Hydrogen absorbing alloys cause internal stress in reaction vessels due to the expansion that occurs when they absorb hydrogen. This stress is affected not only by the amount of reacting hydrogen but also by the ab/desorption cycles. In this paper, we analyze the reaction distribution in a reaction vessel through simulations and show that locally excess internal stress occurs near the heat media inlet. We also show that this excess stress is decreased by unifying the reaction ratio by exchanging heat between the heat media inlet and the outlet in the reaction vessel.