The book, "The Atomic Energy Deskbook" (1963), by John F.
Hogerton, on page 285 under the title section "Los Alamos Water
Boilers (LOPO, HYPO, SUPO)" begins with:
" A series of small nuclear reactors of the aqueous homogeneous, or solution type, built at Los Alamos Scientific Laboratory (see Table 1). The first of the series (LOPO, from " low power") was operated in 1944, and provided the initial experience with the use of a fluid fuel reactor system. LOPO was dismantled and reconstructed as HYPO, which in turn was dismantled and reconstruced as SUPO."
The LOPO, which is now over 50 years old (1944 it began operation), was an amazingly simple device belies the accepted notion that Nuclear Reactors need be extremely complex. LOPO was the approximate size and shape of a beach ball, did not have any control rods and yet was perfectly and safely self-controlling due to its extremely high negative temperature coefficient. It served as the foundation of a group of reactors called "Water Boilers" that served many research institutions well.