On 10 November at 2:18 pm, Japan’s High Temperature Test Reactor (HTTR) achieved criticality for the first time. This 30 MW(thermal) research reactor is a helium cooled, block type HTGR whose core consists of 30 columns of fuel blocks. Fuel loading began by installing the outer ring of columns in order to get some experience of annular core configurations. It had been expected that the core would go critical with 17 columns loaded, but in fact it took 19 columns to reach criticality.

This is the first high temperature gas cooled reactor to start operations since THTR in 1985 and it is also Japan’s first helium cooled reactor.

JAERI will continue to load the remaining columns of fuel and carry out tests to measure the core characteristics. The core should reach its full size at the end of this year. After that JAERI has to carry out some hardware changes before beginning the power escalation tests in the middle of next year. HTTR is expected to reach its full output in 2000.