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XYLENE POWER LTD.

CANDU REACTORS:

By Charles Rhodes, P.Eng., Ph.D.

CANDU REACTORS:
The name CANDU Reactor is short for Canadian Deuterium Uranium Reactor. A CANDU nuclear reactor is a horizontal pressure tube fission power reactor developed by Atomic Energy of Canada Limited (AECL) to meet Canadian energy production needs and circumstances. CANDU reactors provide over 60% of the grid supplied electrical energy consumed in the Province of Ontario.

Enhanced versions of CANDU reactors and related ongoing service support are now provided by CANDU Energy Inc. to CANDU reactor purchasers around the world. The Enhanced CANDU 6E reactor design and performance specifications are available via the SNC Lavalin website.

Unique features of CANDU reactors include ability to operate with unenriched natural uranium fuel and used Light Water Reactor fuel and on-line refuelling while operating at full power.

  
Two views of 8 early CANDU reactors at Pickering, just east of Toronto, Ontario built during the early 1970s. Total reactor capacity was about 4.0 GWe in 8 units. In the right hand photo the vacuum building is in the right foreground.
 
 


The Bruce B Nuclear Generating Station on Lake Huron near Kincardin.
 
 


 
 

 
 

Three views of the 3.5 GWe 4 CANDU unit Darlington NPP 70 km east of Toronto commissioned in 1992.
 
 


Photo showing 2 modern CANDU reactors at Quinshan, China completed in early 2000s.
 
 

A major benefit of the CANDU reactor design is that, because the CANDUs can be fuelled with natural uranium, the owners of CANDU reactors are relatively independent of foreign political control. The importance of this power reactor design feature became obvious when the USA notified Canada that the USA would cease supply of highly enriched uranium to Canada. In a single stroke this notice rendered valueless hundreds of millions of dollars of Canadian taxpayer investment in new medical isotope production reactors (MAPLE Reactors). The design of the MAPLE reactors relied on supply of highly enriched uranium from the USA.

To put the matter simply, for half a century Canada reliably met the world medical isotope market from the NRU reactor at Chalk River, Ontario as set out in the NRU Reactor Video. Greedy parties in the USA with corrupt sway over the US government policy decided that they wanted to control the world medical isotope market. By stopping US export of highly enriched uranium to Canada these parties temporarily succeeded in halting most of Canadian medical isotope production, but in so doing these parties torpedoed US exports of critical nuclear fuel and reactor equipment for at least the next half century. Any country purchasing nuclear fuel or reactor components from the USA today will, during the working life of that reactor, be under the heel of parties who corruptly control the US government.

If a non-US party is serious about taking immediate measures to prevent future climate change and does not want to risk control by the US government, a simple solution is purchase of CANDU reactors. CANDU reactor purchasers should no longer be concerned about long term storage of used CANDU fuel because various parties in Canada are working on implementation of the Ottensmeyer Plan to dispose of used CANDU fuel by recycling this used fuel through liquid sodium cooled fast neutron reactors. The benefit of this fuel recycling is a more than 1000 fold reduction in the toxic lifetime of the spent fuel and a 100 fold improvement in the amount of energy that can be obtained from a kg of natural uranium.
 

CANDU REACTOR POWER CONTROL:
A hidden benefit of CANDU reactors is that their reactivity margins are relatively low, and the nuclear chain reaction can only function in the presence of heavy water. This reliance on heavy water for reactivity makes CANDU reactors very safe as compared to other power reactor technologies. If a CANDU reactor is flooded with light water (normal water) the nuclear reaction stops. As long as there is sufficient liquid water to absorb nuclear decay heat fuel meltdown is physically impossible.

In 50 years of CANDU power reactor operation in many countries the most serious accident has only caused "a puddle on the floor".

Most CANDU power reactors have thermal power ratings in the range 1000 MWt to 3000 MWt (300 MWe to 900 MWe). These reactors normally operate at nearly constant thermal power and are used for base load electricity generation. A typical CANDU reactor has 14 control zones. Each zone has a dedicated radiation flux detector. The output of this flux detector controls the insertion of a control rod so that the radiation flux in each reactor zone remains nearly constant. Thus the reactor produces a nearly constant thermal power and the thermal load distribution within the reactor is nearly uniform.

When it is desired to briefly reduce the electrical power transmitted to the grid a portion of the steam produced is fed directly to the steam condenser without passing through the steam turbine. This electric power modulation methodology gives about a 30% power turndown, which is helpful in balancing uncontrolled renewable electricity generation.
 

FEATURES OF CANDU REACTORS:
Features of CANDU Reactors include:
1. Ability to operate with natural uranium fuel provides independence from foreign parties for U-235 fuel enrichment;

2. Zirconium alloy pressure tubes rather than a pressure vessel provide independence from foreign pressure vessel fabricators;

3. Ability to operate with a variety of fuels, including natural uranium, thorium and and spent fuel from light water reactors;

4. High thermal efficiency through use of direct cold lake or sea water cooling;

5. Unlike most other reactor types a CANDU reactor does not need to be shut down for refueling. On-line refueling allows the reactor to be refueled while operating at full power. Hence in base load service the capacity factor of a CANDU reactor is well over 90%;

6. Inherent safety. If there is a significant loss of heavy water the fission reactions stop because the reactor becomes subcritical. However, some cooling water flow must be maintained to remove fission product decay heat.

7. Potential for use of light water for emergency shut down and emergency heat removal;

8. A high moderator heavy water mass and a large reserve light water mass provide safety protection against thermal transients such as might occur co-incident with a pressure tube rupture;

9. Natural uranium oxide CANDU fuel with zirconium cladding is simple, inexpensive to fabricate and is almost free of combustion risks;

10. Low temperature (< 320 degrees C) operation allows use of elastomer pipe and pump seals which simplify reactor maintenance;

11. In depth safety redundancy and fault tolerance has given CANDU reactors an unparalleled safety record;

12. A long major accident free operating history (> 50 years) has demonstrated all aspects of implementation, electricity production and safety of CANDU technology. The design working life of a properly maintained CANDU reactor is 60 years. Typically there are two fuel and moderator channel changes during the reactor design working life.

CANDU reactors achieve these performance objectives through the use of natural uranium oxide fuel, pressure tubes, moderator tubes and heavy water as both a moderator and a primary coolant.

Like other technologies CANDU reactors went through a series of evolutionary stages. In addition to technical issues there were historic problems with project management and cost control. Many of these problems were caused by politicians with short time horizons. Currently CANDU reactors reliably supply at a competitive cost over 60% of the 140 TWh of electricity used per year in the Province of Ontario. CANDU reactors are also used in New Brunswick and in numerous export markets.

The current design optimized version of the CANDU reactor is the CANDU E6 which is available from CANDU Energy Inc., a subsidiary of SNC Lavalin.

The CANDU reactor fleet is a critical component of Ontario electricity generation, especially in the mid to late summer when renewable electricity production is low due to both reduced river flow and reduced wind generation while the grid load is maximum due to use of summer air conditioning to combat hot humid conditions that have been aggravated by global warming.
 

LIMITATIONS OF CANDU REACTORS:
Fifty years of practical operation have shown that CANDU reactors, while doing an excellent job of meeting the Ontario's electricity base load, have their practical limitations as follows:
1) Formation of a short lived fission product (Xe-135) with a high slow neutron absorption cross section, known as a reactor poison, prevents the fission thermal power tracking rapid changes in grid load. However, a CANDU nuclear generating station can track rapid changes in grid load through the use of steam turbine bypass, which simply means that surplus steam corresponding to constrained electricity generation is fed directly to the steam condenser. This methodology is not an efficient use of reactor fuel or cooling water, but it has proven to be very beneficial in terms of minimizing average CO2 production per kWh of electricity generated by the Ontario electricity system.

2) The use of heavy water both as a primary coolant and as a moderator means that the initial capital cost of a CANDU reactor is relatively high as compared to a light water reactor. A CANDU reactor is also unforgiving of heavy water leaks due to poor maintenance. The heavy water is expensive so CANDU reactors employ elaborate equipment to ensure almost complete recovery of spilled or leaked heavy water.

3) The pressure tube design means that there are a lot of pipe connections and related parts, which collectively contribute to the reactor construction and maintenance costs;

4) The use of heavy water in a high neutron flux leads to production of tritium. Tritium is both an asset and a curse. Tritium production is essential for purposes such as support of nuclear fusion and production of helium-3. However, tritiated water is potentially dangerous because it is easily absorbed by biological systems. Tritium is a hydrogen isotope that if present as a gas in significant quantities can contribute to pressure tube cracking. CANDU reactors incorporate ion exchange equipment to continuously capture, remove and store tritium. Tritium has a 12.6 year half life and spontaneously decays into inert stable helium-3. Helium-3 is essential for detection of neutrons from illicit shipments of fissile materials and for certain cryogenic and medical applications.

5) The use of direct lake water cooling impacts marine species and limits reactor siting choices. Later generation CANDU reactors have cooling water intakes that minimize the impact on marine species. However, direct dissipation of large amounts of heat in the great lakes has increased lake surface temperatures and has reduced lake ice cover in the winter.

6) Absent direct cold lake or sea water cooling the overall thermal efficiency of a CANDU nuclear generating station is poor as compared to a natural gas fired combined cycle power plant. There is opportunity for both raising the thermal efficiency of nuclear generation and reducing related marine enviromental problems by use of liquid sodium as a primary coolant and by use of cooling tower and district heating type heat sinks.

7) CANDU reactors produce relatively large amounts of high level nuclear waste. In principle this waste can be reprocessed and then consumed in a Fast Neutron Reactor (FNR), but for ten years Canadian and Ontario politicians lacked both the scientific education and the moral fiber required to address this issue. The plan by the NWMO to bury unprocessed spent CANDU fuel bundles in a Deep Geologic Repository (DGR) below the water table is not consistent with mitigating climate change and is fraught with major problems. CANDU reactors should be complemented by liquid sodium cooled Fast Neutron Reactors both for efficient utilization of natural uranium and for proper nuclear waste disposal.

8) Use of direct lake or sea water cooling requires that the CANDU reactor be located close to lake or sea level. Hence the reactor structure must be able to safely withstand a tsunami as well as safely contain a release of high pressure radioactive heavy water steam. These two requirements have led to CANDU reactor structures incorporating large amounts of concrete. There is opportunity for substantially reducing concrete requirements by using a liquid sodium cooled reactor design that does not require direct lake water cooling and that cannot produce radio active steam.

9) The CANDU reactor design imposes a combination of neutron stress, pressure stress, erosion and fretting on the fuel channels and related coolant flow path components. These combined stresses limit the fuel channel working life to about 20 years and the reactor working life to about 60 years. More recent reactor designs separate the neutron stress from the mechanical and pressure stresses to reduce maintenance costs, to extend the reactor working life and to reduce the amount of decommissioning waste.

10) Some early CANDU reactors used concrete as a neutron biosafety shield. However, concrete contains Ca-40 which when neutron irradiated becomes Ca-41. Ca-41 has a 80,000 year half life which makes it a decommissioning waste disposal problem. Future nuclear reactor designs should not let materials containing Ca be exposed to the neutron flux.

11) Like other fission reactors, CANDU reactors continue to produce fission product decay heat after reactor shutdown. CANDU reactors are dependent on continuance of coolant pumping to prevent the cooling water becoming very high pressure steam and rupturing the pressure tubes. CANDU reactors are fitted with elaborate safety systems both to prevent over heating and to contain steam releases in the event of a pressure tube rupture. However, this safety in depth adds to the CANDU reactor capital cost.

12) Like other high pressure water cooled power reactors CANDU reactors have potential void issues. If there is a sudden loss of pressure or flow in the primary high pressure water cooling circuit, such as might be caused by loss of pump power or a pipe break, steam can form in the fuel tubes which decreases the reactor reactivity. If in response to the drop in reactor power the reactor power control system automatically withdraws control rods in an attempt to maintain a preset reactor power level there is a risk of an uncontrolled power surge if the steam bubbles suddenly collapse either due to a step drop in the cooling water temperature or due to a step increase in the cooling water flow or pressure. Reactor operators must be trained to be intensely aware of this issue and to not do anything that could lead to a dangerous condition known as prompt neutron criticality. Modern advanced reactor designs avoid this problem through use liquid metals instead of water for primary heat transport.
 

CANDU FUEL:
CANDU reactors use standard size fuel bundles. Each fuel bundle is about 10.2 cm in outside diameter and is about 0.495 m long. Each fuel bundle consists of 37 to 43 parallel zirconium sheathed tubes on approximately 1.5 cm centers that contain uranium oxide pellets. A CANDU-E6 reactor has 380 pressure tubes also known as fuel channels. Each pressure tube contains 12 fuel bundles. In a CANDU reactor operating at full rated power each fuel bundle emits about 457 kWt. Hence the total thermal output of the reactor primary cooling circuit is about:
380 tubes X 12 bundles / tube X 457 kWt / bundle = 2,084,000 kWt
Conversion of this heat into electricity via two stage steam vacuum turbines with cold lake water cooling yields about 740,000 kWe of base load electricity.
 

CANDU USED FUEL:
A CANDU fuel bundle is used until high neutron absorption cross section fission products accumulate in the uranium oxide to the extent that these fission products significantly reduce reactor reactivity. At that point the fuel bundle is expelled from the reactor into a water filled cooling bay. On removal from the reactor the fuel bundle's fission reactions stop and the fuel bundle's thermal output immediately decreases from about 457 kWt to about 37 kWt (8.1 % of full power).

During the first year in wet storage the spent fuel bundle's thermal emission drops from 37 kWt to about 73 Wt. During the next 9 years in wet storage the fuel bundle's thermal emission drops from about 73 W to about 5 Wt.

After about 10 years in wet storage each CANDU spent fuel bundle is removed from the cooling bay and is transferred to Dry Cask storage. During the next 90 years its thermal emission drops from about 5 Wt to about 1 Wt. After about 100 years in storage the thermal output from fission product decay becomes dominated by the thermal output from decay of transuranium actinides which have half lives of up to 25,000 years. Unprocessed spent CANDU fuel will remain radio toxic for about 400,000 years. The processing and safe disposal of spent CANDU fuel after it is removed from dry cask storage is the subject of the Ottensmeyer Plan which is detailed on other web pages on this web site.

At the time of writing there are about 2.4 million spent CANDU fuel bundles (54,000 tonnes) in wet and dry storage at CANDU nuclear generation station sites in Ontario. These sites are on the edges of the Great Lakes and are at constant risk of unlikely but possible major events such as earth quakes, meteorite hits and tidal waves. In terms of public safety an interim priority should be to reduce the used fuel mass by about 90% by selective extraction of weakly radioactive uranium oxide and to move the remaining highly radioactive used fuel mass to a secure high and dry location where it can safely remain with minimal ongoing supervision until it is needed to fuel future Fast Neutron Reactors.

At this time, from both a geophysical perspective and from an international perspective, the most suitable CANDU used fuel storage site in Canada is the 5 million square foot depleted Jersey Emerald mine complex in British Columbia. The water tight granite vaults in this remote mine have 400 m of overhead rock and are more than 300 m above the local water table, enabling both gravity drainage and natural ventilation. These vaults could potentially provide secure, accessible, inexpensive and naturally dry used fuel storage for as long as human beings inhabit planet Earth.

It is contemplated that the used fuel would be stored in double wall porcelain-stainless steel containers. The integrity of each container would be remotely monitored. The storage facility would be naturally ventilated and gravity drained to pumped sumps. There would be ongoing monitoring of container wall integrity, radioactivity in the ventilation exhaust air and the radioactivity in the drains and sumps. The sumps would have sufficient volume to safely capture any leakage of water soluble radio isotopes from the containers.
 

CANDU FUEL NATURAL URANIUM REFERENCE:
It is convenient to use the thermal emission rate of natural uranium as a reference level for the decay emissions of a spent CANDU fuel bundle. Before it is inserted in a CANDU reactor a new natural uranium CANDU fuel bundle consists of about 22 kg of UO2 and about 0.5 kg of zirconium sheath material. The molecular weight of UO2 is about:
238 + 2(16) = 270.
Hence the number of moles of UO2 in a CANDU fuel bundle is:
22,000 gm / (270 gm / mole) = 81.481 moles

The number of uranium atoms is:
81.481 moles X 6.023 X 10^23 UO2 molecules / mole X 1 U atom / UO2 molecule
= 4.90763 X 10^25 U atoms

Natural uranium is primarily composed of 99.276% U-238 and 0.7176% U-235. The number of U-238 atoms in a new natural uranium CANDU fuel bundle is:
4.90763 X 10^25 X 0.99276 = 4.872098 X 10^25 U-238 atoms
and the number of U-235 atoms in a new natural uranium CANDU fuel bundle is:
4.90763 X 10^25 X .007176 = .035217 X 10^25 U-235 atoms

The time T dependent decay of N atoms of a particular isotope is described by:
dN / dT = - K N
or
Ln(Nb / Na) = - K(Tb - Ta)
or
Ln(Na / Nb) = K (Tb - Ta)
or
K = Ln(Na / Nb) / (Tb - Ta)

If:
(Tb - Ta) = one half life
then
(Na / Nb) = 2

Thus:
K = Ln(2) / [half life]

For U-238:
K = (.693147 / 4.51 X 10^9 year)
and for U-235:
K = (.693147 / 7.1 X 10^8 year)

A U-238 alpha decay emits on average about 4.19 MeV and a U-235 alpha decay emits on average about 4.40 MeV. Hence the total thermal emission rate of a new natural uranium CANDU fuel bundle is given by:
(.693147 / 4.51 X 10^9 year)(4.872098 X 10^25 U-238 atoms)(4.19 Mev / U-238 atom)
+ (.693147 / 7.1 X 10^8 year)(.035217 X 10^25 U-235 atoms)( 4.40 MeV / U-235 atom)

 
= (3.137465 X 10^16 MeV year) + (.015127 X 10^17 Mev / year)
 
= (3.288741 X 10^16 MeV / year) X (1 year / 8766 hr) X (1 hr / 3600 s)
X (10^6 eV / MeV) X (1.602 X 10^-19 J / eV)
= .1669 X 10^-3 J / s
= 1.669 X 10^-4 W

Hence the thermal emission of a new natural uranium CANDU fuel bundle is:
1.67 X 10^-4 W
 

USED CANDU FUEL:
The IFPM Report dated 2011 indicates that a used CANDU fuel bundle 10 years after removal from a CANDU reactor emits 4.0 watts of heat. If this data is correct the ratio of thermal emission of a used CANDU fuel bundle 10 years after removal from a CANDU reactor as compared to the thermal emission of a new natural uranium CANDU fuel bundle is about:
4 W / 1.67 X 10^-4 W
= 2.3952 X 10^4
= 23,952

The NWMO indicates that after 10 years in storage the thermal emission per spent CANDU fuel bundle is 5 W, in which case the aforementioned ratio increases to:
5 W / 1.67 X 10^-4 W
= 30,000

2^15 = 32,768
Hence nearly 15 half lives are required to reduce the thermal power output of a radio isotope by a factor of 30,000. If the spent CANDU fuel bundle thermal power output is dominated by natural decay of Pu-239, which has a half life of 24,390 years, the minimum required isolated storage time to reduce the bundle reactivity to twice that of natural uranium is:
24,390 years X 15 = 365,850 years.
Hence, The isolated storage time required to ensure that the Pu-239 thermal emission becomes significantly less than the uranium decay thermal emission is about 400,000 years.

The practical way to reduce this isolated storage time is to fission the Pu-239 and like transuranium actinides to release heat and to convert these isotopes into fission products which have a 1000 fold shorter half life.
 

This web page last updated May 22, 2020.

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