3.5.1   The PWR steam cycle


The PWR power plant features three principal heat transfer processes. In the primary circuit, fuel is irradiated, liberating heat which is transferred to a secondary steam cycle. In a nuclear power station the plant performing this function is known as the Nuclear Steam Supply System (NSSS).

In the second cycle, the steam generated is expanded to drive turbines which generate electrical power. Finally heat is rejected to the Cooling Water (CW) in the condenser — this is the third heat transfer process. The PWR general arrangement is shown in Fig 1.47.

PWR power cycle

The NSSS consists of the reactor, the reactor coolant system (RCS) and a number of auxiliary and safety systems. The RCS consists of the reactor within its pressure vessel and three or four cooling loops con­necting the reactor to the primary/secondary heat exchangers (steam generators). The reactor vessel and cooling loops are filled with demineralised water, which is pumped around the primary circuit. In this system, boiling within the reactor is suppressed by maintaining coolant water pressure in excess of the saturation pressure corresponding to the coolant temperature at outlet from the reactor. The pressure is controlled by a control system and a surge tank, known as the pressuriser, connected to one of the cooling loops.

The secondary circuit is of major interest to this discussion. The feedwater enters the steam generator, where its temperature increases up to its saturation temperature. At the saturation temperature, the water changes state to steam which leaves the steam genera­tor slightly wet. The heat exchange process for both cycles is shown on a Temperature-Enthalpy (T-H) diagram (Fig 1.48).

Temperature plotted against (mass flow x specific enthalpy) for the primary and secondary circuits in the steam generator of a PWR

The wet steam produced in the steam generator is of a lower quality than the superheated steam conditions from both fossil-fired and AGR-powered plant. However, there are limitations to the steam conditions attainable. On the primary side, the maximum temperature is restricted by the desirability of avoiding nucleate boiling in the reactor core, as heat transfer coefficients for steam are considerably lower than that for water. Consequently the primary circuit outlet temperature, T2, is less than the saturation value for that pressure. Having established the primary circuit conditions, the steam generator and secondary circuit steam conditions are designed accordingly. The feedwater is heated and the resulting steam/water mixture passes through moisture separators, which improve the steam quality by reducing wetness. The incorporation of a superheating surface would necessitate an unwarranted complication in the design of the steam generator. The design practice is to reduce the 'pinch point' to a minimum (see Fig 1.48) whilst optimising steam generator cost against improvements in overall cycle efficiency.

Consider now the cycle for the turbine-generator (Fig 1.47). The slightly wet steam leaves the steam generator, passes through governor valves and enters the HP turbine. The steam is expanded through the HP turbine where its pressure reduces and its wetness increases. The wet steam then passes through a moisture separator which returns the steam to a near-dry saturated state. The steam is then reheated in two stages. The first reheat process uses steam bled from a tapping on the HP cylinder, the second uses live steam taken upstream of the governor valves. The steam leaving the reheaters is in a superheated con­dition before entering the LP turbine where it is expanded to a low pressure. The low pressure steam is condensed in the condenser, rejecting heat to the CW. The condensate is pumped through several stages of LP heaters to the de-aerator.

The de-aerator is a large direct contact feedheater which provides for oxygen removal, a buffer store of feedwater and sufficient gravity head to overcome the net positive suction head of the boiler feed pump. The feedwater is pumped through several stages of HP heaters before return to the steam generator.

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