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Nuclear Radiation

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Nuclear Radiation Nuclear Radiation Robert L. Sindelar Savannah River National Laboratory Introduction Fundamental to safe and efficient operations, the effects of radiation must be considered in the design of a nuclear materials separations facility in terms of: • personnel exposure; • adverse effects on the chemical processes; and • degradation of the materials of construction. The first major facility for conducting separations processes for spent nuclear fuel in the United States was the T Plant, a Chemical Separations Building at the Hanford site that began operation December 26, 1944 for the U.S. defense mission. The early separations buildings were massive concrete canyon-like structures, and provided for remote operations for dissolving spent fuel in high acid solutions and processing the highly radioactive solutions in multiple stainless steel tanks (Figure 1). Management of radiation effects have improved since this early era of processing from increased understanding through investigations, and technology development. Figure 1. Photograph of the Savannah River Site F-Canyon, Hot Side, Pre-Operation (circa 1955) A brief overview of radiation effects on materials and systems relevant to nuclear fuel cycle separations is presented in this chapter with an emphasis on the methodologies used to evaluate and/or mitigate the effects. Key references are provided for additional details on the topics. Radiation Shielding Shielding against ionizing radiation must be provided in separations facilities for spent nuclear fuel due to the attendant radiation from radioisotopes produced via fission or capture reactions during irradiation of the fuel. Ionizing radiation in the form of energetic subatomic particles or high energy electromagnetic radiation from unstable or radioactive atoms is radiation that can cause electron removal from atoms or molecules [1]. The primary particles from radioactive decay are the alpha particle, the beta particle, and the neutron. Gamma ray radiation is the term for the high energy electromagnetic radiation from radioactive atoms. A description of the various modes of decay of radioactive atoms is provided in reference 2. Examples of nuclear fuel that are used in research reactors are shown in Figure 2. The fuels shown in Figure 2 are aluminum-clad, uranium-aluminum alloy fuel in plates that are joined by side plates into a box-like assembly. Figure 2. Photograph of Materials Test Reactor Design Nuclear Fuel Assemblies used in Foreign and Domestic Research Reactors including the Massachusetts Institute of Technology Reactor (5 MW); the High Flux Reactor at Petten (50 MW); the Missouri University Research Reactor (10MW); the Oak Ridge Research Reactor (30 MW); and the Omega West Reactor (8 MW) Spent nuclear fuel contains a suite of radioisotopes. As an example of the radioisotopes generated in nuclear fuel during reactor operation, an analysis is performed on assembly #F1369 irradiated in the Petten High Flux Reactor. This fuel assembly initially contained 484 grams of uranium, enriched to 93% uranium-235. The fuel was irradiated for 158 days in the reactor at 50 MW power with 211 MWD/assembly or 58% burn-up. A 28-day cycle with 24.7 days on and 3.3 days off was used in the 158-day irradiation. Figure 3 shows the activity levels with time from the radioisotopes from fission products, and from actinides in the fuel assembly. The burnup simulation of the fuel assembly was modeled with the ORIGEN-S module of SCALE 4.4a. ORIGEN-S [3] computes time-dependent concentrations and source terms of a large number of isotopes, which are simultaneously generated or depleted through neutronic transmutation, fission, radioactive decay, input feed rates, and physical or chemical removal rates. A 28-day cycle with 24.7 days on and 3.3 days off was used to model the 158-day irradiation. Assembly Activity vs. Decay Time 1.E+07 1.E+06 1.E+05 1.E+04 total activity Act 1.E+03 1.E+02 total activity FP Activity (curies) 1.E+01 1.E+00 0 50 100 150 200 250 Decay Time (days) Figure 3. Total Activity from Fission Products (FP) and Actinides (Act) from a Research Reactor Fuel Assembly with 58% Burn-up from158 Day Irradiation or 211 MWD/assembly in the HFR Petten Reactor at 50 MW Table 1A and 1B show the specific high activity actinides and fission product radioisotopes, respectively at the 209-day cool down time. It is noted that the radioisotopes include alpha, beta, and gamma emitters,1 but also spontaneous fission occurs with the emission of neutrons in isotopes such as Cf-252, Am-243, Pu-240, and U-238. In addition, secondary reactions, 1 There are few pure alpha or beta emitters, gamma emission is typically concomitant namely an (α, n) reaction such as Be-9(α, n)C-12, would produce a source of neutrons that would need to be considered in shielding for personnel. Simplified shielding approaches for ionizing radiation are given in Table 2. Table 2 indicates that, without shielding, gamma and neutron radiation travel hundreds of feet in air. Table 1A. Actinides with activity > 10-4 Ci from Table 1B. Fission product radioisotopes with the HFR Petten assembly #F1369 following activity > 102 Ci from the HFR Petten 209 days cool-down assembly #F1369 following 209 days cool-down Fission Actinide Curies Curies Product th231 3.95E-04 sr89 1.21E+03 pa233 6.34E-04 sr90 6.84E+02 u235 3.95E-04 y90 6.84E+02 u236 2.69E-03 y91 2.25E+03 u237 1.34E-04 np237 6.34E-04 zr95 3.09E+03 np239 1.06E-04 nb95 6.28E+03 pu236 1.36E-04 ru103 3.33E+02 pu238 1.57E+00 rh103m 3.33E+02 pu239 3.25E-02 ru106 6.81E+02 pu240 3.49E-02 rh106 6.81E+02 pu241 5.54E+00 cs134 3.35E+02 am241 7.32E-03 am243 1.06E-04 cs137 6.91E+02 cm242 1.42E-01 ba137m 6.53E+02 cm244 2.80E-03 ce141 2.86E+02 total 7.34E+00 ce144 9.59E+03 pr144 9.59E+03 pr144m 1.34E+02 pm147 1.83E+03 total 3.96E+04 Table 2. Range in air and shielding for various ionizing radiation sources Type of Characteristics Range in Shield Hazards Source Ionizing Air Radiation Alpha Large mass, +2 Very short, Paper, skin Internal Pu, U charge 1- 2 inches Beta Small mass, -1 Short, 10 feet Plastic, glass, Internal, external Fission & charge metal skin & eyes activation products Gamma/x-ray No mass or Several 100 Lead, steel, Whole Body Fission & charge, photon feet concrete internal or activation external products Neutron Mass, no charge Several 100 Water, concrete, Whole Body Cf, neutron feet plastic internal or sources external Figure 4a, b, c. The interaction of gamma radiation with matter. Figure 4a depicts the photoelectric effect in which the gamma ray interacts with atom and the initial gamma ray is consumed. Figure 4b depicts the Compton effect or Compton scattering in which an incident photon is scattered through an angle theta and loses energy with a recoil electron. Figure 4c depicts pair-production in which an incident photon is lost in the creation of an electron and positron following an interaction with the atomic nucleus. An initial understanding of shielding processes can be obtained by considering the three primary interactions of gamma radiation with matter. Reference 2 provides a good description of these interaction processes described herein. Figures 4a – 4c are sketches of the three processes: the photoelectric effect; the Compton effect; and pair-production, respectively. In the photoelectric effect, a photon of energy hf where h is Planck’s constant and f is the frequency of the incident gamma ray, is consumed by the binding energy of the electron to the atom, and the kinetic energy of the ejected electron in the following relation: hf = φ + E (1) kmax In Compton scattering, the photon is scattered and loses energy. The energy loss by the photon is transferred to an electron with conservation of energy and momentum. The following relation can be derived for the change in wavelength of the gamma ray after a Compton scattering event where me is the rest-mass of the electron: h λ′ − λ = ()1− cosθ (2) mec In the pair-production process, the photon is consumed, and an electron pair consisting of a positron and electron are created. A threshold energy for the incident photon energy is needed to create the electron pair as identified in the following: 2 hf ≥ 2mec = 1.02MeV (3) Figure 5 shows, in effect, the probability for photoelectric, Compton, and pair-production processes for a gamma ray interaction with lead, that is equivalent to the mass attenuation coefficients, and given as a function of gamma ray energy. The attenuation coefficients for material are dependent on the atomic number of the nucleus [2]. Figure 5. The mass attenuation coefficients of lead as a function of gamma ray energy. The total mass attenuation coefficient and the contributions from the photoelectric effect, Compton effect, and pair-production are shown. [from reference 2] An engineering approach to evaluate gamma radiation shielding is summarized from the description given in Chapter 10 of reference 2. Consideration is first given to a monodirectional beam of monoenergetic gamma rays with initial energy in air with a mass absorption coefficient of and intensity or incident flux of gamma rays/cm2sec. The equation for the exposure rate, in units of milliRoentgens per hour (mR/hr), is given by: Exposure Rate with No Shield: (4) The presence of a shield will reduce the exposure rate, and the introduction of the concept of “buildup flux,” φb , in replacement of the incident flux is used to obtain: Exposure Rate with Shield: X& = 0.0659E0 (μ a / ρ) air φb (5) The concept of buildup flux is based on the phenomenon that a monoenergetic beam of gamma rays will actually become a distribution of gamma rays emergent from the shield as pictured in Figure 6A and 6B.
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