Thermodynamic Cycles

Question 1

Overall nuclear power plant thermal efficiency will decrease if…

Answer 1

the temperature of the steam at the turbine exhaust increases.

Question 2

Which one of the following will cause overall nuclear power plant thermal efficiency to increase?

Answer 2

Changing steam quality from 99.7 to 99.9 percent.

Question 3

Steam turbines X and Y are identical 100 percent efficient turbines that exhaust to a condenser at 1.0
psia. Dry saturated steam at 250 psia enters turbine X. Superheated steam at 250 psia and 500°F
enters turbine Y.
Which one of the following lists the percentage of moisture at the exhaust of turbines X and Y?

Answer 3

24.5% 20.5%

Question 4

Which one of the following actions will decrease overall nuclear power plant thermal efficiency?

Answer 4

Increasing turbine exhaust pressure.

Question 5

To achieve maximum overall nuclear power plant thermal efficiency, feedwater should enter the
steam generator (SG) __________ and the pressure difference between the SG and the condenser
should be as __________ as possible.

Answer 5

close to saturation; great

Question 6

Feedwater heating increases overall nuclear power plant thermal efficiency because…

Answer 6

the average temperature at which heat is transferred in the steam generators is increased.

Question 7

Which one of the following changes will increase the overall nuclear power plant thermal efficiency?

Answer 7

Decreasing the concentration of noncondensible gases in the main condenser.

Question 8

Which one of the following actions will result in a decrease in overall nuclear power plant thermal
efficiency?

Answer 8

Decreasing the amount of turbine steam extracted for feedwater heating.

Question 9

Turbine X and turbine Y are ideal steam turbines that exhaust to a condenser at 1.0 psia. Turbine X is
driven by dry saturated steam at 900 psia. Turbine Y is driven by superheated steam at 500 psia and
620°F.
The greater amount of specific work (Btu/lbm) is being performed by turbine _____, and the greater
moisture content exists in the exhaust of turbine _____.

Answer 9

Y; X

Question 10

Turbine X and turbine Y are ideal steam turbines that exhaust to a condenser at 1.0 psia. Turbine X is
driven by dry saturated steam at 500 psia. Turbine Y is driven by dry saturated steam at 700 psia.
The greater amount of specific work (Btu/lbm) is being performed by turbine _____; and the greater
moisture content exists in the exhaust of turbine _____.

Answer 10

Y; Y

Question 11

A nuclear power plant is operating at 85 percent power when the extraction steam to a high pressure
feedwater heater is isolated. After the transient, the operator returns reactor power to 85 percent and
stabilizes the plant. Compared to the conditions just prior to the transient, the current main generator
output (MW) is…

Answer 11

lower, because the steam cycle thermal efficiency has decreased.

Question 12

Initially, a nuclear power plant was operating at steady-state 85 percent reactor power when the
extraction steam to a high-pressure feedwater heater became isolated. Main generator load was
returned to its initial value. When the plant stabilizes, reactor power will be __________ than 85
percent; and the steam cycle thermal efficiency will be __________.

Answer 12

greater; lower

Question 13

A nuclear power plant is operating at 90 percent power. Main condenser pressure is 1.7 psia and
hotwell condensate temperature is 120°F.
If main condenser cooling water flow rate is reduced by 5 percent, overall steam cycle efficiency
will…

Answer 13

decrease, because the work output of the main turbine will decrease

Question 14

If superheating of the inlet steam to a low pressure (LP) turbine is reduced, LP turbine work output
will __________; and LP turbine exhaust moisture content will __________. (Assume steam mass
flow rate does not change.)

Answer 14

decrease; increase

Question 15

If the moisture content of the steam supplied to a main turbine increases, turbine work will…
(Assume the total mass flow rate does not change.)

Answer 15

decrease, because the enthalpy of the moist steam being supplied to the turbine has decreased.

Question 16

Turbine X is an ideal steam turbine that exhausts to a condenser at 1.0 psia. Turbine X is driven by
dry saturated steam at 500 psia. Which one of the following lists the approximate specific work
output of turbine X and the moisture content of the steam exiting turbine X?

Answer 16

388 Btu/lbm 28%

Question 17

If the moisture content of the steam supplied to a turbine decreases, the steam cycle thermal efficiency
will increase because the…

Answer 17

enthalpy of the steam being supplied to the turbine has increased

Question 18

The theoretical maximum efficiency of a steam cycle is given by the equation:
Effmax = (1 - Tout/Tin) x 100%
where Tout is the absolute temperature for heat rejection and Tin is the absolute temperature for heat
addition. (Fahrenheit temperature is converted to absolute temperature by adding 460°F.)
A nuclear power plant is operating with a stable steam generator pressure of 900 psia. What is the
approximate theoretical maximum steam cycle efficiency this plant can achieve by establishing its
main condenser vacuum at 1.0 psia?

Answer 18

43 percent

Question 19

Which one of the following will be caused by a decrease in main condenser vacuum (higher absolute
pressure) in a nuclear power plant operating at 100 percent power? (Assume that main steam and
main condenser circulating water mass flow rates do not change.)

Answer 19

Decrease in the ideal steam cycle thermal efficiency.

Question 20

A nuclear power plant was initially operating at steady-state 90 percent reactor power when extraction
steam to the feedwater heaters was isolated. With extraction steam still isolated, reactor power was
returned to 90 percent and the plant was stabilized

Answer 20

lower, because the steam cycle is less efficient.

Question 21

Consider the steam cycle thermal efficiency of a nuclear power plant operating at rated power.
If the pressure at which saturated steam is produced in the steam generators is increased, thermal
efficiency will __________; and if the temperature of the feedwater entering the steam generators is
increased, thermal efficiency will __________.

Answer 21

increase; increase

Question 22

A nuclear power plant has a thermal power rating of 3,200 MW. When the plant operates at 100
percent power, the main generator produces 1,200 MW at a 0.95 power factor. Plant modifications
are planned that will upgrade the feedwater heaters and moisture separator/reheaters without changing
the plant’s thermal power rating. If the plant modifications improve plant thermal efficiency by 2
percent, what will be the resulting main generator electrical output at 100 percent reactor power with
the same power factor?

Answer 22

1,264 MW

Question 23

A nuclear reactor has a thermal power rating of 3,200 MW. When the reactor operates at 100 percent
power, the main generator produces 1,200 MW at a 0.95 power factor. Modifications are planned
that will upgrade major power plant equipment without changing the reactor’s thermal power rating.
If the modifications improve the power plant’s thermal efficiency by 3 percent, what will be the
resulting main generator electrical output with the same power factor at 100 percent reactor power?

Answer 23

1,296 MW

Question 24

Initially, a main turbine is being supplied with inlet steam containing 0.25 percent moisture content.
If the inlet steam moisture content increases to 0.5 percent at the same pressure and mass flow rate, the
main turbine work output will…

Answer 24

decrease, due to the increased braking action from water droplets impacting the turbine
blading

Question 25

Initially, a main turbine is being supplied with inlet steam containing 0.5 percent moisture content. If
the inlet steam moisture content decreases to 0.25 percent at the same pressure and mass flow rate, the
main turbine work output will…

Answer 25

increase, due to the decreased braking action from water droplets impacting the turbine blading