal-water and other chemical reactions that may result from degradation but not total failure of emergency core cooling functioning, (2) the limited experience and experimental data available for defining accident phenomena and containment responses, and (3) the conservatism of the calculational model and input parameters."
The bold section can be interpreted as not considering a total cooling loss which should be reviewed. I have a request for clarification from the designer, General Electric, for the design as modified in Fukushima. One design modification I would recommend relates to the proper handling of the steam produced by loss of cooling. The Fukushima Mark I design has a pressure suppression system that condensates relief steam in the suppression pool within the containment building. An external condensation pool recommended by the original designer should be considered in all installations. Improperly timed and diverted steam relief appears to have contributed to the current difficulties in Fukushima.
Second, the design of the Spent Fuel Storage Pools should allow for total loss of mechanical cooling including total loss of water in the pool. Questions of the possibility of renewed criticality and Zircaloy cladding fire potential in the SFSP has been raised. I have reviewed SFSP designs which appear to be adequate with reasonable increase in radioactive material release anticipated for the short term. If the pools are dry, that greatly complicates regaining control of the SFSP radiation levels. In a passive air cooling situation, the spent fuel will become hotter than the boiling point of water. Adding water will produce steam which will carry radiation out of the pool. Some believe that this may also contribute to cladding oxidation and hydrogen production. Recovery from a dry SFSP is not clearly addressed in the literature I have located so far. In a loss of cooling, make up water is designed to maintain radiation containment. The original design allows for boiling of the water in the pool with maximum decay heat. In general situations, the total decay heat of a normal SFSP spent fuel load would warm the water causing increased surface evaporation but not boiling. (As stated by a TEPCO representative.) The make up water should be provided by hard piped make up water systems. Should that system fail, water from tankers or other emergency back up sources can be provided. I have requested more information from the manufacturer better explain design considerations for this worst case scenario.
Added: I don't want the link police after me so here, http://www.osti.gov/bridge/purl.cover.jsp?purl=/6135335-5voofL/
Speaking of link police, I made a comment on another blog that the SFSP do not need cooling. They of course attacked me as an uniformed idiot. Well, SFSP's do not require mechanical cooling, evaporation provides all the cooling they need for safety (The water could boil, so that is a safety risk if you are around the pool). They do require make up water. A hot SFSP can go about 4 days with no cooling and no make up water. A hot pool is one with all fresh rods with less than 96 days of decay. The pool at Fukushima number unit 4 was hotter (deecay heat wise) than the rest but far from a "hot" pool
Small modular reactors like Nuscale modular designs, better address fuel damage which is both an important economic and public health consideration that needs to be considered in future nuclear power plants. The spent fuel storage issue, both SFSP and dry cask storage, is an important issue to be considered before nuclear power in the United States is expanded.
While I have determined that these issues are adequately addressed (contingent on expanded Dry Cask Storage), they are concerns worthy of much more intense scrutiny. I will try to provide more information on the important and complex issue of worst case nuclear power plant design and waste storage.
Small Modular Designs (SMR) include a variety reactor technologies. Light Water Reactor design is the basis for the Nuscale modulars which appears to be more likely available for near term deployment. In my opinion, their design is very similar to US Navy Designs which have a proven safety and reliability record. There are people that disagree and would prefer to wait for the increased safety of Generation IV designs. I totally agree that Generation IV designs, especially Molten Salt Reactors, are inherently safer, but the current generation of reactor designs offer adequate safety for responsible expansion of nuclear power. I will try to get an opposing view to expand the quality of the discussion.