[en] The Sandia ACRR became operational in 1978 and currently serves as the major in-pile fast reactor safety test facility for the US Nuclear Regulatory Commission. The ACRR is an upgrade of the Annular Core Pulse Reactor (ACPR) with the installation of a new flexible control system and a core of uniquely designed BeO-UO₂ fuel elements for increasing the neutron fluence in the experiment cavity. The reactor is now capable of driving multi-pin advanced reactor test fuel into vapor with a pulse width of 5 msec. In the steady state mode, the reactor can simulate post accident decay heat at prototypic levels in fission heated debris beds up to 10 cm in diameter. A number of programmed operating modes including high power square waves, ramps and pulses can produce a multitude of power profiles in order to simulate the power histories in the various accident scenarios. The reactor capabilities and the reactor safety research test program are discussed