May-June 2018 NPJ

Nuclear Plant Journal, May-June 2018 NuclearPlantJournal.com 35 (Continued on page 36) formation. Figure 1 shows the primary system of a BWR. Figure 2 illustrates the steps in Co-60 activation and transport, ultimately leading to the formation of radiation fields. Isotopes and Radiation Fields The gamma-emitting isotopes are responsible for the radiation fields. However, there are differences between the isotopes that are most prolific in the coolant and those that are the major contributors to the out of core radiation fields. How much an isotope contributes to the radiation fields depends on: 1. The quantity of the isotope. 2. The half-life: isotopes with short half-lives (a few hours or less) will decay away quickly, and not have much of an effect on shutdown dose rates. For example, Manganese-56, has a half-life of 2.58 hours, so it will decay quickly. 3. The yield and energy of gamma rays: isotopes with small yields and that emit low energy gammas will have less of an effect on radiation fields. For example, a Chromium-51 decay will emit, on average, 0.1 gamma rays of low energy, so it tends to not be a large contributor to radiation fields. Cobalt-60 (Co-60) is typically the isotope of most concern in BWRs, while in PWRs, Cobalt-58 (Co-58) and Cobalt-60 are of most concern. Table 1 compares Co-58 and Co-60. Co-60 is formed from the neutron adsorption of non-radioactive Co-59, while Co-58 is formed from the neutron adsorption and decay of non-radioactive Nickel-58 (Ni-58). The primary source of non- radioactive Co-59 is the corrosion and wear of materials with Stellite™ hardfacing, as the concentration of Co- 59 in Stellite™ is typically between 50 and 65% by weight. Trace Co-59 is also present in any material that contains nickel. The primary source of Ni-58 is the corrosion of various stainless steels and nickel alloys used in the plant. The Ni- 58 content of typical Type 304 stainless steels is between 8 and 12% by weight. In BWRs, there are many more components with Stellite™ hardfacing that interface with the primary coolant than in PWRs. In PWRs, the large surface area of nickel alloy steam generator tubes results in a substantially greater nickel source than in most BWRs. Accrued outage radiation exposure is typically higher in BWRs than in PWRs because BWRs have higher Co-60 source term. Other isotopes that also contribute to radiation fields include Manganese-54 (Mn-54, which originates from Iron-54), Chromium-51 (Cr-51), and Iron-59 (Fe- 59). In some instances, Silver-110m (Ag- 110m) and Antimony-124 (Sb-124), and Tungsten-187 (W-187) have also been identified as contributing sources. Table 2 lists some of the commonly found activation corrosion products. Figure 1. Figure 2.