The passive safety systems in AP1000 nuclear power plant can effectively remove decay heat for 72 hours without AC power or water supply from the outside. At 72 hours, plant operators will start the Ancillary Diesel Generators and recirculation pumps to pump water to cool the containment shell. The water is supplied from a large storage tank known as the Passive Containment Cooling Ancillary Water Storage Tank (PCCAWST, or "Ancillary Tank"). Estimate the minimum volume of water that is required to ensure that the containment temperature and pressure will not rise from the 72 hour point to approximately 7 days. Assume that water sprayed to the containment shell completely vaporized, and the heat transfer to ambient air is negligible. The AP1000 plant has a thermal power of 3,415 MWth. The water stored in the ancillary tank is at 20C and 1 atm pressure. The reactor has been operated at the rated power for one year. The decay power can be given by the following equation: P0P=0.066[(s)0.20.2]P:decaypowerPo:operatingpower:timeafterstartups:timeoperatedbeforeshutdown