"HEAVY WATER"

History

Who first isolated heavy water?

Francis Aston (photo, right) found evidence for the existence of isotopes in 1913, and he published his findings in 1920. Aston was awarded the Nobel prize for Chemistry in 1922. Soon after, Rutherford suggested the existence of a heavier isotope of hydrogen, which we now call deuterium. It was first detected in 1931 by Harold Clayton Urey (see photo, below), who found that he could enrich liquid hydrogen somewhat by fractional distillation, and confirmed the cause of weak lines in the atomic spectrum of samples of hydrogen as due to the presence of small amounts of deuterium. Using electrolysis, Urey succeeded in enriching samples of water in the heavier isotope. The next step was to isolate pure heavy water. Along with his student Ronald T. MacDonald, the great American chemist Gilbert Newton Lewis (1875-1946) set to the task, using both electrolysis and fractional distillation under reduced pressure (employing a 72-feet-high distillation column). Armed with his supply of deuterium oxide, Lewis set out to investigate its properties - not just the obvious ones, like melting and boiling points - but also whether it would support life (a white mouse drank it and came back for more).

Urey and Lewis

Urey was awarded the 1934 Nobel Prize in Chemistry "for his discovery of heavy hydrogen", Lewis - who had been Urey's PhD supervisor - won nothing. At this, Lewis stopped work on heavy water. Despite his work on heavy water, not to mention inventing the covalent bond (plus dot-and-cross diagrams); coming up with the concept of acids and bases as electron-pair acceptors and donors, respectively; developing a theory of electrolytes and also formulating thermodynamics for chemists (no mean feat), Lewis was never to win a Nobel prize.

Harold Urey Gilbert Lewis

So did Lewis find out that heavy water had different properties to ordinary water?

There are small but measurable differences. One consequence of the D₂O molecule being significantly heavier than H₂O is that an ice cube made from heavy water will sink if placed in liquid H₂O.

	H2O	D₂O
Freezing point (°C )	0.00	3.81
Density of liquid (g/cm³)	0.9999 (277 K)	1.1056 (293 K)
Density of solid at m.p. (g/cm³)	0.917	1.018
Temperature of maximum density (°C)	3.98	11.2
pH (pD) (298K)	7.00	7.43

what happened next?

Researchers continued to examine the properties of D₂O, but obtaining large quantitites of heavy water for research was usually done by electrolysis. This required huge amounts of electricity and was then only achievable at Norsk Hydro's Ryukan plant in Norway (photo, below); they obtained really pure heavy water in early 1935. At first, demand was small, in quantities usually no more than 10 grams, but things were to change after Hahn and Strassmann's discovery in late 1938 that uranium atoms could be split using slow neutrons, releasing energy (and more neutrons). By the end of 1939, Ryukan was receiving orders from the German chemical giant I.G. Farben for up to 100 kg of D₂O a month. By early 1940, a few months after the Second World War had broken out, French intelligence became aware of increased German demand for heavy water, and in early March Lieutenant Jacques Allier, a Deuxième Bureau agent, managed to spirit out of the country the entire Ryukan stock of heavy water, 26 five-litre containers. Shortly afterwards, another 26 containers reached Paris.

Description: Vemork Hydroelectric Plant in 1935

Vemork hydroelectric power station at the Rjukan waterfall in 1935.

Why did the French get involved?

Jean Frédéric Joliot-Curie and his wife Irène shared the 1935 Nobel prize for Chemistry "in recognition of their synthesis of new radioactive elements". By the outbreak of World War II, Joliot-Curie, at that time a professor at the Collège de France, was working actively on nuclear fission, and since the summer of 1939 had been thinking of heavy water as a moderator in a nuclear reactor, to produce neutrons of the right velocity to induce nuclear fission. When the German invasion of France took place in May 1940, the heavy water was removed first to Clermont-Ferrand, in the Auvergne, then, after the collapse of the French Army in mid-June, Joliot-Curie oversaw its movement to Bordeaux, from where it was shipped to England aboard the SS Broompark. The world's supply of heavy water arrived in England on June 21 1940, to be stored first at Wormwood Scrubs prison, then at Windsor Castle. It was safe.

Description: Movie poster for the Film Heroes of Telemark

Why did the Germans want it?

Like the French (and the British and the Americans), German scientists had been following developments in nuclear chemistry. The great Werner Heisenberg (who won the 1932 Nobel prize for Physics aged 31, "for the creation of quantum mechanics") was one of the leading scientists developing the German nuclear project. He had reasoned that since Germany could not separate the uranium isotopes to obtain enrichment in ²³⁵U, a reactor employing natural uranium would require a moderator of either graphite or heavy water. After the German invasion of Norway, I.G. Farben took over Norsk Hydro and the Ryukan plant.

But that wasn't the end of it?

Fed by intelligence from the Norwegian resistance movement, the British followed the increased production of heavy water at Ryukan, and decided to try to destroy the plant. On November 19 November 1942, two RAF Halifax bombers set off from Scotland, heading for Norway; each was towing a glider carrying seventeen commandos and two crew. Operation Freshman was under way. It was a disaster. One Halifax flew into a mountain, killing the crew of six; both the gliders crashed and all those who survived the crash were killed by the Gestapo. Operation Gunnerside followed, when a team of Norwegian commandos struck on the night of February 27-28 1943, causing serious damage to the plant and putting back production. In a final blow, a Norwegian resistance party sank the ferry Hydro when it was transporting heavy water to Germany on 20 February 1944. All these operations helped slow German heavy-water research. The film "Heroes of Telemark", starring Kirk Douglas and Richard Harris, is loosely based upon these events.

Did it matter?

As it happens, German efforts were too half-hearted to bear fruit, whereas the Americans invested huge sums of money in the Manhattan project. In any case, better processes are available to obtain heavy water, such as the Girdler process, simultaneously discovered by Karl-Hermann Geib in Germany and J.S. Spevack at Columbia University. This employs an exchange reaction for which the equilibrium constant is very favourable (K~1.01):

HOH (l) + HSD (g)

HOD (l) + HSH (g)

Post-war, Canada was the only country which invested heavily in plants using natural-abundance uranium which employed heavy water as the moderator.

Introduction

What is heavy water?

Heavy Water (D₂O) is a compound of an isotope of hydrogen called heavy hydrogen or Deuterium (D) and oxygen. This is also known as Deuterium Oxide. Deuterium has an atomic mass of 2, as against 1 for normal hydrogen (H) due to presence of an extra neutron in the nucleus. Deuterium is present in hydrogen and hydrogen bearing compounds like water, hydrocarbons, etc. and has a small natural occurrence (D/D+H) of about 140 to 160 ppm. So it is necessary to process large quantities of the low concentration feed stock to produce the final product which is enriched to the reactor grade i.e. 99.8 mole %. Heavy Water has great similarity in its physical and chemical properties to ordinary water. But its nuclear properties display a significant variation which makes it an extremely efficient material for use as moderator in a nuclear reactor.

What is Moderator?

Moderator is required in a thermal reactor to slow down the neutrons produced in the fission reaction to .025 ev (thermal reaction) so that the chain reaction can be sustained. Different moderators normally in use are Heavy Water, Graphite, Beryllium and Light water. Heavy Water is an excellent moderator. A good moderator should have excellent slowing down power and low absorption cross section for neutrons.

What is Coolant?

Heat energy produced in the fission reactor will be removed by coolant. Water is an excellent coolant that can remove the heat from the feed. Heavy Water is used as a primary coolant to transport heat generated by the fission reaction to secondary coolant, light water. In Gas cooled reactors carbon di-oxide gas is used as coolant. Coolant transports heat to secondary coolant, i.e. water for generation of steam at an appropriate pressure for running steam turbines. Steam turbines drive generators to generate Electricity.

Heavy water is the key to one type of reactor in which plutonium can be bred from natural uranium. As such, the production of heavy water has always been monitored, and the material is export controlled. In addition, a source of deuterium is essential for the production of tritium and 6LiD, two ingredients of thermonuclear weapons. A nation seeking large quantities of heavy water probably wishes to use the material to moderate a reactor, and may be planning to produce plutonium. However, CANDU (CANadian Deuterium Uranium) reactors designed and built in Canada are used for commercial electric power production.

Heavy water, D2O, is water in which both hydrogen atoms have been replaced with deuterium, the isotope of hydrogen containing one proton and one neutron. It is present naturally in water, but in only small amounts, less than 1 part in 5,000. Heavy water is one of the two principal moderators which allow a nuclear reactor to operate with natural uranium as its fuel. The other moderator is reactor-grade graphite (graphite containing less than 5 ppm boron and with a density exceeding 1.50 gm/cm 3 ). The first nuclear reactor built in 1942 used graphite as the moderator; German efforts during World War II concentrated on using heavy water to moderate a reactor using natural uranium.

The importance of heavy water to a nuclear proliferator is that it provides one more route to produce plutonium for use in weapons, entirely bypassing uranium enrichment and all of the related technological infrastructure. In addition, heavy-water-moderated reactors can be used to make tritium.

Although one speaks of "making" heavy water, deuterium is not made in the process; rather, molecules of heavy water are separated from the vast quantity of water consisting of H2O or HDO (singly deuterated water), and the "dross" is discarded. Alternatively, the water may be electrolyzed to make oxygen and hydrogen containing normal gas and deuterium. The hydrogen can then be liquefied and distilled to separate the two species. Finally, the resulting deuterium is reacted with oxygen to form heavy water. No nuclear transformations occur.

Production of Heavy water

The production of heavy water in significant amounts requires a technical infrastructure, but one which has similarities to ammonia production, alcohol distillation, and other common industrial processes. One may separate heavy water directly from natural water or first "enrich" the deuterium content in hydrogen gas. It is possible to take advantage of the different boiling points of heavy water (101.4 °C) and normal water (100 °C) or the difference in boiling points between deuterium (-249.7 °C) and hydrogen (-252.5 °C). However, because of the low abundance of deuterium, an enormous amount of water would have to be boiled to obtain useful amounts of deuterium. Because of the high heat of vaporization of water, this process would use enormous quantities of fuel or electricity. Practical facilities which exploit chemical differences use processes requiring much smaller amounts of energy. Separation methods include distillation of liquid hydrogen and various chemical exchange processes which exploit the differing affinities of deuterium and hydrogen for various compounds. These include the ammonia/hydrogen system, which uses potassium amide as the catalyst, and the hydrogen sulfide/water system (Girdler Sulfide process).

Separation factors per stage are significantly larger for deuterium enrichment than for uranium enrichment because of the larger relative mass difference. However, this is compensated for because the total enrichment needed is much greater. While 235U is 0.72 percent of natural uranium, and must be enriched to 90 percent of the product, deuterium is only .015 percent of the hydrogen in water and must be enriched to greater than 99 percent. If the input stream has at least 5 percent heavy water, vacuum distillation is a preferred way to separate heavy from normal water.

This process is virtually identical to that used to distill brandy from wine. The principal visible difference is the use of a phosphor-bronze packing that has been chemically treated to improve wettability for the distillation column rather than a copper packing. Most organic liquids are non-polar and wet virtually any metal, while water, being a highly polar molecule with a high surface tension, wets very few metals. The process works best at low temperatures where water flows are small, so wetting the packing in the column is of particular importance. Phosphor-bronze is an alloy of copper with .02-.05 percent lead, .05-.15 percent iron, .5-.11 percent tin, and .01-.35 percent phosphorus.

Heavy water is produced in Argentina, Canada, India, and Norway. Presumably, all five declared nuclear weapons states can produce the material. The first commer-cial heavy water plant was the Norsk Hydro facility in Norway (built 1934, capacity 12 metric metric tons per year); this is the plant which was attacked by the Allies to deny heavy water to Germany. As stated above, the largest plant, is the Bruce Plant in Canada (1979; 700 metric tons/year). India's apparent capacity is very high, but its program has been troubled. Accidents and shutdowns have led to effective limitations on production.

The Bruce Heavy Water Plant in Ontario, Canada, is the world's largest producer of D2O. It uses the Girdler Sulfide (GS) process which incorporates a double cascade in each step. In the upper ("cold," 30-40 °C) section, deuterium from hydrogen sulfide preferentially migrates into water. In the lower ("hot," 120-140 °C) section, deuterium preferentially migrates from water into hydrogen sulfide. An appropriate cas-cade arrangement actually accomplishes enrichment. In the first stage the gas is enriched from 0.015% deuterium to 0.07%. The second column enriches this to 0.35% , and the third column achieves an enrichment between 10% and 30% deuterium. This product is sent to a distillation unit for finishing to 99.75% "reactor-grade" heavy water. Only about one-fifth of the deuterium in the plant feed water becomes heavy water product. The production of a single pound of heavy water requires 340,000 pounds of feed water.

Today many nations are considering an expanded role for nuclear power in their energy portfolios. This expansion is driven by concerns about global warming, growth in energy demand, and relative costs of alternative energy sources. In 2008, 435 nuclear reactors in 30 countries provided 16% of the world’s electricity. In January 2009, 43 reactors were under construction in 11 countries, with several hundred more projected to come on line globally by 2030.

A Nuclear Power PlantCourtesy of R2 Controls

Methods of Production

The basis of all methods of heavy water production is the separation of deuterium from a suitable hydrogen-containing raw material by utilising the slight differences in physical and chemical properties which exist between deuterium and hydrogen isotopes. In practice the raw material is almost always natural water or hydrogen. Although the mass ratio of z for hydrogen and deuterium is greater than for any isotopes of the heavier elements the task is made difficult by the low concentration of deuterium in natural water, which requires that large quantities be handled in the initial stages of the separation.

An additional complication is caused by the fact that, when present in such low concentrations, deuterium is distributed in the water as HDO molecules which have properties intermediate between H,O and D,O. The only processes to have been operated on a large scale are the electrolysis of water-in which H,O is decomposed more readily than D,O; water and hydrogen distillation; and chemical exchange, which depends on the isotopic displacement at equilibrium in certain reactions resulting in the concentration of deuterium in one component (I, 2). In selecting suitable processes the following points must be considered: reversibiIity of the reaction, which is a measure of the power required, separation factor, which determines the number of reaction stages, and finally the complexity of the plant.

The highest separation factor, cx = 4 to 7, is given by electrolysis, but this process is highly irreversible and the cost of power is prohibitive except in such places as Norway where very cheap hydroelectric power is available or where there is some form of natural power such as the geothermal springs in New Zealand.

Electrolysis is, however, almost invariably used for the final concentration of deuterium once its level has been raised to the order of z to 5 per cent by other methods, since in this case smaller quantities are involved and the power consumption is much reduced. Distillation methods are highly reversible and therefore have a low power consumption, but water distillation has a low separation factor while hydrogen distillation must be carried out near absolute zero, where knowledge of techniques is very limited. Chemical exchange is probably one of the most promising methods for future large-scale production of heavy water.

DIDO, the experimental high-flux materials-testing reactor at Ilarwell. cooled and moderated by heavy

water. A similar reactor, PLUTO, is also in operation at Harzmll while a new reactor, DMTR, practically

identical with PLUTO, is nearing completion at Dounreay. The Atomic Energy Authority has concluded

that the best possible alternative to the gas-cooled graphite moderated reactor for small land-based

power stations and for ship propulsion may well be a heavy water moderated reactor.

The basis of its operation is that if in the general equation where R and R‘ are

RH+R’D+RD+RH

radicals attached to hydrogen and deuterium, the equilibrium constant K, which is equivalent to the separation factor x, is not equal to I, some concentration of deuterium will be possible. Many reactions are known which satisfy this condition but only those with water as one component are of practical importance. The reaction between water and hydrogen has often been suggested for this purposcit can take place with water in the gaseous or liquid phase:

H,O (gas)-HD (gas)+HDO (gas) 1 H, (gas)

H,O(liquid)+ HD(gas) i=H DO(liquid)+ H,(gas)

The two-phase reaction provides the better separation factor, 4=3.87 at 25°C compared with 3.62 for the single-phase reaction, but it has been found in practice that the hydrogenwater system comes to equilibrium only very slowly, so that a catalyst is required, and until very recently no catalyst was known which would give efficient service in liquid water. Consequently the reaction had to be operated in the gaseous phase, which introduced difficulties into the design of the plant, since the water had to be alternately evaporated and condensed.

Trail Plant

A plant using this process was installed during the last war at the synthetic ammonia plant of the Consolidated Mining and Smelting Company of Canada Ltd. At Trail, B.C. The necessity of using hydrogen as one of the raw materials restricted this process to sites where there was an abundant supply of the gas. In effect the hydrogen, produced by electrolysis, was borrowed for the purpose of extracting its deuterium content and then returned for the synthcsis of ammonia.

The deuterium was concentrated by a combination of catalytic exchange and electrolysis, carried out in the so-called primary and secondary The feed to the primary plant comprised steam and hydrogen gas from the ammonia plant. The deuterium concentrated in the steam and was raised to a proportion of about 2.3 per cent D,O by this method in combination with electrolysis-further concentration to 99.8 per cent D,O was effected in the secondary plant by electrolysis alone.

The most effective catalysts for the exchange reaction were found to be platinum-oncharcoal and nickel-chromium, the former being used in the first three exchange towers and the latter in the fourth. The primary heavy water plant at Trail is shown in the illustration, and a simplified diagram of one of the exchange towers is given.

A stream of water containing H,O and HDO flowed in at the top of the tower and a gas-vapour stream, consisting of a mixture of H,, HD, H,O and HDO flowed into the bottom. This gas stream was obtained from partial electrolysis of the water leaving the column and from stripped gases leaving the top of the subsequent tower. In the exchange tower the HD was partially absorbed in the descending water stream, the remainder of the vapour passing on to the next catalyst layer. Since the catalyst did not operate satisfactorily if wet, the scrubbing water was by-passed around each catalyst section and heaters were provided to dry the gas stream before it entered the catalyst bed. On leaving the tower the water entered an electrolysis cell where it was partially electrolysed; the hydrogen and some of the undissociated water returned to the tower as the vapour stream while the rest of the waterenriched in D,O-entered a subsequent tower for further concentration.

The primary heavy water plant erected at Trail, B.C., by the Consolidated

Mining and Smelting Company of Canada Ltd.

The development of the platinum oncharcoal catalyst for the exchange reaction by G. G. Joris and his co-workers at Princeton University has been described in some detail. Canadian 8-mesh gas-mask charcoal and peach-nut charcoal when completely activated were found to be excellent supports. The most satisfactory combination of economy in platinum with high catalytic efficiency was attained with a catalyst containing 0.4 g platinum per IOO ml charcoal. The catalyst was easily poisoned by carbon monoxide, hydrogen sulphide, sulphur dioxide, mercury and water, even in minute traces. Regeneration was normally very simple; heating in hydrogen removed inactivation due to carbon monoxide and heating

In air or oxygen removed the effects of sulphur poisons. The cost of heavy water produced at Trail was more than $60 per lb, which is comparatively high and which led to the closing of the plant in 1955. However, it is known that by economies such as decreasing the losses of water vapour and hydrogen to a negligible value, and by cutting down the amount of catalyst, the cost of the heavy water could have been reduced to $38 per lb. There are inherent disadvantages in this method.

However, the most serious being the limitation of output imposed by the amount of hydrogen available. The Trail plant is the largest producer of hydrogen in the North American continent and yet its output of heavy water was only 6 tons per year. Since it has bcen estimated that in Canada alone the annual requirements of heavy water will be several hundred tons in a few years’ time, it is evident that some further method must be sought for really large-scale production.

Types of Nuclear Reactors

Description: http://www.chemcases.com/images/9-fig.%201crop.jpg

HEAVY WATER REACTOR

INTRODUCTION

The Heavy Water Reactor (HWR) concept allow the use of natural uranium as a fuel without the need for its enrichment, offering a degree of energy independence,especially if uranium is available for mining or for extraction as a byproduct of another industry such as gold mining or phosphate fertilizer production. However, it needs the installation of a heavy water D2O production capability, which is a much simpler endeavor anyway, since separating the light isotopes (D from H) is much simpler than separating the heavy isotopes (U235 from U238).

HWRs have become a significant proportion of world reactor installations, second only to the Light Water Reactors (LWRs). They provide fuel cycle flexibility for the future and can potentially burn the recycled fuel from LWRs, with no major reactor design changes, thus extending resources and reducing spent fuel storage.

First Generation Douglas Point on a stamp face, first HWR of the Candu

design, now decommissioned, was part of a nuclear complex including the Bruce HWRs.

EVOLUTION OF THE CANDU HWR DESIGN

The HWR concept is primarily represented by the CANDU design which is an acronym for CANada Deuterium Uranium. The CANDU system uses pressurized heavy water D2O as moderator and coolant and natural uranium as fuel in the form of uranium dioxide UO2.

The HWR design, because of its relative technical simplicity is in vogue in

developing countries. New HWR designs are being developed mainly in Canada and

India.

HWR reactors are currently operated in Canada at Ontario, New Brunswick and Québec, in Korea, Argentina and Romania. Other similarly designed Pressurized Heavy Water Reactors (PHWRs) are operated in India and Pakistan. In China, the Qinshan project, a partnership between Atomic Energy of Canada Limited (AECL), Canada and the Third Qinshan Nuclear Power Company (TQNPC), included two units that went into commercial operation in 2002 and 2003 respectively.

In India, a continuing process of evolution of the HWR design has been carried out since the Rajasthan I and II projects. Tarapur 4, an evolutionary 500 MWe HWR achieved criticality in March 2005 and another 500 MWe unit Tarapur 3.

Bruce HWRs Reactors complex, Canada.

Second generation Point Lepreau, Atomic Energy of Canada Limited (AECL)

Candu 6 power plant.

A boiling light water cooled, heavy water moderated Advanced HWR was under development in India. This design is a thermal breeder using the thorium and U233 fuel cycle and incorporates many passive features, including natural circulation cooling.

In Canada, AECL continues to evolve the basic CANDU design to develop the Advanced CANDU Reactor (ACR), focusing on improvements in economics, inherent safety characteristics and performance, while retaining the features of the earlier family of PHWR nuclear power plants that are now decommissioned such as the Douglas Point HWR. The stated goals include lower plant capital and operating costs, plus reduced project schedules, through the use of improved design and construction methods and operational improvements. The design uses Slightly Enriched Uranium (SEU) fuel to reduce the reactor core size, which also reduces the amount of heavy water required to moderate the reactor and allows the use of light water in place of heavy water as the reactor coolant.

At the leading edge of the ongoing AECL program for evolutionary improvements in the CANDU line of reactors is a research and development program, designated as CANDU-X, or innovative CANDU reactors operating at higher thermal efficiencies, which implies a high temperature coolant or supercritical water as a coolant. Such reactors would also incorporate passive high temperature fuel channels, natural circulation heat removal, and passive containment heat removal.

CANDU 6 HEAVY WATER REACTOR DESIGN

The core of the nuclear steam supply system of a CANDU 6 power plant is a large cylindrical vessel called the calandria. This vessel is filled with cool, low-pressure D2O. The vessel houses 380 horizontal tubes, loaded with natural uranium fuel bundles.

Each fuel channel consists of a 104 mm diameter, 4.3 mm thick zirconium niobium alloy pressure tube, inserted into a slightly wider calandria tube, and two stainless steel end fittings at the ends of the fuel channel. The tubes are 6.3 m in length. Garter Spring spacers separate the two tubes.

Heavy water flows through the pressure tubes in a secondary pressurized circuit, removing heat from fuel bundles and transferring it to the steam generators, where secondary circuit light water is being heated and converted into steam to drive the steam turbine and the electrical generator. During reactor operation, the pressure tube material is subject to high pressure at 11.3 MPa and high temperature reaching 310 °C.

Darlington CANDU reactor face showing the horizontal pressure tubes.

Horizontal calandria of HWR showing its components.

Fuel pellets, tube and bundle used in the HWR design.

On line refueling machine in horizontal tubes CANDU design.

Electrical generator and disabled steam turbine in the Douglas Point plant.

CANDU PERFORMANCE

The CANDU 6 reactor design possesses the highest performance rating of all other reactors concepts and is able to be refueled at full power using an online refueling machine. This leads to a power generating capacity factor of 75 percent, even after 13 years of operation making it a more reliable system than other energy options.

The Lifetime Capacity Factor (LCF) is the Total Gross Generation (TGG) figure divided by the capacity (CF) divided by the total number of hours (H) from the time of first synchronization, multiplied by 100.

The CANDU 6 units are reported as having the highest lifetime capacity factors

within their class.

Levelized Unit Energy Cost (LUEC) of gas, coal and nuclear sources in

Ontario, Canada, for units placed in service in 2002, in 1989 Canadian dollars. Ontario

Hydro.

SAFETY ASPECTS

Originally licensed in Canada, the CANDU 6 design has also been licensed in every country where it has been installed over the past two decades. With the Wolsong 2, 3 and 4 plants in the Republic of Korea, it met all requirements for licensing in both Canada and Korea, the latter having licensed both PWR and Candu systems.

The Canadian regulator, the Atomic Energy Control Board (AECB), generally plays a role in the transfer of expertise to international regulatory bodies. Part of the licensing acceptability is its approach to safety. Long operating experience under a wide range of demanding conditions bears this out. The unique defense-in-depth design incorporates tri-level passiveness. Preventative boundaries and independent shut-down systems are built into the design at multiple levels.

The design has established a long history of minimizing radiation doses to the operating staff. This has contributed to ease of plant maintenance which, in turn, has contributed to operating reliability and lower operating costs

The power plants are highly automated, requiring only a minimum of manual operator action. Each plant has two independent digital computers which operate continuously, one operating and one on standby.

The safety systems include two independent and fully capable shutdown systems To provide maximum safety, the systems are separated physically and functionally.

The moderator operates at low pressure and temperature, completely separate from the primary heat transport coolant circuit, providing a large body of water capable of absorbing excess heat.

Power plants have an emergency core cooling system and a containment system. In the event of a Loss Of Coolant Accident (LOCA), the reactor safety system will automatically shut down the reactor, isolate the defective primary heat transport coolant circuit, and assure a supply of cooling water over the fuel.

ENVIRONMENTAL ASPECTS

A CANDU 6 reactor produces about 24 cubic meters of used fuel bundles per year, a volume that would fit into a small room. The design of the power plant is such that these wastes are controlled and represent no threat to station personnel or to the public at large.

Concrete silos temporarily store the used fuel elements allowing cooling

before final recycling or disposal.

The 700 MWe class nuclear power plant saves the burning of about 84 million metric tonnes of coal or about 330 million barrels of oil over a 40-year period and the millions of tonnes of acid-producing NOx and SOx atmospheric pollutants these fossil fuels would generate. It avoids the release of 196 million tonnes of carbon dioxide over its lifetime, an amount that would be produced by a fossil-fuelled plant of equal size.

Air and water discharges are free of such contaminants as heavy metals, organic compounds and acid gases. Radioactivity releases in any form are closely monitored, and are consistently less than 1 percent of those permitted by regulatory standards.

Power stations occupy relatively little land since space is not required for large coal storage and ash disposal areas. As an example, the CANDU 6 at Point Lepreau produces about 300 kgs of used fuel per day. An equivalent coal-fired station produces eight metric tonnes of fly ash and 1,440 metric tonnes of solid ash per day.

WASTE MANAGEMENT

The overall waste handling philosophy and strategy, which has been the subject of 50 years of continuing research and development, is to provide interim storage at the reactor site, followed by permanent disposal underground in geologically stable formations.

Used nuclear fuel has been stored in water filled pools at HWR nuclear generating stations for 25 years and can be stored in this way for many decades. The use of concrete canisters or silos for dry storage of used HWR reactor fuel is now coming into wide use because it provides safe storage at lower cost than water filled storage pools.

At the Point Lepreau plant, after at least seven years under water, the spent fuel which would have seen a decrease in radioactivity of about 99 percent is moved to on-site dry storage canisters, where each year's discharge fills about 10 canisters.

The MACSTOR above-ground dry storage system for spent HWR fuel, suitable as well as fuel from other reactor types such as the PWR, BWR and VVER, has been developed. It features cooling and shielding, easy fuel retrievability for future off site disposal, resistance to earthquakes and tornadoes, and low construction and operating costs. A MACSTOR unit was installed in a record five and a half months at Hydro Quebec's Gentilly-2 CANDU station in 1995.

Cutout diagram showing a Candu 9 plant setup.

Summary of Reactor Types

Reactor Type	Function	Coolant	Moderator	Chemical Form of Fuel	Fuel Enrichment Level
Boiling Water	electricity	light water	light water	uranium dioxide	low enriched uranium
Pressurized Water	electricity, nautical power	light water	light water	uranium dioxide	low enriched uranium
Heavy Water	electricity, plutonium production	heavy water	heavy water	uranium dioxide or uranium metal	natural, unenriched uranium

Properties of Heavy water

CAS number	7789-20-0
molecular formula	2H2O/ D2O
molar mass	20.0276 g/mol
exact mass	20.023118178 g/mol
appearance	pale blue transparent liquid
odor	Odorless
melting point	3.8°C
molecular weight	20.0276 g/mol
vapor pressure	16.4 mm Hg
refractive index	1.328
viscosity at 25°C	0.001095 Pa s
specific heat of fusion	0.3096 kj/g

purpose of using heavy water in nuclear reactor

Heavy water is used as a moderator in a nuclear reactor. It is used to slow the neutrons being directed at the fissionable material, by means of the molecules of the moderator physicaly impacting the incoming neutrons and absorbing some of the kenetic energy they posses, thus slowing them down, in the same way that two billiard balls impacting each other would slow down the incoming one (or both if they were both moving). The reason that the neutrons have to be slowed is that most fissionable materials are more likely to absorb thermal neutrons (2.2km/s) than fast neutrons (14,000km/s).

Light water (the name usually used for regular H₂O when talking about nuclear reactors), is the most common type of moderator, because it is cheap, very available, and is more effecient at slowing the incoming neutrons, due to the fact that the hydorgen atoms in the water posses only one proton and one electron, and thus are almost exactly the same mass as the incoming neutrons (the hydrogen atom weighs only as much as one electron more than the neutrons, and electrons are very light when compard to protons and neutrons, which are equal in mass).

The problem with using light water as a moderator, however, is that the hydrogen atoms may absorb some of the neutrons, thus preventing them from getting through to the fissionable material. Thus, once the percentage of U-235 (the fissionable isotope of uranium) is too low (such as in natural uranium, where the percentage of U-235 is about 0.72%), then the amount of neutrons getting through the moderator without being abosorbed is not high enough to maintain criticality (the point at which the amount of neutrons being produced is equal to the amount escaping the system or being absorbed but not resulting in fission), and the chain reaction can no longer continue, and the reactor can no longer produce power.

Heavy water, however, is deuterium oxide. Deuterium is an isotope of hydrogen with one proton and one neutron. Thus the hydrogen atom already has one extra neutron, and is much less likely to absorb another. This means that when heavy water is used as a moderator, enough neutrons get through that even with very low levels of U-235 (even the very low levels found in natural uranium), criticality can be maintained, and power is produced.

So even though the efficiency of the D₂O (heavy water) molecules at slowing the neutrons is slightly less than that of regualr H₂O (water, or light water) molecules, the use of heavy water as a moderator allows natural uranium to be used as a fuel with little, if any, enrichment (which is a costly process, and controversial, as enriched uranium can be used to make nuclear weapons). This is why CANDU (Canadian Deuterium-Uranium) reactors can use natural uranium, or even the waste uranium from conventional light water reactors as fuel.

Application of Heavy Water

Heavy water is used as tracer in the study of reactions occurring in living organisms and other chemical reactions.

It is used in nuclear reactors as moderator (the substance used to moderate the speed of nuclear reactions) because in nuclear fission process (A nuclear reaction in which a massive nucleus splits into smaller nuclei with the simultaneous release of energy) it slows down the velocity of neutrons.

It has also been used for the preparation of deuterium. All natural water contains a large amount of different salts. In addition to this natural water also contains carbondioxide which forms hydrogen carbonate ion with water.

By directly introducing one microlitre of distillate of urine, blood serum, etc. into the sample reservoir of a mass spectrometer, it is possible to determine the ratio of HDO to H2O molecules in the vapour to within a standard deviation corresponding to 2%. Duplicate determinations can be performed in 5 minutes, not counting the time for distillation which is done in 30 minutes. A rigorous definition of total body water is developed in which attention is given to each appreciable factor required for accurately stating the final volume.

If it isn't used in nuclear power stations, what use is heavy water?

Deuteration of compounds often assists the study of reaction mechanisms. Chemists can use D₂O as a solvent for Nuclear Magnetic Resonance spectra as the different nuclear properties of deuterium means that it does not give resonances in the proton-NMR range. It can also be used to assign the signals due to labile N-H and O-H groups in compounds such as alcohols; just add a drop of heavy water to your solution of the compound in CDCl₃, then the labile protons are exchanged, and the resonance due to O-H (or N-H) disappears

R-O-H + D₂O

R-O-D + D-O-H

This is illustrated in the NMR spectra of menthol, below, where the OH peak at ~1.8 ppm disappears after adding D₂O, showing that the H involved is free to exchange with the solvent.