Dr. George Bereznai
CHAPTER 1
OVERALL UNIT
CHAPTER OBJECTIVES:
At the end of this chapter, you will be able to describe the following for a CANDU nuclear generating station:
1.1. Energy conversions from fission to electricity;
1.2. The main functions and components of each major system;
1.3. How an energy balance is maintained between the reactor and the conventional side of the station;
1.4. How the unit as a whole is controlled;
1.5. The fundamentals of reactor safety;
1.6. The main systems and operating characteristics of a CANDU generating unit.
Nuclear generating stations exist for the purpose of converting the energy obtained from the fission of certain nuclei to electricity. This energy conversion takes place via a number of intermediate stages that require many pieces of equipment organized into several systems under the control and protection of both manual and automatic operations. This chapter presents the main features of a nuclear power plant, so that as each system is studied in greater detail in subsequent chapters and in other courses, the reader should always be able to place such detail into the overall context of an operating station.
1. ENERGY CONVERSION
The basic nuclear generating station energy cycle is shown in Figure 1.1. Fuel containing fissile material (Uranium) is fed to the reactor where fission takes place. The energy liberated appears in the form of heat, which is used to boil water. The steam produced from the boiling water spins a turbine-generator set, where the heat is converted first to kinetic energy in the turbine and to electricity by the generator; the electricity produced (denoted as megawatts) is supplied to the electric power system.
Figure 1.1. Basic flow of energy in a nuclear generating station.
It is important to recognize that while the transport of heat from the reactor to the turbine takes place in one or two closed loop systems that are highly efficient, the transformation of the heat energy of the steam to the kinetic energy of the turbine is accompanied by a large loss of energy as the steam is condensed to water prior to recirculating it back to the steam production system. Approximately 60% of the heat energy removed from the fuel is rejected to the condenser cooling water. As we will see, several other systems are also cooled by water. Under normal operations only a few % of the energy is lost directly to the atmosphere.
As indicated in Figure 1.1 spent fuel is periodically removed from the reactor. On the generator end the flow of electrical energy is shown to be in two directions to indicate the electrical energy consumption of the station itself.
This very much oversimplified representation of a nuclear generating station will become increasingly more complex as we study the details of the many systems involved directly or indirectly in the energy conversion processes, and in ensuring that these processes are always under control and are operated in a safe manner.
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Figure 1.2. Simplified Nuclear Generating Station Control System.
2. WATER MODERATED REACTORS
Most of the nuclear power plants in operation around the globe use reactors that are both moderated and cooled by water. Reactors that use enriched uranium use ordinary (or light) water as both moderator and coolant. The reactor core is contained in a pressure vessel with no separation between moderator and coolant. Two main types of light water reactors have been developed, the Pressurized Water Reactor (PWR) and the Boiling Water Reactor (BWR). In the former the reactor coolant forms a closed primary loop in which it is not permitted to boil under normal operating conditions, and the steam is produced in a secondary loop. In a BWR the coolant is allowed to boil and the steam is fed directly to the turbine. The main characteristics of PWR and BWR reactors are shown in Figure 1.3 and Figure 1.4. The next two sections outline some of the main design and operating features of these types of reactors.
Reactors fueled with natural uranium must use heavy water instead of light water as the moderator, and in order to achieve maximum neutron economy, many heavy water moderated reactors also use heavy water as the coolant. The currently used designs are of the pressurized primary loop type similar to PWRs, but instead of a pressure vessel, pressure tubes contain the coolant and the fuel, while the moderator is in a low pressure, low temperature calandria vessel. Since this text deals extensively with the CANDU (CANadian Deuterium Uranium) type of pressurized heavy water reactors, the illustration of a CANDU in Figure 1.5 is provided only as a means of easy comparison with the PWR and BWR reactor types.
3. ENERGY BALANCE
Nuclear generating stations are designed to operate for extended periods at a constant power level, requiring that a steady state balance is maintained between the rate of energy released from the fuel in the reactor and the electrical output of the generator. This must be achieved despite inherent variations in the burn-up of fuel in the reactor, disturbances in the energy conversion processes, in the demands of the electrical power system and in the energy exchanges between the environment and the station.
As a minimum the plant control system must be able to adjust reactor power to produce the desired amount of electricity. Since under normal operating conditions the generator is synchronized to the electric power grid, the electrical energy produced by the generator is determined by the energy of the steam admitted to the turbine. A mis-match between the energy produced by the reactor and the steam energy required to produce the desired electrical output will result in a change of steam temperature and pressure. Because steam pressure measurements respond more quickly than temperature measurements, it is steam pressure that is used to indicate an imbalance of energy between reactor and generator, and is therefore the parameter chosen as an input to the control system to maintain the required balance.
A very much simplified plant control system is shown in Figure 1.2. The key inputs to the control system are:
reactor power
steam pressure
generator output (MW).
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The station control system is designed to keep steam pressure constant while matching the stations output to the desired setpoint. If the setpoint is the desired level of megawatts, then the control system adjusts reactor power by changing the position of the reactivity control devices, and the control system is said to be in 'reactor lagging' mode. If the setpoint is the desired reactor power output, then the control system adjusts the steam flow to the turbine by changing the opening of the governor valve, thereby altering the generator's output, and the control system is said to be in 'reactor leading' mode. The choice of which type of setpoint to specify depends on the operating status of the generating station and the requirements of the electrical power grid, and input to the control system by the authorized station operator.
4.
1. - .
| . . . . . . . . . | Important safety instructions Children do not realize the dangers associated with electrical appliances. Never allow children to work with electrical equipment without supervision. An appliance should never be left unattended when plugged in. In the case of obvious damage to the appliance or the cable consult an electrician. Repairs to electrical equipment may only be carried out by properly trained person. Repairs which are improperly carried out can result in considerable danger for user. Use only a dry anti-slip level surface. Only suitable for water - never put other liquids in the jug kettle. Never use the appliance in the open air. |
2. .
3. .
Important Safeguards
Before using the iron, read the use instructions completely. Keep the use instructions during the entire life of the iron.
Make sure your voltage corresponds to the voltage printed on the iron. Connect the iron to alternating current only.
Always unplug the iron before filling with water and make sure the steam button is in its lower position (steam off). Always pull the plug, not the cord. The cord should never come in contact with hot objects or with the hot sole plate.
Never immerse the iron in water or other liquids.
During ironing pauses, always place the iron upright on its heel rest. Unplug it when leaving the room, even if only for a short time.
Never pull out the steam button (3) during ironing.
Keep the iron away from children. Electric irons combine high temperatures and hot steam that could lead to burns.
Check the cord regularly for possible damage.
If the appliance (including cord) shows any defect, stop using it and take it to a Braun Service Centre for repair.
Faulty or unqualified repair work may cause accidents or injury to the user.
4. . .
How to Prevent a Fire
1. Arrange for furnace inspections every year before heating season starts. Regular inspections are important in detecting problems in your furnace that could cause a fire. Keep newspapers, boxes and other combustible items far from your furnace.
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2. Exercise caution when using a space heater. Turn off the heater when you leave the room, and place the heater 3 feet away from anything that could burn. Don't try to extend the reach of your space heater with an extension cord. If the cord overheats, a fire could occur.
3. Buy a fireplace screen and use it to prevent sparks from igniting carpeting, flooring, kindling, furniture or other items in the room. Before you begin to use your fireplace each year, have it professionally cleaned to remove creosote, a byproduct of wood fires that can cause a fire in your chimney.
4. Avoid overloading outlets with power strips that can accommodate multiple items, and don't use a higher wattage light bulb than is recommended. Don't use extension cords if possible, as they can easily overheat. Never use a frayed extension cord. Don't push your television up against the wall. If you don't allow air space around the TV, it can overheat and cause a fire.
5. Smoke your last cigarette before you go to bed. Smoking is the leading cause of home fire deaths, according to the National Fire Protection Association. Don't smoke when you are tired, have been drinking or are in bed.
6. Keep matches and lighters away from children, and don't leave kids alone when there is a fire in the fireplace or a space heater is running. Make sure your children know the escape plan if a fire should occur.
Store gasoline in red safety containers that are kept outside the house. Make sure you wait for your lawnmower to cool down before adding more gasoline, and don't add gasoline in an enclosed area.
5. .
University of Cambridge Department of Physics Health and Safety Safety information for new Personnel
| When do you need this? | What do you need? |
| On your first day | A copy of the single-page handout 'Cavendish Laboratory Safety Notes'. You will have been given a printed copy by the Department Secretary on your first day. Click here for a copy, if you have lost it. Familiarisation with the department emergency arrangements. Click here to look at them. |
| Within three months of joining | You need to attend an induction course in the Department. These are usually held on the second Tuesday of each month. Check on the training page here for the list of dates. You can also look at the handouts before the session. The course lasts one hour for the basics, and an additional one hour to learn about risk assessment, which is required by all who do experimental work or are going to supervise or manage the work of others. |
| Before you start any experimental work | You must carry out a risk assessment; the blank form is here. The skills for risk assessment part of the Induction Course. Much of the guidance that you need in order to be able to make decisions about control measures is to be found in the Department codes of practice, found listed in the 'hazards' section of the index page. Don't forget that there are also a number of specialists to help you. A list of names is found here. |
Some Department rules:
| Working at night | You must not work alone at night on anything that is more risky than studying at your desk. If you wish to do experimental work, make arrangements so that you are never alone. The highest risk activities, such as work with HF, are NOT PERMITTED at night. Before working at night, be sure that you have a good knowledge of the emergency arrangements. See here. You must sign in if you are here after 6pm, before 8am and at weekends. The sign-in facility is web-based. |
| Electrical work | If you make or modify any electrical equipment you need to have it checked by a NAMED COMPETENT person. This is to ensure that it meets all the safety requirements required by law. Your research group should be able to put you in touch with a person who can do this for you. |
| Radiation work | We have a number of radioactive sources. Please ensure that you notify the Safety Officer, Jane Blunt WELL IN ADVANCE of bringing another to the site. This is to ensure that we do not violate our licensing agreement. There are also several security arrangements to be made. Before using radioactive sources or radiation generators you will need to undertake a short course. |
| Lasers | For all lasers of Class 3R, 3B and Class 4, new users need to register with the Department Laser Safety Officer, Richard Gymer, telephone 37264. Users of Classes 3B and 4 will need a simple eye test. If a new laser is brought on site Richard must be informed and risk assessments MUST be carried out. The Department laser page is here. |
| Biological work | If you wish to work with tissue or other biological materials please contact the Department Biological Safety Officer, Dr Kristian Franz, as soon as possible, to ensure that we have the necessary capabilities and we do not violate our permissions with the HSE. |
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REACTOR SAFETY
In order to minimize the potential threat to the public from the radioactive materials contained within a nuclear station, a number principles have been and incorporated into the design and operation of nuclear generating stations. Collectively, these principles have been incorporated in the golden rule of Reactor Safety, which can be stated as:
There is a minimum risk to the public and the environment from reactor fuel, provided that at all times:
The reactor power is controlled;
The fuel is cooled;
The radioactivity is contained.
This rule is often shortened to CONTROL, COOL, and CONTAIN.
DEFENSE IN DEPTH
There are different ways of achieving the golden rule (CONTROL, COOL and CONTAIN). Many of these have been incorporated into an important concept known as Defense in Depth. This underlies the whole process of design, construction, commissioning, and operation of a nuclear power plant. This concept is illustrated by the five part model shown in Figure 1.6.
Figure 1.6, Defense in Depth Model.
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The Defense in Depth concept assumes the following:
nuclear station design will have some flaws;
equipment will occasionally fail;
operating personnel will occasionally make mistakes.
The key is to ensure sufficient depth of defense that flaws, failures and mistakes can be accommodated without increasing the risk or consequences of an accident. If we look at each of the major blocks of the model in turn, we can see how this is realized.
7. .
CITIZEN
SR-281 / SR-282 Scientific Calculator
HDBMR17EE01 D25
General Guide
Turning on or off
To turn the calculator on, press [ ON/C ]; To turn the calculator off, press [ 2nd ] [ OFF ].
Battery replacement
SR-281 is powered by two alkaline batteries (GP76A or LR44).
SR-282 is powered two AA-size (UM-3) batteries. If the display becomes dim and difficult to read, the batteries should be replaced as soon as possible.
To replace batteries:
1) Slide the battery compartment cover in the direction indicated by the arrow and remove it.
2) Remove the old batteries and install new ones with polarity in correct directions, then replace the battery compartment cover and press [ ON/C ].
Auto power-off function
This calculator automatically turns it off when not operated for approximately 6~9 minutes. It can be reactivated by pressing [ ON/C ] key and the display, memory, settings are retained.
Reset operation
If the calculator is on but you get unexpected results, press [ MODE ] [ 4 ]
(RESET) in sequence. A message appears on the display to confirm whether you want to reset the calculator and clear memory contents.
| RESET: N Y |
Move the cursor to " Y " by [→ ], then press [ENTER] to clear all variables, pending operations, statistical data, answers, all previous entries, and memory; To cancel the reset operation without clearing the calculator, please choose " N ".
If the calculator is lock and further key operations becomes impossible, please press [ 0 ] [ CE ] at the same time to release the condition. It will return all settings to default settings.
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Contrast adjustment
Pressing the [ - ] or [ + ] following [MODE] key can make the contrast of the screen lighter or darker. Holding either key down will make the display become respectively lighter or darker.
Display readout
The display comprises two lines and indicators. The upper line is a dot display up to 128 characters. The lower line is capable of displaying a result of up to 12 digits, as well as 2-digits positive or negative exponent.
When formulas are input and executed the calculation by [ENTER], they are displayed on the upper line, and then results are shown on the lower line.
The following indictors appear on the display to indicate you the current status of the calculator.
| Indicator | Meaning |
| M | |
| - | |
| E | |
| STO | |
| RCL | |
| 2nd | |
| HYP | |
| ENG | |
| CPLX | |
| CONST | |
| DEGRAD | |
| BIN | |
| OCT | |
| HEX | |
| () | |
| TAB | |
| STAT | |
| REG | |
| EDIT | |
| CPK | |
| USL | |
| LSL | |
| i | |
| ← |
8. .
Function Setup
Function Setup
1. Press the SETUP button and the screen displays the selection menu of the function setup.
Common Setup Professional Setup
Exit Menu
2. Press the CURSOR button to select the menu to be entered and press the SELECT button to confirm; or press the CUSOR button to exit the setup menu and then press the SELECT button to exit. If you just need general setup, please select Common Setup. If you need professional setup, please select Professional Setup.
3. Press UP/DOWN arrow to select the desired item and press SELECT. For example, press UP/DOWN arrow to select Sharpness, and press SELECT. The sharpness appears on the screen.
4. Press UP/DOWN arrow to select the desired values. Press SELECT to confirm. For example, press UP/DOWN arrow to select Medium, and then press SELECT.
5. Press SETUP to exit the setup menu.
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