CANDU Fuel and Reactor Specifics


This photo illustrates the preparation of the fuel pellets before they are loaded into the individual fuel rods as shown below.
Photo courtesy AECL
There are 380, 390 or 480 horizontal fuel channels in each CANDU. The number of channels is related to the Megawatt output of the reactor. In India, AECL- and Indian-designed reactors each have 306 fuel channels.

Each fuel channel consists of an inner pressure tube, which contains the fuel bundles and the heavy water primary coolant, and an outer calandria tube.

The tubes are concentric, and the gap between them (8-9 mm) is filled with a slow purge of carbon dioxide.

The calandria tube keeps the heavy water moderator (<70 C) away from the pressure tube (~310 C). The moderator (in the calandria vessel) is essentially at atmospheric pressure (the head of heavy water plus a cover gas pressure).

Data courtesy Morgan Brown of AECL

Photo courtesy AECL

This graphic shows how fuel pellets are loaded into fuel rods in a fuel bundle (fuel assembly). Then shown is the location of the fuel bundle within the pressure tube and calandria.
Photo courtesy AECL
This graphic illustrates how fueling machines load the fuel bundles into the reactor and how a defueling machine removes the spent bundles. The spent fuel is pushed by one fueling machine into another fuelling machine attached to the same pressure tube. After the fueling machines are detached from a channel (after the end plugs are refitted), the receiving machine offloads the spent bundles to the cooling pond.

Graphic courtesy AECL

This photo shows the size of the fuel bundle used in the CANDU reactor.
Photo courtesy AECL
As in the PWR, control rods can shut down the reactor. Similar to the PWR, a bank of shutdown rods, normally held up against gravity and springs by electro-magnetic clutches, can drop into an unpressurized calandria vessel - they don't have to be driven in.

Graphic courtesy AECL

A second shutdown system is via gadolinium nitrate liquid "neutron poison" injection. Both systems operate via separate and independent trip logic.

The gadolinium nitrate system function is similar to that of the PWR high pressure safety injection system or the BWR standby liquid control system.

Graphic courtesy AECL

Copyright 1996-2005.  Joseph Gonyeau, P.E.. The Virtual Nuclear Tourist. All rights reserved. Revised: March 15, 2001.