What is CANDU?

Overall safety concept and licensing approach

Comparison between conventional candu reactor and fossil fueled plants

CANDU

Comparison between conventional candu reactor and fossil fueled plants

Disadvantage of conventional candu reactor

a) Heavy water is expensive.

b) The core is large and complex.

A good conventional plant produces hot, high-pressure (dry) steam.

A CANDU reactor delivers steam at lower temperature and pressure (that is, saturated almost wet, steam). This requires larger volumes of steam flow to transfer the same amount of energy

As a result, the steam piping and the steam turbine are physically larger than similar equipment in an equivalent fossil-fueled plant.

In the CANDU reactor, heavy water coolant is pumped over hot uranium dioxide fuel and becomes hot. It then flows into a boiler where it gives this heat to ordinary water, converting it to steam

In a conventional plant, heat to make steam comes from burning coal or oil. In each case, the steam drives a turbine that turns a generator

Description of nuclear system

Reactor control

Reactor core design

Fuel channel assembly

Reactor assembly

The reactor regulating system includes mechanical control absorber units .Control absorber rods can either be inserted slowly under motor control or dropped into the core by gravity. All reactivity mechanisms operate in the water filled reactor vault or the low‑temperature, low‑pressure moderator

The reactor core designed to produce thermal power at a reduced capital cost, with specific characteristics:

1-A more compact size.

2-A significant reduction in heavy water inventory .

3-substantial simplification in reactor control by operating with negative feedbacks in reactor power.

The fuel channel assembly comprises of a zirconium alloy pressure tube. Each pressure tube is thermally insulated from the low‑temperature, low‑pressure moderator by the carbon dioxide.

The major nuclear systems of each ACR‑1000 unit are located in the Reactor Building and the Reactor Auxiliary Building. These nuclear systems include the following:

1-Reactor assembly

2-LEU fuel

3-Heat transport system (HTS) with light water coolant, steam generators, four heat transport pumps.

The ACR-1000 reactor assembly consists of a horizontal, cylindrical calandria and end shield assembly enclosed and supported by a steel‑lined concrete vault.

-The calandria contain heavy water modearator.

-the vault contains light water which serve as thermal and biological shield.

calandria

moderator

Defense-in-depth

Fuel

Main components of Candu

coolant

Candu Reactor

Heat transport systems

concentration of U-235 in natural uranium is low, so the number of neutrons bombarding the fuel must be high. So neutron losses must reduced.

D2O moderator is required. Any other moderator would absorb too many neutron

CANDU is unique in using natural uranium. Natural uranium is 99.3% U-238, which is not fissile, and 0.7% fissile U-235. In most reactors

collision between a neutron and a fissile atom is greater in enriched fuel, so a chain reaction

continues with fewer neutrons Which does not occur in candu. Candu was designed that did not waste neutrons.

The concept of defense-in-depth is applied throughout the ACR-1000 plant design including measures to prevent accidents and measures to provide protection in the event that prevention fails.

key parts of a reactor are the fuel, moderator and coolant

A large tank with hundreds of passageways (channels) through it contains the moderator. This complicated tank is called the calandria. It is about 6 m long and 7 m across. Heavy water absorbs few neutrons, but is not as effective as light water in slowing them down. For the same power output, a heavy water reactor is larger than a light water moderated reactor

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In a light water moderated reactor, the moderator also serves as the coolant. Liquid water in contact with hot fuel becomes very hot

. A lot of heavy water is needed to slow neutrons down. Which makes heavy water moderated reactor large. A large pressure vessel is difficult to build and very expensive. A pressure tube reactor design solved this problem. This design separates the moderator and coolant. Pressure tubes running horizontally through the reactor contain the fuel. High-pressure heavy water coolant passes through the pressure tube and over the fuel.

The objectives of safety design

• Mahrosa Mohamed
• Mahmud DiaaHo
• Mahmoud Atef
• Karem Hossam

The HTS consists of two figure‑of‑eight loops circulating pressurized light water coolant through the reactor fuel channels to remove heat produced by nuclear fission in the core.

:-The major components of the HTS

1-Fuel channel

2-four sream generators

3-four heat transport pumps

Safety and design philosophy

What is candu ?

prevent accidents and to mitigate their consequences

Ensure that, for all postulated accidents considered in the design including those of very low would be below limits;

Ensure that the likelihood of accidents with serious radiological consequences is extremely low.

CANDU-specific features and advantages

Levels of defense in depth

Use of natural uranium as a fuel

CANDU is the most efficient of all reactors in using uranium: it uses about 15% less uranium than a pwr for each megawatt of electricity produced.

Use of natural uranium makes fuel fabrication easier.

There is no need for uranium enrichment facility.

Fuel reprocessing is not needed, so costs, facilities and waste disposal associated with reprocessing are avoided .

Early efforts

the first CANDU-type reactor, the Nuclear Power Demonstration (NPD), in Rolphton, Ontario.

22MWe, a very low power for a commercial power reactor

NPD produced the first nuclear-generated electricity in Canada, and ran successfully from 1962 to 1987.

The second CANDU was the Douglas Point reactor, a more powerful version rated at roughly 200 Mwe.

The ACR-1000 design utilizes passive, stored-energy, natural circulation and gravity features for:

Reactor shutdown,

Cooling of the HTS when forced circulation is unavailable,

Core refill and fuel cooling following a LOCA,

Post-accident pressure and temperature suppression inside containment,

Emergency feedwater supply to the steam generators, and

Mitigation of postulated beyond design basis accidents.

ACR-1000 plant aims to prevent, as far as practicable, challenges to the integrity of physical barriers; failure of a barrier when challenged, and failure of a barrier as a consequence of the failure of another barrier. This approach is

structured in five levels, as presented below:

Level 1 – Prevention of abnormal operation and of failures of SSCs.

Level 2 – Detection and control of deviations from normal operation

Level 3 – Control of accidents within the design basis

Level 4 – Control of severe plant conditions to manage accidents and mitigate their consequences, as far as practicable, by the

robust containment design, complementary design features, and severe accident management procedures;

Level 5 – Mitigation of radiological consequences of significant releases of radioactive materials by emergency support centre and plans for on-site and off-site emergency response.

What is CANDU?

 There are four fundamental safety functions

 Control of reactivity (control)

Removal of heat from the core (cool)

Confinement of radioactive materials and control of operational discharges, and limitation of accident releases

Monitoring of critical safety parameters to guide operator actions (monitor).

CANDU ® stands for "CANada Deuterium Uranium".

It is a Canadian-designed power reactor of PHWR type (Pressurized Heavy Water Reactor) that uses heavy water (deuterium oxide) for moderator and coolant, and natural uranium for fuel.

All of Canada's 20 nuclear reactors are of the CANDU design. Other nations with CANDU reactors include Argentina, China, India, South Korea, Pakistan, and Romania.

Accident resistance

Design features reduce the probability and severity of accidents. The design minimizes the occurrence and propagation of initiating events that could lead to events of greater severity and resulting challenges to safety systems

Economics

Waste management costs

People who predicted cheap nuclear power overlooked costs that offset fuel savings. Nuclear plants are expensive to finance, build and maintain. Capital cost is at least three times that of an equivalent fossil plant Also, nuclear plants take 3 or 4 years longer to construct, commission and bring into service.

Predicting the cost of both conventional and nuclear power production is difficult. Financing for construction must be in place years before revenue from power sales begin. The cost of nuclear power, with its high front-end cost, depends strongly on interest rates. Fuel cost is a

much more important part of the price of electricity from fossil fuels.

From initial construction to in-service presently takes 8-10 years for a nuclear plant. The designer is expecting that a modular construction technique will allow the newest design of small reactor to be built in half that time.

Cost estimates for each kind of power generation depends on how prices change during the life of the plant. Fossil generated power becomes expensive if fuel costs increase sharply after the plant is built Cost increases after the plant starts operating do not affect the cost of nuclear power as much. Borrowing money to build a nuclear plant is a good strategy when costs are expected to increase, especially if interest rates are low. When fuel costs are stable or expected to fall or the cost of borrowing money is high, coal may be a better choice

The inventors of nuclear power expected it to be a much cheaper energy source than energy from fossil fuels (coal, gas or oil). The economic advantage of electricity from nuclear power compared to fossil fuel comes from the cost of the uranium fuel. For comparison, in1990, nuclear fuel costs in Ontario were about 10% of the fuel cost for an equivalent sized fossil plant. A large coal fired plant uses about 20,000 tons of coal a day. The equivalent nuclear plant uses about20 fuel bundles, each with less than 20 kg of uranium

The politics of CANDU

Environmental Benefits of Nuclear Energy

 About 25% of the global carbon dioxide release to the atmosphere is caused by electricity generation using fossil fuels

 Each year the emission of 100 million tons of carbon dioxide is avoided owing to Canada's existing nuclear power plants

 During the last 30 years the nuclear generating stations in Ontario avoided the release of a billion of tons of carbon dioxide, 32 million tons of acid gas, and 80 million tons of ash to the atmosphere that would have occurred if their electricity had been produced by fossil-fueled plants

 The high energy density of nuclear fuel is an environmental advantage. The energy from one CANDU fuel bundle would require 400 tons of coal or 270,000 liters of oil or 300 million liters of natural gas

 In contrast to the large areas required by solar or wind generating systems, the nuclear generating station sits on 2-3 sq. km of land

In The Politics of CANDU Exports,We provide a comprehensive history of the export of the Canada Deuterium-Uranium (CANDU) reactor - a pressurized heavy water natural-uranium power reactor designed and marketed by Atomic Energy of Canada Limited. We examine every CANDU sale, as well as some important unsuccessful sales attempts, from 1956 to the present. He also outlines the impact that changes in the international political climate such as the creation and strengthening of the international nuclear non-proliferation regime, and the increasing importance of human rights and environmental protection, have had on CANDU exports over the last fifty years. The studies attempt to develop a framework for understanding

flow of the influences of different foreign policy objectives on Canada’s decision-making process. There are litanies of economic and political interests that Canadian governments have hoped to serve by exporting CANDUs, interests such as economic gain, containing communism, and assisting the developing world. Yet, Canada has additional foreign policy objectives such as national security, the protection of human rights, and preservation of the environment, which constrain the desire to export CANDUs. Furthermore, the nature of the debate surrounding CANDU exports has changed over time. showing that while the traditional debate over CANDU exports was between Canada’s commercial interests and its security concerns, since the early 1990s a new debate focused on two separate planes of argument has emerged. The economic benefits of exporting the CANDU reactors are now weighed against the economic cost of extensive government subsidies; while the environmental benefits of CANDU exports are measured against the environmental costs of building and promoting nuclear power.

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Experience with CANDU reactors shows nuclear power is much less expensive than fossil fuel power when nuclear plants are kept running 80% or more of the time. New CANDUs routinely do this well or better. Good performance over the life of a reactor will result in a significant cost advantage of nuclear over fossil fuel. Poor operation can make nuclear power very expensive.

A capacity factor of 60% is sometimes given as a crude estimate of the break-even point between nuclear and fossil fueled plants.

The advantage of uranium over fossil fuel will likely increase as world oil and gas supplies dwindle and concerns about greenhouse gases grow. The supply of uranium is finite too. Canada is fortunate to have supplies for itself and for a large export market for at least 50 to 60 years. It is possible, but not now economic, to use thorium as the nuclear fuel in a CANDU reactor. When uranium is used up, thorium could extend fuel supplies for 100 years or more.

Nuclear energy is a clean form of energy, particularly in comparison to thermal sources such as coal and oil where fossil fuel is burned to create steam and energy. Because there is no combustion during the nuclear reaction, nuclear energy does not emit acid gases or carbon dioxide. Carbon dioxide is considered the major contributor to the "greenhouse" effect causing climate change, and acid gases cause acid rain and smog.

Consider this:

CANDU-specific features and advantages

Core damage prevention

Moderator system

Accident mitigation

Proliferation resistance

Safety systems

Safety and security

engineered systems are provided to prevent significant damage to the reactor core as follows:

Independent safety systems are provided for the key functions of reactor shutdown, core decay heat removal and containment of radioactive releases;

Two diverse shutdown systems, independent from each other and the reactor regulating system, are provided;

The safety system responses are automated to the extent that no operator action is needed for a minimum of eight hours;

Each unit can withstand loss of off-site electrical power and loss of one set of diesels1 for at least 24 hours without fuel damage.

Provisions are made in the design of the ACR-1000 plant for the installation of safeguard systems. The supply and installation of safeguard instrumentation is the responsibility of the IAEA through its agreements with the utility. The provisions made for this equipment are based on the installation of systems similar to the ones that have been previously accepted by the IAEA on the CANDU plants, with support for increased remote monitoring and real time accounting.

The anticipated instrumentation for safeguard purposes includes

• spent fuel bundle counters,
• surveillance cameras, and other devices

Design basis threat and beyond design basis threat

The ACR-1000 plant resists a set of threats . Threats identified as design basis threats have credible attributes and characteristics of potential insider or external adversaries who might attempt sabotage against which a physical protection system is designed and evaluated. Beyond design basis threats are less frequent and more severe than design basis threats and their consequences are assessed in order to establish means of mitigation to the extent practicable

. The five safety systems are as follows:

Shutdown system No. 1 (SDS1) quickly terminates reactor power operation and brings the reactor into a safe shutdown condition by Dropping shutoff rods,

The Shutdown System No. 2 (SDS2) quickly terminating reactor power operation by injecting a strong neutron-absorbing solution (gadolinium nitrate) into the moderator.

Emergency core cooling system

The ECC system supply cooling water to the reactor core in the event of a LOCA.

The ECC function is accomplished by two subsystems:

The emergency coolant injection system, .

The long-term cooling (LTC) system provides long-term injection

Emergency feedwater system heat removal function is accomplished by the EFW system. This system is designed to provide cooling water to the secondary side of the steam generators

Containment system : the containment system minimizes release of resultant radioactive materials to the external environment

An accident may progress to the release of radioactive material from the fuel. To prevent the release of these materials to the public and the environment

The ACR-1000 design:

Provides a robust containment building and associated systems for heat removal

The containment design pressure is based on the limiting design basis event;

Provides a hydrogen control system so that the concentrations of combustible hydrogen under design basis events and limited

Core damage accidents do not exceed the hydrogen detonation limits commensurate with the probability and severity of the event.

Use of heavy water as a moderator

Heavy water (deuterium oxide) is highly efficient because of its low neutron absorption and affords the highest neutron economy .

Heavy water used in CANDU reactors is readily available.

It can be produced locally, using proven technology.

Heavy water lasts beyond the life of the plant and can be re-used.

The moderator system is a low‑pressure and low‑temperature system.it used the heavy water

To remove the heat generated from the fuel and can work as neutron reflector.

The moderator system components:

1-pumps

2-heat exchangers

3-heavy water

CANDU-specific features and advantages

The ACR-1000, features and improvement

Overview

CANDU reactor core design

Reactor core comprising small diameter fuel channels rather that one large pressure vessel.

Allows on-power refueling - extremely high capability factors are possible

The moveable fuel bundles in the pressure tubes allow maximum burn-up of all the fuel in the reactor core

The following key differences from the traditional CANDU

the CANDU plant design:including horizontal fuel channel core, a low temperature heavy-water moderator, a water filled reactor vault, two independent safety shutdown systems, a highly automated control system, on-power fueling and a reactor building that is accessible for on-power maintenance and testing.

• Introduction
• conventional Reactor
• Design
• Safty of Enhanced Reactor
• Effects
• Improvments

The use of LEU fuel contained in CANFLEX-ACR®[3] fuel bundles.

• The use of light water instead of heavy water as the reactor coolant.

• Lower moderator volume to fuel ratio

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