The ARMF-I, a reactor located in a small pool in a building east of the MTR in the Test Reactor Area, was used to determine the nuclear characteristics of reactor fuels and other materials subject to testing in the MTR. Together with the MTR, the reactor helped improve the performance, reliability, and quality of reactor core components. Until the next generation reactor, the ARMF-II, this was considered the most sensitive device for reactivity determinations then in existence.