**3. E**ff**ects of LPwC**

### *3.1. E*ff*ects on Residual Stress*

The effect of LPwC on RS was studied through experiments. As shown in Figure 2, a sample was immersed in water and driven two-dimensionally with an X-Y stage during consecutive irradiation of laser pulses. Samples were cut out from a type-304 austenitic stainless steel plate after 20% cold-working which simulated the irradiation hardening due to fast neutrons during long-term operation of NPRs (2 × 10<sup>25</sup> neutrons/m2, neutron energy >1 MeV). The size of the samples was 40 mm × 60 mm with 10 mm thickness and an area of 20 mm × 20 mm was processed. Laser irradiation conditions were 200 mJ pulse energy, 8 ns pulse duration, 0.8 mm focal spot diameter and 36 pulses/mm<sup>2</sup> pulse density. This corresponds to 50 TW/m<sup>2</sup> laser peak power density on the sample. Prior to LPwC, the sample surface was ground in the rolling direction of the original plate to introduce a tensile RS on the surface. X-ray diffraction (XRD; sin2Ψ method) was used to measure the surface RS, and the in-depth profile was estimated by alternately repeating the XRD and electrolytic polishing.

**Figure 2.** Experiment of underwater LPwC.

Figure 3 exhibits the RS in-depth profiles with and without LPwC together with those predicted by time-dependent elasto-plastic simulation based on finite element method (FEM) [23–25], which reproduced experimental results quite well in terms of magnitude and profile. It is obvious that LPwC can induce compressive RSs in the surface layer of material, typically up to around 1 mm depth.

**Figure 3.** Residual stress in-depth profiles of 20% cold-worked type-304 austenitic stainless steel. Time-dependent elasto-plastic simulation based on a finite element method (FEM) well reproduces the experimental result.

The simulation of LPwC was made in two steps. The first one is to calculate temporal evolution of plasma pressure based on Fabbro's model [26] in which the plasma was assumed to be an ideal gas. To calibrate the plasma pressure, we measured the expansion velocity of the plasma generated on the sample surface underwater [3], then the velocity was converted to the pressure with Fabbro's model. The second step is to calculate the RS in-depth profile by using a home-made FEM program SAFFRON developed in a framework of a non-linear displacement-based incremental scheme [27]. The calculation system was discretized with 20-node isoparametric solid elements [23–25]. The plasma pressure calculated in the first step of the simulation was used as the time-dependent external load working on the sample. Stress-strain relation was modeled by the data obtained from static tensile test of the sample material. The Poisson's ratio was assumed to be 0.28. The von Mises yield criterion and a combined hardening rule were applied in the second step of the simulation.

### *3.2. E*ff*ects on Fatigue Properties*

Fatigue test samples were prepared from a low carbon type austenitic stainless steel (type-316L) as shown in Figure 4 [28]. Two types of heat treatments were applied to the samples before LPwC, namely full heat (FH; 1373 K, 3600 s in vacuum) treatment and stress relieving (SR; 1173 K, 3600 s). Figure 5 shows the microstructure of the materials after the heat treatments. The grain sizes of the materials after FH and SR treatments were 88 μm and 24 μm, respectively. LPwC was made with 200 mJ pulse energy, 0.8 mm spot diameter and 36 pulses/mm<sup>2</sup> pulse density. Then, rotating bending fatigue testing (R = −1) were made with a frequency of 2820 rpm. During fatigue loading, the samples were cooled by flowing distilled water. The micro-vickers hardness (Hv) and RS were measured for the samples with and without LPwC [28].

**Figure 4.** Type-316L austenitic stainless steel sample: (**a**) Dimensions; (**b**) External appearance. The color of the center part changed from metallic to grayish due to direct laser irradiation.

**Figure 5.** Microstructure of type-316L austenitic stainless steel: (**a**) Full heat-treated (FH); (**b**) Stress-relieved (SR).

The results showed that LPwC hardened the surface of both FH and SR materials down to about 0.6 mm from the surface. The hardness of both materials was increased by about 140 Hv with LPwC and reached about 300 Hv at just below the surfaces. The RS in-depth profiles exhibited anisotropy between longitudinal (z) and circumferential (θ) directions; σz on the surface was about −400 MPa, on the other hand σθ was about −200 MPa. The maximum compressive RSs were about −600 MPa (σz) and −400 MPa (<sup>σ</sup>θ) at 60–100 μm depth.

Figure 6 shows the fatigue test results. Fatigue strengths of FH and SR materials with LPwC were 300 MPa and 340 MPa at 10<sup>8</sup> cycles, respectively, i.e., LPwC enhanced the fatigue strengths by 1.7 and 1.4 times as grea<sup>t</sup> as those of the reference materials. Fatigue properties enhancement was also confirmed in uniaxial fatigue of steel [16,29,30], aluminum alloy [31] and titanium alloy [15].

**Figure 6.** Rotating bending fatigue test results of type-316L austenitic stainless steel. Fatigue strengths of FH and SR materials were increased by LPwC by 70% and 40%, respectively.

### *3.3. E*ff*ects on SCC Susceptibility and Application to NPRs*

Creviced bent beam (CBB) type testing was performed to evaluate the effect of LPwC on SCC susceptibility [24]. Samples of 10 mm × 50 mm and 2 mm thick were cut out from a plate of type-304 austenitic stainless steel with thermal sensitization (893 K, 8.64 × 10<sup>4</sup> s) followed by 20% cold working. As shown in Figure 7, samples were bent to make 1% tensile strain on the surface by using a curved fixture. After LPwC on the sample surface, crevices were made with graphite wool, and then the samples were immersed in 561 K water with 8 ppm dissolved oxygen and 10−<sup>4</sup> S/m electrical conductivity for 1.8 × 10<sup>6</sup> s duration by using autoclaves.

**Figure 7.** Procedure of accelerating stress corrosion cracking (SCC) test: (**a**) Sample setting and LPwC; (**b**) Preparation of crevices on sample surface for immersion in autoclave.

After the immersion, the surface and cross-section of all samples were precisely observed with microscopes. Inter-granular type SCC appeared in all reference samples, however no cracks were found out in samples with LPwC. Typical cross-sectional micrographs are shown in Figure 8. LPwC induced compressive RSs on the surfaces of austenitic stainless steels, nickel-based alloys and their weld metals, and prevented SCC in all tested materials [32].

**Figure 8.** SCC test results of type-304 austenitic stainless steel: (**a**) Cross-section of reference material (unpeened); (**b**) Material with LPwC.

Figure 9 illustrates LPwC in a boiling water reactor (BWR) [1]. Laser pulses are delivered from the laser system on the top floor of the reactor building to weld lines of the reactor core shroud with waterproof guide pipes and mirrors at corners of the piping. An elaborate beam tracking/alignment system with a fast-responding anti-vibration function was developed and implemented to control laser irradiation point within accuracy of 0.1 mm at about 50 m away from the laser system.

Fiber delivery technology was also developed to extend the applicability of LPwC [4,5]. The intense laser pulses sometimes cause damage on the inlet surface of optical fiber and, if not, the incoming laser pulses tend to converge and lead to damage inside the optical fiber due to reflection at the curved boundary between core and cladding and/or the non-linear effect of refractive index. To avoid this situation, an inlet optics with a homogenizer consist of micro lens arrays was developed, which flatten the spatial distribution of laser intensity and eliminated conceivable hot spots. Thus, the technology was established for delivering frequency-doubled Nd:YAG laser pulses with 100 mJ energy and 5 ns duration with a single optical fiber, which improves the applicability to 3D structures, together with a tiny optical head as presented in Figure 10.

**Figure 9.** Schematic of LPwC for weld lines of a reactor core shroud in a boiling water reactor (BWR).

**Figure 10.** Fiber-delivered LPwC: (**a**) Optical head; (**b**) Mockup experiment for the bottom of a BWR.

After the completion of the system and personnel training, LPwC has been applied to reactor core shrouds, bottom-mounted nozzles, etc. of NPRs since 1999 [1,2].

### **4. Palmtop-Sized Handheld Laser Development**

The effect of low-energy LPwC on fatigue properties was investigated for HT780 welded joints around 2013. In the course of the investigation, the pulse energy was reduced from 200 mJ to 100 mJ and then 50 mJ, the fatigue lives were significantly prolonged nevertheless [33]. Further experiments showed LPwC with the pulse energy even down to 20 mJ or 10 mJ has sufficient effects to enhance fatigue properties as shown in Figure 11 [16].

Considering such progress on the low-energy LPwC, the development of 20 mJ-class palmtop-sized handheld lasers was initiated in 2014 in a five-year Japanese national program, ImPACT [17]. A near-infrared (λ = 1.06 μm), sub-nanosecond (<1 ns) and passively Q-switched Nd:YAG laser with a weight of less than 1 kg was developed in IMS (Institute for Molecular Science ) led by Prof. Taira [34,35], as shown in Figure 12.

**Figure 11.** Fatigue test results of 780 MPa grade high-strength steel (HT780) welded joints. LPwC with 10 mJ and 20 mJ pulse energies significantly extends the fatigue life.

**Figure 12.** Palmtop-sized handheld laser: (**a**) External appearance; (**b**) Handheld laser manipulated by a robotic arm along a pipe object. Neither the movement nor vibration affects the function of the handheld laser.

A concept of LPwC system with the handheld laser is illustrated in Figure 13. A miniaturized optical head containing the laser is manipulated by a multi-axes robotic arm. Such a simple LPwC system could certainly extend the applicability and drastically reduce the time required in all phases of applications, i.e., designing, manufacturing, system integration, testing, training, transportation, installation, operation, quality assurance and dismantling.

Compared to earlier LPwC systems with current massive lasers, the system proposed above would be much smaller and simpler taking full advantage of ultra-compact handheld lasers. The pronounced characteristics expected are as follows:


**Figure 13.** Schematic of LPwC using a handheld laser for SCC mitigation in a BWR: (**a**) Concept to apply LPwC to hidden weld lines; (**b**) Cutaway view of a reactor pressure vessel in outage. The concept reduces the scale of LPwC system and laser transmission distance from tens of meters (~50 m) to tens of millimeters (~0.05 m).
