36 Environmental Nuclear Chemistry

Dr. Y. P. Singh

Contents

Introduction
Radioactivity

Radioactive emanations

Radionuclide

Disintegration rate, rate constant, average life

Measurement unit and permissible dosage

Sources of radionuclide Natural Artificial
Sources of radioactive waste
Disposal of radioactive waste
Radionuclide hazards
Nuclear disasters
References

Introduction

The explosion of nuclear bombs at Hiroshima and Nagasaki in Japan led to serious loss of life and property. Not only this, it had serious long term hazardous environmental consequences. Moreover recent incident of Fukushima nuclear disaster led to serious concern regarding the use of radio nuclides in power generation etc. Because of wide spread use of radionuclides in industry and medical fields, it is necessary to understand the environmental effects of radio nuclides.

Radioactivity

Henry Becquerel; a French physicist in1895, observed that the photographic plate, wrapped in a black paper and placed in drawer containing potassium uranyl sulphate was blackened. This led Becquerel to conclude that the uranium salt emitted certain high energy radiations which were capable of affecting photographic plate. He also observed that these rays were capable of ionizing air. He called these rays “radioactive rays” and the properties of spontaneous emission of rays from a substance as radioactivity. Radioactivity is the spontaneous disintegration of high energy rays from the nucleus of the radioactive element transforming thereby into different elements.

Radioactive emanation

Becquerel and P. Villard investigated the properties of the radiation emanated from radioactive material. On the basis of the penetration power of these radiations, the emanations were found to be of three types. These were named as alpha(α) and beta(β) particles and gamma(γ) rays. Characteristics of these radiations are as follows:

[1] Alpha rays

α -particles consist of positively charged particles, 42He2+,as indicated by their

deflection in electrical and magnetic field. In fact α-particles are formed from helium atom by the loss of two electrons. The velocity of α-particles when emanated from radioactive nuclides ranges between 1.5 to 2.0 × 109 cm/s. One such example of α-emanation is the disintegration of 226Ra56.

Due to high mass and velocity and hence momentum they can pass through thin metal foil and can also ionize the air. This is the reason of their hazardous nature.

[2] Beta rays

β -particles are electrons ejected from the radio nuclides, having the velocity light much higher than α-particles.

The following nuclear radiation represents emission of β-particles.

However due to lower mass, the ionizing power of β-particles is less than those of α-particles. The penetration power of β-particles due to their light weight and high velocity is more than α-particles

[3] Gamma rays-

γ -rays are actually electromagnetic radiation of high energy. The emission of α or β-particles from radioactive nuclides usually, but not always, is accompanied by the emission of γ-rays. Although the emission α- or β-particles is accompanied by the energy, the transformed nucleus may be still in a excited state. It may decay to its ground state by emitting γ-rays. With the exception to cosmic rays, these rays have shortest wave length. Due to the neutral nature, these rays cannot be defected even by the strongest electric or magnetic field. These rays are harmful to living tissues.

It is noteworthy that all the three type of rays are not emitted simultaneously from any radionuclide.

Radionuclide

Radioactive nuclides are intrinsically unstable, and hence undergo spontaneous change forming new nuclides by one or the other way of rearranging or losing some of their protons and neutrons. About 700-800 nuclides including a very large number of manmade ones are known to be radioactive of one type or the other.

Disintegration rate, rate constant and half life period –

Radioactive disintegration follows first order equation. Half life period, t1/2, is defined as the time needed for a given number of atoms of radionuclide to disintegration exactly to half its initial value. It is related disintegration constant, λ, by the equation:

It may be important to note here that the half life period vary over extremely wide limits for different radionuclides for example from 10-6 s for 214At to 106 y (≈ 1017 s) for 232Th.

Average life or mean life

It is convenient to use the average life or mean life of radionuclides. The average life is defined as the sum of the life times of all the individual atoms of a radionuclide divided by the total number of atoms present originally.

It has been shown the average or mean life, τ, to be dined by the equation:

Measurement unit

Radioactivity is measured in unit of Becquerel (Bq). By definition: 1 Bq = 1 decay or disintegration per second.

Curie (Ci) was the unit used earlier. It is related to Bq by the relation:

Dosage

The radiation dose, i. e., the amount of radiation exposure, is defined by the amount of energy deposited in a unit mass in human tissue or other media. Originally, rad [100 erg/g] was the unit used. Currently, the SI unit, the gray (Gy) [1 J/kg], where 1 gray = 100 rad is in use.

The biological dose, also known as dose equivalent, H, is expressed in units of rem or, in the SI system, sievert (Sv). The biological damage caused by a particle depends not only on the total energy deposited but also on the rate of energy loss per unit distance traversed by the particle. Alpha particles are much more damaging per unit energy deposited than β-particles. A quality factor, Q, is used to express this. Q is taken to be 1.0 for electrons and for x-rays and gamma rays, 20 for alpha particles, and from 5 to 20 for neutrons depending on neutron energy.

The dose equivalent, H, is the product of the absorbed dose D and the quality factor Q, i. e.

H = Q × D.

The unit for the H is the rem if the absorbed dose is in rad and the sievert (Sv) if the absorbed dose is in grays. Thus, 1 Sv = 100 rem. As discussed below, 1 rem is roughly the average dose received in 3 years of exposure to natural radiation. 1 Sv is at the bottom of the range of doses that, if received over a short period of time, are likely to cause noticeable symptoms of radiation sickness.

Natural Radionuclides

In view of the long life of the earth, the most unstable isotopes have since disappeared from the earth. So only those decaying at an extremely low rate are present in significant amount. Among the notable radionuclides, the important ones are: 232Th (t1/2 = 1.4 x 1010y), 235U (t1/2 = 7.0 x 108y), 238U (t1/2 = 4.5 x 109y), 40K (t1/2 = 1.5 x 109y) and 37Rb (t1/2 = 4.9 x 1010). Out of these five radionuclides first three 232Th, 235U and 238U are α emitter where as 40K and 37Rb are β emitter, although the abundance of 40K is low (0.001 %) but because it is an important constituent of biological tissues, 40K provides significant fraction of the background radiation to which we are normally subjected.

Nature is a source of radioactive materials. Small amounts are found in n mineral springs, sand mound and volcanic eruptions. Radon (222), Ra226, Ra(228) and U(234) are found in groundwater.

Radon

Certain mineral springs are reported to emit fairly large amounts of Rn (222). Due to its noble nature it does not form chemical bond and are free to escape from their production sites i.e. Uranium containing rocks. 223Rn travel an appreciable distance (t1/2 = 382 days) before decay. It can enter nearby building by seeping through foundations and can accumulate to significant levels in air. It can also infiltrate well’s and enter water supply. The natural flow of 222Rn is the highest of the average atmospheric flows of all radionuclides from all sources and its daughter contribute the generated part of natural radiation dose received by the population of world.

Radium

Owing to its relatively stable long life, isotopes e.g., the concentration of 226Ra (t1/2 = 1600 y) is significant in uranium deposits. Ra(226) and Ra(228) are trace natural ground water contaminants.

Uranium

Uranium is naturally occurring contaminant found in both groundwater and surface water.

Anthropogenic Sources of Radionuclide

There are three major anthropogenic sources of radionuclide into atmosphere, viz., (i) production and testing of nuclear weapons (ii) Nuclear reactions and (iii) coal- fuel cycle.

Production and tasting of radionuclides- 3H, 137Cs, 239Pu, 235U, 238U and other radionuclide

formed during the fission of U235 and U238 are important radionuclide released into atmosphere during the production and testing of nuclear weapons like atom bomb and hydrogen bomb with an ever increasing demand of natural U238 and U235 for war heads (nearly 5040 tons of uranium) between the

year 1972-81 when cold war between USA and USSR was on peak. During the mining, milling

conversion and enrichment processes, it has been estimated by United Nations Scientific Committee on Effect of Atomic Radiation (UNSCEAR) that a large amount of radiation due to U238, Rn222, Ra226, U235 and Pb210 has been introduced into atmosphere.

Nuclear power

Fission reactors

Currently, the most important application of nuclear reactions is on the large scale production of power. India has several nuclear power stations in operation based on fission reactions. The latest to join the list is Kudankulam Nuclear Power Plant in Tamil Nadu. Nuclear power (fission) reactions are based on the fission of U235 by thermal neutrons. An example of fission pathway is:

The energy obtained from these nuclear reactions is used to heat water in the reactor and produces steam to derive turbine.

Nuclear reactors operate at about 625 K compared to 800 K in fossil fuel plants. The thermal efficiency for the production of electricity does not exceed 30%. Thus 70% of the fission energy is released into environment.

Besides the low efficiency of the fission reaction the major constrains in the wide spread use of nuclear fission power is the yield of large quantity of radioactive fission waste products, the latter remain lethal for thousands of years. No fool proof disposal method has yet been devised. Human error and/or negligence are responsible for the release of radioactive waste into atmosphere.

Fusion Reactor

Energy released in the fusion reactors is based on the fusion of isotopes of hydrogen, viz., 2H1 and 3H1.as shown below-

Fusion’s greater appeal compared to nuclear fission rests mainly in greater safety. The only radioactive material that could be released in an accident is tritium. However the technological problems for harnessing fusion energy will take several years to solve.

Sources of radioactive waste

There are two types of nuclear wastes, low level and high level radioactive wastes.

The sources of low level wastes are glove boxes, shielding materials air filters and laboratory equipments, components of decommissioned reactors.etc. The high level radioactive waste is generated by spent fuel, liquid effluents generated by reprocessing of spent fuel. These wastes include uranium, plutonium and other highly radioactive elements generated during nuclear reactions.

Disposal of radioactive waste

Low level waste is generated during the operation of atomic particle accelerator for medical, research or industrial applications. The radioactivity of waste is in general has low life, i. e., less than one year. It is stored safely until it is no longer radioactive.

High level waste generated by nuclear reactors as spent fuel needs special treatment for disposal.

Environmental Monitoring

For the protection of general population from release of radioactive radiation, protection of individuals during the operation and inadvertent intrusion of radiation, the monitoring of environmental radioactivity is essential. For this, the sampling of flora and fauna, seawater, groundwater, river water, sediments and soil is done and then analyzed for radioactivity.

Hazards of Radioactive radiation

The radioactive emanations due to their high energy are ionizing. When these radiations strike a living organism’s cell, it is likely to injure them. If the number of cells is significant it may eventually develop cancer. And extremely high dose may even cause death.

Radon is known to cause lung cancer. Its source can be drinking water and is found indoors air. Radium can cause bone, stomach, lungs and other cancers also. Its source is natural groundwater contaminants that occur usually in trace amounts.

Uranium is found in both groundwater and surface water. It is believed to cause bone and other cancers in humans. It is toxic to kidneys also.

In general all α, β and γ-emissions are responsible for various types of cancers and other ailments.

Nuclear Disasters

Under normal operating conditions, the radiation released by nuclear reactor is very low. The concentration of fissionable material in reactor is too dilute to become explosive itself. But a great deal of heat is generated by an operating fuel rod even after control rods have been lowered to stop the chain reaction (which happens automatically in case of an accident). The heat is carried away by water circulating through the reactor. However if the water leaks out or boils away and is not replenished, then the temperature can rise to disastrous levels and allow highly radioactive material to be released. There are emergency backup systems to replenish the water, but these systems can fail through equipment malfunction or human error.

Three Mile Island nuclear Accident

The Three Mile Island power station is near Harrisburg, Pennsylvania in USA. It had two pressurized water reactors. Unit one was of 800 MW and had entered service in 1974 and remains one of the best-performing units in USA. Unit 2 was brand new unit of 906 MW.

In 1979 at Three Mile Island nuclear power plant an accident occurred. At that time, the reactor was operating at 97% power. There was a minor malfunction in the secondary cooling circuit which caused the temperature in the primary coolant to rise consequently the reactor was shut down automatically for about one second. At this point a relief valve failed to close, but instrument failed to indicate the fault of the valve and the coolant continue to drain .So much of the primary coolant drained away and the residual decay heat in the reactor core was not removed. The core suffered severe damage as a result of overheating. A large bubble of hydrogen was formed due to the reaction of water and zirconium cladding. The cooling water could not reach the fuel rod due to this hydrogen bubble as a result the fuel rod partially melted. Fortunately the hydrogen did not explode because sufficient oxygen was not present and major accident was averted. This hydrogen gas “bubble” was removed by periodically opening the vent valve on the reactor cooling system press riser. Radioactive gases from the reactor cooling system built up in the makeup tank in the auxiliary building. During March 29 and 30, operators used a system of pipes and compressors to move the gas to waste gas decay tanks. The compressors leaked, and some radioactive gas was released to the environment. However most of the waste gas went through high-efficiency particulate air filters and charcoal filters which removed most of the radio nuclides, except for the noble gases. With short half-life and being biologically inert, these did not pose a health hazard.

The Chernobyl disaster

The Chernobyl disaster was a catastrophic nuclear accident that occurred on 26 April 1986 at the Chernobyl Nuclear Power Plant in Ukraine. An explosion and fire released a large quantities of radioactive particles into the atmosphere, which spread over much of the western USSR and Europe.

The Chernobyl disaster was the worst nuclear power plant accident in history in terms of cost and casualties. It is classified as a level 7 event (the maximum classification) on the International Nuclear Event Scale. The battle to contain the contamination and avert a greater catastrophe ultimately involved over five lakh workers and cost an estimated 18 billion rubles. During the accident itself, 31 people died, and long-term effects such as cancers are still being investigated.

The disaster in number four unit of the Chernobyl plant, which is near the city of Pripyat began during a systems test on Saturday, 26 April 1986 at reactor. There was a sudden and unexpected power surge, and when an emergency shutdown was attempted, an exponentially larger spike in power output occurred, which led to a reactor vessel rupture and a series of steam explosions. These events exposed the graphite moderator of the reactor to air, causing it to ignite. The resulting fire sent a plume of highly radioactive fallout into the atmosphere and over an extensive geographical area, including Pripyat. The plume drifted over large parts of the western Soviet Union and Europe. The amount of radioactive material released was 400 times more than the amount of atomic bombing of Hiroshima released. The fall out could be detected in almost all part of Europe From 1986 to 2000, 350,400 people were evacuated and resettled from the most severely contaminated areas of Belarus, Russia, and Ukraine.

Large quantities of radioactive fuel, core materials and was released into the atmosphere and ignited the combustible graphite moderator. The burning graphite moderator increased the emission of radioactive particles which was carried by the smoke into the atmosphere, as the reactor had not been encased by any kind of hard containment vessel.

The radiation levels in the worst-hit areas of the reactor building have been estimated to be 5.6 roentgens per second (R/s), equivalent to more than 20,000 roentgens per hour. In some areas, unprotected workers received fatal doses in less than a minute.

The Fukushima Daiichi nuclear disaster

The Fukushima nuclear disaster was the largest nuclear incident since the Chernobyl disaster in April 1986 and the only (after Chernobyl) to measure Level 7 on the International Nuclear Event Scale.

The plant was located in an active seismic zone in Japan; The International Atomic Energy Agency (IAEA) had expressed concern about the ability of Japan’s nuclear plants to withstand seismic activity. At a 2008 meeting of the G8’s Nuclear Safety and Security Group in Tokyo, an IAEA expert had warned that a strong earthquake with a magnitude above 7.0 could pose a “serious problem” for Japan’s nuclear power stations. Earlier too the region had experienced three earthquakes of magnitude greater than 8.

The Fukushima nuclear disaster at the Fukushima I Nuclear Power Plant which began on 11 March 2011 resulted in a nuclear meltdown of three of the plant’s six nuclear reactors.

At the time of the earthquake11 March 2011, Reactor 4 had been de-fueled and Reactors 5 and 6 were in cold shutdown for planned maintenance.

Immediately after the earthquake, following government regulations, the remaining reactors, 1–3, were automatically shut down by inserting the control rods to shut down the sustained fission reactions. although fission stopped almost immediately. The fission products in the fuel continue to release decay heat. . Coincident with the shut down emergency generators were automatically activated to power electronics and cooling systems. The tsunami arrived some 50 minutes after the initial earthquake. The 14 meter high tsunami overwhelmed the plant’s 10meter high seawall with the the tsunami water quickly flooded the low-lying rooms in which the emergency generators were housed. The diesel generators were flooded and began to fail soon after, their job being taken over by emergency battery-powered systems. When the batteries ran out the next day on 12 March, active cooling systems stopped, and the reactors began to heat up Overheating caused a reaction between the water and zircoaloy creating hydrogen gas. On march 12 a large amount of hydrogen gas was produced which was vented out of the reactor pressure vessel and mixed with the ambient air. The gas eventually reached explosive concentration limits in Units 1 and 3. Either through piping connections between Units 3 and 4 or from the zirconium reaction in Unit 4 itself unit 4 was also filled with hydrogen. Explosions occurred in the upper secondary containment building in all three reactors.

There were several spills of contaminated water at the plant, some into the sea. Plant workers tried to contain the leaks using measures such as building chemical underground walls, but they were ultimately unsuccessful at improving the situation.

Although no fatalities due to short-term radiation exposure were reported, some 300,000 people evacuated the area. The World Health Organization indicated that evacuees were exposed to so little radiation that radiation-induced health impacts are likely to be below detectable levels, and that any additional cancer risk from radiation was extremely small, for the most part—and chiefly limited to those living closest to the nuclear power plant.

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