NUCLEAR POWER PLANT- PART:2 - PRESSURIZED WATER REACTOR (PWR)

    The excellent properties of water as a moderator and coolant make it a natural choice for power reactor, and the PWR has been extensively developed in USA. The most important limitation in PWR is the critical temperature of water, 374* C. This is the maximum possible temperature of the coolant in the reactor, and in practice it is considerably less, possibly at about 300*C . In PWR, the coolant pressure must be greater than the saturation pressure at, say, 300*C (85.93 bar) to suppress boiling. The pressure is maintained at about 155 bar so as to prevent bulk boiling.

                      

   A PWR power plant is composed of two loops in series , the coolant loops called as the PRIMARY LOOP, and WATER- STEAM or WORKING FLUID LOOP. The coolant picks up heat in the reactor and transfers it to the working fluid in the steam generator. The steam is then used in a Rankine type cycle to produce electricity.

    The fuel in PWR is slightly ENRICHED URANIUM in the form of thin rods or plates. The cladding is either of stainless steel or zircaloy. because of very high coolant pressure, the steel pressure vessel containing the core must be about 20 to 25  cm thick. A typical PWR contain  about 200 fuel assemblies , each assembly being an array of rods. In a typical fuel assembly, there are 264 fuel rods and 24 guide tubes for control rods. Grid spacers maintain a separation between the fuel rods to prevent excessive vibration and allow some axial thermal expansion.

   The coolant leave the reactor and enter the steam generator which can be either shell and tube type with U- tube bundles or once-through type, the former being more common. In the U- tube steam generator, the hot coolant enters an inlet channel head at the bottom, flows through the U-tubes, and reverse the direction to an outlet at the bottom. It can produce only saturated steam. In the once-through design, the primary coolant enters at top, flows downward through tubes and exits at the bottom to the main pumps. feedwater is on the shell side. A dry or low degree of superheat stream is possible.

    The first land based PWR for power generation was built at SHIPPINGPORT, USA in 1957. Its thermal output is 231 MW, the pressure in the primary circuit is 141 bar, and the water temperature at outlet from the reactor is 283*C . Dry saturated steam is generated in the heat exchangers at 41 bar, 252*C. For a gross electrical output of 68 MW, the efficiency is 29.4%.

                             

   The shippingport cycle has been modified in the Indian point (USA) PWR by the inclusion of an oil-fired superheater between the main heat exchangers and the turbines. There is also an economizer along with some feedwater heaters. The steam condition improves to 25.5 bar and 538*C at turbine inlet, and so the cycle efficiency increases.

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