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CANDU Reactor

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Are you interested in CANDU technology? This on-line interactive 3D model contains educational descriptions and specifications, discover everything from Calandria to Feeder pipes.

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A larger cylindrical vessel containing heavy water that acts purely as a moderator. The vessel is easy to fabricate because it is not pressurized and moderator temperature in it is quite low. The heavy water moderator in the calandria is cooled by a dedicated circuit.

Calandria vault

The calandria vault is a six-sided structure of reinforced concrete. It is supported on reinforced concrete bearing foundation walls and contains a large volume of light water for cooling.

End shield

There is an end shield at each end of the calandria vessel. It is a horizontal cylindrical shell enclosed by two tubesheets and spanned horizontally by lattice tubes. On the calandria side, a calandria tubesheet is welded to the shell, into which the calandria tubes are fixed. On the outside of the end shield facing one of the fueling machines is the fueling tubesheet.

Fuel channels

The fuel channels consist of a large number of horizontally installed pressure tubes, located in the calandria with cold heavy water. These tubes perform the function that a pressure vessel performs in a light water reactor. The fuel channel is a combination of four main components: the pressure tube with fuel bundles, the calandria tube, the end fittings and the spacers.

Feeder pipes

In CANDU reactors, there are two types of feed lines: inlet and outlet pipes. In each fuel channel, an inlet feeder pipe on one side carries cooling heavy water from the inlet header through the channel fitting to the fuel. The outlet feed line then conveys the heated water from the channel to the outlet header and to the steam generator. The feeders are an essential part of the primary heat transfer system.

Headers

There are two inlet and two outlet headers at each end of the CANDU reactor. One inlet and one outlet header provide flow through the feed lines for approximately 100 fuel channels.

Heat transport pump

The four heat transport pumps in CANDU plants are vertical, single stage centrifugal pumps. Typically pumps have single suction, single or double discharge and have multi-stage mechanical shaft seals. Each pump is driven by a vertical, air-water cooled induction motor.

Steam Generator

The steam generator transfers heat from the heavy water of the primary coolant to the light water on the secondary side of the generator, causing it to boil and produce steam that drives a turbine with an attached electric generator. The primary side of the steam generator consists of an inverted vertical U-tube bundle. On the secondary side, residual moisture is removed at the top of the steam generator by means of a separator.

CANDU Fuel Channel

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CANDU Fuel Channel

Fuel channel

Fuel pellets

The cylindrical fuel pellets are made of high-density UO₂, which gives them very good dimensional stability. The small chamfers and profile of the end-dishes reduce inter-pellet stress on the cladding and create additional volume for fission gas.

Fuel element

The fuel element or fuel rod consists of a stack of ceramic fuel pellets enclosed in a thin zirconium cladding. The element has a diameter of about 13 mm and is approximately 0.5 m long. There are 37 fuel elements in one bundle (CANDU 6).

Endplates

The plates at both ends of the fuel bundle are made of zircaloy. They must be thin to minimize neutron absorption, strong to provide structural support and separation of the elements, and flexible to allow the plates to accommodate fuel element differential expansion.

Split spacers

Inter-element spacers provide element separation at the bundle midplane.

Bearing pads

Bearing pads brazed to the outer elements provide element-to-pressure tube separation and keep the bundle centered in the fuel channel. This means that bearing pads are only points of contact between fuel bundle and pressure-tube under NOC.

End caps

Thin end caps at both ends hermetically seal the fuel element. Like the element cladding, they are made of zircaloy, so they have low neutron absorption and very good heat transfer. The end caps are mechanically connected to the endplates.

Cladding

The fuel element cladding is made of a thin (0.4 mm) neutron-transparent zirconium tube with an outer diameter of 13 mm. It has excellent heat transfer to the coolant and, thanks to the material, low neutron absorption.

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