A better perception of nuclear radiation

The central theme of our discussions pertaining to nuclear power, NP, so far has been focused on the fact that mankind is living in a sea of radiation including ionizing radiation. It is also a well-established fact that the share of NP, the most perceived source of hazardous ionizing radiation, is even negligible in the USA, which has the largest fleet nuclear power reactors.

The Electromagnetic Radiation

Radiation in the environment is derived from two basic sources, namely (1) Electromagnetic Radiation and (2) Nuclear Radiation, which is described below. It is clear from Fig1 that the electromagnetic radiation is characterized by wavelength and frequency of the radiation. Note where emissions from radio, radar, cell phones and micro-wave are found in Figure 1. I

f we could extrapolate the figure further to the left, into the area of longer wavelength and lower frequency, that would be the area for low radiation emissions from power transmission lines, household electric wiring and electricity appliances such as TVs and computers.

As seen in Figure 1, when the wavelength of radiation gets shorter and its frequency gets higher, radiation begins to build energy to ionize atoms. That is why ultraviolet rays with shorter wavelengths and higher frequencies, can cause skin burns and skin cancer.

The ionization energies for X-, gamma- and cosmic rays get progressively higher. Most of the  X-rays we encounter in the environment are man-made, principally from hospitals. X-rays are produced when high voltage electrons interact with materials such as tungsten.

Gamma radiations, are one of the three kinds nuclear radiation. Gamma and X-rays have almost the same properties, but gamma-rays originate from atomic nucleus. Both X- and gamma-rays are used for several purposes in hospitals, however gamma rays from cobolt-60 (Co-60), with 2 high ionization energies of 1.172 and 1.332Mev, stand out as the most effective source for tele-therapy as well as sterilization of surgical and heat-sensitive items in hospitals.

Nuclear Radiation

The word ‘nuclear’ is from the Greek word ‘nucleus’, which means “a nut of a fruit”. It was used for the first time in 1911 with the discovery of the atomic nucleus by Ernest Rutherford, a New Zealand-born British physicist. He postulated from his experiments on firing alpha particles on a 0.00004cm thick gold foil that, an atom is a miniature solar system with mostly empty space with a very tiny dense and hard positive nucleus, made up of protons and neutrons, and surrounded negative electrons.

It is remarkable to add that, a neutrally charged neutron was so elusive that it took over two decades of meticulous research in several countries before they were discovered by James Chadwick at the Cavendish Lab, Cambridge in 1932, when Ernest Rutherford was heading the laboratory.

Rutherford coined special names or terms such as alpha, beta and gamma particles, and discovered the half-life, and the atomic nucleus. He is generally regarded as the father of nuclear physics. But Ernest Rutherford unfortunately died in 1937 before Lise Meitner, an Austrian physicist, published her discovery of nuclear energy, which will be discussed Katter.

Though the spontaneous nuclear radiation, began from the time of the Big Bang, the mankind became aware of that only after the sensational discovery of X-ray in Germany on the 8^th of November 1895 by Wilhelm Rontgen. This discovery was published on the 28^th of December 1895, and Henri Becquerel in France, buoyed with renewed expectations, and a bit of luck, discovered this spontaneous uranium radiation during the springtime of 1896.

In 1897, J. J. Thomson in Cavendish laboratory at Cambridge University in the United Kingdom (Rutherford was then working under Thomson) discovered electrons emitting from his studies on cathode r ays. The discovery of electrons, together with the 2 earlier discoveries significantly disrupted the accepted norms of science at that time.

Note also that Dalton’s atomic theory says among others that: (1) Atoms of a given element are the same, and (2) Atoms cannot be created or destroyed. For instance, Uranium isotopes, U-235 and U-238 may be chemically the same, but physically, U-238 is heavier than U-235. When U-238 and 235 atoms they are bombarded with neutrons, U-238 is transmuted into plutonium-239, Pu-239, while U-235 fissions to release a colossal amount of energy.

Marie Currie, who coined the word ‘radioactivity’ and performed pioneering work on the uranium or Becquerel radiation, which led her to discover radioactive elements, radium and polonium.

Before the beginning of the 1900s, Ernest Rutherford, found from his studies at McGill University in Montreal, Canada that, Becquerel radiation was made up of two types radioactive particles. The heavy and positively charged particles were called alpha particles, while the light and negatively charged particles, beta particles were electrons ejected from atomic nuclei.

The third type of Becquerel radiation, which was discovered in 1900 by Paul Villard, a French chemist, was in the form of electromagnetic radiation, named gamma rays.

Half-Life the Main Characteristics of Nuclear Radiation

After 1911, Becquerel radiation was replaced by nuclear radiation. It b ecame clear that nuclear radiation, spontaneous radiation, started when the earth was created. There are several primordial radionuclides (from the time of creation) with very long half-lives in our environment. It is now known that the heat in the earth’s mantle comes mainly from the decay heat of the primordial radionuclides, of which the top four are thorium-232, (Th-232), uranium-238 (U-238), potassium-40 (K-40), and uranium-235 (U-235). They account for 44, 35, 15 and 2% of the generated heat respectively.

As a matter of interest, let’s note that: (1) U-235 is the only naturally occurring fissile material, which, when bombarded by neutrons, fissions to produce a colossal amount of energy in power reactors; (2) The largest source for radioactivity in human body comes from K-40 in foods, especially bananas; and (3) The half-lives of Th-232 and U-238 are the longest half-lives, and they are14.0 and 4.5 billion years respectively. Since Th-232 is decaying more slowly than U-238, it follows that (1) its radioactivity is less than that of U-238, and (2) Th-232 is about 3-4 times more abundant than U-238.

Fresh nuclear fuel from (1) natural uranium, where the content of U-235 and U-238 are 0.7 and 99.3% respectively, or (2) say 3.5% low enriched uranium, where U-235 and U-238 account for 3.5 and 96.5% of the fuel respectively, is very mildly radioactive. A few alpha particles released are readily absorbed in the fuel itself or by its cladding material.

When a reactor is in operation, most of its numerous fission products (FPs) have very short half-lives, ranging from days to seconds, producing tremendously high radiation. As such, the functions of the containment building of a reactor are: (1) To protect the reactor from a direct hit of a missile or a falling plane, and (2) To prevent inadvertent releases of radiation into the atmosphere.

Since many of FPs are short lived as stated above, the decay heat in the spent fuel is quickly reduced in the first 1hour to about 1.5% of the operating temperature. The heat then reduces slowly to 0.4 % after a day, and 0.2% after a week. After 10 years of cooling in a pond of water, the spent fuel can be safely kept in casks and stored in dry storage in the operating sites.

Ionizing and Penetration Power of Nuclear Radiation

The ionizing power for alpha, beta and gamma are usually described as very high, intermediate and very low respectively. When it comes to penetration power, alpha particles can be stopped by a sheet of paper, beta particles by aluminium foil, while gamma radiation is stopped with very thick and dense materials like lead and concrete. Gamma radiation is the most hazardous for external radiation, while alpha particles posse very serious problems in the case internal radiation, which can be via ingestion (of foods and drinks) or inhalation.  Alpha radiation via inhalation poses more serious problems than ingestion.

It is quite clear from Table1 that human beings are naturally radioactive, and we have all the 3 kinds nuclear radiation in our bodies. It has been stated above that the ionization power is very high for alpha radiation, but very low for gamma radiation. So we should not be too anxious about the huge emission rates from K-40, which emits beta and gamma ray, while U-238 and radium-226 (Ra-226) emit very few alpha particles.

Another major source of radiation in our bodies is carbon-14 (C-14), a product of interaction between nitrogen atoms and cosmic rays in the atmosphere. C-14 emits beta particles, and it accounts for about 38% of radiation in our bodies, as compared to about 54% from K-40. Note that none of the radioactive elements ingested into our bodies originates from NP.

Table 1: RADIOACTIVE ELEMENTS IN THE HUMAN BODY

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