*3.3. Effects of Dislocations Formed by Cold Work*

To determine the effects of the dislocations on the desorption spectrum, TDS experiments were conducted on cold-worked specimens in which dense dislocations were introduced. Figure 10a–d shows the desorption spectrum for the cold-worked specimens after irradiation with 5.0 keV D2+ ion to the fluence of 3.0 <sup>×</sup> <sup>10</sup><sup>21</sup> ions/m<sup>2</sup> at room temperature. In Figure 10a,b, the desorption stage A shifted to the lower temperature side when the level of cold work increased. However, the desorption stage B remained consistent when the level of cold work increased. Table 2 summarizes the total amounts of HD and D2 . These values became saturated at 5% cold work and did not show any dependance on

the level of cold work. Table 3 summarizes the desorption temperature range and their responsible trapping site for deuterium obtained by the present study. respectively. A new stage appeared in the position close to Peak A because of the Ni3+ ion irradiation (up to 3.0 dpa). mation was already saturated, and the loops were connected. Figure 9a,b shows the desorption spectrum before and after the 3.2 MeV Ni3+ ion irradiation at room temperature, respectively. A new stage appeared in the position close to Peak A because of the Ni3+ ion

Figure 8a,b shows the microstructure after Ni3+ ion irradiation at room temperature. After the irradiation (up to 3.0 dpa), high-density dislocation loops (approximately 4.1 × 1020 m−3) were detected. In the Zr alloys, interstitial-type dislocation loops (a-loops) and vacancy-type dislocation loops (c-loops) are known to form. Nakamichi et al. investigated the formation and growth process of a-loops in Zry-2 under electron irradiation using a high voltage electron irradiation (HVEM) [20]. Estimated migration energy for interstitial and vacancy were 0.17 eV and 1.0 eV, respectively. These dislocations formed in this study were identified as a-loops because of vacancy mobility at room temperature. A-loop for-

Figure 8a,b shows the microstructure after Ni3+ ion irradiation at room temperature. After the irradiation (up to 3.0 dpa), high-density dislocation loops (approximately 4.1 × 1020 m−3) were detected. In the Zr alloys, interstitial-type dislocation loops (a-loops) and vacancy-type dislocation loops (c-loops) are known to form. Nakamichi et al. investigated the formation and growth process of a-loops in Zry-2 under electron irradiation using a high voltage electron irradiation (HVEM) [20]. Estimated migration energy for interstitial and vacancy were 0.17 eV and 1.0 eV, respectively. These dislocations formed in this study were identified as a-loops because of vacancy mobility at room temperature. A-loop formation was already saturated, and the loops were connected. Figure 9a,b shows the desorption spectrum before and after the 3.2 MeV Ni3+ ion irradiation at room temperature,

*Quantum Beam Sci.* **2021**, *5*, x FOR PEER REVIEW 6 of 10

*Quantum Beam Sci.* **2021**, *5*, x FOR PEER REVIEW 6 of 10

*3.2. Effects of Nickel Ion Irradiation* 

*3.2. Effects of Nickel Ion Irradiation* 

**Figure 8.** (**a**,**b**) The dislocation loops formed after 3.2 MeV Ni3+ ion irradiation at room temperature. The irradiation dose was 3.0 dpa. Ion irradiation and TEM observation were conducted perpendicular to the <C> direction. **Figure 8.** (**a**,**b**) The dislocation loops formed after 3.2 MeV Ni3+ ion irradiation at room temperature. The irradiation dose was 3.0 dpa. Ion irradiation and TEM observation were conducted perpendicular to the <C> direction. **Figure 8.** (**a**,**b**) The dislocation loops formed after 3.2 MeV Ni3+ ion irradiation at room temperature. The irradiation dose was 3.0 dpa. Ion irradiation and TEM observation were conducted perpendicular to the <C> direction.

**Figure 9.** The thermal desorption spectra (**a**) before and (**b**) after 3.2 MeV Ni3+ ion irradiation with a dose of 3.0 dpa at room temperature. **Figure 9.** The thermal desorption spectra (**a**) before and (**b**) after 3.2 MeV Ni3+ ion irradiation with a dose of 3.0 dpa at room temperature.

**Figure 9.** The thermal desorption spectra (**a**) before and (**b**) after 3.2 MeV Ni3+ ion irradiation with a dose of 3.0 dpa at room temperature. As was discussed in Section 3.2, Ni3+ ion irradiation at room temperature induced a-loops into the samples. By increasing the level of cold work, stage A moved to the lower temperature side. Stage A corresponded to the recovery stage for weak trap sites as dislocation loops and hydrides formed in the surface region. Tangled dislocations (formed by cold work) and hydrides (formed in the thick region) contributed to stage B in the higher-temperature region.

In the Zircaloy-2 samples, a relatively large number (approximately 30 nm) of Zr2(Fe,Ni) precipitates were formed in the matrix [19]. Small Zr(Fe,Cr)<sup>2</sup> precipitates were also found in the vicinity of the Zr2(Fe,Ni) precipitates. The number density of the Zr(Fe,Cr)<sup>2</sup> precipitates was 2.0 <sup>×</sup> <sup>10</sup><sup>19</sup> <sup>m</sup>−<sup>3</sup> and that of the Zr2(Fe,Ni) precipitates was 3.2 <sup>×</sup> <sup>10</sup><sup>18</sup> <sup>m</sup>−<sup>3</sup> . The stability of these SPPs is necessary at higher dose levels because it is at these levels that the formation of c-loops and the dissolution of the SPPs are known to occur simultaneously [17,19].

Among these SPPs, the Zr(Fe,Cr)<sup>2</sup> precipitates are unstable during irradiation and undergo an amorphous transformation resulting in the decomposition and redistribution of other precipitates into defect sinks [15–17]. In this study, low-dose irradiation was chosen where phase stability of the SPPs was not essential. The irradiation dose was estimated to be approximately 18 dpa [21] at a burn-up of 50 GWd/t. This corresponded with the end of the life of the fuel rods for BWRs. The role of the redistributed precipitates and the number of c-loops formed at higher dose levels for hydrogen pickup are essential for the degradation of LWR fuel-cladding tubes during operation. *Quantum Beam Sci.* **2021**, *5*, x FOR PEER REVIEW 8 of 10

**Figure 10.** The amount of cold work dependance of the thermal desorption spectrum after 5.0 keV D2+ ion irradiation at room temperature: (**a**,**b**) lower temperature region and (**c**,**d**) higher-temperature region. **Figure 10.** The amount of cold work dependance of the thermal desorption spectrum after 5.0 keV D<sup>2</sup> + ion irradiation at room temperature: (**a**,**b**) lower temperature region and (**c**,**d**) highertemperature region.

As was discussed in Section 3.2, Ni3+ ion irradiation at room temperature induced a-**Table 2.** Total desorption of HD and D<sup>2</sup> obtained in cold-worked samples.


also found in the vicinity of the Zr2(Fe,Ni) precipitates. The number density of the Zr(Fe,Cr)2 precipitates was 2.0 × 1019 m−3 and that of the Zr2(Fe,Ni) precipitates was 3.2 × 1018 m−3. The stability of these SPPs is necessary at higher dose levels because it is at these levels that the formation of c-loops and the dissolution of the SPPs are known to occur

irradiation and undergo an amorphous transformation resulting in the decomposition and redistribution of other precipitates into defect sinks [15–17]. In this study, low-dose irradiation was chosen where phase stability of the SPPs was not essential. The irradiation dose was estimated to be approximately 18 dpa [21] at a burn-up of 50 GWd/t. This corresponded with the end of the life of the fuel rods for BWRs. The role of the redistributed precipitates and the number of c-loops formed at higher dose levels for hydrogen pickup

are essential for the degradation of LWR fuel-cladding tubes during operation.


**Table 3.** Desorption temperature range and their responsible trapping site for deuterium.
