CANDU reactor

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The CANDU reactor is a Pressurized Heavy Water Reactor designed in the late 1950s and 1960s by a partnership between Atomic Energy of Canada Limited (AECL) and the Hydro-Electric Power Commission of Ontario (now known as Ontario Power Generation), as well as several private industry participants. The acronym "CANDU", a registered trademark of Atomic Energy of Canada Limited, stands for "CANada Deuterium Uranium". This is a reference to its deuterium oxide (heavy water) moderator and its use of natural uranium fuel. All current power reactors in Canada are of the CANDU type. Canada markets this power reactor abroad.

Contents 1 Design features 2 Chronology 3 Active CANDU reactors 4 Economic and political concerns 5 Measures that address concerns 6 Notes 7 References 8 See also 9 External links

Design features

Image:CANDU reactor schematic.pngCANDU reactors have some unique design features that may provide advantages over other reactor designs:

• CANDU uses natural uranium as fuel (in the form of UO₂). This means that it can be operated without expensive uranium enrichment facilities. Most less-developed countries find this attractive because they cannot afford the enrichment facilities. However, a more efficient moderator is needed,in this case heavy water (D₂O).

• The moderator is in a large tank called calandria, penetrated by several hundred horizontal pressure tubes which form channels for the fuel, cooled by a flow of heavy water under high pressure in the primary cooling circuit, reaching 290°C. As in the pressurized water reactor, the primary coolant generates steam in a secondary circuit to drive the turbines. The tubes design means that the reactor can be refueled continuously, without shutting down, as the fuel channels can be accessed individually.

• CANDU reactors do not need large pressure vessels commonly used in light water reactors. Such vessels are extremely expensive and require heavy industry that is lacking in many countries and this was the case with Canada too. The reactor pressurizes only small tubes that actually contain the fuel. These tubes are constructed of a special zirconium alloy (zircaloy) that is relatively transparent to neutrons.

• A CANDU fuel assembly consists of a bundle of 37 half-metre long fuel rods: Ceramic fuel pellets in zircaloy tubes plus a support structure, with 12 bundles lying end to end in a fuel channel. Control rods penetrate the calandria vertically and a secondary shutdown system involves injecting gadolinium nitrate solution to the moderator.¹

• Since the bulk moderator of the reactor is maintained at relatively low temperature and pressure, the equipment to monitor and act on the core is quite a bit less complex. It only has to cope with high radiation and high neutron flux. In particular, the control rods and emergency equipment are simpler and more reliable than in other reactor types.

• The reactor has the least down time of any known type. This is partly because so much of the reactor operates at low temperatures and pressures. It is also caused by the unique fuel handling system. The pressure tubes containing the fuel rods can be individually opened, and the fuel rods changed without taking the reactor out of service.

• Another advantage is that fuel use is the most efficient known. This is due to the use of heavy water as the moderator. The efficiency is also increased because of the online refuelling mechanism permitting the fuel assemblies to be shuffled to the most efficient parts of the reactor core as their reactivity changes. Most other reactor designs need to insert degradable poisons in order to lower the high reactivity of their initial fresh fuel load. This is not necessary in a CANDU.

• Another advantage of the fuel management system is that the reactors can potentially be operated as low temperature breeder reactors. CANDU can operate very efficiently because their neutron economy is so good. They can breed fuel from natural thorium, if uranium is unavailable. CANDU can even be operated to "burn" former nuclear weapons material (in the form of mixed oxide) to a less reactive state effectively rendering it useless for warheads, while at the same time turning the relatively easily handled weapons grade material into highly radioactive waste. Fuel cycle tests also have included the "DUPIC" fuel cycle, or "direct use of spent PWR fuel in CANDU", where used fuel from a PWR reactor is packaged into a CANDU fuel bundle with only physical reprocessing (cut into pieces) but no chemical reprocessing. Where BWR designs require the reactivity associated with enriched fuel the DUPIC fuel cycle is possible in a CANDU due to the neutron economy which allows for the low-reactivity of natural uranium and used enriched fuel.

• After the classic CANDU design was certified, an experimental reactor was developed that used oil as the primary coolant. The oil passed through a heat-exchanger to heat steam. This reactor operated successfully for many years, and may be less expensive, more reliable and even safer than a classic CANDU reactor because the oil circulated at much lower pressures than the steam, and was less corrosive. This was the now shutdown Whiteshell Reactor One or WR-1. Gentilly-1 was also an experimental version of CANDU using a boiling water design but was not considered successful.

• CANDUs have a small positive void coefficient which is mitigated by two independent, fast-acting shutdown systems in loss-of-coolant accident scenarios. This coefficient is much smaller than that of the RBMK design, as well as being insensitive to reactor power level (unlike the RBMK).

• For more information on CANDU technical details, see the CANTEACH web site

Chronology

The first CANDU-type reactor was the Nuclear Power Demonstrator (NPD), in Rolphton, Ontario. It was intented as a proof-of-concept design, and was rated for only 22 MWe, a very low power for a commercial power reactor. It produced the first nuclear-generated electricity in Canada, and ran successfully from 1962 to 1987.[1], [2]

The second CANDU was the Douglas Point reactor, a more powerful version rated at roughly 200MWe and located near Kincardine, Ontario. Somewhat controversially, the Douglas Point project was started in 1959, even before NPD, the prototype CANDU, went on-line. Douglas Point went into service in 1968, and ran until 1984. Uniquely among CANDU stations, Douglas Point incorporated an oil-filled window which offered a view of the east reactor face, even when the reactor was operating. The Douglas Point type was exported to India and Pakistan, and is the basis for India's domestic 'CANDU-derivatives'. Douglas Point was originally planned to be a two-unit station, but the second unit was cancelled because of the success of the larger 515 MWe units at Pickering.[3], [4]

The successes at NPD lead to the decision to construct the first multi-unit station in Pickering, Ontario. Pickering A, consisting of units 1 to 4, went into service in 1971. Pickering B, consisting of units 5 to 8, went into service in 1983, giving a full-station capacity of 4120MWe. The station is placed very close to the city of Toronto, in order to reduce transmission costs. The location of the station has long been a concern for activists, who feel it puts Toronto at risk should an accident and radioactive release occur.

Pickering A was placed into voluntary lay-up in 1997, as a part of Ontario Hydro's Nuclear Improvement plan. Units 1 and 4 have since been returned to service, although not without considerable controversy regarding significant cost-overruns, especially on Unit 4. (The refurbishment of Unit 1 was essentially on-time and on-budget, accounting for delays in project startup imposed by the Ontario provincial government.)

In 2005, Ontario Power Generation announced that refurbishment of Units 2 and 3 at Pickering A would not be pursued, contrary to expectations. The reason for this change in plan was economic: the material condition of these units was much poorer than had existed for Units 1 and 4, particularly the condition of the steam generators, and thus the refurbishment costs would be much higher. This rendered a return-to-service of Units 2 and 3 uneconomical. A project to decommission these units is currently in the early stages of planning.

Active CANDU reactors

Today there are 29 CANDU reactors in use around the world, and a further 11 "CANDU-Derivatives" in use in India (these reactors were developed from the CANDU design after India detonated a nuclear bomb and Canada stopped nuclear dealings with it). The countries the reactors are located in are:

• Canada - 18 (+2 refurbishing, +6 decommissioned)
• South Korea - 4
• China - 2
• India - 2
• Argentina - 1
• Romania - 1
• Pakistan - 1

Economic and political concerns

One economic disadvantage of the CANDU reactor is the initial, one time cost of the heavy water. This is offset by the lower fuelling cost as it does not require enriched uranium. The heavy water required must be more than 99.75% pure² and tonnes of this are required to fill the calandria and the heat transfer system. High purity heavy water is expensive because it is almost indistinguishable from normal water, while roughly only one in 3,200 water molecules is just semi-heavy water. The next generation reactor (the Advanced CANDU Reactor, also called the "ACR") mitigates this disadvantage by having a smaller moderator size and by using light water as a coolant.

There is a common misconception that the plutonium for India's Operation Smiling Buddha nuclear test was produced in a CANDU design. In fact, the plutonium was produced in the unsafeguarded CIRUS reactor that is based on the NRX design, a different Canadian reactor design. India has some unsafeguarded reactors based on the Pressurized Heavy Water Reactor design, used for power generation, and some spent fuel from the Madras Atomic Power Station (MAPS) was reprocessed for plutonium in the late 1980's. (Reference: Albright & Hibbs.) While these reactors could in principle be used for plutonium production, India has a locally-designed military plutonium production reactor called Dhruva which is a scaled-up version of the CIRUS designed for plutonium production. It is this reactor which is thought to have produced the plutonium for India's more recent Operation Shakti nuclear tests.

{{cite journal

| first = David
| last = Albright
| authorlink = 
| coauthors = and Mark Hibbs
| year = 1992
| month = September
| title = India's Silent Bomb
| journal = Bulletin of the Atomic Scientist
| volume = 48
| issue = 7
| pages = pp. 27-31
| id = 
| url = http://www.thebulletin.org/article.php?art_ofn=sep92albright
}}

Measures that address concerns

Efficient CANDU installations are careful to control heavy water losses from the calandria, and also actively separate tritium from the moderator to sell in the secondary medical market. Some large CANDU installations use surplus power to operate their own small deuterium separation plants, to upgrade the heavy water inventory and reduce costs.

The large thermal mass of the cool calandria acts as a substantial safety mechanism. If a fuel assembly were to overheat and melt, it would be cooled in the very process of changing the reactor geometry. Furthermore, due to the use of natural uranium as the fuel, the reactor cannot sustain a chain reaction if its original fuel channel geometry is altered in any significant manner.

As mentioned above, by burning it as fuel, CANDU could actually render existing stocks of weapons-derived plutonium unsuitable for further use in weapons. A proposal to do this submitted by Atomic Energy of Canada to the United States Department of Energy is currently being debated by government agencies and non-governmental organizations ³.

Notes

• ¹ Whitlock A. CANDU Nuclear Power Technology A.12 How are CANDU reactors controlled? Shutdown System 2 (SDS 2), in most CANDU designs, works by high-pressure injection of a liquid poison (gadolinium nitrate) into the low-pressure moderator.
• ² Whitlock A. CANDU Nuclear Power Technology A.3 What is "heavy water"? "reactor-grade" heavy water, nominally 99.75 wt% deuterium content.
• ³ CCNR

References

• Whitlock:Template:Cite web