Nuclear power generation: Safety and some important facts to be considered

Saturday, 2 July 2011 01:09 -     - {{hitsCtrl.values.hits}}

Prior to 1956, energy dependence of world was mainly involved with wind power, solar power, coal, gas, etc., as sources of electricity. Even though we still use these sources, since 1956 nuclear power too has become an emerging energy source.

Both the coal and gas sources which are known as fossil fuels release significant amounts of carbon dioxide to the atmosphere. They are also predicted to be in short supply in the future as world fuel needs continue to grow exponentially.

Environmental pollution is the major disadvantage of these sources because they give off carbon dioxide when burned, which in turn causes green house effect. This is also the main contributory factor to the global warming experienced by the earth today.

It releases other harmful wastes such as sulphur dioxide, a gas that could create acid rain, nitrogen oxides, sulphuric acids, arsenic and ash. Carbon dioxide emissions from the burning of fossil fuels now account for about 65% of the extra carbon dioxide in our atmosphere.

If we take solar and wind power, most environmentalists believe that it is possible to control the amount of carbon in the atmosphere by using these. But clean electricity from these renewables – solar, wind, biomass and geothermal power need strong support for their effective utilisation.

However, the collective capacity of these technologies to produce electricity in the next decade is limited. The International Energy Agency projects that even with continued subsidy and research support these renewable can only provide around 6% of world electricity needs by 2030.

New energy sources

So, to overcome the increasing energy needs in the future, we have to switch to new energy sources which can reduce the emission of harmful gases and thereby minimise pollution. For reasons mentioned above, nuclear power appears to be a good candidate for fulfilling energy requirements in the coming decade.

Nuclear power is clean and effective. Like wind, hydropower and solar energy, it can generate electricity with minimal emission of carbon dioxide or other green house gases. And what counts most is its capability of generating energy in larger quantities using just a small amount of fuel when compared to the other energy sources such as coal, natural gas and so on.

Nuclear energy is released by a nuclear reaction. This energy can be produced in two ways; as natural energy or man-made energy. For example, the sun and the stars make heat and light by natural nuclear reactions and nuclear power can be generated using manmade nuclear reactors.

At present, the topic ‘nuclear energy’ is being discussed all over the world because of the nuclear reactor explosions which took place in Japan’s Fukushima Daiichi nuclear power plant. Since the Government of Sri Lanka also has intentions concerning nuclear power as a viable power generation option for the exponential growth of energy demand in future, public awareness of the topic seems to be growing rapidly. So the frequently asked questions would be:

Isn’t nuclear power dangerous? As a developing country, can we manage to reach the required security standards? Do we have enough technology to overcome the problems generated by those so-called nuclear power plants? After all, will there be any advantages of reaching it?

Nuclear power

Before answering these questions, we should learn how nuclear power is being generated and the history of nuclear power stations.

If we go through the beginning of nuclear power stations, it takes us back to 1956. The first large-scale nuclear power station was opened at Calder Hall in Cumbria, England in 1956. That is, nuclear generation began more than 50 years ago and now generates as much global electricity as was produced then by all the other sources.

Today nuclear energy provides 16% of world electricity. With sound public policy, this percentage could grow rapidly, supporting global economic prosperity without green house gases and pollution.

The first generation of nuclear plants was justified by the need to alleviate urban smog caused by coal-fired power plants. Nuclear power was also considered as an economical source of base load electricity which reduced the dependence on overseas imports of fossil fuels.

Today’s reasons for moving towards nuclear energy can be mentioned as increasing energy demand, climate change, security of supply (of other fuels), economics and insurance against future price exposure.

How nuclear power is generated

As we can see, nuclear power holds a main role in fulfilling energy needs at present and it will continue to be so in the future. As in the case of Sri Lanka, we expect to construct a nuclear power station as energy demand is increasing year by year. Therefore, it would be more important to learn how nuclear power is generated in a nuclear power station, before discussing its effects.

Nuclear energy is released as a result of a nuclear reaction. One can identify that there are two types of nuclear reactions which generate huge amounts of energy, where no other conventional fuel sources can match them by any means. These two reactions are nuclear fission and nuclear fusion.

In nuclear fission, the nuclei of a relatively large atom split into two, releasing energy. The atomic bombs and nuclear reactors function with fission reactions. In nuclear fusion, the nuclei of relatively smaller atoms are fused together and release large amounts of energy. Fusion takes place only at very high temperatures.

The sun, like all other stars, creates heat and light through nuclear fusion. The hydrogen bomb, humanity’s most powerful and destructive weapon, also works by fusion. The heat energy required to start the fusion reaction is so great and the large energy released in fission reaction is used to provide the heat energy required to initiate the fusion reaction.

So far scientists supposedly have control only over the nuclear fission, they are yet to find a way to tame nuclear fusion, thus making it the most destructive type. Thus, scientists use nuclear fission to generate nuclear power within a device designed and named the nuclear reactor. It is in the nuclear reactor that the nuclear fission reaction takes place to attain the goal of harnessing the energy in the nuclei.

The thermal energy generated in a nuclear fission reactor is used to produce steam, which is then used to drive a turbine. After the production of steam, the process of generation of electricity by a nuclear reactor is similar to that of a coal generator. The major difference between the two methods is the way steam is generated. In a nuclear reactor it is done by the energy which is released due to nuclear fission.

Uranium

Uranium (U) is the main fuel source that is used to undergo nuclear fission to produce nuclear energy since it has many favourable properties. Uranium is a radioactive isotope and hence it can be easily split by shooting neutrons at it. Also, once a uranium nucleus is split, multiple neutrons are released which are used to split other uranium nuclei. This phenomenon is known as a chain reaction.

Uranium is a common element on earth and has existed since the planet formed. While there are several varieties of uranium, uranium-235 (U-235) is the one most important to the production of nuclear power.

U-235 decays naturally by alpha radiation: It throws off an alpha particle (42He nucleus), or two neutrons and two protons bound together. It’s also one of the few elements that can undergo induced fission. By firing a free neutron into a U-235 nucleus, the nucleus will absorb the neutron, which becomes unstable and splits immediately.

As soon as the nucleus captures the neutron, it splits into two lighter atoms and throws off two or three new neutrons. The process of capturing the neutron and splitting happens very quickly. The decay of a single U-235 atom releases approximately 200 MeV (mega electron volts). That may not seem like much, but there are lots of uranium atoms in the sample we use inside the nuclear reactor.

Plutonium

Uranium-235 isn’t the only possible fuel for a power plant. Another material is Plutonium-239. Plutonium-239 is created by bombarding U-238 with neutrons, a common occurrence in a nuclear reactor.

If we take a sample of uranium from mine, it contains 0.7% of U-235 and the remaining 99.3% mostly contains with U-238 which does not contribute directly to the fission process. When this is included in a nuclear reactor, and bombarded with neutrons, U-238 converts to plutonium by capturing one neutron and hence it contributes to the fission cycle. And U-235 captures neutrons and directly contributes to the fission cycle.

The directly mined uranium can’t be used as the fuel in many reactors. So it has to undergo an enrichment process to make uranium useful at nuclear reactors. There are separate plants to carry out those processes in Europe, Russia and North America.

How it works

Enriched uranium is typically formed into inch-long pellets, each with approximately the same diameter. Next, the pellets are arranged into long rods or metal tubes, and the rods are collected together into bundles to form the core of the nuclear reactor. Then placing it as the core of the reactor and assembling the other required parts, a complete nuclear power plant can be formed. The figure shows the structure of a nuclear power plant.

The core of the reactor is pointed by the letter K. Inside the reactor, there are fuel rods, control rods and water to be heated up. Fuel rods are the sealed containers of the radioactive uranium and to control the fission reaction, the control rods are used between the fuel rods. Control rods are made out of neutron absorbing material such as cadmium or boron and are inserted or withdrawn from the core to control the rate of reaction, or to halt it.

When the fission chain reaction is taking place, lots of neutrons are emitted as a result. Those are captured by uranium isotopes to continue the nuclear reaction. Because of that, the heat is released continuously until there are no neutrons to capture. So if we want to stop releasing heat, the only thing we have to do is to control the number of neutrons involved in the process. It is done by the control rods.

When the rods are lowered towards the reactor, they absorb more neutrons and the fission process slows down. To generate more power, the rods are raised and more neutrons can crash into Uranium atoms. The rods can also be lowered completely into the uranium bundle to shut the reactor down in the event of an accident or to change the fuel.

The uranium bundle acts as an extremely high-energy source of heat. It heats the water and turns it into steam. The steam drives a turbine, which spins a generator to produce power. Again the steam is condensed into water and re-circulated through the same cycle.

As the source can emit harmful levels of radiation, extra precautions are required to be taken to avoid any accident. A concrete liner typically houses the reactor’s pressure vessel and acts as a radiation shield. That liner, in turn, is housed within a much larger steel containment vessel. This vessel contains the reactor core. The steel containment vessel serves as a barrier to prevent leakage of any radioactive gases or fluids from the plant.

An outer concrete building serves as the final layer, protecting the steel containment vessel. This concrete structure is designed to be strong enough (in generally its having a thickness of 3m) to survive a massive damage that might result from an earthquake or a crashing jet airliner. These secondary containment structures are necessary to prevent the escape of radiation/radioactive steam in the event of an accident.

The fuel rods can be used for about two to three years and after that the reactors have to be refuelled. For this purpose, whole nuclear reactor has to be temporarily shut down. A major problem associated with nuclear energy generation is nuclear waste management.

Though the resulting elements do not have a capability of splitting further and generating energy, they still remain radioactive in harmful levels. In that case, the used fuel rods can’t be thrown out just to the environment by itself. It has to be placed somewhere until it becomes less in radioactivity and until it can be satisfied that the radioactive levels of them aren’t harmful at all. And the countries which use nuclear power, uses large digs excavated in underground to place such used fuel rods.

Harmful aspect

Then what would be harmful in this process of generating energy? This will be the next question.

Uranium 235 is a naturally radioactive element. Radioactive means that it is not a stable nuclide and hence it releases several types of radiation to release the energy and to become stable. There are three types of radiations as “alpha radiation,” “beta radiation” and “gamma radiation”. The most destructive type of radiation is the gamma radiation since the released energy from the nuclear reaction is of the form of gamma radiation. However, all types of radiation can be harmful for the human body if exposed directly.

In the natural decay of uranium the speed of the reaction is very low and, inside the nuclear reactor the reaction is accelerated. And hence there are large amounts of radiation released inside the reactor core. This won’t be harmful until radiation is inside the reactor. Since the entire structure of the plant is covered by several shields of concrete and metals the possibility of any leakage is obviated.

What happened in Fukushima?

Then what happened in Fukushima Daiichi nuclear power plant? On 11 March 2011 Japan was hit by the largest earthquake recorded in the most recent past. After the earthquakes strike, the nuclear reactors were shut down as it was programmed in Fukushima Daiichi nuclear power plant. The control rods were fully inserted to the fuel rods to stop the fission reaction.

Unfortunately, though the entire fission process was shut down, the nuclear waste continues to generate heat from the decay process. So in order to protect the reactor from this heat, pumping cool water to the reactor should be continuously done to absorb the heat and to cool the reactor.

But unfortunately the generators which supported to pumping water into the reactor were damaged from the tsunami that hit Japan after the earthquake. In addition to that, all of the backup plans for supplying electricity to the plant were damaged and hence apparently every measure that was in place to support pumping of water to the reactor has failed. These pumps circulate water through the reactor to remove heat emission from the reactor. Without the circulation of water, both the water temperature and water vapour pressure inside the reactor continued to rise.

With no fresh coolant flowing into the reactor core, the water that kept it cool, began to boil off. As the water boiled away, the tops of the fuel rods were exposed, and the metal tubes holding the Uranium fuel pellets overheated and cracked. The cracks allowed water to enter the tubes and get to the fuel pellets, where it began generating hydrogen gas. That is, if water gets hot enough, it breaks down into its constituent hydrogen and oxygen atoms.

Then the pressure from the hydrogen built up, and the gas had to be vented. Unfortunately, so much hydrogen vented so quickly that it exploded inside the reactor building. This same chain of events occurred in several different reactors.

The explosions did not rupture the pressure vessels holding the nuclear cores, nor did they release any significant amounts of radiation. These were simple hydrogen explosions, not nuclear explosions. The explosions damaged the concrete and steel buildings surrounding the pressure vessels.

And then since there is no water to cool the reactor core, the fuel rods were heated up and melted releasing radiation to the environment, which is the most devastative thing that can ever happen in a nuclear power plant. However trying nearly for about a month, they were successful in repairing the generators to pump water into the reactor. So it would help to stop the radiation leakage to the environment.

Though Japan is a technologically far more advanced country than ours; they couldn’t defeat the natural disasters, and today it has caused immeasurable damages to human lives especially to the workers who are still working on in the nuclear plant to overcome the problem. Anyone can’t be guaranteed about their lives. Even with such advanced technology Japan couldn’t avert the disaster.

Having discussed all of these doesn’t imply nuclear power is 100% harmful. This accident which occurred due to the presence of a tsunami and an earthquake does not exactly make any fact for us to get afraid of nuclear power, because Sri Lanka is positioned in the Indian Ocean where most of the active plate boundaries are lying far away from us. Nevertheless, there are great risks associated with it, in many other ways other than the natural disasters like earthquakes and the tsunami.

Advantages and disadvantages

In summary, if we talk about the advantages of it; it is not as expensive as is coal power and it does not directly participate to produce smoke or carbon dioxide, so it does not excessively contribute to the greenhouse effect. It produces significant amounts of energy with a small amount of fuel which making it as a reliable energy source.

On the other hand, the risks and uncertainties associated with it are phenomenal. Even with many counter measures to avoid a meltdown, with significant technological infrastructure, Japan had to face a partial meltdown and subsequent radiation exposure incidents. The disposing of spent fuel rods is still not a solved issue. Therefore, currently, nuclear waste management is not perfect.

Within this context enters the prospect of a nuclear power plant in Sri Lanka. Do we have the necessary potential to have one here? For instance, we have to wonder whether we have sufficient technological infrastructure to support and maintain such a power source. Add to that the likelihood of having to face a natural disaster, which is not that slim either. We have to make certain that a subsequent nuclear meltdown can be avoided.

Do we have the required technology or the confidence in technology that such disasters can be avoided? Apparently, Japan doesn’t seem to.

(The writer is a physics special student (3rd year) at the Department of Physics, University of Sri Jayewardenepura.)

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