sign in sign up
<<
Home , Background radiation

APPENDIX B Understanding

This section introduces the general reader to some basic concepts of radioactivity and an understanding of the radiation emitted as radioactive materials decay to a stable state. To better comprehend the radiological information in the Site Environmental Report (SER) it, is important to remember that not all are the same and that different kinds of radiation affect living beings differently. This appendix includes discussions on the common sources of radioactivity in the environment, types of radiation, the analyses used to quantify radioactive material, and how radiation sources contribute to radiation dose. Some general statistical concepts are also presented, along with a discussion of that are of environmental interest at BNL. The discussion begins with some definitions and background information on scientific notation and numerical prefixes used when measuring dose and radioactivity. The definitions of commonly used radiological terms are found in the Technical Topics section of the glossary, Appendix A, and are indicated in boldface type here only when the definition in the glossary provides additional details. radioactivity and radiation ionizing or not, may pose health risks. In the All substances are composed of atoms that SER, radiation refers to . are made of subatomic particles: , neu- Radioactive elements (or radionuclides) trons, and . The protons and are referred to by name followed by a number, are tightly bound together in the positively such as cesium-137. The number indicates the charged nucleus (plural: nuclei) at the center of mass of that element and the total number of the atom. The nucleus is surrounded by a cloud neutrons and protons contained in the nucleus of negatively charged electrons. Most nuclei of the atom. Another way to specify cesium-137 are stable because the forces holding the pro- is Cs-137, where Cs is the chemical symbol for tons and neutrons together are strong enough to cesium in the Periodic Table of the Elements. overcome the electrical energy that tries to push This type of abbreviation is used in the SER. them apart. When the number of neutrons in the nucleus exceeds a threshold, then the nucleus Scientific Notation becomes unstable and will spontaneously “de- Most numbers used for measurement and cay,” or emit excess energy (“nuclear” energy) quantification in the SER are either very large or in the form of charged particles or electromag- very small, and many zeroes would be required netic waves. Radiation is the excess energy to express their value. To avoid this, scientific released by unstable atoms. Radioactivity and notation is used, with numbers represented in radioactive refer to the unstable nuclear prop- multiples of 10. For example, the number two erty of a substance (e.g., radioactive ). million five hundred thousand (two and a half When a charged particle or electromagnetic million, or 2,500,000) is written in scientific wave is detected by radiation-sensing equip- notation as 2.5 x 106, which represents “2.5 ment, this is referred to as a radiation event. multiplied by (10 raised to the power of 6).” Radiation that has enough energy to remove Since even “2.5 x 106” can be cumbersome, the electrons from atoms within material (a pro- capital letter E is substituted for the phrase “10 cess called ionization) is classified as ionizing raised to the power of ….” Using this format, radiation. Radiation that does not have enough 2,500,000 is represented as 2.5E+06. The “+06” energy to remove electrons is called nonionizing refers to the number of places the decimal point radiation. Examples of nonionizing radiation was moved to the left to create the shorter ver- include most visible , light, micro- sion. Scientific notation is also used to represent waves, and radio waves. All radiation, whether numbers smaller than zero, in which case a

B- 2006 Site Environmental Report APPENDIX B: understanding radiation

of cosmic radiation from outer space, radiation from radioactive elements in and rocks, and radiation from and its decay products in Radon, air. Some people use the term background when 200 Medical, 39 referring to all non-occupational sources com- Manmade monly present. Other people use natural to refer Nuclear only to cosmic and terrestrial sources, and back- Medicine, 14 ground to refer to common man-made sources Consumer such as medical procedures, consumer products, Internal, Products, 10 40 and radioactivity present in the atmosphere from former nuclear testing. In the SER, the term Cosmic, natural background is used to refer to radiation 26 Terrestrial, from cosmic and terrestrial radiation. 28 Cosmic – Cosmic radiation primarily consists of Figure B-1. Typical Annual Radiation Doses from Natural and Man-Made Sources (mrem). Source: NCRP Report No. 93 (NCRP 1987) charged particles that originate in space, beyond the earth’s atmosphere. This includes ionizing minus sign follows the E rather than a plus. For radiation from the sun, and secondary radia- example, 0.00025 can be written as 2.5 x 10-4 tion generated by the entry of charged particles or 2.5E-04. Here, “-04” indicates the number of into the earth’s atmosphere at high speeds and places the decimal point was moved to the right. energies. Radioactive elements such as hydro- gen-3 (tritium), beryllium-7, carbon-14, and NUMERICAL Prefixes sodium-22 are produced in the atmosphere by Another method of representing very large cosmic radiation. Exposure to cosmic radiation or small numbers without using many zeroes is increases with altitude, because at higher eleva- to use prefixes to represent multiples of ten. For tions the atmosphere and the earth’s magnetic example, the prefixmilli (abbreviated m) means field provide less shielding. Therefore, people that the value being represented is one-thou- who live in the mountains are exposed to more sandth of a whole unit; 3 mg (milligrams) is 3 cosmic radiation than people who live at sea thousandths of a gram or E-03. See Appendix level. The average dose from cosmic radiation C for additional common prefixes, including to a person living in the United States is ap- pico (p), which means trillionth or E-12, giga proximately 26 mrem per year. (For an expla- (G), which means billion or E+09, and tera (T), nation of dose, see effective dose equivalent in which means trillion, E+12. Appendix A. The units rem and also are explained in Appendix A.) Sources of Ionizing Radiation Terrestrial – Terrestrial radiation is released Radiation is energy that has both natural by radioactive elements that have been pres- and manmade sources. Some radiation is essen- ent in the soil since the formation of the earth. tial to life, such as heat and light from the sun. Common radioactive elements that contribute to Exposure to high-energy (ionizing) radiation terrestrial exposure include of potas- has to be managed, as it can pose serious health sium, , actinium, and uranium. The risks at large doses. Living things are exposed average dose from terrestrial radiation to a per- to radiation from natural background sources: son living in the United States is approximately the atmosphere, soil, water, food, and even our 28 mrem per year, but may vary considerably own bodies. Humans are exposed to ionizing depending on the local geology. radiation from a variety of common sources, the Internal – Internal exposure occurs when most significant of which follow. radionuclides are ingested, inhaled, or absorbed Background Radiation – Radiation that occurs through the skin. Radioactive material may be naturally in the environment is also called back- incorporated into food through the uptake of ter- ground activity. Background radiation consists restrial radionuclides by plant roots. People can

2006 Site Environmental Report B- APPENDIX B: understanding radiation ingest radionuclides when they eat contaminat- of particles that are identical to electrons. ed plant matter or meat from animals that have Therefore, beta particles have a negative charge. consumed contaminated plants. The average Beta radiation is slightly more penetrating than dose from food for a person living in the United alpha radiation, but most beta radiation can be States is about 40 mrem per year. A larger expo- stopped by materials such as aluminum foil and sure, for most people, comes from breathing the plexiglass panels. Beta radiation has a range in decay products of naturally occurring radon gas. air of several feet. Naturally occurring radioac- The average dose from breathing air with radon tive elements such as -40 emit beta byproducts is about 200 mrem per year, but that radiation. Some beta particles present a hazard amount varies depending on geographical loca- to the skin and eyes. tion. An EPA map shows that BNL is located Gamma Radiation – Gamma radiation is a form in one of the regions with the lowest potential of electromagnetic radiation, like radio waves radon risk. or visible light, but with a much shorter wave- Medical – Every year in the United States, length. Gamma rays are emitted from a radioac- millions of people undergo medical procedures tive nucleus along with alpha or beta particles. that use ionizing radiation. Such procedures Gamma radiation is more penetrating than alpha include chest and dental x-rays, mammography, or beta radiation, capable of passing through thallium heart stress tests, and tumor irradia- dense materials such as concrete. Gamma radia- tion therapies. The average doses from nuclear tion is identical to x-rays except that x-rays medicine and x-ray examination procedures are are more energetic. Only a fraction of the total about 14 and 39 mrem per year, respectively. gamma rays a person is exposed to will interact Anthropogenic – Sources of anthropogenic with the human body. (man-made) radiation include consumer prod- ucts such as static eliminators (containing Types of Radiological AnalysEs polonium-210), smoke detectors (containing The amount of radioactive material in a americium-241), cardiac pacemakers (contain- sample of air, water, soil, or other material can ing plutonium-238), fertilizers (containing iso- be assessed using several analyses, the most topes from uranium and thorium decay series), common of which are described below. and products (containing polonium-210 Gross alpha – Alpha particles are emitted from and lead-210). The average dose from consumer radioactive material in a range of different products to a person living in the United States energies. An analysis that measures all alpha is 10 mrem per year (excluding tobacco contri- particles simultaneously, without regard to their butions). particular energy, is known as a gross alpha ac- tivity measurement. This type of measurement COMMON TYPES OF Ionizing RADIATION is valuable as a screening tool to indicate the The three most common types of ionizing total amount but not the type of alpha-emitting radiation are described below. radionuclides that may be present in a sample. Alpha Radiation – An is identi- Gross beta – This is the same concept as that for cal in makeup to the nucleus of a helium atom, gross alpha analysis, except that it applies to the consisting of two neutrons and two protons. measurement of gross activity. Alpha particles have a positive charge and have Tritium – Tritium radiation consists of low-en- little or no penetrating power in matter. They ergy beta particles. It is detected and quantified are easily stopped by materials such as paper by liquid scintillation counting. More infor- and have a range in air of only an inch or so. mation on tritium is presented in the section However, if alpha-emitting material is ingested, Radionuclides of Environmental Interest, later alpha particles can pose a health risk inside the in this appendix. body. Naturally occurring radioactive elements Strontium-90 – Due to the properties of the such as uranium emit alpha radiation. radiation emitted by strontium-90 (Sr-90), Beta Radiation – Beta radiation is composed a special analysis is required. Samples are

B- 2006 Site Environmental Report APPENDIX B: understanding radiation chemically processed to separate and collect any Radiation events occur randomly; if a strontium atoms that may be present. The col- radioactive sample is counted multiple times, a lected atoms are then analyzed separately. More spread, or distribution, of results will be ob- information on Sr-90 is presented in the section tained. This spread, known as a Poisson dis- Radionuclides of Environmental Interest. tribution, is centered about a mean (average) Gamma – This analysis technique identifies value. Similarly, if background activity (the specific radionuclides. It measures the particu- number of radiation events observed when no lar energy of a ’s gamma radiation sample is present) is counted multiple times, it emission. The energy of these emissions is also will have a Poisson distribution. The goal unique for each radionuclide, acting as a “fin- of a radiological analysis is to determine wheth- gerprint” to identify it. er a sample contains activity greater than the background reading detected by the instrument. Statistics Because the sample activity and the background Two important statistical aspects of measur- activity readings are both Poisson distributed, ing radioactivity are uncertainty in results, and subtraction of background activity from the negative values. measured sample activity may result in values Uncertainty – Because the emission of that vary slightly from one analysis to the next. radiation from an atom is a random process, a Therefore, the concept of a minimum detection sample counted several times usually yields a limit (MDL) was established to determine the slightly different result each time; therefore, a statistical likelihood that a sample’s activity is single measurement is not definitive. To account greater than the background reading recorded by for this variability, the concept of uncertainty the instrument. is applied to radiological data. In the SER, Identifying a sample as containing activity analysis results are presented in an x ± y format, greater than background, when it actually does where “x” is the analysis result and “± y” is the not have activity present, is known as a Type I 95 percent “confidence interval” of that result. error. Most laboratories set their acceptance of That means there is a 95 percent probability a Type I error at 5 percent when calculating the that the true value of x lies between (x + y) and MDL for a given analysis. That is, for any value (x – y). that is greater than or equal to the MDL, there is Negative values – There is always a small 95 percent confidence that it represents the de- amount of natural background radiation. The tection of true activity. Values that are less than laboratory instruments used to measure radioac- the MDL may be valid, but they have a reduced tivity in samples are sensitive enough to mea- confidence associated with them. Therefore, sure the background radiation along with any all radiological data are reported, regardless of contaminant radiation in the sample. To obtain whether they are positive or negative a true measure of the contaminant level in a At very low sample activity levels that are sample, the background radiation level must be close to the instrument’s background reading, it subtracted from the total amount of radioactivity is possible to obtain a sample result that is less measured. Due to the randomness of radioac- than zero. This occurs when the background tive emissions and the very low concentrations activity is subtracted from the sample activ- of some contaminants, it is possible to obtain ity to obtain a net value, and a negative value a background measurement that is larger than results. Due to this situation, a single radia- the actual contaminant measurement. When the tion event observed during a counting period larger background measurement is subtracted could have a significant effect on the mean from the smaller contaminant measurement, (average) value result. Subsequent analysis a negative result is generated. The negative may produce a sample result that is positive. results are reported, even though doing so may When the annual data for the SER are com- seem illogical, because they are essential when piled, results may be averaged; therefore, all conducting statistical evaluations of data. negative values are retained for reporting as

2006 Site Environmental Report B- APPENDIX B: understanding radiation well. This data handling practice is consistent Cs-137 is associated with the following areas with the guidance provided in the Handbook of that have been, or are being, addressed as part Radioactivity Measurements Procedures (NCRP of the Environmental Remediation Program: 1985) and the Environmental Regulatory former Hazardous Waste Management Facility, Guide for Radiological Effluent Monitoring Waste Concentration Facility, Building 650 and Environmental Surveillance (DOE 1991). Reclamation Facility and Sump Outfall Area, Average values are calculated using actual and the Brookhaven Graphite Research Reactor analytical results, regardless of whether they are (BGRR). above or below the MDL, or even equal to zero. Strontium-90 – Sr-90 is a beta-emitting radio- The uncertainty of the mean, or the 95 percent with a half-life of 28 years. Sr-90 is confidence interval, is determined by multiply- found in the environment principally as a result ing the population standard deviation of the of fallout from aboveground nuclear weapons mean by the t(0.05) statistic. testing. Sr-90 released by weapons testing in the 1950s and early 1960s is still present in the en- Radionuclides of Environmental vironment today. Additionally, nations that were Interest not signatories of the Nuclear Test Ban Treaty Several types of radionuclides are found in of 1963 have contributed to the global inventory the environment at BNL due to historical opera- of fission products (Sr-90 and Cs-137). This tions. radionuclide was also released as a result of the Cesium-137 – Cs-137 is a fission-produced 1986 accident in the former Soviet radionuclide with a half-life of 30 years (after Union. 30 years, only one half of the original activ- Sr-90 is present at BNL in the soil and ity level remains). It is found in the worldwide groundwater. As in the case of Cs-137, some environment as a result of past aboveground Sr-90 at BNL results from worldwide nuclear and can be observed in testing; the remaining contamination is a by- near-surface at very low concentrations, product of reactor operations. The following usually less than 1 pCi/g (0.004 Bq/g). Cs-137 areas with Sr-90 contamination have been or are is a beta-emitting radionuclide, but it can be being addressed as part of the Environmental detected by because its Remediation Program: former Hazardous Waste , barium-137m, emits gamma Management Facility, Waste Concentration radiation. Facility, Building 650 Reclamation Facility and Cs-137 is found in the environment at BNL Sump Outfall Area, the BGRR, Former and mainly as a soil contaminant, from two main Interim Landfills, Chemical and Glass Holes sources. The first source is the worldwide depo- Area, and the STP. sition from nuclear accidents and fallout from The information in SER tables is arranged weapons testing programs. The second source by method of analysis. Because Sr-90 requires is deposition from spills or releases from BNL a unique method of analysis, it is reported as a operations. operations produce separate entry. Methods for detecting Sr-90 us- Cs-137 as a byproduct. In the past, wastewater ing state-of-the-art equipment are quite sensitive containing small amounts of Cs-137 generated (detecting concentrations less than 1 pCi/L), at the reactor facilities was routinely discharged which makes it possible to detect background to the Sewage Treatment Plant (STP), result- levels of Sr-90. ing in low-level contamination of the STP Tritium – Among the radioactive materials that and the Peconic River. In 2002/2003, under are used or produced at BNL, tritium has re- the Environmental Restoration Program, sand ceived the most public attention. Approximately and its debris containing low levels of Cs-137, 4 million Ci (1.5E+5 TBq) per year are pro- Sr-90, and heavy metals were removed, assur- duced in the atmosphere naturally (NCRP ing that future discharges from the STP are free 1979). As a result aboveground weapons testing of these contaminants. Soil contaminated with in the 1950s and early 1960s in the United

B- 2006 Site Environmental Report APPENDIX B: understanding radiation

States, the global atmospheric tritium inventory has been replaced by a tritium atom (hence, its was increased by a factor of about 200. Other shorthand notation, HTO). Most of the tritium human activities such as consumer product released from BNL sources is in the form of manufacturing and reactor opera- HTO, none as elemental tritium. Sources of tions have also released tritium into the environ- tritium at BNL include the reactor facilities (all ment. Commercially, tritium is used in products now non-operational), where residual water such as self-illuminating wristwatches and exit (either heavy or light) is converted to tritium via signs (the signs may each contain as much as bombardment; the accelerator facilities, 25 Ci [925 GBq] of tritium). Tritium also has where tritium is produced by secondary radia- many uses in medical and biological research tion interactions with soil and water; and facili- as a labeling agent in chemical compounds, ties like the Brookhaven Linac Producer and is frequently used in universities and other (BLIP), where tritium is formed from secondary research settings such as BNL and the other radiation interaction with cooling water. Tritium national laboratories. has been found in the environment at BNL as Of the sources mentioned above, the most a groundwater contaminant from operations significant contributor to tritium in the environ- in the following areas: Current Landfill, BLIP, ment has been aboveground nuclear weapons Alternating Gradient Synchrotron, and the High testing. In the early 1960s, the average tritium Flux Beam Reactor. Although small quantities concentration in surface streams in the United of tritium are still being released to the envi- States reached a value of 4,000 pCi/L (148 Bq/ ronment through BNL emissions and effluents, L; NCRP 1979). Approximately the same con- the concentrations and total quantity have been centration was measured in precipitation. Today, drastically reduced, compared with historical the level of tritium in surface waters in New operational releases as discussed in Chapters 4 York State is less than one-twentieth of that and 5. amount, below 200 pCi/L (7.4 Bq/L; NYSDOH 1993). This is less than the detection limit of most analytical laboratories. REFERENCES and bibliography Tritium has a half-life of 12.3 years. When DOE Order 5400.5. 1993. of the Public an atom of tritium decays, it releases a beta par- and the Environment. U.S. Department of Energy, Washington, ticle, causing transformation of the tritium atom DC. Change 2: 1-7-93. into stable (nonradioactive) helium. The beta DOE. 1991. Environmental Regulatory Guide for Radiological radiation that tritium releases has a very low Effluent Monitoring and Environmental Surveillance. DOE/EH- 0173T. U.S. Department of Energy, Washington, DC. energy, compared to the emissions of most other NCRP. 1979. Tritium in the Environment. NCRP Report No. 62. radioactive elements. In humans, the outer layer National Council on Radiation Protection and Measurements. of dead skin cells easily stops the beta radia- Bethesda, MD. tion from tritium; therefore, only when tritium NCRP. 1985. Handbook of Radioactivity Measurements is taken into the body can it cause an exposure. Procedures, NCRP Report No. 58. National Council on Radiation Protection and Measurements, Bethesda, MD. Tritium may be taken into the body by inhala- NCRP. 1987. Ionizing of the Population of tion, ingestion, or absorption of tritiated water the United States. NCRP Report No. 93. National Council on through the skin. Because of its low energy Radiation Protection and Measurements. Bethesda, MD. radiation and short residence time in the body, NYSDOH. 1996. Radioactive Contamination in the Peconic the health threat posed by tritium is very small River. Bureau of Environmental Radiation Protection, New for most exposures. York State Department of Health, Albany, NY. NYSDOH. 1993. Environmental Radiation in New York State. Environmental tritium is found in two Bureau of Environmental Radiation Protection, New York forms: gaseous elemental tritium, and tritiated State Department of Health, Albany, NY. water or water vapor, in which at least one of Radiochemistry Society Online. www.radiochemistry.org/ nomenclature/index/html, accessed 3-25-04. the hydrogen atoms in the H2O water molecule

2006 Site Environmental Report B-