Radiation Protection Program: Atomic Structure and Radiation Theory, Slides of Physics

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Jan Burnham spent most of his career with the New Brunswick Power Corporation. Among other things, he was responsible for designing and implementing the radiation protection program for Point Lepreau Generating Station in New Brunswick, Canada.

Since he retired from NB Power in 1997, he has worked as a consultant to the nuclear industry in Canada, primarily in the areas of technical and management review of health physics activities.

Jan is an avid sailor, and enjoys bridge, photography, and making and drinking wine.

CONTENTS

CHAPTER 1 – ATOMIC STRUCTURE 1

Atomic Structure 1 The Atom; Protons, Neutrons, Electrons; Atomic Number; Elements; Atomic Mass; Atomic Size Isotopes 8 Separation of Isotopes; Isotopes of Hydrogen and Uranium; Nuclides

CHAPTER 2 – RADIATION THEORY 17

Radioactivity 17 Stable and Radioactive Nuclides; Alpha, Beta, Gamma, and Neutron Radiation; Energy Of Radiation Ionisation^21 Ion Pairs; Ionisation By Alpha and Beta Particles; Ionisation by Gamma Photons (Photo-Electric Effect, Compton Scattering, Pair Production); Ionisation by Neutrons Penetrating Ability of Radiation 27 Comparison of Alpha, Beta, Gamma, and Neutron Range; Alpha and Beta Energies; Bremsstrahlung; Beta Range in Various Materials; Gamma Half-Value Layer; Neutron Range Decay Schemes 32 Alpha, Beta, Positron, and Gamma Decays; Excited and Ground States; Decay Series Rate of Decay 38 Activity; Half-Life; Decay Curve; Becquerel Neutrons 45 Fast, Slow, Thermal Neutrons; Neutron Sources; Neutron Reactions; Fission, Elastic And Inelastic Scattering; Neutron Activation (n,p), (n,α), (n,γ); Radiative Capture, Neutron Reactions in Tissue Radiation Sources in a Nuclear Plant 53 Activation and Contamination; Radiation from the Core; Fission Products; Fuel Failure; Activation Products In Moderator and Heat Transport Systems

CHAPTER 3 – RADIATION DOSE 72

Ionisation and Radiation Dose 72 Effect of Ionisation on Tissue; Penetration of Radiation in the Body, Absorbed Radiation Dose; Gray; Quality Factor; Equivalent Dose; Sievert Natural Background Radiation 75 Cosmic Rays; Radioactivity in the Earth’s Crust; Radon Daughters; External Gamma Radiation; Natural Radioactivity in the Body

Practical Considerations 142 Mechanical Design; Energy Dependence; Special Operating Techniques; Electronic Design; Scalers and Ratemeters, Analogue and Digital Displays, Response Time, Efficiency; Dead-Time; Instrument Checks, Calibration Scintillation Counters 152 Functions of Phosphor, Phototube, Amplifier, Discriminator, Alpha Scintillation Counter; Wide Range Gamma Survey Meter, Vehicle Monitor; Gamma Spec- trometry; Whole-Body Counter; Liquid Scintillation Counters (for Bioassay, Portal Monitor) External Dosimetry 162 Thermoluminescence; Description of our TLD System; Neutron Response; TLD Accuracy; Operational TLD Procedures; Personal Alarming Dosimeters (PADs), Description of our PAD System; Operational PAD Procedures; Difference in TLD and PAD Results

CHAPTER 7 – PROTECTION FROM EXTERNAL EXPOSURES 177

Minimise the Hazards 177 Safety Precedence Sequence; Minimise the Source; Reduce Exposure Time; Increase Distance; Inverse Square Law; Point, Line, and Plane Sources; Alpha, Beta, Gamma, and Neutron Shielding; Permanent Shielding Install Physical Barriers 194 Access Control System Install Warning Devices 196 Alarming Area Gamma Monitoring System (AAGM); Portable Alarming Gamma Monitor Establish Procedures 200 Minimise Human Error Potential; Routine and Job Radiation Surveys (Gamma, Beta, Neutron); Signposting Train, Motivate, and Supervise Personnel 206 Recognising Radiation Hazards; Hazards in PHT System; Moderator System, Liquid Zone Control System; Fuel Handling System; Assessing Magnitude of Radiation Hazards; Typical Gamma and Neutron Fields; Anticipating Hazard Changes

CHAPTER 8 – INTERNAL DOSE 277

Behaviour of Internal Sources 227 Problems with Internal Exposure; Entry and Distribution of Radionuclides in the Body; Target Tissue; Biological and Effective Half-Lives; Physical Form of Internal Contaminants Annual Limit on Intake 230 Intake and Uptake; Reference Man; ALI; DAC; Internal Exposure and Committed Dose

Tritium 236 Production; Physical Form; Entry and Behaviour in the Body; ALI and DAC; Exposure and Committed Dose; Bioassay; Dose Calculations; Infinity Dose; Bioassay Update Reports; Systems Containing Tritium Radioiodines 247 Important Isotopes; Sources; Behaviour in the Body; Dose Commitment; Thyroid Blocking Particulates 251 Short-Lived and Long-Lived; Transportable and Non-Transportable; DAC for Unidentified Particulates; Sources and Locations of Particulates; Carbon- Bioassay 257 Excretion Analysis, Whole-Body Counting

CHAPTER 9 – CONTAMINATION CONTROL 269

Surface Contamination 265 Sources; Loose and Fixed Contamination; Contamination Control Limits (CCL); Direct and Indirect Measurements; Contamination Surveys Airborne Contamination 278 Particulates; Continuous Air Monitor (CAM); Spot Sampler; Nobles Gases; Radioiodine; Tritium; Alarming Area Tritium Monitor (AATM); Scintrex Tritium Monitor; Tritium Bubbler Contamination Control by Design 290 Zoning; Release Permits; Rubber Areas; Rubber Change Areas; Change Rooms; Ventilation Systems; Local Ventilation Protective Clothing 303 Browns; Disposable Coveralls; Gloves; Footwear; Plastic Suits; Protection Factor Respirators 310 Air-Purifying, Tritium, Air-Supplied Respirators; Self-Contained Breathing Apparatus (SCBA); Suggested Protection Factors; Respiratory Hazards; Fit Testing; Maintenance Decontamination 318 Chemical and Physical Methods; General Principles; Decontamination Centre; Decontamination Of Equipment, Work Areas, Heavy Water Spills, Clothing, Skin

CHAPTER 10 – DOSE RECORDS 333

Dosimetry Information Form (DIF), External Dose; Internal Dose; Monitoring Period Dose Report; Bioassay Update Report; Dose Check Point; Radiation Work; Occupational Dose Report; Work Group and Station Dose

CHAPTER 1

ATOMIC STRUCTURE

INTRODUCTION

In the operation of a nuclear power station you will deal with "atomic radiation" every day. To understand what radiation is, you must first learn a bit about the atom.

Why does each substance possess its own characteristic properties? Why are salt and iron solids under ordinary conditions? Why are oxygen and hydrogen gases? Why does sulphur melt at a much lower temperature than salt? Why do metals conduct electricity whereas non-metals generally do not?

These and countless other questions remained unanswerable until theories were developed about the structure of matter.

BUILDING BLOCKS OF MATTER

Greek philosophers used to speculate as to whether matter was continuous or discontinuous. (I guess they had their slack days like everyone else.) By continuous they meant that, if it were possible, a piece of iron could be divided into two, the two parts divided into two again and this process carried on forever, the two divided parts always being iron. By discontinuous they meant that somewhere in this dividing process you would reach a point when the two halves were not iron.

Thanks to the efforts of many scientists who followed the Greeks, we now know that matter is discontinuous. In the case of iron, there comes a time when on dividing what is undoubtedly a piece of iron, the parts are no longer iron. This last piece of matter with iron characteristics is known as an atom of iron. The same applies to any other primary material; the last tiny piece of it is called an atom of that material.

Certain facts have been discovered about these atoms. Here are three of them:

1. Atoms are composed of three basic particles. These particles are called subatomic and are

a) protons b) neutrons c) electrons These subatomic particles are the same in all atoms.

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2. All atoms of one kind of material are similar.
3. Atoms of different materials differ only in the number and arrangement of their subatomic particles.

THE NUCLEUS AND ORBITAL ELECTRONS

The simplest kind of atom is the hydrogen atom. It is so simple that it has only two of the three subatomic particles. The hydrogen atom consists of a proton in the centre and one electron circling around it as shown in Fig. 1.1. The second simplest atom is that of helium. It has two protons and two neutrons in the centre of each atom with two electrons circling around it (Fig. 1.2).

electron

proton

Fig. 1.1. Hydrogen Atom The neutrons and protons in an atom are so closely packed together that they form a single cluster called the nucleus of the atom. The helium nucleus contains two protons and two neutrons. When the two electrons are included, the whole thing is the helium atom.

In fact, we may say that an atom consists of a nucleus and the associated electrons that are called orbital electrons. The reason for this name is that the electrons can be pictured as little particles whirling around the nucleus much as the planets orbit about the sun.

electron

proton

neutron

Fig. 1.2. Helium Atom

Why do the electrons stay in their orbits rather than fly off into space? The answer to this is electrical attraction. First, we'll review briefly the nature of electrical force

ELECTRICAL BALANCE IN THE ATOM

There are two kinds of electric charge, positive (+) and negative (-). When two objects are both charged with a positive charge, they repel one another. Similarly, two negatively charged bodies repel one another. But when one object is positively charged and another is negatively charged, the two objects will attract one another. All this can be said very briefly in the statement: "like charges repel, unlike charges attract".

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The number of protons in the nucleus, which makes atoms different from one another in their structure and character, is such an important number that it is given a special name, the atomic number.

The ATOMIC NUMBER of any atom is the number of protons in its nucleus. The letter Z represents this number.

Therefore, we say that the atomic number of hydrogen is one (Z = 1), the atomic number of helium is two (Z = 2), and the atomic number of uranium is 92 (Z = 92). So the atomic number tells us how many positive charges there are in the nucleus, and of course, it also tells us how many orbital electrons the atom should have. Neutrons are uncharged particles; they don’t affect the electrical state of the atom. But they do add mass to the nucleus and we’ll discuss them later.

THE ELEMENTS

If a large number of atoms, all with the same number of protons, are assembled together in one place, we call the substance formed by these atoms an element.

An ELEMENT is a form of matter whose atoms all have the same atomic number.

We’ve already met the elements hydrogen and helium. Hydrogen has the simplest structure with only one proton in the nucleus. Then comes helium with two, lithium with three, and so on.

Over 100 different elements have been identified. Ninety of these occur naturally, and the remaining elements are man-made in nuclear reactors or particle accelerators.

The table on the next page lists all of the elements. Apart from technetium (Z = 43)

electron proton neutron

Fig. 1.5. Lithium Atom

and promethium (Z = 61), all the elements from hydrogen to uranium (Z = 92) occur in nature. You’ll recognise many of the names of the elements, but others are quite uncommon. Fortunately, we don't have to know many of these names – those with which you will become familiar during this course are shown in italics in Table 1.1.

Atomic Radiation 5

TABLE 1.1. LIST OF THE ELEMENTS

Z SYMBOL NAME Z SYMBOL NAME Z SYMBOL NAME

1 H hydrogen 36 Kr krypton 71 Lu lutetium 2 He helium 37 Rb rubidium 72 Hf hafnium 3 Li lithium 38 Sr strontium 73 Ta tantalum 4 Be beryllium 39 Y yttrium 74 W tungsten 5 B boron 40 Zr zirconium 75 Re rhenium 6 C carbon 41 Nb niobium 76 Os osmium 7 N nitrogen 42 Mo molybdenum 77 Ir iridium 8 O oxygen 43 Tc technetium 78 Pt platinum 9 F fluorine 44 Ru ruthenium 79 Au gold

10 Ne neon 45 Rh rhodium 80 Hg mercury

11 Na sodium 46 Pd palladium 81 Tl thallium

12 Mg magnesium 47 Ag silver 82 Pb lead

13 Al aluminium 48 Cd cadmium 83 Bi bismuth

14 Si silicon 49 In indium 84 Po polonium

15 P phosphorus 50 Sn tin 85 At astatine

16 S sulphur 51 Sb antimony 86 Rn radon

17 Cl chlorine 52 Te tellurium 87 Fr francium

18 Ar argon 53 I iodine 88 Ra radium

19 K potassium 54 Xe xenon 89 Ac actinium

20 Ca calcium 55 Cs cesium 90 Th thorium

21 Sc scandium 56 Ba barium 91 Pa protactinium

22 Ti titanium 57 La lanthanum 92 U uranium

23 V vanadium 58 Ce cerium 93 Np neptunium

24 Cr chromium 59 Pr praseodymium 94 Pu plutonium

25 Mn manganese 60 Nd neodymium 95 Am americium

26 Fe iron 61 Pm promethium 96 Cm curium

27 Co cobalt 62 Sm samarium 97 Bk berkelium

28 Ni nickel 63 Eu europium 98 Cf californium

29 Cu copper 64 Gd gadolinium 99 Es einsteinium

30 Zn zinc 65 Tb terbium 100 Fm fermium

31 Ga gallium 66 Dy dysprosium 101 Md mendelevium

32 Ge germanium 67 Ho holmium 102 No nobelium

33 As arsenic 68 Er erbium 103 Lr lawrencium

34 Se selenium 69 Tm thulium 104 Rf rutherfordium

35 Br bromine 70 Yb ytterbium 105 Ha hahnium

Atomic Radiation 7

If you are given the mass number (A) and the atomic number (Z) of an atom, you don't have to be a Rhodes Scholar to work out the number of neutrons. If you prefer to work with a formula, you let the number of neutrons equal N, and then

A = Z + N, and N = A – Z

For example, the mass number of an oxygen atom is 16 and the atomic number is 8. How many neutrons are there in the nucleus of this oxygen atom?

N = A – Z = 16 – 8 = 8

What is the mass number A of a uranium atom with 92 protons and 146 neutrons? Your turn.

TABLE 1.3. SOME ATOMS

Name Z N A Hydrogen 1 0 1 Helium 2 2 4 Lithium 3 4 7 Beryllium 4 5 9 Boron 5 6 11 Carbon 6 6 12 Nitrogen 7 7 14 Oxygen 8 8 16 Fluorine 9 10 19 Neon 10 10 20 Sodium 11 12 23 Magnesium 12 12 24 Aluminum 13 14 27 Iron 26 30 56 Iodine 53 74 127

We have considered the structure of the first three atoms in some detail. The nuclei of the other atoms are built up of protons and neutrons in exactly the same manner.

The relationship between the mass number, the atomic number and the number of neutrons in some individual atoms is shown in Table 1.3. It gives the first 13 atoms and a few others.

THE SIZE OF THE ATOM

We haven’t said much yet about the size of atoms or about the sizes of the nucleus and its electrons, and the distance between them. Actually, our illustrations of the atom couldn't possibly be accurate, for if you mag- nified the size of an atom so that the nucleus were a barely visible dot, the electrons would be so far away that you couldn't get them on the page. To illustrate the great amount of empty space in an atom, if you con- sider a golf ball to represent the size of the nucleus (its diameter is really only about 10-15^ m), the electrons would then be orbiting it at a distance of several km. Uranium^92 146

You’ll appreciate that subatomic particles of about the same size as the neutron can pass through an atom with little chance of colliding with the nucleus or with an electron.

The actual distance between a nucleus and an orbital electron is of course very small. Ten million hydrogen atoms side by side would form a line one millimetre long. But small as the hydrogen atom is, its diameter is still 50,000 times greater than the diameter of its nucleus. Ten million is often written as 10^7. It can also be written as E7. Similarly 10-7^ would be written as E-7. Five million or 5 x 10^6 would be 5E6. We are going to use the exponential E notation for the rest of this book, because it is easier to read. And besides, most calculators use the same notation.

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ISOTOPES

Careful studies of the atoms of an element show that they don't all have the same mass. How come? Let's use lithium as an example. Most of its atoms have a mass of 7, but some of them have a mass of only 6. Atoms of the same element with different masses are called isotopes. To understand this clearly, let’s repeat the definitions for "Atomic Number" and "Mass Number".

The ATOMIC NUMBER Z of an atom is the number of protons in the nucleus.

The MASS NUMBER A of an atom is the number of protons plus the number of neutrons in its nucleus.

Obviously both isotopes of lithium must have the same atomic number, which for lithium is Z = 3, or one or the other would not be lithium.

For the two lithium isotopes to have different masses, they must have a different number of neutrons in their nuclei: the nucleus of the lighter isotope has three neutrons to give it a total mass of six — the other has four neutrons and a total mass of seven.

Z = 3 N = 3 A = 6

Z = 3 N = 4 A = 7

Fig 1.6. Two Lithium Isotopes

The drawing of the lithium isotopes shows that the number of orbital electrons and their arrange- ment in the outer structure of the two isotopes are exactly the same. The only difference is in the mass of the nucleus (different number of neutrons).

The chemical properties of an element describe how an element will react with any other element. It is the number of orbital electrons in the atom that determines the chemical properties of an element, and this number is equal to the atomic number. So, isotopes of the same element have the same chemical properties.

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THE ISOTOPES OF HYDROGEN

Normal hydrogen has a mass of one amu but about one out of every 7,000 hydrogen atoms has a mass of two amu, and one in about 1E17 hydrogen atoms has a mass of three amu. The light isotope has only one particle in its nucleus, a proton. The heavier isotopes also have one proton in the nucleus plus one or two neutrons to give it a mass of two, or three amu. Fig. 1.7 gives you the general idea. As a rule, the difference in mass between two isotopes is small compared to the total mass of the atom, as in Fe-56 and Fe-57, for example. But this is not the case for hydrogen. Hydrogen-2 has twice the mass of hydrogen-1.

Z = 1, N = 0, A = 1 Hydrogen-

Z =1, N = 1, A = 2 Hydrogen-

Z = 1, N = 2, A = 3 Hydrogen- Fig. 1.7. The Hydrogen Isotopes

For this and other reasons, hydrogen-2 was given the name "deuterium", with the symbol D, and hydrogen-3 was called "tritium", with the symbol T. These are exceptions. For all other elements, the isotopes have the same symbol. They are distinguished from each other by the mass number at the upper right of the symbol. You can use the symbols H^2 , H-2 and D to represent the same thing, which is one atom of the isotope hydrogen-2.

Since the CANDU reactor uses heavy water, let us compare the properties of light and heavy water.

First of all, water is not an element, but a compound. A compound is a chemical combination of two or more elements combined in a fixed proportion. The smallest part of a compound, which still has the properties of that compound, is called a molecule. So a molecule is to a compound what an atom is to an element.

The symbol for the compound called water is H 2 O. This means that its molecule consists of two hydrogen atoms and one oxygen atom. In light water, the two hydrogen atoms are hydrogen- isotopes, and in heavy water they are hydrogen-2 isotopes. Hydrogen-2 is produced from normal hydrogen by separating the hydrogen-2 atoms (1 in 7,000) from the hydrogen-1 atoms.

The properties of light and heavy water are summarized on the next page in Table 1.4. What’s missing from the table is the cost! Heavy water costs about 15 times as much as kitchen scotch.

Atomic Radiation 11

TABLE 1.4. LIGHT AND HEAVY WATER

Properties Light Water Heavy Water

Common Names Water Heavy Water or Deuterium Oxide

Formula H 2 O D 2 O Taste Same Appearance Same Chemical Properties Same Density (kg/L) 1.0 1. Freezing Point 0 oC 3.8 oC Boiling Point 100 oC 101.4 oC

THE NATURAL ISOTOPES OF URANIUM

Naturally occurring uranium ore contains the three uranium isotopes U-234, U-235, and U-238. Since the refining of uranium metal from uranium ore is a chemical process, these same isotopes are present in the same concentrations in the pure metal and in the ore. Table 1.5 shows that almost all of the uranium is made up of U-238 (99.28%).

TABLE 1.5. ISOTOPES OF NATURAL URANIUM

Isotope Z N A % Abundance U-234 92 142 234 0. U-235 92 143 235 0. U-238 92 146 238 99.

U-235, which accounts for only 0.72% of natural uranium, has the best nuclear properties for nuclear power generation. Most types of power reactors need fuel with a higher concentration of U-235 than normal. In that case, you have to put the natural uranium through an isotope separation plant. This is super expensive, and you’ll be pleased to hear that for CANDU fuel we don’t need to do this. Uranium that has a higher than normal percentage of U-235 is said to be enriched. The concentration of U-235 in enriched fuel can vary all the way from just greater than 0.72% to 100%.