Elements that undergo radioactive decay as a result of an unstable nuclear structure are referred to as radionuclides. Isotopes have nucleus with the same number of protons, but different numbers of neutrons. Radionuclides are typically written with the chemical symbol and a superscript to the left of the symbol that denotes the atomic mass (total number of protons and neutrons) of that isotope. For example, 12C and 14C are isotopes of the element carbon; each has 6 protons, but are associated with 6 and 8 neutrons, respectively.
As part of the decay process, radionuclides release ionizing radiation in the form of alpha particles, beta particles , and gamma radiation.
Radionuclides Present in Ground water
There exist a variety of sources of radionuclides which can be dissolved in groundwater. Sources are typically categorized as natural or anthropogenic. Natural sources of radionuclides include formation in the atmosphere caused by cosmic irradiation, and presence in mineral deposits of long-lived radioisotopes of heavy elements and their associated daughters. Anthropogenic sources consist of nuclear power plants, nuclear weapons explosions and testing, medical, research, industrial, and pharmaceutical radionuclide applications, and mining wastes. A number of specific radionuclides are known to represent potential problems in groundwater systems. Each is described below.
The only long-lived isotope of radon is 222Rn, which undergoes adecay with a half-life of 3.82 days. This radionuclide is produced by decay of 226Ra, and is often associated with deposits that have significant uranium concentrations (226Ra and 222Rn are part of the 238U decay chain). 222Rn represents a significant health concern in some homes, entering as a dissolved species in drinking water or directly from soil as a gas. Additional radiation is emitted from daughter products of 222Rn as it decays to a stable element. 222Rn is a naturally occurring radioisotope, and can be significant for uranium mining in terms of worker safety and waste disposal (Fetter, 1993).
Radium (226Ra and 228Ra)
226Ra (t1/2 = 1600 years) is a daughter of 230Th, part of the 238U decay chain, and decays subsequently to 222Rn. Because 228Ra (t1/2 = 5.8 years) is a daughter of 232Th, both radium radionuclides are associated with uranium- and thorium-containing deposits. Radium is more soluble than either thorium or uranium, and can therefore be transported in higher concentrations with an associated higher activity, particularly when considering the relatively short t1/2 of 228Ra. Because radium is associated with uranium deposits, it is therefore a potentially important problem in mining wastes (Freeze and Cherry, 1979).
Uranium (238U and 235U)
Uranium in mineral deposits is composed of 99.3% 238U (t1/2 = 4.5 X109 years) and 0.7% 235U (t1/2 = 7.1 X 108 years). Because 235U is fissile (split by low-energy neutrons), it is used as fuel for nuclear power reactors worldwide. Both uranium isotopes are long lived and part of long decay chains. Each has numerous radioactive daughter nuclei that produce emissions before decaying to stable states. Uranium is typically mined in open pits or underground and concentrated to 65 to 70% uranium via flotation, dissolution, and solvent extraction processes. Further purification to U3O8 and enrichment to increase the fraction of the 235U isotope can achieve desired levels of 235U, with 3 to 4% being typical for nuclear power reactors (Choppin et al., 1995).
237Np can be found in nuclear waste as the product of nuclear reactions (in a nuclear power reactor) converting 238U to 237U, which then beta decays to 237Np. It has a t1/2 of 2.2 X 106 years and undergoes a decay to 233Pa, with associated daughter decays from that element and others. 237Np is of great concern in high level and spent nuclear fuel disposal because it is only slightly sorbed (i.e., high mobility) in many groundwater environments and has a long half-life (USDOE, 1995).
Tritium undergoes beta decay with a half-life of 12.43 years, producing 3He as a daughter element. Because tritium can combine with hydrogen and oxygen to form tritiated water, it poses a significant threat in terms of groundwater transport. Tritium is especially useful in delineating groundwater recharge occurring since the 1963 spike of tritium released by the
U.S. and Soviet Union prior to the implementation of an above-ground testing treaty (Fritz et al., 1991). Tritium concentrations in natural waters have increased nearly tenfold since the advent of the nuclear age (Fetter, 1993). Tritium is formed naturally in the atmosphere when high-energy neutrons bombard 14N, producing 12C and 3H. It is also produced in large quantities at nuclear power plants, and in nuclear weapons testing and production.
Like tritium, 14C can be used for dating purposes, in this case for organic material. It decays by beta emission to 14N, and has a t1/2 of 5715 years. 14C labeling of organic compounds is often done to provide sensitive detection methods for experiments with organic compounds. Because it can be oxidized to soluble or gaseous 14CO2, or form a variety of organic compounds, 14C can pose a significant transport threat as a radionuclide. It can be produced naturally in the atmosphere when low-energy neutrons bombard 14N, producing 14C and 1H. Anthropogenic sources include nuclear power plants, nuclear weapons testing, and research laboratories.
Fission Fragments (90Sr 99Tc, 129I, and 137Cs)
Fission fragments are radioactive materials produced by fission of heavy isotopes such as 235U. The fission reaction typically splits the heavy atom into two (sometimes three) lighter isotopes, with associated neutron, neutrino, and gamma radiation emissions as well. The lighter isotopes are produced primarily in the atomic mass range of 80 to 155. Although a variety of fission fragments are produced, 90Sr 99Tc, 129I, and 137Cs are of primary concern due to the significant amounts that are produced and their relatively long half-lives.
90Sr decays by beta emission to 90Y with a t1/2 of 29.1 years. 90Y then decays to stable 90Zr via beta emission.Strontium is a prominent fission fragment with no associated gamma emissions. In high-level nuclear waste and spent fuel, it is one of the highest activity radioisotopes for the first 200 years after fission ceases. It can be particularly dangerous when ingested by humans because it is known to accumulate in or near the bones, resulting in greater irradiation of sensitive bone marrow.
99Tc undergoes beta decay to 99Ru with a t1/2 of 2.1X105 years. In addition to being a fission fragment,99Tc is also associated with medical and research wastes where it is used in diagnostic testing. 129I is an important fission fragment because it beta decays to stable 129Xe with a long t1/2 of 1.7 X107 years. It is also used in medical diagnostic tests and can therefore be found in low-level waste as well as spent fuel.
137Cs emits gamma and beta radiation while decaying to stable 137Ba and has a t1/2 of 30.3 years. Along with 90Sr, 137Cs is one of the high-activity isotopes in high-level nuclear waste for the first 200 years after fission ceases.