[en] Fort St. Vrain high temperature gas-cooled reactor in the USA has exhibited two types of anomalous behavior called core temperature fluctuations and redistributions during the rise-to-power test program. These originally unexpected phenomena pose an important technical issue to the design, research and development works for JAERI experimental VHTR consisting of prismatic block type fuel elements similar to those of Fort St. Vrain. This report reviews the fluctuation/redistribution problem encountered in Fort St. Vrain along with its resolution. (author)