Fukushima Daiichi: A nuclear incident that might have been avoided

A recent article by Frank Von Hippel, a nuclear physicist at Princeton, is illuminating because it explains some of the problems that led to the incident at Fukushima Daiichi and suggests that some of the problems might have been avoided.

Hydrogen

Three Mile Island was similar to Fukushima Daiichi in that in both cases reactors suffered exposed fuel rods and elevated temperatures causing the zirconium fuel casings to react with steam to produce hydrogen.  At TMI this was referred to as a hydrogen bubble, and there was serious concern that the bubble might cause the reactor to explode during President Carter's visit to the plant.  Ultimately the hydrogen was vented over a period of a month to avoid the kind of explosion that occurred at every operating reactor at Fukushima Daiichi. 

Von Hippel points out that even before Three Mile Island, engineers had proposed a system of filtered vents that would reduce significantly the amount of radiation, comprised mostly of radioactive Iodine and Cesium, that is released into the atmosphere. If the pressure inside containment increases dangerously, venting results in much less release of radiation and presumably without the hydrogen explosions that occurred at Fukushima Daiichi.  Apparently,  France and Germany installed filtering vents in their plants, but the Nuclear Regulatory Commission (NRC) in the US decided against mandating this upgrade.  Apparently it was also not mandated in Japan.

Spent Fuel

Von Hippel also says that independent nuclear analysts have made the case that it is dangerous to pack five times more spent fuel into spent fuel pools than they were designed to hold.  A case has also been made that 80% of the spent fuel could safely be stored elsewhere.  The NRC also rejected this proposal, Von Hippel says because it would have been more expensive.  Apparently the recommendation was not implemented in Japan either.

Defense in Depth for Backup Power

Early nuclear reactors had some major failings including control rods that required power to be inserted and no containment vessels.  Modern plants have corrected these design defects, but others remain.  After the March 11 earthquake off Sendai in Japan all affected nuclear reactors were successfully shut down, which means that control rods were inserted that stopped the uranium fission reaction.  However, after shut down radioactive products of the uranium fission reaction remain and continue to decay and generate a lot of heat.  The major remaining design defect in most modern reactors is that they have to be actively cooled to remove this heat. 

At Daiini off-site power was restored very quickly, cooling restarted and all four reactors were successfully brought to a cold shut down. 

At Daiichi, two weeks after the tsunami stopped both off-site power and diesel backup generators and washed away fuel tanks, power is just beginning to be restored and reactor and spent fuel pool cooling restarted.  Based on the experience at Fukushima Daiichi, another Japanese power utility, Tohoku Electric Power, has already announced that it plans to deploy backup generators on trucks as well as backup cooling units for cases of catastrophic power loss.

Passive Safety

Von Hippel describes a reactor design that fails to a safe condition even if active cooling fails after shut down.  The high-temperature gas-cooled graphite reactor uses fuel comprised of small particles surrounded by a material that would contain their radioactivity in case cooling fails. Von Hippel says that the United States built two prototypes in the 1960s, and Germany built one in the 1980s. China is planning two prototypes and, if these work, 36 more.

Simulation

I would add that the more I have learned about what has happened and is happening at Fukushima, the more this seems like an example where the convergence of engineering design, geospatial technology, physical modeling and simulation, and 3d visualization would help the designers of these plants design for extraordinary events like a 10 meter tsunami.

Comments

[Suggestivo]

The brief comments made about NRC decisions and decisions in Japan as compared to safety decisions made in France and Germany area also a substantive change in engineering process. It appears industry "value engineering" may be the hole in the boat.