Far-Field Influences Shadow the Effects of a Nuclear Power Plant’s Discharges in a Semi-Enclosed Bay

Abstract

The sustainable development of society depends on the reliable supply of electricity while keeping impacts on the environment to a minimum. A 951 MWe nuclear power plant in the semi-enclosed Nanwan Bay at the southern tip of Taiwan began operating in May 1984. Part of the bay is in Kenting National Park, which is known for its coral reefs and abundant marine life; thus, thermal pollution from the cooling water discharge is a great concern. Fortunately, the bay opens south to face the Luzon Strait, where the world’s strongest internal tides are generated. Because the bay is deep enough, internal waves bring up cold deep water and reduce the surface temperature by as much as 10 °C for a few hours every day. These internal waves and topographically generated upwelling also bring nutrients to the euphotic layer from the depths, but the upwelled waters quickly leave the bay along with the cooling water. As a result, a thermal plume with a temperature of 1 °C or higher than the ambient temperature only covers 1 km. By way of comparison, El Niño—Southern Oscillation- or Pacific Decadal Oscillation-related interannual variations in temperature are as high as 5 °C. The rapid turnover of the upwelled waters also helps to prevent heat released by the power plant from accumulating and diminishes the thermal stress, thus sustaining corals and other marine life forms. Typhoons, even hundreds of kilometers away, could also induce the upwelling of cold subsurface water. Consecutive typhoons have been observed to reduce the water surface temperature by up to 10 °C for two weeks or longer. Furthermore, the currents are such that the thermal plume flows out of the bay most of the time. All of these factors make the surface waters in the bay about 0.5 °C cooler than the waters outside of the bay, despite the operation of a nearby nuclear power plant.

Monsoons and Tide-Induced Eddies Deflect the Dispersion of the Thermal Plume in Nan Wan Bay

“…Irrespective of whether it is summer or winter, the duration of the thermal plume staying at the east station is approximately 10 h, which is much shorter than its duration at the south station. This result corresponds with Chen et al's [3] study of the flow field in Nan Wan Bay. Two selected model sites to the east and south of the outlet at a distance of approximately 500 m were sampled to illustrate their time series of temperature profiles in both summer and winter, and the results are shown in Figure 12.…”

“…Between the seamount to the south and the landmass to the north, the terrain is deep, and this depth extends toward the two capes, forming an open-ended, arc-shaped channel. The movement of water masses in Nan Wan Bay is primarily governed by robust tidal forces, including diurnal and semidiurnal tides, and is additionally influenced by seasonal monsoons [3,4]. In the offshore parts of Nan Wan Bay, during ebb tides, tidal currents typically flow eastward from the southern end of the Taiwan Strait to the western Pacific Ocean through the bay; conversely, they flow westward during flood tides.…”

“…Seawater is circulated through a nuclear reactor to transfer or remove heat, and the output of this process is thermal waste seawater. This seawater is frequently drained through a long diversion dike into an area near the shore, such as into a bay or an estuary [3,7]. The Third Nuclear Power Plant (NP_No.…”

“…Thermal wastewater, which is less dense than seawater, typically floats on the sea surface layer and flows back and forth due to tidal currents. The fate of a thermal plume's dispersal depends on the strength and direction of tidal currents, which features a fortnightly The movement of water masses in Nan Wan Bay is primarily governed by robust tidal forces, including diurnal and semidiurnal tides, and is additionally influenced by seasonal monsoons [3,4]. In the offshore parts of Nan Wan Bay, during ebb tides, tidal currents typically flow eastward from the southern end of the Taiwan Strait to the western Pacific Ocean through the bay; conversely, they flow westward during flood tides.…”

Structural Equation Modeling of the Marine Ecological System in Nanwan Bay Using SPSS Amos

“…The operation of the nuclear plant has since raised significant environmental concerns, leading to active research in the area. Long-term water quality monitoring data indicate that the water temperature changes in the Bay are mainly affected by weather, seasonality, and large-scale ocean events, and are not directly related to the discharge of heated water [9,10]. In an attempt to gain a deeper understanding of how these complex natural phenomena can impact marine ecosystems, it is necessary to investigate the unique influence of strongly interrelated factors.…”