A thermal accumulator assembly (TAA) for a vehicle waste heat recovery system (WHRS) utilizing a two-phase coolant includes a hermetically sealed housing having a separator plate dividing an interior of the housing into a higher pressure first chamber and a lower pressure second chamber, and an expander generator disposed between the first and second chambers and including an expander operably coupled to a motor generator. The first chamber is configured as a reservoir for the two-phase coolant, and the expander is configured to receive a flow of vapor coolant from the first chamber to generate power via the motor generator.