[en] The HECTR (Hydrogen Event: Containment Transient Response) code was assessed against two large-scale thermal-hydraulic and hydrogen mixing tests (Tests 31.1 and 31.5) in the German Heiss Dampf Reaktor (HDR) decommissioned reactor facility. The HDR primary containment with a total volume of 11,300 m³ is enclosed by a cylindrical steel shell with an overall height of 60 m and a diameter of 20 m. Unlike most US power reactor containments, the HDR containment consists of a large number (∼71) of relatively small interconnected chambers. A steam and water source is injected into one of the chambers through a discharge pipe connected to the reactor pressure vessel to simulate the source in a loss of coolant accident in Tests 31.1 and 31.5. A hydrogen and helium mixture was introduced 36 minutes after the steam blowdown in Test 31.5. The specific objective of this work is to assess the transport and mixing models in the HECTR code. 9 refs., 25 figs., 1 tab