Structural, physical, and radiation absorption properties of a signi ﬁ cant nuclear power plant component: A comparison between REX - 734 and 316L SS austenitic stainless steels

: Austenitic stainless steels ( SSs ) are commonly used as in - core and surrounding structural materials in today ’ s industrial BWR and PWR systems. Such adap - table steels have also been the primary materials studied and used in several advanced nuclear reactor technolo - gies, such as fast breeding and magnetic fusion reactors. In this study, some critical material properties, such as structural, physical, and radiation - shielding properties of REX - 734 and 316L SS, were experimentally evaluated and compared to those of a number of other alloys. In addi - tion to homogeneous element distribution, both alloys exhibit strong crystal orientation. The REX - 734 alloy has a tensile strength of 1,259 MPa, whereas the 316L SS alloy has a tensile strength of 495 MPa. Moreover, nitrogen in the REX - 734 alloy formed ultra - hard nitrides with Cr, Nb, and Si and precipitated into the structure and increased the strength. According to our ﬁ ndings, the mass attenua - tion coe ﬃ cient values of the 316L SS sample were slightly higher than those of the REX - 734 sample at all energies. It can be concluded that the REX - 734 sample, with its excep - tional strength qualities and excellent radiation attenua - tion capabilities, may be a viable nuclear power plant material for future investigations.


Introduction
It is well-known that shielding materials for X-ray and gamma-ray radiation are crucial [1][2][3][4][5][6][7][8] for reducing the harmful effects of exposure and severe consequences.On the other hand, experts in the nuclear industry are well aware of the essential role that shielding materials, like steel, play in protecting both the public and nuclear equipment in nuclear power plants.In a while, in the post-Fukushima era, even industry insiders are interested in understanding how to secure their operations.Stainless steel (SS) and other super alloys play a role in this objective.Effective operation of a normal nuclear power plant requires considerable usage of SS, which may be found in practically every system component, large or small.For instance, the pressure tubes used to circulate coolant around the reactor core are crucial to the plant's safety procedures since they pump coolant around the reactor core.Interestingly, they are almost always constructed of SS.As one of the major components of a nuclear power station, containment vessels are typically composed of SS.They serve an essential purpose in protecting the reactor from external elements and the employees from radiation exposure.However, certain properties must be provided by such materials used in engineering applications to better understand their employability in nuclear applications.These characteristics include a high density, a strong corrosion resistance, a high mechanical strength, a high melting temperature, and a low cost [9].As radiation-shielding materials, numerous materials, such as lead (Pb), concrete, various alloys, and barium, are currently employed.Lead is the most extensively utilized of these materials.Although lead has a high density and a cheap cost, its melting point and strength are insufficient [1].In addition, it is a poisonous metal that is hazardous to human health [10,11].Fe alloys have begun to garner interest in this field because they are inexpensive and have promising radiation-shielding capabilities [12,13].Due to their properties, SSs have a unique technical and economic significance among ferrous alloys.They might be used as SS radiation-shielding material.SSs have high melting temperatures, strong corrosion resistance, adequate mechanical strength, and high density [14].Moreover, their costs are affordable.SSs contain more than 12% chromium, and chromium increases corrosion resistance.With the inclusion of nickel in addition to chromium, the corrosion resistance and high-temperature oxidation resistance of the resulting alloy are increased.The most corrosion-resistant SSs are austenitic SSs containing chromium.Fe-Cr-Mn SS alloys have been developed and compared to Fe-Cr-Ni austenitic SSs [15,16] for radiation shielding in fusion reactors.There are several varieties of SS, including REX-734 [17].Although this alloy is like SS 316L, there are significant differences.Due to their composition, they demonstrate a greater corrosion resistance and higher mechanical strength than 316L SSs [18].The radiation-shielding capabilities of the widely available REX-734 alloy were examined in this study.Although this alloy has a similar composition to 316L SS, it has 3% more Cr and 3% more Mn in its structure.Other alloying elements, such as Ni, C, and Mo, are present in the same proportion in both alloys.It is anticipated that REX-734 would have a stronger corrosion resistance to its increased Cr and Mn contents [19,20].However, the effect of increasing quantities of these metals on the radiation-shielding characteristics of REX-734 alloy has not been investigated in the literature.An increase in Cr may result in a greater proportion of chromium oxide layer forming on the surface of the alloy, hence enhancing its radiation absorption capabilities.In a previous study, Susoy et al. [21] have observed that increasing the proportion of chromium oxide enhances the shielding material's radiation absorption characteristics.Meanwhile, Mn is a significant alloying element in steels from a metallurgical viewpoint.For instance, after carbon, the alloying element that enhances steel hardness the greatest is [22].It performs a crucial function in rendering the steel's sulfur harmless.Moreover, it decreases the critical cooling rate [23].An excessive amount of manganese has harmful effects.The lower produced long-term radioactivity of Fe-Cr-Mn austenitic SSs compared to typical Fe-Cr-Ni austenitic SSs [16,24] makes them a feasible material for fusion reactor structural components.However, there are insufficient investigations on the influence of manganese increase on the radiation-absorbent characteristics of the alloy.The literature review showed that high Mn SSs have the potential to be used as a radiation-shielding material, according to the authors.The study of Eissa et al. [24] is lacking in understanding the radiation absorption characteristics of specific alloys like REX-734.We intended to undertake a multi-phase characterization research on REX-734 and 316L SS in view of the demands of this concern and the current development strategies and efforts for fusion reactors.In this study, the structural, physical, and radiation-shielding properties of REX-734 and 316L SS were evaluated and compared to those of several other alloys.The results of the present study may be valuable for a better knowledge of the structural, physical, and gamma-ray attenuation characteristics of REX-734, where the relevance of this type of material for potential applications, such as fusion reactors, has been addressed.

Experimental procedure 2.1 Experimental gamma-ray transmission measurements
In this study, an experimental gamma-ray transmission setup was designed for the determination of absorption properties of REX-734 and 316L SS samples.Figure 1 shows the overall appearance of the modeled transmission setup.First, 133 Ba point isotropic source, which can emit various gamma-ray energies between 81 and 383 keV, was placed in a suitable place.Through a lead (Pb) collimator pair, the primary gamma rays were directed onto the samples.Another Pb collimator pair was also placed behind the sample in terms of absorbing scattered secondary gamma rays.A high purity germanium detector was placed in a Pb shield block.The Genie-200 program was utilized for the generation of spectra, which was processed in Origin 2021 software.Meanwhile, Figure 2 depicts the measured absorption spectra of 133 Ba point isotropic source.Absorption spectra were created by merging three distinct spectra acquired with no attenuator sample, REX-734 and 316L SS, respectively.Without an attenuator sample, the maximum number of gamma rays was counted.This is expected; yet, there is no material that may reduce the initial gamma-ray emission from a 133 Ba point isotropic source.Next, we located REX-734 and 316L SS samples individually between the source and detector.As can be observed, placing an absorber between the source and detector reduces the total number of gamma rays counted.However, significantly different counting values were found across samples of REX-734 and 316L SS.The quantity counted for REX-734 was lower than that of 316L SS.This may be a result of the greater attenuation of initial gamma rays in the REX-734 sample compared to the 316L SS sample.Consequently, we have shown that the designed gamma-ray transmission setup was able to produce a logical reaction against energetic gamma rays as well as the attenuation tendency of the REX-734 and 316L SS samples.Two samples at eight various gamma-ray energies were examined to figure out the important attenuation coefficients and other parameters as a function of the calculated attenuation coefficients through the counted gamma-ray quantities.was used.The chemical composition of these two alloys is given in Table 1.Both alloys (REX-734, 316L SS) were structurally characterized by applying X-ray diffraction (XRD; RIGAKU Miniflex600) analysis.In addition, microstructural characterizations of the alloys were made by scanning electron microscopy (SEM; Hitachi SU3500).

Structural and physical properties
In addition, energy-dispersive spectroscopy (EDX; Oxford AZtech) was used for elemental identification.They were subjected to a tensile test to determine the mechanical properties of the alloys.The tensile test was performed on a Shimadzu AG-X brand device with a crosshead speed of 2 mm/min at room temperature.
3 Results and discussion

Structural and physical properties
The XRD pattern of the alloys is given in Figure 3.Both alloys exhibit strong crystal orientation.It can be said that the austenitic phase with FCC crystal lattice dominates the structure of the alloys.Figure 4 depicts the SEM image and EDX analysis of REX-734.A homogeneous element distribution is observed, and there are 18% Cr, 8% Ni, 3.8% Mn, and 0.6% Mo in the region where the analysis was taken.In addition, there is an oxide film of 10% on the surface, which is an expected result.Figure 5 shows the SEM image and EDX analysis of 316L SS.A homogeneous element distribution is observed, and there are 17.5% Cr, 10.5% Ni, and 1.3% Mo in the region where the analysis was taken.The results of the tensile test applied to the samples are given in Figure 6.As can be seen from the figure, both the yield strength and tensile strength of the REX-734 alloy are very high compared to that of 316L SS.The yield strength of the REX-734 alloy is 1,054 MPa, while that of the 316L SS alloy is 208 MPa.The difference is more than five times.Likewise, the tensile strength of the REX-734 alloy is 1,259 MPa, while that of the 316L SS alloy is 495 MPa.Looking at the elongation values, it can be said that 316L SS alloy is more ductile.The fact that the strength values of the REX-734 alloy are higher than the 316L SS alloy is due to the composition difference.REX-734 alloy has 3% more Mn.This is because Mn causes the strength increment by 100 MPa for every 1% increase in steel up to 3% [25].One of the factors in the high strength of the REX-734 alloy is Mn contribution in the structure.Another effect is the difference in the Si ratio.REX-734 alloy contains 0.40% higher Si.It has contributed to the increase in strength.One of the most important reasons for the increase in strength is that there is 0.3% nitrogen in the REX-734 alloy.It is a well-known fact that nitrogen, which is present in a certain amount in steels, increases the strength [26][27][28].The presence of a nitriding alloying element in a steel together with nitrogen causes a significant increase in strength.Nitrogen in the REX-734 alloy formed ultra-hard nitrides with Cr, Nb, and Si and precipitated into the structure and increased the strength [29,30].

Gamma-ray and neutron-shielding properties
The percentage of photons eliminated from a monochromatic X-ray beam by a homogeneous absorber per unit mass is described by a constant called the mass attenuation coefficient (µ ρ ).This density-independent parameter is crucial for the determination of attenuation properties of materials against ionizing gamma rays or X-rays.In this study, we calculated the µ ρ values of REX-734 and 316L SS samples at 81, 162, 223, 276, 303, 356, 384, and 661 keV gamma-ray energies.Figure 7 shows the variation in µ ρ values as a function of abovementioned energy values.As it is seen, µ ρ values decreased with increasing gamma-ray energy.Hence, the number of gamma rays absorbed by the absorber material decreases with increasing energy.This may be explained by the differences in penetrability between low-and high-energy gamma rays.In other words, a significant portion of low-energy gamma rays are absorbed per unit mass of absorber, and this portion decreases drastically with  increasing gamma-ray energy.The measured gamma-ray energy values are shown in Table 2.Meanwhile, the measured µ ρ values were compared using the EpiXS code [31].
As shown in Table 2, the theoretical and experimental values for REX-734 and 316L SS samples are in good agreement.According to our findings, the µ ρ values of the 316L SS sample were slightly higher than the REX-734 sample at all energies.According to these findings, the number of gamma rays that will be absorbed per unit mass of the 316L SS sample will be higher than that of the REX-734 sample.When a photon beam passes through a material with a thickness of the half value layer (HVL), the quantitative intensity of the beam is reduced by a factor of two.For materials whose absorption properties are being studied, this parameter is significant.The practical and theoretical HVL values for REX-734 and 316L SS samples were shown in Figure 8. HVL values were found to be at their lowest point at low energies.However, there was a noticeable increase in HVL values when gamma-ray energy increased from 81 to 661 keV.This means that even at very thin thicknesses, the material can absorb photons with relatively low energies.HVLS values, on the other hand, increased as energy levels increased, as seen in Figure 8.
A crucial indicator that more material thickness is required since this will reduce the quantity of penetrating photons by half as their energy increases.At 661 keV, both samples' HVL values were at their highest.The sample 316L SS has the lowest T0.5 values, as shown in the figure.The point here is to emphasize that a 316L SS sample of thinner thickness might accomplish quantitative reduction more effectively than REX-734.The fluctuation in effective atomic  numbers (Z eff ) as a function of incident photon energy is seen in Figure 9.The difference in mass between these compounds has a direct effect on the total atomic weight, which is the consequence of changes in chemical composition.Between REX-734 and 316L SS, each compositional modification (see Table 1) affected the atomic weights.This increase also modified the effective atomic number values during the absorption of energetic photons, allowing the 316L SS sample with the greatest concentration of the heavier atoms to attain the highest values.

Benchmarking of mean free path values
During the benchmark phase of this study, the mean free path (mfp) values were computed and compared to other materials such as FeCoNiMnCr, Fe86B5C8Pa1, Inconnel 718, and FeSiMnCrNi [12,[32][33][34].The mfp is the distance gamma rays must travel for two consecutive interactions and is measured in centimetres.By producing consecutive contacts at small distances, this material absorbs the absorption process more efficiently because of the direct contact.A low mfp rating often implies the superiority of absorption capabilities over comparable materials.Figure 10 illustrates the changing trends of the mfp values of bioactive glasses created.As seen in the figure, the mfp values of photons with a high energy are higher for all samples.Nonetheless, the mfp value of the Inconnel 718 sample was the lowest among the samples examined.On the other hand, 316L SS was found to have the second lowest mfp values.This demonstrates that the 316L SS sample has slightly higher absorption properties than the REX-734 sample, as well as better absorption properties compared to similar materials in the literature.

Conclusions
The great majority of nuclear scientists and industry experts concur that nuclear energy will continue to be in demand soon owing to the desire for clean energy supplies.Like the nuclear energy sector grows and  nuclear engineers construct more high-temperature reactors, the demand for technology and products that can endure more adverse and hazardous conditions grows.Equipment will need to be more durable, corrosion resistant, and efficient than ever.Accordingly, it is necessary to manufacture SS grades.Nevertheless, the SS sector is rising to the challenge of making nuclear energy safe and more efficient.The primary objective of this work was to characterize these steel types and compare them comprehensively to other comparable materials, given the significance of these steel types in nuclear applications.The following is a summary of the study's resulting conclusions.
• Both REX-734 and 316L SS alloys exhibit strong crystal orientation.
• The strength values of the REX-734 alloy are higher than the 316L SS alloy due to the composition difference due to 3% more Mn contribution in REX-734.• The yield strength and tensile strength of the REX-734 alloy are very high compared to those of 316L SS. • The nitrogen in the REX-734 alloy formed ultra-hard nitrides with Cr, Nb, and Si, precipitated into the structure, and increased the strength.• It can be concluded that the controlled level of nitrogen contribution in the REX-734 alloy can enhance the strength.• REX-734 and 316L SS samples have similar attenuation properties against gamma-rays emitted from 133 Ba radioactive isotope.• The µ ρ values of the 316L SS sample were slightly higher than those of the REX-734 sample at all energies.
Manufacturers of SS generate new alloys to fulfill more demanding criteria as demand rises.This comprises more durable steam generators, tube sheets, pump castings, and pressure vessels resistant to corrosion.These are all essential components of the nuclear industry.Therefore, it can be concluded that the REX-734 sample, with its exceptional strength qualities and excellent radiation attenuation capabilities, may be a viable nuclear material for future nuclear technology enhancement studies.

Figure 1 :
Figure 1: Experimental setup for gamma transmission measurements.

Table 1 :
Chemical composition of 316L SS and REX-734

Table 2 :
Experimental and theoretical mass attenuation coefficients (µ ρ , cm 2 /g) for the alloy samples