Lecture 2

Question 1

Control Rods

Answer 1

Quick shutdown by gravity and as a backup injection of Poison in primary water

Question 2

Steam Generator

Answer 2

Heat is used to produced high pressure steam

Question 3

Turbine

Answer 3

Steam energy is transformed into mechanical rotation energy

Question 4

Electric Generator

Answer 4

Produces electric energy to supply to the grid

Question 5

Pressuriser

Answer 5

Expansion of volume and pressurization of components and suppressing the steam production in primary system

Question 6

Reactor

Answer 6

Nuclear reaction takes place and heat produced

Question 7

Emergency cooling of reactor - Function

Answer 7

Radioactive products accumulated in reactor continue to generate heat after shutdown. The emergency cooling system is both passive and active

Question 8

Containment

Answer 8

Provides protection after release of radioactive products, made with concrete and steel

Question 9

Reactor Control System of PWR involves the control of:

Answer 9

• Reactivity
• Coolant volume
• Water level of the steam generators
• Steam flow to the turbine

Question 10

Reactivity Control

Answer 10

Fast Control: Full-length control rods
Slow Control: Boric Acid concentrate in coolant

Question 11

Volume Control - Purpose

Answer 11

1. offset changes to coolant volume due to changes in temperature
2. Replace lost coolant due to minor leakage in the primary system
3. Adjust the boron concentration in the primary coolant

Question 12

Volume Control - Components

Answer 12

Volume control tank
3 parallel charging pumps
Storage tanks with boric acid and deionised water

Question 13

Volume Control - System

Answer 13

• Manually controlled in the control room
• “Automatic Makeup” is the mode of operation: Boric acid and deionised water are blended to the same composition as that of the reactor coolant and fed to the suction side of the charging pump

Question 14

Feedwater Control System - Purpose

Answer 14

Balance the feedwater flow to the steam generators and the steam flow to the turbine.
Achieved by regulating the water level on the secondary side of the steam generators.

Question 15

Steam flow to the turbine

Answer 15

• Generator power is adjusted the grid demand by the regulation of the steam to the turbine.
• Turbine generator speed is kept constant.

Question 16

If more steam is generated than required by the turbine-

Answer 16

Excess steam is led directly to the turbine condenser by the use of bypass valves

Question 17

Central Control room

Answer 17

Where the primary process systems are monitored and controlled

Question 18

Special computer

Answer 18

Registers, processes and presents data for core monitoring.
The computer calculates and proposes adjustments of control rod positions.

Question 19

Alarm signals and annunciators

Answer 19

Used to attract the reactor operator’s attention in the event of malfunction or if bounding values for process variables are exceeded.

Question 20

Control of neutron flux

Answer 20

Monitored by a large number of measuring channels with neutron detectors inside the core.
In order to cover the entire range from start-up to full power, three overlapping systems are used.

Question 21

Three overlapping systems for neutron control

Answer 21

Source range monitoring (10e-6)
Intermediate range monitoring (20% of relative power)
Power range monitoring (instantaneous)

Question 22

Insertion of control rods

Answer 22

By actuating the electromechanical transmission of the drive mechanisms or by hydraulic insertion.

Question 23

Time taken for control rods insertion

Answer 23

It takes about 4 minutes to screw the control rods into the core from a fully withdrawn position.
During scram, the control rods are fully inserted within 4-6 seconds.

Question 24

Power Control

Answer 24

Utilises the coolant flow as means to control the power.
Coolant flow ↓, the void content of the core↑, reactivity and power ↓

Question 25

3 operating modes for a power control system

Answer 25

1. Pump speed regulation 2. Power control 3. Power and frequency control

Question 26

Pump speed regulation

Answer 26

When all main recirculation pumps are controlled by the operator.

Question 27

Power control

Answer 27

When the electrical power generated by the unit is maintained at a preset value.

Question 28

Power and frequency control

Answer 28

When the electric power generated is automatically adjusted to the frequency of the grid

Question 29

Plant components

Answer 29

- Reactor pressure vessel - Piping

Question 30

Major components of the reactor pressure vessel

Answer 30

- Reactor vessel - Core barrel - Reactor core - Upper internals package

Question 31

Reactor vessel anatomy

Answer 31

Cylindrical vessel with a hemispherical bottom head and a removable hemispherical top head

Question 32

Top head removable - Why?

Answer 32

Allow for refueling of the reactor

Question 33

Irradiation specimen holders

Answer 33

Samples of the material used to manufacture the vessel will be placed there. At periodic time intervals, some of these samples will be removed and tested to see how the radiation from the fuel has affected the strength of the material.

Question 34

Upper internals package

Answer 34

- Contains the guide columns to guide the control rods when they are pulled from the fuel. - Prevents the core from trying to move up during operation due to the force from the coolant flowing through the assemblies.

Question 35

Describe the flow path for the reactor coolant.

Answer 35

1. The coolant enters the reactor vessel at the inlet nozzle and hits against the core barrel. 2. The core barrel forces the water to flow downward in the space between the reactor vessel wall and the core barrel. 3. After reaching the bottom of the reactor vessel, the flow is turned upward to pass through the fuel assemblies. 4. The coolant flows all around and through the fuel assemblies, removing the heat produced by the fission process. 5. The now hotter water enters the upper internals region, where it is routed out the outlet nozzle and goes on to the steam generator.

Question 36

What are the consequences of "the break"?

Answer 36

- Pipe whip - Fuel jet

Question 37

Fuel jets

Answer 37

Due to decompression waves propagating inside the broken system. Causes the generation of loads on the internal components.

Question 38

Cracks in the primary system

Answer 38

Small pipes have the probability of 0.01 breaks per reactor year

Question 39

Thermal fatigue

Answer 39

Caused by the defective closure of isolation valves and by consequent seepage of a fluid at different temperatures within the pipes.

Question 40

Where does mechanical vibration fatigue occur?

Answer 40

small pipes and in ‘socket welds'

Question 41

What is the cause of 'socket welds'?

Answer 41

Stress concentration points are caused due to unwanted but real notches, that are prone to initiate and propagate fatigue cracks. The presence of pressure pulses due to pumps or due to ‘cavitation’ phenomena with rapid evaporation tends to enhance this tendency