Alternative designs for a modular test reactor are presented. In one aspect, a molten fuel salt nuclear reactor includes a vessel defining a reactor volume, the vessel being open-topped and otherwise having no penetrations. A neutron reflector is provided within the vessel and displacing at least some of the reactor volume, the neutron reflector defining a reactor core volume. A plurality of heat exchangers are contained within the vessel above the neutron reflector. A flow guide assembly is provided within the neutron reflector that includes a draft tube draft tube separating a central portion of the reactor core volume from an annular downcomer duct. Fuel salt circulates from the reactor core volume, through the heat exchangers, into the downcomer duct and then back into the reactor core volume.