Nuclear power

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Description

Nuclear power is principally the generation of electricity using the heat produced in a nuclear fission reactor. This is used to produce steam feeding conventional steam turbines, as with any other thermal power station. There are many types of "atomic pile", the current ones being mainly Generation IIIa reactors. Fusion reactors are under development, but they are unlikely to be in commercial service before 2050. Nuclear power has a "bad press" in some quarters, mainly as a reaction to the Chernobyl disaster and because some governments amd other authorities have not fully addressed the disposal of nuclear waste. On the other hand, it is an economical means of generating electricity safely and with almost zero emissions of greenhouse gases. Much of the opposition to nuclear power is engineered by some NGOs using emotive, rather than scientific, reasoning.

Detailed description

Nuclear power has been shown to be perfectly safe and almost carbon emission free in a number of countries. For example, nearly 80% of France's electricity is generated from nuclear power; nearly 40% of Switzerland's energy comes from the same source. However, in many places, the general public is afraid of nuclear power, mainly because of Chernobyl but also because of Nagasaki and Hiroshima, which has nothing to do with the generation of electricity, of course. In order to allay these fears, which are largely irrelevant, it is proposed to present some of the popular misconceptions and the reality. However, this is just a very brief description. For further details, the Areva consortium have published a more detailed 26 page brochure^[1] plus many other useful documents.

Terrorism

The fear of terrorists may be divided into two categories

• the fear of terrorist activity causing malfunction which would allow radionuclides to escape
• the fear of terrorists stealing radioactive material for use in a dirty bomb

Modern nuclear reactors are housed in massive shielded steel constructions, themselves protected in a double steel and concrete containment shell. Even if someone exploded a large bomb within the containment shell, the latter would not be destroyed and nothing would escape. Equally, the shell is designed to withstand the impact of very large aircraft diving into it. In addition, in the event of a mechanical shock, the control rods would be automatically dropped into the reactor core, by gravity, to immediately stop the chain reaction and the generation of heat. Of course, the normal safety devices in the reactor are mostly driven by electricity and are generally quadruplicated. Malicious activity could interrupt the electricity supply, in which case four diesel generators, in separate buildings, any one of which could supply all the electricity needed to control the reactor, would take over. If this happened the control rods would also stop the reaction immediately

Nuclear power stations are probably equipped with the most sophisticated security devices to prevent unauthorised persons from entering the site or from leaving the site with radioactive materials. Of course, there is always the possibility of a human error, as was recently illustrated in a Canadian nuclear power station when someone entered the the site with a small amount of explosive materials, although he was stopped long before he reached a reactor. This is an illustration that even if one security system fails, the overall system is reliable.

Earthquakes

The protection against earthquakes is similar to that against terrorism in that the construction of the containment shell and the reactor is such that even powerful earthquakes will not cause physical damage. This has been illustrated in Japan with two recent earthquakes. The first developed a fault line right under Japan's largest nuclear power station and the second, of magnitude 7.2 Richter, affected three nuclear power stations in the vicinity of the epicentre. In neither case were the reactor housings or containment shells damaged. In both cases, there were very minor escapes of water from the cooling ponds due to slurping occasioned by the shock waves. This water, of a volume of about one cubic metre, was slightly radioactive but presented no danger to the environment, at either location.

There are many other locations of nuclear power stations subject to severe seismic activity, including in France and Switzerland.

Meltdown

The fear of a meltdown or the "China syndrome" has been eliminated in the Generation IIIa reactors. Apart from the security measures mentioned previously, rendering the overheating of the reactor core almost impossible, additional emergency cooling circuits would be activated in the unlikely event of it happening. If this was insufficient, the whole of the containment shell interior would be cooled by massive quantities of gravity fed water sprayed onto the reactor housing. Either of these security devices would be sufficient to halt the snowball effect of overheating. Nevertheless, there is an additional security device in that if the reactor core did melt, all the molten material would be collected in very large ceramic trays, dimensioned to receive the whole volume. Such an event would be catastrophic in the sense that the reactor would be condemned but no radioactive materials could escape from the bounds of the containment shell.

Nuclear waste

There are several kinds of nuclear waste, each of which is treated differently. As a general rule, the higher the level of radioactivity in waste, the shorter is the half life. This means that high level waste decays fairly rapidly, while some of the low-level waste may be radioactive for very long periods. However, there are various types of radioactivity depending on the particles emitted and each type must be treated accordingly.

The most important types of waste are the used fuel rods, each of which contains a small amount of highly radioactive materials. The normal disposal system is to immerse them into a water coolant pond for a number of years. This eliminates any risk from overheating and allows the fissile material to decay. At the end of this process, the rods may be sent for recycling; the MOX process recycles 96% of the fissile material for reuse. The remaining 4% is medium level waste.

Very lightly contaminated material, such as contaminated clothing, may contain isotopes with a half life of tens of thousands of years. In some cases, if the contamination is water-soluble, they will be washed and the water filtered through an ion-exchange bed, which will capture the fissile material. In other cases, it may be preferable to burn the clothing and carefully collect the ashes. Yet again, it may be better to guard the contaminated material in its original form.

In order to finally dispose of radioactive waste, one method is to vitrify it. This means that it is encased in a solid block of lead glass which is very carefully annealed to avoid stress cracking. This is placed in single or double drums of carefully chosen materials to reduce external radiation. The outer container is often in stainless steel or titanium and is hermetically welded.

The controversial problem is not technical, in any way. The technology is clear and well known. The problem is that no one really desires to have radioactive waste buried close to where they live! The ideal repository would be in a thick layer of anhydride rock. This would have to be situated in a geologically and seismically stable area. Unfortunately, such sites are not available everywhere and sometimes the choice has to be made in a less than ideal situation. Generally speaking, the repository should be adequately sized with separate rooms for different kinds of radioactive waste. It should be exempt from any water flow under any conditions. In reality, there are indeed many suitable places throughout the world for very large storage facilities. Some countries have taken the bull by the horns and have repositories under very safe conditions already in service; this has not stopped some eco-political bodies from protesting under all sorts of pretexts.

Uranium enrichment

The uranium ore is purified into uranium oxide. The uranium in this consists of 99.28% of U²³⁸, 0.71% of U²³⁵, the rest being other isotopes. U²³⁸ is an isotope with a half life of about 4 1/2 billion years but is not fissionable. On the other hand, U²³⁵ is fissionable with slow neutrons and chain reactions can occur. However, the quantity in natural uranium is too small to maintain a chain reaction and some form of enrichment is necessary. This is a very difficult process and may be done by diffusion or in centrifuges. As a general rule, the enrichment is done to a level of between 4 and 5% of U²³⁵ for use in nuclear power station reactors.

Note that to make uranium nuclear weapons requires an enrichment of more than 95%, which is infinitely more difficult than that to provide uranium for civilian use.

Recycling nuclear fuel

In order to minimise the quantities of high level waste, it is desirable to recycle it, as much as possible. There are several ways of doing this. The most usual way of doing this is the Mixed OXide (MOX) process. The following is an extract (with permission) from a World Nuclear Association document^[2], which may be consulted for fuller details:

In every nuclear reactor there is both fission of isotopes such as uranium-235, and the formation of new, heavier isotopes due to neutron capture, primarily by U-238. Most of the fuel mass in a reactor is U-238. This can become plutonium-239 and by successive neutron capture Pu-240, Pu-241 and Pu-242 as well as other transuranic or actinide isotopes. Pu-239 is fissile, like U-235. (Very small quantities of Pu-236 and Pu-238 are formed similarly from U-235.)

Normally, with the fuel being changed every three years or so, most of the Pu-239 is "burned" in the reactor. It behaves like U-235 and its fission releases a similar amount of energy . The higher the burn-up, the less plutonium remains in the spent fuel, but typically about one percent of that discharged from a reactor is plutonium, and some two thirds of this is Pu-239. Worldwide, almost 100 tonnes of plutonium in spent fuel is generated each year.

A single recycle of plutonium increases the energy derived from the original uranium by some 12%, and if the uranium is also recycled this becomes about 20% or more.

Recycling Fuel

The first step is separating the plutonium from the remaining uranium (about 96% of the spent fuel) and the fission products with other wastes (together about 3%). This is undertaken at a reprocessing plant.

The plutonium, as an oxide, is then mixed with depleted uranium left over from an enrichment plant to form fresh mixed oxide fuel (MOX, which is UO2+PuO2). MOX fuel, consisting of about 7% plutonium mixed with depleted uranium, is equivalent to uranium oxide fuel enriched to about 4.5% U-235, assuming that the plutonium has about 60-65% Pu-239. If weapons plutonium were used (>90% Pu-239), only about 5% Pu would be needed in the mix.

Plutonium from reprocessed fuel is usually fabricated into MOX as soon as possible to avoid problems with the decay of short-lived isotopes of Pu. In particular, Pu-241 decays to Am-241 which is a strong gamma emitter, giving rise to a potential occupational health hazard if the separated plutonium over five years old is used in a normal MOX plant. The Am-241 level in stored plutonium increases about 0.5% per year. Pu-239, Pu-240 and Pu-242 are long-lived and hence little changed with prolonged storage.

Pu-238 becomes significant in high-burnup fuel. It is a strong alpha emitter and a source of spontaneous neutrons.

While fast neutron reactors allow unlimited recycle of plutonium, since all transuranic isotopes there are fissionable, in thermal reactors isotopic degradation limits the plutonium recycle potential and most spent MOX fuel is stored pending the greater deployment of fast reactors. Along with about 40% Pu-239, there may be 32% Pu-240, 18% Pu-241, 8% Pu-242 and 2% Pu-238.

(Recycled uranium from a reprocessing plant is re-enriched on its own for use as fresh fuel. Because it contains some neutron-absorbing U-234 and U-236, the enrichment level is slightly greater than for mined uranium providing equivalent fuel.)

Recycled MOX fuel is used in many countries, including China, France, India, Japan, Switzerland, UK etc. The USA has one facility that could use it and others are being planned, but a legal quirk forbids recycled fuels from being used, for the moment. The USA uses a "straight-through" uranium process with the result that they have massive quantities of used fuel in cooling ponds and in repositories. Some of this could be recycled at a later date if the legislation is changed, but changes in the isotope composition over time would render this more costly.

Most modern light water reactors can accept up to about 35% of MOX in their fuel load, but modern Generation IIIa reactors can accept up to a full 100%.

Nuclear fuel resources

Some opponents of nuclear energy have argued that the amount of accessible uranium for nuclear fuel is limited to a few decades of generation at the current rate of use. This argument is specious for a number of reasons. It is true that the quantity of uranium in the most easily exploited mines is limited. What is not true is that there is no further uranium available and it is assumed, in their calculations, that there will be no recycling of nuclear fuel. In fact, uranium ore is quite common throughout the world, but the extraction is not cost effective, compared with the prices of uranium from old stockpiles and easily accessible ore. If the price of uranium were to double from today's prices, then the quantities available would be very significant. Furthermore, if the price were to double, the effect on the price of generated energy would be almost negligible, typically about 1 cent per kilowatt hour. The cost of fuel in a nuclear power station is a very low part of the overall exploitation costs.

Uranium is considered to be more than double the mercury, antimony, silver or cadmium and is about as abundant as molybdenum or arsenic, in the earth's crust. However, there are an estimated 4,000,000,000 tonnes of uranium in the oceans. The concentration is very low, so that active extraction would not be energetically viable. Japanese scientists have developed a method of passive extraction which appears promising. The cost of doing this is estimated at about $200/kilogram, which is about three times higher than current prices. It is probable that economy of scale could reduce the costs considerably, in which case we have an almost infinite source of energy available.

There is another source of nuclear energy besides uranium. Thorium is an even more common metal than uranium, about as abundant as lead in the earth's crust. Natural thorium is essentially Th232 and is non-fissile alpha emitter with a half life of about 14 billion years. However, it is easily converted into uranium 233 by neutron bombardment. This latter is fissile and nuclear reactors could be manufactured to use it efficiently.

It is estimated that, even without recycling, there is sufficient uranium and thorium economically exploitable to satisfy the world's energy requirements for about 1000 years. With recycling and, particularly, the use of fast breeder reactors, then this figure could be decupled. Hopefully, nuclear fission will become obsolete as a means of a power generation before the end of the century.

Advantages of nuclear power

Despite its disadvantages which have been largely used as propaganda by the opponents of nuclear power, it does have undeniable advantages which is why there is an enormous worldwide resurgence of interest in the technology.

• Even with the increased costs of construction, insurance and decommissioning, the overall cost of energy production is very competitive to that of electricity generation from fossil fuels or renewable sources.
• Cost of electricity produced little dependent on the cost of the fuel.
• The "cradle to grave" carbon emissions per kilowatt hour are lower than most other methods of electricity generation, including some hydroelectric schemes and other renewable sources. They are typically between 1 and 2% of those from fossil fuels.
• From the point of view of health and safety, nuclear power is very much safer than either fossil fuel combustion or hydroelectric generation.
• It is a proven and mature technology with long experience.
• It is capable of providing constant electricity at the required rate over long periods, without any relation to external events such as the weather.
• Supply of nuclear fuels assured over longer periods than fossil fuels with no dependence on states with unstable economies or political entities.

References

1. ↑ 1600 MWe advanced pressurised water reactor (EPR)[1]
2. ↑ Mixed Oxide Fuel (MOX)[2]