Heavy water reactors

Pressurized Heavy-Water Reactor (CANDU)

We will here describe the reactor currently available in Canada. Theterm "CANDU" applies specifically to the fact that this reactor is a deuterium-moderated uranium-fueled device.

Instead of moderating neutrons with “"light” water, containing hydrogenwith an atomic mass of 1, this design uses “heavy” water (containingdeuterium, mass 2), which absorbs fewer neutrons than light waterdoes.

The fuel of a CANDU is very similar to that of an LWR in that fuelpins containing uranium dioxide pellets are arranged into bundles whichcan be inserted into the reactor fuel channels.

Each of these channels is a pressure tube, containing a single line offuel bundles arranged end-to-end. The core actually consists of a largenumber of these tubes, all of which pass through a lattice of tubes whichare part of a “Calandria” which contains the moderator. The pressurizedcoolant is pumped through the pressure tubes and boilers (steam generators)much like the coolant of a PWR, except that the CANDU coolant’ssealed off from the moderating fluid, which is maintained at a muchlower pressure and temperature than the coolant. The lower pressureremoves the necessity of fabricating a single large pressure vessel. Atthe boilers, steam is raised in a light water system which drives the turbogenerators.Both the moderator and coolant fluids of current CANDUare heavy water; a possible variant uses light water as the coolant.

One result of the individual pressure tube structure of the CANDU isthat the reactor can be refueled without shutdown. A single channel canbe opened and partially refueled by two refueling machines. The factthat the reactor is refueled only a little at a time results in continuous

(on-line) refueling, as opposed to the abrupt once-yearly loading. Thelatter scheme causes abrupt increases in the fissile load, thus increasingthe neutron multiplication factor. In this case substantial amounts ofcontrol are required. A reactor such as the CANDU does not need thismuch control since the fissile load remains essentially constant andmore neutrons are available for useful purposes such as conversion offertile material to fissile. Decreased control thus improves conversion.

A basic feature of the CANDU, permitted by the use of a moderatorthat absorbs few neutrons, is the use of natural uranium as the fuel. Uraniumfed into the CANDU contains only 0.7 % 235U and this is reduced toabout 0. 2% by the time a bundle is removed. (The irradiated bundle alsocontains plutonium, as a result of neutron capture by 238U.) This removesthe need for costly enrichment of uranium to the 2 to 4 % 235U contentneeded by LWR’s. However, an equivalent cost is production of heavywater by separation from ordinary water, of which 0,016 % is heavy. Thelower fuel enrichment means that excess fissile material is not available inthe fuel. As a result, the design must be based on a minimum of controland on an associated continuous refueling procedure, as just described.

The pressure maintained in the primary coolant system, again by apressurizer as in a PWR, is 1500 psi, and the coolant is allowed to riseto 590 °F, below the boiling point for that pressure. The moderator, onthe other hand, is maintained at approximately atmospheric pressure,under a cover of helium gas. Operating temperatures are in the range110 — 160 °F. As a result, the calandria, which has a basically cylindricalshape, 25 ft in length, with an outer diameter of about 25 ft, is constructedof much thinner steel than the pressure vessels for light-waterreactors. The tubes through which the fuel pressure tubes pass are madeof zircaloy.

Control of the reactor is maintained by several systems including anumber of variable neutron absorbers (actually compartments intowhich a specified amount of light water can be pumped) and a numberof adjusting rods (typically consisting of an absorber such as stainlesssteel, although these have sometimes been booster rods, consisting ofhighly enriched fuel). Shutdown capability exists in the form of gravityoperatedabsorbing rods, backed up by a system for injecting poisonsinto the moderator. It is also possible to dump the moderator out of thecalandria into a storage tank.

The CANDU has an additional cooling system (also heavy water) ofsufficient capacity to cool the reactor after shutdown. Connected to thisshutdown system is an emergency-core-cooling supply of light waterwhich may be injected should the basic system fail.

The calandria is actually in a vault that is filled with light water, whichserves as shielding and helps to maintain the calandria at a constant temperature.The vault is itself in a sealed containment building and in somedesigns can be maintained at a negative pressure with respect to the externalsurroundings. A spray system exists within the containment building.The overall thermal efficiency of a CANDU system is about 29 %, significantlylower than that of most commercial nuclear power plants.

The primary coolant flow pattern is relatively simple: coolant from aprimary pump passes through a distribution header to the individualtubes, goes once through the header at the stem generator, andthrough the U-tube steam generator to the primary pump. The flowrate (600 MWe CANDU) is about 60 million pounds per hour.

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