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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. 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 6 reactor design and performance specifications are available on the CANDU Energy Inc. website.

A major non-obvious benefit of the CANDU reactor design is that it makes owners of CANDU reactors relatively independent of US 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 by the USA.

To put the matter simply, for half a century Canada reliably met the world medical isotope market from a reactor at Chalk River, Ontario. Greedy parties in the USA with sway over the US government decided that they wanted to control the world medical isotope market. These parties succeeded in halting much of Canadian medical isotope production, but in so doing so these parties torpedoed US exports of critical nuclear reactor equipment for at least the next half century. Any country purchasing a nuclear reactor 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 corrupt remote 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 spent CANDU fuel because various parties in Canada are working on implementation of the Ottensmeyer Plan to dispose of spent CANDU fuel by recycling this material through liquid sodium cooled fast neutron reactors.
 

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

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 water cooling;

5. On-line refueling allows the reactor to be refueled while operating at full power. Unlike most other reactor types a CANDU reactor does not need to be shut down for refueling. Hence in base load service the capacity factor of a CANDU reactor is 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 pressure tube 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 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 is simple and inexpensive to fabricate and is free of combustion risks;

10. Low temperature (< 300 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 working life of a properly maintained CANDU reactor is 60 years.

CANDU reactors achieve these performance objectives through the use of pressure tubes, oxide fuel and heavy water as both a moderator and as 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 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. These limitations are 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 capacity, 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 and a CANDU reactor is unforgiving of heavy water leaks due to poor maintenance.

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 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 winter ice formation.

6) Absent direct lake 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 a cooling tower heat sink.

7) CANDU reactors produce large amounts of high level nuclear waste. In principle this waste can be chemically treated 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 ill thought and is fraught with major problems. CANDU reactors should be complemented by liquid sodium cooled Fast Neutron Reactors 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 high 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. A more advanced reactor design would separate the neutron stress from the mechanical and pressure stresses to reduce maintenance costs, to extend the reactor life and to reduce the amount of decommissioning waste.

10) Some early CANDU reactors dconcrete 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 reactors should avoid use of Ca in materials exposed to neutrons.

11) Like other fission reactors, CANDU reactors continue to produce heat after nuclear power shutdown. The reactor is dependent on continuance of coolant pumping to prevent the cooling water beecoming very high pressure steam and rupturing the pressure tubes. CANDU reactors are equiped with elaborate safety measures 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.
 

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. 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 kW of base load electricity.
 

CANDU SPENT 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.

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 one million 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 over 2 million spent CANDU fuel bundles (45,000 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, tidal waves and terrorist attacks. In terms of public safety the number one priority should be to move the spent CANDU fuel to a secure high and dry location where it can safely remain with little 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 spent 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 provide secure, accessible, inexpensive and naturally dry spent fuel storage for as long as human beings inhabit planet Earth.

It is contemplated that the spent fuel would be stored in double wall porcelain-stainless steel containers, each container holding up to 180 fuel bundles. 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 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 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
 

SPENT CANDU FUEL:
The IFPM Report dated 2011 indicates that a spent 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 spent 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 is 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 convert these isotopes into fission products which have a 1000 fold shorter half life.
 

This web page last updated May 21, 2017.

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