Introduction
As the number of reactors being built and in operation increases, the generation and accumulation of spent fuel will result in potential radiation risks. There are two main treatment protocols for the spent fuel. The closed-loop treatment of offloaded spent fuel is more effective than traditional deep-burial treatment. The realization of a nuclear fuel closed-loop cycle reduces the pollution of nuclear waste and improves the utilization rate of Actinide element materials. The main post-treatment technology is to use boiling concentrated HNO3 to dissolve the spent fuel, and then the Purex process is used to extract uranium and plutonium (for recycling) from the spent fuel by an organic extractant [1, 2]. Therefore, the container materials used in spent-fuel reprocessing need to be robust under the harsh environment of high temperature, strong acid, and intense irradiation. The consequences of corrosion penetration are wide ranging, therefore alloys (with neutron absorption properties) that can be used at high temperatures and display a resistance to corrosion by concentrated HNO3 have become necessary materials for key equipment used during and after spent fuel treatment. Adding neutron poisonous elements to some corrosion-resistant alloy systems is expected to result in good neutron absorption properties, excellent mechanical properties, and corrosion resistance.
Currently, nuclear-class stainless steel 316 L, Ti-Ta alloy Ti35, and Zr alloys have been extensively adopted for use in the nuclear industry [3-11]. Boron is frequently added to alloys or other shielding materials as the preferred neutron poison element owing to its low cost and good neutron absorption capacity. Boron stainless steel (BSS) has excellent mechanical properties [12-14]; however, it exhibits severe intracrystalline corrosion in concentrated HNO3 solutions containing oxidation ions [15, 16]. Even more severe intercrystalline cracking can occur in a high-radiation environment [17-19]. Boron-titanium alloys exhibit excellent wear resistance and strength. However, the plasticity displays a marked reduction under irradiation [20], and the alloy displays even the loss of malleability in severe cases [21]. The alloy also swells owing to the transmutation of B, similar to BSS [22]. It appears that the addition of non-metallic boron to the alloy is self-limiting. As for zirconium and titanium alloys, in contrast to zirconium alloys, titanium alloys appear to exhibit an evident condensate corrosion phenomenon [23]. At high temperatures, the corrosion resistance of zirconium alloys is preferable to that of titanium alloys.
Mattern et al. [24, 25] reported the relationship between the solubility of Gd in Ti and Ti in Gd, and the temperature based on Vegard's law. The solubility of Gd in body centered cubic BCC-Ti at 1800 K was ~0.6–0.3 at. %, and the maximum solubility of Ti in BCC-Gd is ~0.8–0.3 at. %. Combined with the phase diagram of Gd-Ti and the above results, it can be seen that the solubility of Gd and Ti in the corresponding solid alloys is minimal. According to the Zr-Gd phase diagram [26], the solubility of Zr in the HCP-Gd base phase is 3.8 at.% (6.38 wt. %) at 1160 ℃, and the solubility of Zr in the HCP-Gd base phase decreases with a decrease in temperature. The maximum solubility of Gd in a BCC-Zr base phase appears to be approximately 1500 ℃, which is approximately 6.5 at. %, and then decreases with increasing or decreasing temperature. The maximum solubility of Gd in the HCP Zr phase was 2.8 at. % (4.73 wt. %) at 880 ℃, and the solubility decreases with a decrease in temperature. In addition, the solid alloy system is in a mixed crystal state of Gd and Zr.
Therefore, zirconium alloys, which exhibit excellent corrosion resistance, good mechanical properties [27], and superior thermal conductivity [28], have become the preferred materials for nuclear chemical equipment. Zirconium alloys do not possess neutron absorption capacity; therefore, it is necessary to add neutron toxic elements to improve the neutron shielding performance. However, the corrosion resistance of boron–zirconium alloys becomes markedly worse, and alloy swelling occurs during irradiation [29]. Gadolinium is another element, which may be considered. Gadolinium has the highest neutron shielding capability performance and can be added to zirconium alloys as a neutron poison. There are a few reasons for selecting gadolinium. Firstly, natural gadolinium has two isotopes, 155Gd-60600b and 157Gd-139000b, with extremely high neutron absorption cross-sections, which are considerably higher than that of 10B-3837b. Secondly, under long-term corrosion conditions, the degradation rate of gadolinium in water is lower than that of the borides [30, 31]. However, studies on the addition of the neutron toxicant gadolinium to zirconium alloys are still lacking.
This study aims to explore the influence of adding gadolinium to zirconium alloys on their corrosion resistance and neutron properties. In this study, Zr-Gd alloys with different Gd contents were prepared by non-consumable arc melting. To simulate the actual service conditions, 6 mol/L concentrated HNO3 at 95 ℃ was used for the test. The relationship between the Gd content in the Zr-Gd alloy and the corresponding corrosion resistance properties was studied. Neutronic calculations of the shielding of the Zr-Gd alloy were performed using the MCNPX-2.7.0 Monte Carlo method.
Experiments
Preparation of Zr-Gd alloy
A vacuum non-consumable arc melting furnace was employed to smelt 150 g of ingot castings. Varying amounts (1 wt. %, 3 wt. %, 5 wt. %, 7 wt. %, and 9 wt. %) of gadolinium were added to zirconium sponge; zirconium sponge (Zr-702) was used as a control. The zirconium sponge was mixed with the gadolinium chips and placed in a crucible. To obtain uniform Zr-Gd alloy ingots, the melting current was set at 300–500 A; each ingot was flipped and melted six times. The Zr-Gd alloy ingot underwent solid solution (1050 ℃, 1 h) treatment, quenching, hot rolling (800 ℃ insulation for 30 min, 20–50% of channel pressure), annealing (500 ℃; 45 min), and scale removal. Finally, the plate was machined to the required size.
Electrochemical corrosion test
The Zr-Gd alloy plates were cut into 12.5 mm×9.5 mm×1.5 mm-size samples. The surfaces and cross-sections of the samples were polished with water abrasive papers of 60 #, 240 #, 600 #, 600 #, 1200 #, and 2000 #, successively, to avoid gap corrosion during electrochemical corrosion. Zirconium wire was used as a conductive wire to avoid concentrated HNO3 infiltration into the plates.
Electrochemical performance tests were performed using a Koster CS350M Electrochemical Workstation (Wuhan, China). The Zr-Gd alloy was used as the working electrode, the reference electrode was a saturated calomel electrode (Ag/AgCl saturated KCl solution), and the counter electrode was a platinum electrode placed in the three-electrode system electrolyte. A schematic diagram of the device is shown in Fig. 1. The solution temperature was maintained at 95±1 ℃. The corrosion medium was a 6 mol/L concentrated HNO3 solution. Zr-Gd alloys with different Gd concentrations were placed in this system for dynamic potential scanning measurements. Finally, the polarization curve and alternating current impedance spectrum were obtained.
The electrochemical parameters were set as follows: scan from the initial potential of -1 V to the termination potential of 3 V, at a scanning rate of 1 mV/s. The open-circuit potential measurement occurs before the electrochemical impedance spectroscopy (EIS) measurement. The sample was immersed in the electrolyte for approximately 3,600 s until its open-circuit potential was stable. The scanning frequency ranged from 100 kHz to 0.1 Hz, with a sinusoidal perturbation potential amplitude of 10 mV.
The surface morphology was observed using a desktop SEM (COXEM EM30AX+, 12 kV, Korea) after corrosion. The laser intensity data were reconstructed using a 3D laser micro-imaging system (VK-150K, Kez, Japan), and the surface 3D images and height distributions of the samples were obtained. To detect the chemical compositions on the Zr-Gd alloy surface, XPS (Thermo Fischer, ESCALAB 250Xi) analysis was performed with a monochromatic Al Kα (hv = 1486.6 eV) source under a pressure of 4×l0-9 mbar, with a working voltage of 14.6 kV, a filament current of 13.5 mA, and signal accumulation for 20 cycles. The test pass-energy was 20 eV, the step size was 0.1 eV, and the charge correction was performed with a C 1s = 284.8 eV combined energy standard.
Results and discussion
Microstructure analyses
Fig. 2 shows the X-ray diffraction patterns of the Zr-Gd alloys with different Gd concentrations. The acronym hcp indicates a close-packed hexagonal crystal structure. The numbers in brackets indicate the crystal indexes. Zr-702 mainly exists in the α-Zr phase with an HCP crystal structure; the (002) and (101) lattice planes grow in competition. The overall left-shift of the crystal plane is due to the addition of the solid solution of Gd atoms in the zirconium lattice. With the addition of gadolinium, the gadolinium signal gradually increases but is not obvious. The Gd (102) plane is evident at 42.6°, wherein the existence of the hcp phase is completely demonstrated. However, the content is low because the main phase Zr signal is so strong that it overwhelms the Gd peak. Up to 7 wt.% or higher, (100), (002), and (101) can be found at 28.8°, 31.137°, and 32.411°, respectively. Gadolinium is insoluble in the zirconium HCP lattice at low temperatures; it can only melt into Zr at approximately 3.8 at.% (6.38 wt.%) at 1160 ℃. Although quenching retains the solution state at high temperatures, it also precipitates gadolinium atoms. Therefore, gadolinium phase precipitation can be clearly observed when the content is above 7 wt. %.
Metallographic corrosion photos and EDS surface scanning were prepared to determine the distribution of gadolinium within the HCP structure in the Zr-Gd alloy, as shown in Fig. 3. Metallography (Fig. 3a-1-d-1) shows that the grain size increases significantly in the Zr-Gd alloy with an increase in the content of gadolinium. As gadolinium is added, it is uniformly dissolved in the zirconium lattice, thus inhibiting the nucleation rate of the Zr metal. Gadolinium increases the viscosity of the alloy liquid, reduces the condenser depression of the zirconium alloy, and provides more crystallization time for the grain growth of the zirconium alloy. EDS mapping analysis (Fig. 3a-3-d-3), indicates that HCP Gd is evenly distributed in the grains of the zirconium alloy. When the concentration is less than 7 wt. %, the gadolinium is uniformly distributed in the grains and no visible precipitation occurs. However, when the content is ≥7 wt. %, uniform precipitation occurs while the gadolinium is uniformly distributed in the grains, but not concentrated at the grain boundaries.
Open-circuit potential
The open-circuit potential is the electrode potential at zero current density, and the positive and negative values of the electrode potential determine the tendency of the alloy corrosion. The electrode potential of the alloy in the electrolyte gradually stabilized. The gadolinium content was inversely proportional to the open-circuit potential, and the addition of gadolinium decreased the open-circuit potential.
As shown in Fig. 4, when the electrode potential is stable, the electrode potentials of Zr, 1 wt. %, and 3 wt. % are 0.98 V, 0.96 V, and 0.93 V, respectively. The electrode potential gradually decreases, indicating that the corrosion tendency gradually increases. However, the decrease in the electrode potential is very low, and its corrosion tendency is the same.
When the gadolinium content in the alloy reaches 5 wt. % and 7 wt. %, the electrode potential is maintained at the 0.85 V horizontal line, proving that the corrosion tendency under this composition is constant. The electrode potential of 9 wt. % is 0.75 V, and is more pronounced, so the component Zr-Gd alloy is the most prone to corrosion. It can be assumed that the 7 wt. % alloy composition belongs to the inflection point of the alloy.
This may be because under the high-temperature solution condition of 1050 ℃, the HCP-Gd atom can maintain approximately 6 wt. % content in α-Zr (HCP) and fully enter the zirconium lattice. The gadolinium atom is fixed in the α-Zr phase by cold quenching, forming a composite crystal structure of the α-Zr (Gd) and β-Zr phases. Excess Gd atoms beyond the solution range precipitate and become a preferential point for corrosion.
Polarization curve and corrosion morphology
Fig. 5 shows the corrosion polarization curves of Zr-Gd alloys with different Gd concentrations at 95 ℃ (6 mol/L HNO3). The polarization curve is divided into four potential zones: the active dissolution zone, transition passivation zone, stable passivation zone, and over-passivation zone, where the point corresponding to the peak is the critical passivation point. When the curve exceeds the critical passivation region, the current density increases with the potential. This shows that the corrosion and regeneration rate of the passivation film reaches dynamic equilibrium at this time. The passivation zones corresponding to Zr (1 wt. %, 3 wt. % and, 5 wt. %) are stable at the corrosion current density of 1×10-5 A/cm2. The corrosion current density of 7 wt. % is slightly higher than those of the other concentrations, indicating that the corrosion rate increases slightly but is generally consistent, while the corrosion current density of 9 wt.% rises to 1×10-4 A/cm2. The passivation films of Zr (1 wt. %, 3 wt. %, 5 wt. % and 7 wt. %) are more stable and have better protective performance than the passivation films of the 9 wt. % alloy when the Zr-Gd alloy is formed.
This is related to the maximum solubility of gadolinium in zirconium; gadolinium destroys the corrosion resistance of zirconium and gadolinium alloys when its content exceeds a reasonable level (about 6.38 wt. %).
The critical potential of the destruction of the passivation film of the Zr-Gd alloy is approximately 1.75 V. When the potential exceeds 1.75 V at high temperature, the corrosion current density of the Zr-Gd alloy rises sharply into the over-passivation zone. The passivation film is destroyed and ruptured because the passivation film generated by the zirconium alloy in concentrated HNO3 or high-temperature HNO3 is porous and unstable [32, 33].
The corrosion potential and current density can be calculated in the transition zone using the Tafel method [34], as shown in Table 1. Fig. 6 shows the variation diagram of the corrosion potential and current density of Zr-Gd alloys with different gadolinium concentrations in a 6 mol/L HNO3 (95 ℃) solution. With an increase in the gadolinium concentration, the corrosion current density of the Zr-Gd alloy gradually increases, and positively correlates with the gadolinium concentration. The corrosion current density of the 9 wt. % alloy rises sharply compared with that of the other concentrations, once again, confirming that the 7 wt. % alloy composition is the corrosion inflection point. However, the corrosion potential is inversely correlated with the gadolinium concentration. This tendency for corrosion is consistent with the results of the opening circuit potential. The stability of the Zr-Gd alloy oxidation film worsens and it is prone to damage and spot corrosion [35]. Spot corrosion undergoes self-catalysis and accelerates the reaction, and the dissolution rate in the hole is expected to be significant, resulting in accidents [36, 37].
| Alloy | Ecorr(mV) | Icorr(A/cm²) |
|---|---|---|
| Zr | 981.6 | 1.04×10–7 |
| 1 wt. % | 976.5 | 5.23×10–7 |
| 3 wt. % | 945.8 | 4.01×10–6 |
| 5 wt. % | 903.3 | 8.82×10–6 |
| 7 wt. % | 854.7 | 1.18×10–5 |
| 9 wt. % | 740.0 | 1.75×10–4 |
This is shown in Fig. 7 and Fig. 8. The residual passivation film on the alloy surface has many small holes and severe peeling marks after the polarization treatment. It is indicated that the passivation film generated on Zr-Gd alloy surface is an unstable porous passivation film. When the voltage exceeds the critical voltage, the passivation film is damaged and peels off because the corrosion rate is greater than the regeneration rate. The alloy corrosion rate accelerates and the corrosion current density increases sharply.
As shown in Fig. 8, the surface of the 5 wt. % Zr-Gd alloy produces evenly distributed micro porous spot corrosion. This is caused by the corrosion and shedding of Gd atoms or precipitated atomic groups. The evenly distributed shedding has a destructive effect on the oxide film. The corrosion liquid can enter and pass through the oxide film to continue the ion exchange, producing a continuous corrosion effect. The sample with a concentration of 9 wt. % (5.43 at. %) performs poorly in terms of its mechanical properties and total immersion corrosion; the micropores become more extensive, and signs of deterioration occur in overlapping sheets. Although not detached, it can be assumed that an extension has occurred internally. Corrosion is accelerated by continuously destroying the alloy surface via perforation corrosion.
To determine the specific composition of the residual oxide film generated on the surface after electrochemical corrosion, the surface composition was tested using XPS and the curve was calibrated. Gd was detected using 1 wt. %–9 wt. % through full spectrum analysis. To fit and divide the peaks of the O element, three peaks must correspond, as shown in Fig. 9.
The Zr-Gd alloy mainly contains Zr and Hf from Zr-702 and also contains 1–9 wt. % Gd. According to a comparison with the NIST XPS database, the binding energies of ZrO2, Gd2O3, and HfO2 are ~529.90–531.30 [38, 39], ~529.5–531.4, [40] and ~530.0–530.40 [41], respectively.
The residual oxide film was determined to be a mixture of ZrO2, Gd2O3, and HfO2.
With an increase in the Gd content, the proportion of the integral area of ZrO2 to the that of the oxide film decreases, which proves that the dense oxide film is gradually corroded and penetrated, and reduces the corrosion resistance of the alloy.
The three-dimensional morphology characterization of the 3 wt.%, 5 wt.%, and 7 wt.% alloys were tested by laser microscopy, as shown in Fig. 10. Sdr is the interface expansion area ratio; the surface roughness increases as Sdr increases. Fig. 10 shows the three-dimensional topography of the specified area with a magnification of 200×. where Sdr is the surface roughness of the shot area. With an increase in the gadolinium content, the Sdr value of the 3 wt.%, 5 wt.%, and 7 wt.% alloys are 0.6477, 1.194, and 1.748, respectively; the Sdr values of the Zr-Gd alloys increase gradually after polarization. Fig. 10 shows that the surface of the Zr-Gd alloy exhibits uniform corrosion and low roughness. With the increase in the gadolinium content, the stability of the passivation film produced by the Zr-Gd alloy also gradually decreases. Combined with the surface morphology, a local range of cracking and accelerated corrosion is observed.
Alternating current impedance spectrum of the Zr-Gd alloy
Fig. 11 shows the Nyquist and locally amplified plots of the Zr-Gd alloy with different gadolinium concentrations in a 6 mol/L HNO3 solution at 95 ℃. The radius of the capacitive arc represents the corrosion resistance of an alloy in a corrosive medium. The larger the radius, the better is the corrosion resistance of the alloy. Zirconium has the maximum arc-resistance radius, indicating the best corrosion resistance. The capacitive arc radius gradually decreases with an increasing gadolinium content, indicating that the corrosion resistance gradually decreases. This is consistent with the experimental results of the polarization curve.
Fig. 12 shows the Bode plots of the Zr-Gd alloy in high-temperature-concentrated HNO3. Fig. 12(a) shows the change curve of the impedance amplitude and disturbance frequency. The larger the mold value, the better is the corrosion resistance of the alloy. With an increase in the gadolinium content, the mode value of the Zr (1 wt. %, 3 wt. %, 5 wt. %, and 7 wt. %) alloys gradually decreases to a minimum, and its corrosion resistance does not mean much difference. The 9 wt. % component alloy has a mode value that is significantly lower than those of the alloys comprising the other gadolinium concentrations, which agrees with the experimental results of the polarization curve. Fig. 12(b) shows the change curve of the relationship between the phase angle and the disturbance frequency. The larger the phase angle and the wider the curve, the better the corrosion resistance of the alloy. The phase angle and frequency relationship curve are divided into two arcs, and the second arc of the 1 wt. %, 3 wt. %, 5 wt. %, and 7 wt.% alloys is roughly the same size. The alloy arc of the 9 wt. % Zr alloy is significantly smaller than those of the other alloys containing differing concentrations of gadolinium.
Fig. 13 shows the equivalent circuit-diagram model used for the EIS analysis. The equivalent circuit can be described as a resistor and constant phase angle element. The equivalent impedance spectral circuit of the Zr-Gd alloy in 6 mol/L HNO3 (95 ℃) solution fitted the corrosion potential by using ZSimpWin software tool. The impedance spectra provided the best equivalent fitting to the circuit R (C(R(CR))). Here, Rs is the solution resistance, representing the electrode impedance between the reference electrode and working electrode, Rpor is the oxide film resistance generated by the alloy, Rct is the charge transfer resistance of the electrode during the charge transfer process, and CPE is a constant phase angle element (understood as a capacitor) formed on the electric bilayer capacitor surface between the working electrode and corrosion medium. The continuous phase-angle element CPE expression is as follows: [42]
Fig. 13 shows that the polarization resistance is Rp = Rpor + Rct. The electrochemical parameters of Zr-Gd alloys in 6 mol/L HNO3 (95 ℃) solution were obtained using an equivalent circuit, as shown in Table 2. Because the corrosion resistance of the Zr alloy is realized by the generated passivation film ZrO2, the corrosion resistance of the alloy can be determined by the polarization resistance, Rp. With an increase in the Gd content, the polarization resistance and Rct of the alloy gradually decrease. It has been proven that with the addition of excess gadolinium, the Zr-Gd alloy passivation film will be porous, unstable, can burst, and even fall off. The difficulty in ion exchange between the passivation films is notably reduced. The charge transfer resistance Rct decreases, resulting in the transport of ions, and the charge of the corrosion resistance is reduced, accelerating the passivation film failure. As the corrosion rate of the alloy increases, the corrosion resistance of the alloy decreases. The corrosion resistance of Zr-702 is the same as that of 1 wt.%, 3 wt.%, 5 wt.%, and 7 wt.%, and a sudden drop occurs at 9 wt.%, which is also consistent with the results expressed by the polarization curve.
| Alloying constituent | Rs(Ω cm-2) | CPEpor(μFcm-2) | n1 | Rpor(Ω cm-2) | CPEdl(μFcm-2) | n2 | Rct(Ω cm-2) |
|---|---|---|---|---|---|---|---|
| Zr | 1.176 | 5.573 | 0.98 | 1134 | 37.07 | 0.82 | 10,974 |
| 1 wt. % | 1.029 | 6.779 | 0.96 | 1588 | 41.17 | 0.82 | 8059 |
| 3 wt. % | 1.022 | 4.157 | 0.99 | 2783 | 64.66 | 0.80 | 4695 |
| 5 wt. % | 0.992 | 5.481 | 0.97 | 2590 | 74.84 | 0.73 | 4049 |
| 7 wt. % | 1.244 | 7.854 | 0.96 | 1883 | 88.92 | 0.75 | 2758 |
| 9 wt. % | 0.896 | 16.05 | 0.92 | 1546 | 66.33 | 0.93 | 2288 |
Thermal neutron absorption properties of Zr-Gd alloy
The composition and thickness of the shielding material affected the shielding performance. The Monte Carlo simulation method is currently a widely used critical and shielding calculation method. The MCNP is a universal geometric, time-correlated, combined neutron-photon-electron transport Monte Carlo particle transport program. It can create continuous/multi-group energy cross-sections and solve the problems of neutrons, photons, electronic transportation, and joint transportation.
The Monte Carlo model requires the density of the Zr-Gd alloy. The theoretical density of metallic zirconium is 6.49 g/cm³, and that of metallic gadolinium is 7.901 g/cm³. The theoretical density of the Zr-Gd alloy for each component is listed in Table 3.
| at% | wt.% | ρ(g/cm3) | |
|---|---|---|---|
| 1 wt. % | Gd0.58Zr99.42 | Gd1Zr99 | 6.501611 |
| 3 wt. % | Gd1.76Zr98.24 | Gd3Zr97 | 6.524958 |
| 5 wt. % | Gd2.96Zr97.04 | Gd5Zr95 | 6.548473 |
| 7 wt. % | Gd4.18Zr95.82 | Gd7Zr93 | 6.572158 |
| 9 wt. % | Gd5.43Zr94.57 | Gd9Zr91 | 6.596015 |
| 11 wt. % | Gd6.69Zr93.31 | Gd11Zr89 | 6.620047 |
Fig. 14 simulates the thermal neutron transmittance, x, of Zr-Gd alloys with different Gd concentrations, where the shielding rate (absorption rate) is 1-x. The Gd concentration calculated by neutron shielding simulation was extended to a broader range, that is, 1 wt. % to 19 wt. %. According to the local magnification diagram, when the thickness of the Zr-Gd plate is 1 mm, the 1 wt.% Zr-Gd alloy will pass through 31.5 % thermal neutrons, to shield 99.9 % thermal neutrons; the thickness of the plate must be 7 mm, more and thicker Zr-Gd alloy material are needed to achieve the neutron absorption effect of 99.9%; therefore, the shielding effect is not good. When the Gd content is increased to 3 wt. %, the alloy can reduce the neutron transmittance to 3 % by a 1 mm plate, and the decreasing effect is significant; a 3 mm plate can absorb 99.9 % thermal neutrons. A 2 mm plate can completely shield neutrons when the concentration is 5 wt. %, and a 1 mm plate thickness can completely shield neutrons when the concentration is 7 wt. % or more.
Conclusion
In this study, Zr with excellent corrosion resistance and Gd with good neutron absorption performance were selected and developed to obtain Zr-Gd alloys for monitoring the corrosion resistance and neutron shielding function. The following conclusions are drawn.
(1) The corrosion resistance of the Zr-Gd alloy gradually decreases with increasing Gd content.
(2) When the Gd content in the alloy was ≤7 wt.%, the corrosion resistance of the alloy remained almost constant when the Gd content in the alloy was ≤7 wt.%, although there was a negligible decreaseThe corrosion resistance rapidly decreases between 7 wt.% and 9 wt.%, proving that ~7–8 wt.% is the inflection point of the alloy corrosion resistance.
(3) A porous and unstable passivation film was formed on the Zr-Gd alloy by applying an overpass voltage in boiling concentrated HNO3.
(4) When the Gd content was 3 wt.%, 99.9% of the hot neutrons can be absorbed without the need for a plate of 2 mm thickness. With an increase in the Gd content, the absorption effect of the hot neutrons becomes more apparent.
(5) The thermal neutrons are absorbed after elastic collision with Gd atoms, and produce secondary gamma-ray hazards [43, 44].
In future studies, the neutron absorption and secondary gamma-ray absorption properties of Zr-Gd alloys must be considered for optimization by adding heavy elements, such as W, Ta, and Bi.
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