High-temperature engineering test reactor
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The high-temperature test reactor (HTTR) is a graphite-moderated gas-cooled research reactor in Ōarai, Ibaraki, Japan operated by the Japan Atomic Energy Agency. It uses long hexagonal fuel assemblies, unlike the competing pebble bed reactor designs.
The primary coolant is helium gas at a pressure of about 4 MPa, the inlet temperature of 395 °C, and the outlet temperature of 850–950 °C. The fuel is uranium oxide (enriched to an average of about 6%).