With an opening statement, “Nuclear, the dreaded “N” word, the BOMB!” guest speaker at last week’s Forum for Astronomy, Science and Technology meeting, retired chemist Allen Lubbe immediately grabbed the attention of his audience.
He also unequivocally said that a nuclear power station cannot explode while reserving the Chernobyl and Fukushima issues for later.
According to Lubbe most of the electricity generated today comes from fossil fuel power stations burning coal or oil. Those which burn waste gas from oil wells or so-called renewable bio-fuels like bagasse from sugar production or methane gas from rubbish tips are thinly scattered.
All, however, had the same drawback; they emit carbon dioxide as a waste product which is thought to give rise to global warming. What is required is a means of generating electricity as required by modern living without harmful emissions. Hydroelectric schemes do but good dam sites are far and few between.
Nuclear power plants do not produce any gaseous products at all and only a relatively small amount of production waste. In fact, a 1000 Mw/h station only produces some six cubic metres of waste per annum about the mass of two motor cars. In comparison, a similar size coal-fired unit produces 900 000 tons of ash and dumps many tons of sulphur and nitrous oxides into the atmosphere resulting in acid rain. It also dumps about 5.2 tons of radioactive products, mainly uranium, over the surrounding countryside every year.
The New Scientist recently published a 50-year safety record comparison of the different means of power generation as reported by the lnternational Electrical Association.
Their findings were that coal-fired stations caused 32.7 deaths, hydroelectric stations 230 000 deaths. (Mainly burst dams in China in 1975), Natural gas 1.6 deaths, and nuclear 0.2 deaths,
Fatalities at Chernobyl were not considered as it was not a power station as such but an experimental reactor; the spare heat being used to generate electricity. It is anticipated that some 9000 people will eventually die as a result of that accident.
But many people are concerned and say nuclear power generating methods are dangerous and present an unacceptably high risk. They argue the many forms of renewable energy should be used instead. These sources all suffer from the same two shortcomings; lack of capacity to deal with demand and not reliable as it depends on favourable conditions.
A nuclear power plant works very similarly to a fossil fuel plant. The only difference is the fuel used. It is essentially a boiler that produces high-pressure steam which is used to turn a generator in a conventional manner.
The fuel in a commercial reactor is always uranium which is a metal which is often machined into rods about the size and shape of an ordinary torch and these are inserted into the pile. The “raw” form of uranium oxide straight from the mine (yellow cake) can also be used in which case the rods are about 1cm in diameter and about a metre and a half long. The whole caboodle is inside a containment vessel which is made from stainless steel and reinforced concrete. These vessels are strong enough to withstand the impact of a fully loaded Jumbo jet or an earthquake. Steam generated by the hot reaction is used to turn a steam turbine in exactly the same way as a fossil fuel power station. And that’s all there is to it.
Until recently the latest development in reactors was that of the pebble bed reactor. Unfortunately, the government has pulled the plug on it and the research team has been disbanded. However, it is quite the cleverest idea. It totally eliminates the possibility of thermal runaway and hence a meltdown.
In this design, the uranium is encapsulated in the centre of a tennis ball sized sphere of pyrolytic graphite which in turn is surrounded by silicon carbide. This structure is a very stable and inert structure. Because the ball is solid any piece of uranium cannot approach closer to another piece than the diameter of the ball. This limits the amount of reaction possible.
Furthermore, if the temperature in the reactor rises, the neutrons released by the fuel will be self-absorbed by the uranium. This is called Doppler Broadening and has the effect of slowing the reaction down. So the temperature will rise to a predetermined point and stay there. Even if there was a total breakdown of all station functions including cooling, core damping, ball circulation or any or all of the station controls. The reactor will settle to a stable state and remain there indefinitely. In other words, it gets the sulks. This is more than even the most well behaved conventional power station is capable of.
Objections to nuclear plants are based on a number of misconceptions as well as some well-founded concerns. Firstly, people associate the word nuclear with the bomb but a nuclear power station cannot explode. Making a bomb is also not an easy task and if it were easy, every tin-pot dictatorship in the world would have had a bomb years ago.
Another fear is that the power station could be used to manufacture other radioactive materials and this does happen if the reactor is configured as a fast breeder, i.e. the waste from the reactor is plutonium and not depleted uranium. The energy potential of the plutonium is higher than the uranium required to make it, so more fuel has been made that was used. In addition, plutonium is the material used to make the hydrogen bomb. However, the uranium used in this type of reactor has to be highly enriched, far higher than the power generating set-up which uses uranium not far removed from the raw “yellowcake” as it comes out of the mines. Again the technology required to enrich uranium to this degree is very advanced.
Thermal runaway, however, will result in a meltdown in which the core becomes so hot that everything turns to molten metal. This has happened.
There are two known famous or infamous nuclear accidents; the first is Three Mile Island and the second is Chernobyl. Now we have a third, Fukushima in Japan.
Chernobyl wasn’t only a power station but an experimental reactor used by the Russians for research purposes, some of it probably military, and it was designed in a way that would never be permitted in the west. It was not even inside a containment building, a requirement of even the crudest western reactor. It was also old and of superseded technology.
Three Mile Island, on the other hand, was a well-designed modern reactor with all the safety and operational systems known at the time. The station entered the initial stages of a meltdown due to errors on the part of the operational staff. However, instead of a major catastrophe, all the shutdown procedures operated as designed and the core was damped and went into stasis mode.
A small amount of radioactive gas was released to the atmosphere via the stack but that was the only uncontrolled emission. Not one single person on the station was subject to excessive radiation and no one was hurt.
Gaseous emissions have also not resulted in a single injury to any person on the plant or in the surrounding area. Monitoring is still going on to this day, and there have been no radioactive illnesses recorded. Instead of looking on Three Mile Island as a tragedy it should be looked upon as a triumph of good design and excellent construction.
Fukushima is also a well-designed and well-operated unit. It is however 40 years old and suffered the shortcomings of its time. It was to be decommissioned this year but had been given a new lease of life by the Japanese authorities due to a shortage of power in Japan.
The Japan was hit by was the biggest earthquake ever recorded at 9.0 Mw (Moment Magnitude Scale has replaced the Richter scale) and the station performed exactly to specification in that it shut down when the earthquake struck just as it was designed to do and everything would have been fine except for one thing – the designers had not anticipated, a tsunami of the magnitude that struck them. The plant had been designed for a wave height of 6 metres and the one that hit them was an unprecedented 14 metres high (that’s 46ft in old terms).
The tsunami wrecked the power infrastructure of the area to the extent that all electrical power to the power station was cut off. The reactors were water cooled and required electricity for the cooling pumps but the tsunami wiped out the power and backup generators. With no cooling, the reactor core heated up to the extent that the water boiled off exposing the core resulting in even more overheating. With the heat of this magnitude, the steam around the core disassociated into hydrogen and oxygen. The extreme temperature caused the reactor vessel to crack and superheated steam and hydrogen was released into the containment building where it exploded. So the damage was caused by a chemical explosion and not a nuclear one.
ln the final analysis, the major objection to nuclear power is the waste matter left over after as much energy as possible has been extracted from the fuel. Radioactive material cannot be deactivated by any means presently known although there have been some interesting experiments done recently using lasers. The radioactivity can only be allowed to run down in its own good time (known as the half-life) and that time can easily be a thousand years.
There are techniques to reprocess spent uranium in which the radioactive portion can be reconcentrated and used as a fuel again but there are only three plants in the world that offer this service, and they are in France, Russia and America respectively. It is a very expensive process and it is considerably cheaper to use the freshly mined material.
As for now the only practical way of dealing with the waste is to store it for however long is required in some safe depository. In fact, it is not that difficult. An area 100km square could be allocated in some remote area such as the Sahara, Atacama or Gobi deserts and the material kept there in the centre ten kilometres until it is safe. It has been calculated that the waste from all the present and projected power stations for the foreseeable future could be stored in an area of this size. Under the Canadian Cambrian shield is another place. There is no shortage of suitable sites.
But what if an unfriendly power raided these sites and helped them to what is there? As mentioned before, it is easier and more economical to start from scratch and refine newly mined uranium.
There has recently been a suggestion to use thorium instead of uranium as a fuel source as it is more plentiful than uranium and only slightly radioactive. In use. it would be dissolved in liquid Lithium Fluoride, a salt, at several hundred degrees Celsius and circulated through the reactor vessel as a molten liquid and the heat removed during the recirculation process to generate electrical power. In the event of an accident the circulation would stop, the salts solidify and the reactor would become inert.
All the plants discussed so far are fission reactors where an atom is broken down into smaller atoms; uranium ends up as lead, for example. Another form of nuclear reaction exists and that is fusion. This is the process that the sun uses to provide its heat. Here hydrogen is combined with itself to create energy. Such a power station is highly desirable as the fuel used, hydrogen is the most plentiful element in the universe. There is literally an inexhaustible supply. The fusion product is helium a valuable and saleable material. The technical problems containing the tremendous energy developed have so far not been solved. Despite the enormous benefits of such a power source there seems to be a strange reluctance of governments to supply money for research. So, nuclear fusion is still some 20 or 30 years away.
There is really no reason why most of our electricity requirements cannot be met by nuclear generation and certainly, all baseload power should be generated in this way. The advantages so outweigh the disadvantages that there is really no argument. In time the fission reactors will be superseded by better methods such as possibly fusion technology or perhaps something unknown at the moment. That is normal. But in the meantime, we are in real danger of damaging our atmosphere and are rapidly depleting our hydrocarbon stocks in the most wasteful way imaginable.
Radiation exposure is measured in Sieverts; it is specifically calculated for a human.
Symptoms of radiation exposure for 24hrs in milli-sieverts (mSv) (not instantaneous exposure)
1 -250 None, 250 – 1000 some marrow, lymph node, spleen damage, 1000 – 3000 more severe damage but recovery probable, 3000 – 6000 Death if untreated, 6000 – 10 000 Death expected, and Above 10 000 Death certain
Some single dose examples
Chest CT scans 6 – 18mSv and Gastrointestinal X-ray 14mSv
International max recommendation exposure during an emergency – 500mSv
International max recommendation exposure during a rescue – 1000mSv
City of Fukushima levels at height of emergency – 1.6pSv/h (14mSviy)
Finland after Chernobyl – SpSv/h
Yearly dose examples
Average background radiation 2mSv/y, Living near a nuclear power station 0.0001 – 0.01mSv/y, Sleeping next to a human for 8hrs/d 0.02mSv/y and smoking 30 cigarettes per day 60 – 160mSv/y
Current max for nuclear workers is 50mSv over and above background and medical exposure.
Deaths per billion Kw/h globally
Coal 100 000, Oil 36 000, Bio-fuel 24 000, Hydro 1 400 (dam failures), Solar 440, Wind 150 and Nuclear 90