Reply #17: "Fire by Sodium Coolant Leak at Prototype Fast Breeder reactor, Monju"

[Junkdrawer]

On December 8, 1995, a prototype fast breeder reactor, Monju, located in Tsuruga City, 350km west of Tokyo, was operating at 40% power. The Power Reactor and Nuclear Fuel Development Corp (PNC), a government controlled organization, operated this reactor. At 19:47 high temperature liquid sodium coolant at one of the three secondary heat exchangers started leaking through a broken thermometer sheath (designed by IH Company, OD: 10mm, ID: 4mm, Length: 150mm) on the piping and it ignited on contact with air. As shown in Fig. 1, the primary heat exchangers are designed to take heat out of the core of the reactor to the secondary heat exchangers, which then transfer the heat to steam generators for power. Because of this design, it was a simple fire caused by the leakage of chemically reactive but non-radioactive sodium coolant. But due to the delay in shutting down of the reactor, 640kg of the sodium leaked in 3 hours and caused some unexpected damage by the fire and chemical reactions to the surrounding structure.

The sheath was found to have been broken by following design errors such as,

The sharp cut edge design of the sheath as shown in Fig. 2, which caused stress concentration and breakage,
Neglect of newly established mechanical design analysis of vibration of the sheath parallel to the sodium flow and
The IH company's failure to consider the fact that liquid sodium is 120 times more heat conductive than water and to use the another contractor's (the H company's) better and simpler design for those of the primary heat exchangers. See Fig. 3.

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http://www.onlineethics.org/cms/22335.aspx