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Next-generation alternatives
Published in Peter M. Schwarz, Energy Economics, 2023
One motivation for encouraging new nuclear plants is to evaluate changes in technology since the 1970s. New construction includes the plants at Vogtle in the southeast United States as well as the Olkiluoto 3 plant in Finland. Both plants have experienced years of delay and dramatic cost overruns. Vogtle uses the Westinghouse (now majority owned by Toshiba) AP1000. The AP1000 is a Gen III+ design (there was also a Gen III AP600, but very few Gen IIIs were built). Gen II technology refers to plants built in the 1970s. The new plants require less land and promote cheaper, safer, and simpler design with fewer components such as pumps, valves, and cables. One prominent feature is passive safety, where the plant proceeds to shut down automatically if it detects a malfunction rather than requiring human intervention. The Summer plant located in South Carolina also used the AP1000 design, but was abandoned in 2017 due to delays and cost overruns.
Reactor Accidents, DBAs, and LOCAs
Published in Robert E. Masterson, Nuclear Reactor Thermal Hydraulics, 2019
In the long-term cooling stage, heat removal can be achieved in a variety of different ways. Modern PWRs rely on what is called a passive containment cooling system (PCCS) to remove this heat. The PCCS uses the process of natural convection to cool the containment building, and therefore, it requires no active systems to function. The driving force for the flow through the PCCS is the pressure difference between the dry well and the wet well. Normally, the PCCS is designed to keep the containment pressure below 0.50 MPa (~72 PSI) for a LB LOCA for at least 72 hours. To do this, it must transfer the energy inside of the containment building through the process of natural convection to the outside atmosphere. As energy is removed from inside of the containment building, the air–steam mixture inside of the building condenses, and in most cases, it flows back into the wet well. The internal steel liner absorbs some of this heat, which is then passed on to the PCCS. The timing of these stages is shown in Table 30.5, and the peak cladding temperatures during each stage are summarized in Table 30.6. In Westinghouse AP600’s and AP1000’s, heat is removed by the evaporation of a thin liquid film on the outside surface of the steel containment liner, which lowers the temperature of the liner so that steam can condense on its inner surface. As more energy is removed from the containment building, the pressure of the gaseous mixture inside the building is lowered and so is its temperature. The evaporation rate of the film is increased by having moist air flow over the film through an annular air space between the outside of the steel liner and the missile defense shield. Inlets are provided at the top of the containment building to let the moist air in. A picture of the PCCS is shown in Figure 30.2. AREVA uses a slightly different approach to cool the containment building. However, their system is also a passive design.
Water-Cooled Reactors
Published in William J. Nuttall, Nuclear Renaissance, 2022
Extensive work has been done in ensuring the resilience of both AP600 and AP1000 in the face of moderate LOCAs. Even for an 8-inch (200 mm) break in the vessel injection lines of the reactor coolant system, there is predicted to be no possibility that the reactor core could become uncovered in either the AP600 or AP1000 designs [18].
Severe Accident Phenomena: A Comparison Among the NuScale SMR, Other Advanced LWR Designs, and Operating LWRs
Published in Nuclear Technology, 2020
Scott J. Weber, Etienne M. Mullin
Certain ALWRs have employed design strategies to increase the likelihood of retaining core debris in the reactor vessel, namely, by providing effective external cooling of the reactor vessel lower head. Internationally, this strategy has already been implemented in several operating reactors including most VVER-440 reactors in Europe. In the United States, AP600/AP1000 is currently the only certified reactor design to credit in-vessel retention of core melt by external reactor vessel cooling as a key severe accident management feature. Several unique design measures were required to demonstrate success, namely, the development of a reactor cavity flooding system using the in-containment refueling water storage tank and an engineered reflective insulation system that induces greater superficial flow rates on the lower head surface and increases the critical heat flux (CHF) above the heat flux imposed by the heat-generating debris at all locations on the lower head.10 The CHF corresponds to the maximum boiling heat flux above which a vapor film forms on the heated surface resulting in a drastic reduction in the heat transfer coefficient and an increase in temperature of the surface. By demonstrating that the CHF on the outside of the lower head is in excess of the heat fluxes imposed on the inside of the lower head, the lower head is shown by extension to maintain structural integrity and retain the core debris.