Mumbai University > Mechanical Engineering > Sem 7 > Power Plant Engineering

Marks : 10M

Year: May 2016

USA. The most important limitation on a PWR is the critical temperature of water, 3740 C. This is the maximum possible temperature of the coolant in the reactor, and in practice it is considerably less, possibly about 300 0C, to allow a margin of safety. In a PWR, the coolant pressure must be greater than the satuThe ration pressure at, say, 3000 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 loop, called the primary loop, and the water-steam or working fluid loop. The coolant picks up heat in the reactor and transfer 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 PWRs is slightly enriched uranium in the form of thin rods or plates. The cladding is either strainless 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 contains 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 first land-based PWR for power generation was built at Shippingport, USA in 1957. Its thermal output is 231MW, the pressure in the primary circuit is 141 bar, and the water temperature at outlet from the reactor is 282 0C. Dry saturated steam is generated in the heat exchangers at 41 bar, 252 0C. For a gross electrical output of 68 MW, the thermal 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

Radiations from nuclear-power plant effluents are low-dose-level types of radiations. The effluents are mainly gases and liquids. Mainly the effects of these radiations on the populations living near the plants prompt environmental concerns about nuclear power plants. Sources of effluents vary with the type of reactor.

In both pressurized-water reactors (PWR) and boiling-water reactors (BWR), two important sources of effluents are

(1) The condenser steam-jet air ejectors and

(2) The turbine gland-seal system.

The ejector uses high-pressure steam in a series of nozzles to create a vacuum, higher than that in the condenser, and thus draws air and other non-condensable gases from it. The mixture of steam and gases is collected, the steam portion condenses, and the gases are vented to the atmosphere. In the gland seal, high-pressure steam is used to seal the turbine bearings by passing through a labyrinth from the outside in so that no turbine steam leaks out and, in the case of low-pressure turbines, no air leaks in. The escaping gland-seal steam is also collected and removed. In the BWR, the effluents come directly from the primary system. In the PWR, they come from the secondary system, so there is less likelihood of radio-active material being exhausted from a PWR than a BWR from these sources.

A BWR differs from the PWR in that the steam flowing to the turbine is produced directly in the reactor core. Steam is seperated and dried by mechanical devices located in the upper part of the pressure vessel assembly. The dried steam is sent directly to the high pressure turbine thus eliminating the need for steam generators. The coolant thus serves the triple function of coolant, moderator and working fluid. Since the ccolant boils in the reactor itself, its pressure is much less than that in a PWR and it is maintained at about 70 bar with steam temperature around 285 0C. However, an increase in the boiling rate displaces water (moderator) in the core and reduces the ability of the moderator to above 60% of the nominal, the fraction of steam in the core can be kept nearly constant by varying the coolant circulation rate. The saturated liquid that seperates from the vapour at the top of the reactor in a steam separator flows downward either internally within the reactor or externally outside the reactor and mixes with the return condensate. This recirculating coolant again either flows naturally due to density difference or by a forced circulation pump. The ratio of the recirculated coolant to the saturated vapour produced is called the circulation (or recirculation) ratio. It is a function of the core average exit quality. The BWR core exit quality varies from 10 to 14%, so that circulation ratio is of the range 6-10. Thjis is necessary to avoid large void fractions in the core, which would reduce the moderating power of the coolant resulting in low heat transfer coefficient or vapour blanketing and burnout.