A breeder reactor is not restricted to using recycled plutonium and uranium. Water increases the oxidation rate of plutonium and uranium metals. Metal actinide fuel is typically an alloy of zirconium, uranium, plutonium, and minor actinides. Twenty radioactive isotopes of plutonium have been characterized. Some nuclear reactors can operate with a mixture of plutonium and uranium (see MOX). Ru is a product of fission of a nucleus of uranium or plutonium. In practice, bombs do not contain hundreds of tons of uranium or plutonium. Algae cultivation is under study for uranium/plutonium sequestration and purifying fertilizer runoff. It is radioactive and can accumulate in bones, which makes the handling of plutonium dangerous. Fuels with plutonium and thorium are also an option. However it was not until 1952 that a tangible goal of building plutonium reactors progressed. Its inclusion in reactor-bred plutonium appeared unavoidable. Its toxicity is comparable to, but slightly lower than that of americium and plutonium. The fuel capsule contained about of plutonium oxide. More wreckage along with plutonium contamination was found on nearby Sand Island.