A second nuclear power plant in Bangladesh: How do we choose it?
The Rooppur Nuclear Power Plant, the first ever nuclear power plant in Bangladesh, is being built. The plant capacity is 2400 MW, with two Russian reactors (model VVER-1200 of III+ generation). It is a fast track project scheduled to be completed by 2024. Meanwhile, Bangladesh plans to build a second nuclear power plant on the seaside, and that is a prudent decision. The source of cooling water is the Bay of Bengal, whose summer water temperature is 31 degrees Celsius. Preliminary work is already underway and a few tentative sites have been selected in the coastal districts on the south. Now is the time for necessary homework to settle on a judicious choice for the second plant. The aim is to select a thermally efficient plant suitable for the weather conditions of Bangladesh.
Before we do so, we must evaluate the thermal performance of a plant and see how cooling water temperature influences the plant output. Observing modern design approaches for improvement of plant capacity and learning from the experiences of other nuclear power plants in Asia can contribute towards this.
The nuclear power plants up to generation III+ are basically subcritical steam power plants. Unlike coal-based steam power plants, they cannot attain the supercritical or ultra supercritical temperature level and achieve higher efficiency of 40 to 45 percent. The reason for this is that the primary coolant water temperature in the reactor core itself has a ceiling to about 330 degrees Celsius, much less than the critical temperature (374°C), and the steam temperature at turbine inlet is limited to 285 degrees Celsius. Also, there is no scope to superheat the steam inside the reactor.
As per rule of thermodynamics, the higher the steam temperature at turbine inlet and lower the temperature in the condenser, the higher will be the thermal efficiency. So, the advanced designs of nuclear power plants try to extract as much work as possible from steam, at far below the atmospheric pressure to increase output and efficiency. As the condenser pressure decreases, so does the steam condensing temperature. The condensing temperature is selected depending on the cooling water available near plant sites. The standard design cooling water temperature is 20 degrees Celsius, which will condense steam at as low as 32 degrees Celsius. So the plants can achieve the designed capacity if situated at seasides in cold countries, but not when situated in hot countries, where seawater temperature can go as high as 35 degrees Celsius. Again, only one-third of the heat energy generated in the reactor core can be converted into electricity for the grid. The remaining two-thirds of heat will be ejected into the environment. It is the cooling water that carries this unused heat to the sea or releases into the atmosphere through cooling towers. The efficiency of nuclear power plants is thus limited within 33 to 35 percent.
Cooling water temperature is very important for the thermal performance of steam power plants, whether they use nuclear fuel or fossil fuel. High cooling water temperatures reduce thermal efficiency. That is why plants in cold countries are more efficient than those in hot countries. Again, the thermal performance is better in winter than in summer. For example, a plant operates at full capacity when the cooling water temperature is 20 degrees Celsius and steam passes through turbines at a designed flow rate. As the cooling water temperature rises to 32 degrees Celsius, the turbine will experience a back pressure for inadequate condensation of steam. The steam flow through turbines will reduce and the duty point will adjust itself at a point of higher condensing temperature and pressure. As a result, plant capacity will reduce. A previous study says that for each degree rise of cooling water temperature, the capacity is reduced by 0.45 percent and efficiency by 0.12 percent.
How do nuclear power plants in other regions compare? At the Barakah Nuclear Power Plant (BNPP) at Abu Dhabi in UAE, there are four nuclear power plants with Reactor model APR-1400 of generation 3+ from Korea. They are still under construction on the coast of the Gulf, although one of them has started operating. The total nominal capacity here is 5600 MW.
These plants will use once-through condenser cooling water systems. The summer temperature of Gulf seawater is 35 degrees Celsius. On the other hand, two nuclear power plants of the same model APR-1400 are in operation at Shin Kori, Korea, where seawater temperature in summer is 20 degrees Celsius. Due to the cooling water inlet temperature difference of 15 degrees C, a unit of BNPP will produce 95 MW less than that in Korea. Because of high cooling water temperature, the plants will be highly derated (reduction of capacity).
BNPP also has some environmental prohibitions. The plant will shut down if the cooling water inlet temperature becomes 38.5 degrees Celsius. Also, there is a bar for water discharge temperature to sea (above 40 degrees Celsius). For compliance, the plant cooling system will have a bypass mode for mixing normal seawater with hot leaving water to lower the temperature below this bar. This restriction is likely to reduce output further.
Again, the standard condenser model designed for the 10 degrees Celsius temperature range will not be suitable for the Gulf site. A larger condenser of 7 degrees Celsius range can be used with higher pumping capacity for increased water flow. That will call for a customised design of the cooling system.
Another plant that we can refer to is the Kudankulam Nuclear Power Plant (KNPP) in India, built with two reactors, model VVER-1000 of generation III (made in Russia). Both plants are operational with a total nominal capacity of 2000 MW. The plants have seawater condenser cooling systems with the temperature range of 7 degrees Celsius. The summer seawater temperature is 31 degrees Celsius, and the discharge of hot water to sea is 38 degrees Celsius, which is below the limit of 40 degrees Celsius.
According to a report, the plant KNPP-2 reached the nominal capacity of 1000 MW on January 21, 2017. It is noteworthy that the plant at least achieved the nominal capacity in winter, if not in summer. It is also evident that the design condenser temperature is near 42 degrees Celsius. Such a model is suitable for Indian Ocean belt as far as thermal performance is concerned.
The examples of these plants make it clear that an energy efficient plant for Bangladesh will be one that is designed for tropical conditions, with cooling water temperature at 31 degrees Celsius and steam condensing temperature of 42 degrees Celsius. Higher capacity plants (like AP1400) are definitely alluring, but will be highly derated if used in Bangladesh. This is not desirable as they add up and create a big gap between installed capacity and actual generation. This difference between capacity and generation is what gives the wrong impression of overcapacity in the power generation sector.
KM Mahbubur Rahman is a senior mechanical engineer and energy consultant.
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