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. A pressure control valve is disposed between the first and second chambers. A first inlet port is connected to the first chamber and configured to receive a first flow of coolant. A first outlet port is connected to the first chamber and configured to a second flow of liquid coolant to a heat generating component in the WHRS. A second inlet port is connected to the first chamber and configured to receive a third flow of heated coolant from the heat generating component. A second outlet port is connected to the second chamber and configured to supply a fourth flow of vapor coolant to the WHRS.