Once when I was in junior school the school I attended offered a science demonstration in assembly. The whole idea was absurd, because many of our parents were scientists, and the demonstrator was a science teacher, who was far less versed in real science than my father. The demonstrator showed us a few tricks with chemistry, but the highlight of the show came when he chucked a small piece of carefully stored sodium metal into a container filled with water. The sodium bounced on the surface of the water as it burned strongly, illustrating that water – or for that matter air – does not mix well with sodium. Had the demonstrator chucked a larger piece of sodium into the water a violent explosion would have occurred.
The demonstration left a vivid impression on my mind, and similar demonstrations probably effected the views of early reactor scientists like Eugene Wigner and Alvin Weinberg. In 1966 Alvin Weinberg gave a lecture on nuclear technology to the Autumn meeting of the National Academy of Science.
Weinberg noted, “in spite of the great emphasis on fast breeders that the world now displays,
there are some difficulties that must be overcome before fast breeders become commercially successful.”
Weinberg did not comment on the safety of sodium cooled reactors on that occasion, but in a lecture delivered at Argonne National Laboratory ten years later, Weinberg observed:
“We have no real estimates of accident probabilities for liquid metal fast breeder reactors (LMFBR’s). The Rasmussen estimate (one in 20,000 per reactor year with an uncertainty of five either way) would lead to a meltdown every 3 years. This is probably an unacceptable rate; an accident rate at least ten times lower, and possibly 100 times lower may be needed if the system is to be acceptable.”
Later in the same lecture Weinberg added, “the acceptable accident rate will probably have to be much lower than the Rasmussen report suggests. If one uncontained core meltdown per 100 years is acceptable (and we have no way of knowing what an acceptable rate really is), then the probability of such an accident will have to be reduced to about one in 1 million per reactor per year.”
The basic problem with sodium cooled reactors like the Liquid Metal Fast Breeder Reactor is the safety problem inherent in the use of sodium as a coolant. Sodium reacts chemically with both air and water, and will burn strongly with either. Hence sodium leaks become a significant issue with sodium cooled reactors. The history of sodium cooled reactors give scant comfort to those who argue that they are safe.
Perhaps the best known Internet video related to reactor safety is the video of Japanese reactor workers responding to a sodium leak at the Monju Sodium cooled breeder reactor. The Monju reactor has been shutdown since the 1995 accident although reportedly the Japanese plan to reopen it this year. The Japanese were fortunate that the leak occurred in a secondary sodium coolant system, and that no radiation was leaked, however the danger of working with sodium are best illustrated by a 1996 attempt by Japanese researchers to recreate the conditions that lead to the Monju accident. Researchers concluded that the liquid sodium released during the accident, could have melted steel doors, and come into contact with a cement floor. A reaction between the liquid sodium and water in the cement would have caused a violent explosion. What would have happen next is not reported but the leaked sodium was not the only sodium that could have potentially been involved in the accident. Not only does primary coolant sodium burn easily in contact with air, it is also highly radioactive.
Weinberg, who had common sense, and who worried about nuclear safety, thought that the safety risk from using sodium as a reactor coolant was too great.
Like all reactors with solid fuels, sodium cooled reactors requite extensive piping in order to move the molten sodium fluid from the reactor to the heat exchange and back. Secondary sodium systems carry the heat to a steam generating systems, or to a gas turbine generating system. The movement of sodium through a system of pipes, coupled with the existence of two heat exchange systems, create an inherent safety danger for sodium cooled reactors.
The amount of sodium involved, and its radioactivity, potentially makes for a catastrophic accident.
In addition other fluid coolants, for example fluoride salts are superior to liquid sodium in many ways:
* Fluoride salts do not burn in contact with water or air.
* Fluoride salts boil at a much higher temperature than sodium, thus a fluoride salts cooled reactor can operate at a much higher temperature, hence with greater thermal efficiency.
* Nuclear fuel can be dissolved in Fluoride salts eliminating the need to fabricate nuclear fuel.
* Chemical operations involving fluoride compounds are well known in the nuclear industry and are relatively simple.
* Some fluoride salts have lower neutron cross sections than sodium, thus facilitating the transformation of fertile isotopes like Th232 into fissionable U233.
* Fluoride salts reactors have many features that make them inherently safe.
- Charles Barton
