[en] Monju, Japan’s prototype sodium cooled fast reactor of 280 MW(e) class, restarted its test operation in 2010 after a long stoppage since the sodium leak accident in 1995. The zero power system startup tests were successfully conducted. The major achievement of the tests was an accurate prediction of reactor physics parameters with a core having a complex fuel composition that includes americium-rich fuel. The hardware troubles recently experienced, none of them being safety significant, have been restored to make the plant ready for the next power increase tests. The reactor, however, has been put into a standby mode again since the Fukushima-Daiichi accident of 11 March 2011, at least until the national energy and environment programme is revised and a research plan using Monju is developed through 2013. We believe the roles of Monju will not change and comprise further enhancement of safety against severe accidents; demonstration of stable power generation and actinide burning; provision of technology and knowledge base for future sodium cooled fast reactors; and use of the plant as an international research facility. (author)