NOVELTY - A chemical thermal storage medium comprises particles (1) containing hydroxide of an alkali metal or alkaline-earth metal as a main component and by which a water absorbing substance (2) is conjugated. The particles form a structure component (3) having structure which holds the particles.    USE - Chemical thermal storage medium is used for reactor for thermal accumulator used in vehicle (all claimed). Uses include but are not limited to solar heat engine, internal combustion engine, fuel cell, battery, hot-water supply apparatus, and heating apparatus.    ADVANTAGE - The invertibility of dehydration reaction and a hydration reaction of chemical thermal storage medium is enabled.    DETAILED DESCRIPTION - INDEPENDENT CLAIMS are included for the following:    (1) reactor, which consists of chemical thermal storage medium, and a heat exchanger in which the chemical thermal storage medium and a heat exchange medium are heat-exchanged. The heat exchanger is provided such that the heat exchange medium is heated by thermal radiation emitted from the hydration reaction of the chemical thermal storage medium. The dehydration reaction of the chemical thermal storage medium is achieved by the heat supply from heat exchange medium to the chemical thermal storage medium; and    (2) thermal accumulator, which has the reactor, a condenser which condenses the water vapor supplied from the reactor, and an evaporator which supplies the generated water vapor to the reactor. During a thermal storage operation in the reactor, the dehydration reaction of chemical thermal storage medium is carried out and the generated water vapor is condensed in the condenser, and a hydration reaction of chemical thermal storage medium is carried out by supplying water vapor to the reactor from the evaporator.    DESCRIPTION OF DRAWING(S) - The drawing shows schematic view of chemical thermal storage medium. (Drawing includes non-English language text)    Particles (1)    Water absorbing substance (2)    Structure component (3)