Generating Electrical Power: NUCLEAR STATIONS

Nuclear stations produce electricity from the heat released by a nuclear reaction. When the nucleus of an atom splits in two (a process called atomic fission). a considerable amount of energy is released.

Note that a chemical reaction, such as the combustion of coal produces only a rearrangement of the atoms, without in any way affecting their nuclei.

A nuclear station is identical to a thermal station, except that the boiler is replaced by a nuclear reactor. The reactor contains the fissile material that generates the heat. A nuclear station, therefore, contains a synchronous generator, steam turbine, condenser. and so on, similar to those found in a conventional thermal station. The overall efficiency is also similar (between 30 and 40%), and a cooling system must be provided for. Consequently, nuclear stations are also located close to rivers and lakes. In dry areas, cooling towers are installed.

Owing to these similarities, we will only examine the operating principle of the reactor itself.

Composition of an atomic nucleus; isotopes

The nucleus of an atom contains two types of particles-protons and neutrons. The proton carries a positive charge, equal to the negative charge on an electron. The neutron, as its name implies, has no electric charge. Neutrons are, therefore, neither attracted to nor repelled by protons and electrons.

Protons and neutrons have about the same mass, and both weigh 1840 times as much as electrons do. The mass of an atom is concentrated in its nucleus.

The number of protons and neutrons in the nucleus depends upon the element. Furthermore, because an atom is electrically neutral, the number of electrons is equal to the number of protons. Table A gives the atomic structure of a few important elements used in nuclear reactors. For example, there are two types of hydrogen atoms that can be distinguished from each other only by the makeup of the nucleus. First, there is ordinary hydrogen (H), whose nucleus contains 1 proton and no neutrons.

Next, there is a rare form, deuterium (D), whose nuclei contain 1 neutron, in addition to the usual pro ton. This rare form is called an isotope of hydrogen.

When two atoms of ordinary hydrogen unite with one atom of oxygen, we obtain ordinary water (H2 O) called light water. On the other hand, if 2 atoms of deuterium unite with 1 atom of oxygen, we obtain a molecule of heavy water (D2O). The oceans contain about] kg of heavy water for every 7000 kg of sea water.

In the same way, two isotopes of uranium are found in nature: uranium 238 e38U and uranium 235 e35U. Each contains 92 protons, but has 146 neutrons and 235U has 143. Uranium 238 is very common, but the isotope 235U is rare.

Uranium 235 and heavy water deserve our attention because both are essential to the operation of the nuclear reactors we are about to discuss.

-- 28. Various steps in the manufacture of nuclear fuel for heavy-water and light-water reactors. This extremely simplified diagram shows that in the process of enriching uranium dioxide, it’s inevitable that large amounts of 238_UO2 remain as a byproduct.

The source of uranium

Where does uranium come from? It’s obtained from the ore found in uranium mines. This ore contains the compound U30 8 (3 atoms of uranium and 8 atoms of oxygen). It so happens that U3O8 is actually composed of 238_UO8 and 235_UO8 in the relatively precise ratio of 1398: 10.

In other words, the ore contains 1398 parts of the less interesting 238U for every 10 parts of the isotope 235U. It’s very difficult to separate 238_UO8 from 235_UO8 because they possess identical chemical properties.

In order to use these substances in nuclear reactors, they are processed into uranium dioxide (UO2). The natural UO2 again contains 238_UO2 and 235_UO2 in the ratio of 1398: 10.

Some nuclear reactors require UO2 that has more of the isotope 238 U than natural UO2 does. This is produced by an enrichment process whereby the ratio of 235_UO2 to 2311UO 2 is raised to 50: 1398 rather than the natural ratio of 10: 1398. In this enrichment process a lot of 238UO2 is obtained as a byproduct that must be stored. As we shall see, this byproduct also has useful applications.

The process of converting uranium ore into these uranium derivatives is shown in highly simplified form.

Energy released by atomic fission

When the nucleus of an atom fissions, it splits in two. The total mass of the two atoms formed in this way is usually less than that of the original atom. If there is a loss in mass, energy is released according to Einstein's equation: where E energy released [J] m = loss of mass rkg 1 c = speed of light [3 x 10^8 m/s]

An enormous amount of energy is released be cause, according to this formula, a loss in mass of only one gram produces 9 X 10^13 Joules, which is equivalent to the heat given off by burning 3 thousand tons of coal. Uranium is one of those elements that loses mass when it fissions. However, uranium 235 is fissionable, whereas uranium 238 is not, and so large separating plants have been built to isolate molecules containing U_235 from those containing U_238.

Chain reaction

How can we provoke the fission of a uranium atom? One way is to bombard its nucleus with neutrons. A neutron makes an excellent projectile because it’s not repelled as it approaches the nucleus and, if its speed is not too great, it has a good chance of scoring a hit. If the impact is strong enough, the nucleus will split in two, releasing energy. The fission of one atom of 235_U releases 218 MeV of energy, mainly in the form of heat. Fission is a very violent reaction on an atomic scale, and it produces a second important effect: It ejects 2 or 3 neutrons that move at high speed away from the broken nucleus.

These neutrons collide with other uranium atoms, breaking them up, and a chain reaction quickly takes place, releasing a tremendous amount of heat.

This is the principle that causes atomic bombs to explode. Although a uranium mine also releases neutrons, the concentration of 235U atoms is too low to produce a chain reaction.

In the case of a nuclear reactor, we have to slow down the neutrons to increase their chance of striking other uranium nuclei. Toward this end, small fissionable masses of uranium fuel (UO2) are immersed in a moderator. The moderator may be ordinary water, heavy water, graphite, or any other material that can slow down neutrons without absorbing them. By using an appropriate geometrical distribution of the uranium fuel within the moderator, the speed of the neutrons can be reduced so they have the required velocity to initiate other fusions. Only then will a chain reaction take place, causing the reactor to go critical.

As soon as the chain reaction starts, the temperature rises rapidly. To keep it at an acceptable level, a liquid or gas has to t10w rapidly through the reactor to carry away the heat. This coolant may be heavy water, ordinary water, liquid sodium, or a gas like helium or carbon dioxide. The hot coolant moves in a closed circuit which includes a heat ex changer. The latter transfers the heat to a steam generator that drives the turbines. Thus, contrary to what its name would lead us to believe, the coolant is not cool but searingly hot.

Types of nuclear reactors

---29. Schematic diagram of a nuclear power station; nuclear fuel; reactor moderator; heat exchanger; coolant circulating pump; feed water pump

--- 30. Aerial view of a light-water nuclear generating station. The large rectangular building in the foreground houses a 667 MVA, 90-percent power factor, 19 kV, 60 Hz, 1800 r/min turbo generator set; the circular building surrounds the reactor.

-- 31. Looking down into the water-filled refueling cavity of the reactor.

There are several types of reactors, but the following are the most important:

1. Pressure-Water Reactor (PWR). Water is used as a coolant and it’s kept under such high pres sure that it cannot boil off into steam. Ordinary water, as in light-water reactors may be used, or heavy water, as in CANDU* reactors. [* CANDU: Canada Deuterium Uranium. developed by the Atomic Energy Commission of Canada.]

2. Boiling-Water Reactors (BWR). The coolant in this reactor is ordinary water boiling under high pressure and releasing steam. This eliminates the need for a heat exchanger, because the steam circulates directly through the turbines. However, as in all light-water reactors, enriched uranium dioxide must be used containing about 3 % 235_U.

3. High-Temperature Gas Reactor (HTGR). This reactor uses an inert gas coolant such as helium or carbon dioxide. Due to the high operating temperature (typically 750°C), graphite is used as a moderator. The steam created by the heat ex changer is as hot as that produced in a conventional coal-fired steam boiler. Consequently, the overall efficiency of HTGR stations is about 40%.

4. Fast Breeder Reactor (FBR). This reactor has the remarkable ability to both generate heat and create additional nuclear fuel while it’s in operation.

Example of a light-water reactor

Reactors that use ordinary water as a moderator are similar to those using heavy water, but the uranium dioxide fuel has to be enriched. Enrichment means that the fuel bundles contain between 2 and 4% of 235_U, the remainder being 235_U. This enables us to reduce the size of the reactor for a given power out put. On the other hand, the reactor has to be shut down about once a year to replace the expended fuel.

The generated heat, created mainly by the fission of uranium 235, is carried away by a coolant such as ordinary water, liquid sodium, or a gas such as CO2 , As it flows through the heat exchanger. the coolant creates the steam that drives the turbine.

A typical nuclear power station possesses a light-water reactor that is composed of a massive vertical steel tank having an external diameter of 4.5 m and a height of 12.5 m. The tank contains 157 vertical tubes, which can lodge 157 large fuel assemblies. Each assembly is 3 m long and groups 204 fuel rods containing a total of 477 kg of enriched UO2 . The nuclear reaction is kept under control by 45 special-alloy control rods.

When these rods are gradually lowered into the moderator, they absorb more and more neutrons.

Consequently, they control the rate of the nuclear reaction and. hence, the amount of heat released by the reactor.

The nuclear station drives a 3-phase, 667 MVA, 90% power factor, 19 kV, 60 Hz, 1800 r/min synchronous generator.

Example of a heavy-water reactor

--- 32. Simplified schematic diagram of a CANDU nuclear generating unit composed of one heavy-water reactor driving one alternator.

The CANDU reactor uses heavy water, both as moderator and coolant. It differs from all other re actors in that it uses natural uranium dioxide as a fuel. One of the biggest installations of its kind is located at Pickering, a few kilometers east of Toronto, Canada. The nuclear station has 4 reactors.

Each reactor is coupled to 12 heat exchangers that provide the interface between the heavy-water coolant and the ordinary steam that drives the turbines. Each reactor is enclosed in a large horizontal vessel (calandria) having a diameter of 8 m and a length of 8.25 m. The calandria possesses 390 horizontal tubes, each housing 12 fuel bundles containing 22.2 kg of UO2. Each bundle releases about 372.5 kW while it’s in operation. Because there is a total of 4680 bundles, the reactor develops 1740 MW of thermal power.

Twelve pumps. each driven by an 1100 kW motor, push the heavy-water coolant through the reactor and the heat exchangers in a closed loop.

The heat exchangers produce the steam to drive the four turbines. The steam exhausts into a con denser that is cooled by water drawn from Lake Ontario.

Each turbine drives a 3-phase, 635 MVA, 85% power factor, 24 kV, 1800 r/min, 60 Hz alternator.

The fuel bundles are inserted at one end of the calandria and, after a 19-month stay in the tubes, they are withdrawn from the other end. The bundles are inserted and removed on a continuous basis-an average of nine bundles per day.

Table B compares the typical characteristics of light-water and heavy-water reactors.

TABLE: TYPICAL LIGHT-WATER and HEAVY-WATER REACTORS

Reactor Vessel: external diameter length vessel thickness weight empty position

Light-Water Reactor

4.5 m 12.5 m 274mm 416 t vertical

Heavy-Water Reactor: number of fuel canals type of fuel total mass of fuel 157 enriched U02 (3.3%) 75 t

8m 8.25 m 25.4 mm 604 t horizontal 390 natural UO2 104 t

Moderator type; volume

Reactor Cooling :heat produced in reactor coolant volume flow rate coolant temperature entering the reactor coolant temperature leaving the reactor coolant pumps total pump power

Electrical Output: 3-phase. 1800 r/min. 60 Hz synchronous generator

light-water 13.3 nr'

1825 MW light-water 249 m' 128 t/s 285°C 306°C 4 12MW 600MW heavy-water 242 m" 1661 MW heavy-water 130 m] 7.73 t/s 249°C 294°C 12 14MW 540MW

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Principle of the fast breeder reactor

A fast breeder reactor differs from other reactors because it can extract more of the available energy in the nuclear fuel. It possesses a central core containing fissionable plutonium 239 Pu.

The core is surrounded by a blanket composed of substances containing non-fissionable uranium 238 U_238. No moderator is used; consequently, the high-speed (fast) neutrons generated by the fissioning 239_Pu bombard the non-fissionable atoms of 238_U. This nuclear reaction produces two important results:

a. The heat released by the fissioning core can be used to drive a steam turbine.

b. Some atoms of 238_U in the surrounding blanket capture the flying neutrons, thereby becoming fissionable 239_Pu. In other words, the passive atoms of uranium 238 are transmuted into fissionable atoms of plutonium 239.

As time goes by, the blanket of non fissionable 238U is gradually transmuted to fissionable 239 Pu and waste products. The blanket is periodically re moved and the materials are processed to recover the substances containing 239 Pu. The nuclear fuel re covered is placed in the central core to generate heat and to produce still more fuel in a newly relined blanket of substances containing uranium 238.

This process can be repeated until nearly 80% of the available energy in the uranium is extracted. This is much more efficient than the 2% now being extracted by conventional reactors.

The breeder reactor is particularly well adapted to complement existing light-water reactors. The reason is that a great deal of 238U is available as a byproduct in the manufacture of enriched 235U. This otherwise useless material (now being stored) could be used to surround the core of a fast breeder reactor. By capturing fast neutrons, it could be rejuvenated, as explained above, until most of the potential energy in the uranium is used up.

Nuclear fusion

We have seen that splitting the nucleus of a heavy element such as uranium results in a decrease in mass and a release of energy. We can also produce energy by combining the nuclei of two light elements in a process called nuclear fusion. For example, energy is released by the fusion of an atom of deuterium with an atom of tritium. However, owing to the strong repulsion between the two nuclei (both are positive), they only unite (fuse) when they approach each other at high speed. The required velocity is close to the speed of light and corresponds to a thermodynamic temperature of several million degrees. If both the atomic concentration and speed are high enough, a self-sustaining chain reaction will result.

We can, therefore, produce heat by the fusion of two elements, and the hydrogen bomb is a good ex ample of this principle. Unfortunately, we run into almost insurmountable problems when we try to control the fusion reaction, as we must do in a nuclear reactor. Basically, scientists have not yet succeeded in confining and controlling high-speed particles without at the same time slowing them down.

A major worldwide research effort is being de voted to solve this problem. If scientists succeed in domesticating nuclear fusion, it could mean the end of the energy shortage because hydrogen is the most common element on earth.