Nuclear Hazards and Safety Issues

Recently there has been much apprehension about the dangers inherent in nuclear plants—fears of radiation hazard, waste disposal, disastrous accidents. While some of the hazards are real, nuclear scientists point out that many of them are not based on scientific facts and unbiased observation.

Radiation Hazard:

There is no doubt that radiation causes damage to living cells—but this depends on the intensity of radiation and the time of exposure. When an atom of a complex organic cell is exposed to radiation, ionization takes place and molecules disintegrate, adversely affecting the biological system, sometimes even destroying the cell.
While high doses are fatal, low doses may have cumulative effect and cause cancers, especially of the skin, and leukemia. It may affect lymphatic tissues, the nervous system, and the reproductive organs. However, die adverse effects take place after considerably high and constant doses of radiation.
The release of radioactivity into air and water from reactors does take place, but it is kept well within the limits prescribed by the AERB. The earth is being constantly bombarded by cosmic ray nuclear particles (65 per cent of natural radiation experienced by a human being is due to this).
Background radiation from terrestrial and extra-terrestrial sources is much higher than radiation from nuclear plants. In the circumstances, the radiation exposures from nuclear plants is of a negligible quantity. The fear of radiation arises because most people are unwilling to believe in any “safe level” for radiation exposure.
Hazard from Nuclear Waste:
Another aspect of nuclear hazard is waste management. The general technique of dealing with radioactive wastes is to concentrate and contain as much radioactivity as possible, and discharge to the environment only effluent of as low a concentration level as is possible.
At inland sites like Narora and Rawatbhatta, low level liquid wastes are discharged into the environment at a minimum level. At coastal sites such as Tarapur and Chennai significant dilution in the sea is possible. For solid wastes, different types of containments are used and located at sites selected on the basis of geological and geohydrological evaluation.
The fissioning of U-235 produces many radioactive isotopes, such as strontium 90, caesium 137, and barium 140. These wastes remain radioactive and dangerous for about 600 years because of the strontium and caesium isotopes. If these get into food or water supplies, they can be taken into people’s bodies where they can cause harm.
The body is unable to distinguish between radioactive strontium and calcium, for instance. The plutonium and other artificially created elements in the wastes remain radioactive for thousands of years. Even in small amounts, plutonium can cause cancer or genetic (reproductive) damage in humans.
Larger amounts can cause radiation sickness and death. Safe disposal of these wastes is one of the problems involved in nuclear power production. The wastes are carefully managed by incorporating them in inert solid matrices and placing them in canisters which are kept under cooling till the radioactivity comes to desired level. Finally, the canisters are stored in suitable geological media. However, the problem is not entirely resolved.

Effects of a Nuclear Explosion:

The effects that a nuclear explosion has on people, buildings, and the environment can vary greatly, depending on a number of factors. These factors include weather, terrain, the point of explosion in relation to the earth’s surface, and the weapon’s yield.
The weapon’s explosion would produce four basic effects:

(i) Blast Wave:

The explosion begins with the formation of a fireball, which consists of a cloud of dust and of extremely hot gases under very high pressure. A fraction of a second after the explosion, the gases begin to expand and form a blast wave, also called a shock wave.
The blast wave and wind probably would kill the majority of people within 5 kilometers of ground zero and some of the people between 5 and 10 kilometers from ground zero. Many other people within 10 kilometers of groupd zero would be injured.

(ii) Thermal radiation:

This consists of ultraviolet, visible, and infra¬red radiation given off by the fireball. The ultraviolet radiation is rapidly absorbed by particles in the air, and so it does little harm. However, the visible and infrared radiation can cause eye injuries as well as skin burns called flash burns.
Between 20 and 30 per cent of the deaths of Hiroshima and Nagasaki resulted from flash burns. Thermal radiation also can ignite such highly flammable materials as newspapers and dry leaves. The burning of these materials can lead to large fires.

(iii) Initial nuclear radiation:

This is given off within the first minute after the explosion. It consists of neutrons and gamma rays. The neutrons and some of the gamma rays are emitted from the fireball almost instantaneously. The rest of the gamma rays are given off by a huge mushroom-shaped cloud of radioactive material that is formed by the explosion. Nuclear radiation can cause the swelling and destruction of human cells and prevent normal cell replacement.
Large doses of radiation can cause death. The amount of harm a person would suffer from initial nuclear radiation depends in part on the person’s location in relation to ground zero. Initial radiation decreases rapidly in strength as it moves away from ground zero.

(iv) Residual Nuclear Radiation:

This comes later than one minute after the explosion. Residual radiation created by fission consists of gamma rays and beta particles. Residual radiation produced by fusion is made up primarily of neutrons. It strikes particles of rock, soil, water, and other materials that make up the mushroom-shaped cloud. As a result, these particles become radioactive. When the particles fall back to earth, they are known as fallout. The closer an explosion occurs to the earth’s surface, the more fallout it produces.
Early fallout consists of heavier particles that reach the ground during the first 24 hours after the explosion. These particles fall mostly downwind from ground zero. Early fallout is highly radioactive and will kill or severely damage living things.
Delayed fallout reaches the ground from 24 hours to a number of years after the explosion. It consists of tiny, often invisible, particles that may eventually fall in small amounts over large areas of the earth. Delayed fallout causes only long-term radiation damage to living things. However, this damage can be serious for certain individuals.
Safety Measures:
The chief hazards of nuclear power production result from the great quantities of radioactive material that a reactor produces. These materials give off radiation in the form of alpha, beta and gamma rays. Hence, the sites for nuclear plants are chosen with safety parameters in mind. The plants are designed for safe operation through a series of protective measures. Recognising the possibilities of human error, equipments’ malfunctioning, and extreme natural phenomena, the plants are designed on the concept of “defence-in depth”
A reactor vessel is surrounded by thick concrete blocks called a shield, which normally prevents almost all radiation from escaping.
In countries with nuclear energy, regulations limit the amount of radiation allowed from nuclear plants. Every plant has instruments that continuously measure the radioactivity in and around the plant. They automatically set off an alarm if the radioactivity rises above a prede¬termined level. If necessary, the reactor is shut down.
A plant’s routine safety measures greatly reduce the possibility of a serious accident. Nevertheless, every plant has emergency safety systems. Possible emergencies range from a break in a reactor water pipe to a leak of radiation from the reactor vessel. Any such emergency automati¬cally activates a system that instantly shuts down the reactor, a process called scramming. Scramming is usually accomplished by the rapid insertion of the control rods into the core.
A leak or break in a reactor water pipe could have serious consequences if it results in a loss of coolant. Even after a reactor has been shut down, the radioactive materials remaining in the reactor core can become so hot without sufficient coolant that the core would melt. This condition, called a meltdown, could result in the release of dangerous amounts of radiation.
In most cases, the large containment structure that houses a reactor would prevent the radioactivity from escaping into the atmosphere. However, there is a small possibility that the molten core could become hot enough to burn through the floor of the containment structure and go deep into the earth.
Nuclear engineers call this type of situation the “China Syndrome.” To prevent such an accident from occurring, all reactors are equipped with an emergency core cooling system, which automatically floods the core with water in case of a loss of coolant.
The external radiation doses received by occupational workers from all over the country are monitored on a monthly basis. Film monitoring service is provided to people working in medical, industrial and research institutions. Thermo-luminescent dosimeter monitoring service and fast neutron monitoring service are provided to the people working in reactors, fuel reprocessing plants and accelerators.
The International Commission on Radiological Protection (ICRP) has recommended for radiation workers an effective dose limit of 20 MSV per year averaged over five years with the further provision that the effective dose should not exceed 50 MSV in any one year.
The IAEA classifies events on the International Nuclear Event Scale—a scale of 0-7 depending on the severity. Events which may be termed ‘accidents’—level 4 and above on the scale—have all so far happened in the West (Chernobyl was 7 on the scale; the Narora fire was put at level 3). What is more, weapons complexes have a far greater degree of safety-related problems.