Strontium-90 has been used in radioisotope thermoelectric generators (RTGs) in the past because of its relatively high power density (0.95 W thermal /g for the metal, 0.46 W thermal /g for the commonly used inert perovskite form Strontium titanate) and because it is easily extracted from spent fuel (both native Strontium metal and Strontium oxide react with water by forming soluble Strontium hydroxide). However, the increased availability of renewable energy for off-grid applications formerly served by RTGs as well as concern about orphan sources has led to a nigh-total abandonment of Sr in RTGs. The few (largely space based) applications for RTGs that still exist are largely supplied by Pu despite its higher cost, as it has a higher power density, longer half life and is easier shielded since it is an alpha emitter while Strontium-90 is a beta emitter.