Another method of extraction is using supercritical and subcritical fluids. It can automatically adapt to subcritical flow and supercritical flow. They can be subcritical or supercritical, in which case significant radiation fluxes can be produced. Devices that produce engineered but non-self-sustaining fission reactions are subcritical fission reactors. At the critical value, both equilibrium points lose stability through a subcritical Hopf bifurcation. One promising alternative from this perspective is an accelerator-driven sub-critical reactor / subcritical reactor. Neutron-producing accelerators have been proposed for some super-safe subcritical experimental designs. Otherwise it is unstable and the bifurcation is subcritical. A population of neutrons introduced to a subcritical assembly will exponentially decrease. Non-destructive X-ray, gamma ray, and subcritical detonation tests continue to be conducted in Area 1. So concrete obtains hardness from subcritical failure. The main fact in the subcritical phase is "exponential decay". Known as a subcritical inlet condition, this was stable, but produced inefficient spillage drag. As of 2011, another direction for development was an RTG assisted by subcritical nuclear reactions. The hollow cylindrical shape made it subcritical.