A passively-cooled spent nuclear fuel pool system in one embodiment includes a containment vessel comprising a thermally conductive shell and an annular reservoir surrounding the shell that holds a liquid coolant forming a heat sink. A spent fuel pool is disposed inside the containment vessel and includes a body of water in contact with a peripheral sidewall of the fuel pool. At least one spent nuclear fuel rod submerged in the body of water heats the water. The peripheral sidewall of the spent fuel pool is formed by a portion of the shell of the containment vessel adjacent to the fuel pool, thereby defining a shared common heat transfer wall. The heat transfer wall operates to transfer heat from the body of water in the spent fuel pool to the heat sink to cool the body of water. The heat transfer wall comprises metal in one embodiment.