The BM (Russian БМ ) was a nuclear reactor, which was used in Soviet submarines of project 705. The reactor was cooled with a lead-bismuth alloy (LMC = Liquid metal cooled).
The thermal power of the reactor was 155 MWth. The degree of enrichment of uranium used was 90 %. The melting point of the alloy used was 124 ° C, the boiling point at 1679 ° C.
Therefore, the cooling circuits of the reactors had to be continuously heated at about 124 ° C by means of steam generators to prevent the solidification of the coolant. When precipitated by the malfunction of this heat, solidified coolant and made the reactor unusable. A subsequent liquefaction of solidified coolant has not been possible, so that in two cases the reactor in the manner described " froze " and broke.
The reactor core with fuel elements was enclosed with a beryllium layer to slow down the neutrons. The control rods contain boron and europium.
The BM- 40A reactor had a total of two cooling circuits. In the modified OK -550 were, however, three cycles.
The reactors were used 1968-1997 on submarines. The fuel rods were removed between 1997 and 2007 from the reactors and transferred into long-term storage facilities.