The Attenuation Capability of Selected Steel Alloys for Nuclear Reactor Applications

Neutronsand gamma ray attenuation of different steel grades (SS304, SS304L, SS316L, SS430, a modified high manganese-nitrogen austenitic stainless steel, and developed cobalt-free Maraging steel) was measured to study their capability to be used as nuclear reactor materials. The hardness and microstructure of the studied steel alloys were carried out using Vickers hardness and optical microscope respectively.Neutron and gamma rays measurements were carried out using a narrow beam transmissions geometry method. Measurements and calculations of gamma ray attenuation coefficients were carried out at energies 238.63, 338.28, 583.19, 911.2, 968.97, 1173.23, 1332.49, and 2614.51 keV. The transmitted gamma rays were detected by the Hyper Pure Germanium detector (HPGe), while, the neutron flux emitted from 241 Am-Be neutron source was used to measure the neutron removal cross section for both slow and total neutrons. The transmitted beam of neutrons was measured under a good geometric conditions using 3 He counter.A good agreement between experimental data of mass attenuation coefficients and theoretical results calculated by the WinXcom computer program (version 3.1) was obtained.


Introduction
In high-temperature medium, such as this of nuclear reactors, suitable radiation resistance materialsare needed [1][2] .In this regard, owing to the superior characteristics of stainless steels (SSs) alloys such as excellent mechanical properties, high radiation resistance, and the corrosion resistance, stainless steel alloys were considered as promising candidate materials for nuclear reactor systems [3][4] .Steels are being utilized extensively for structural components in many Reactor designs and also in other high temperatureapplications [5] .The stainless steel alloys are the common structural materials in-core and out-of-core components of nuclear power plants.Their high resistance to degradation by irradiation is particularly important for nuclear applications [6][7] .Nickel-free austenitic stainless steels having a large amount of chromium, manganese, nitrogen, and small amount of molybdenum are currently developed to enhance the strength and corrosion resistance of stainless steel.Hence, manganese and nitrogen are employed instead of nickel to obtain austenitic phase, because nitrogen and manganese also stabilize austenitic phase and control mechanical properties of stainless steel as same as nickel [8][9] .Commercial cobalt free Maraging steels alloys, with molybdenum, titanium, and aluminum additions, are used in industrial applications that demand high strength steels, such as in aerospace and nucleartechnologies [10][11] .
In this work, the neutron and gamma ray shielding properties of SS304, SS304L, SS316L, SS430, a modified high manganese-nitrogen austenitic stainless steel and a developed chromium-titanium containing cobalt-free Maraging steelwerestudied experimentally under good geometry conditions.Neutron and gamma rays attenuation parameters were deduced.Additionally, Vickers hardness and microstructure were carried out.

Experimental measurements and theoretical calculations
Different stainless steel grades including three austenitic stainless steels SS304, SS304L, and SS316L and one ferritic stainless steel SS430were prepared using a pilot plant induction furnace.In addition, modifiedchromium-titanium containing cobalt free Maraging steel(MRG-MoCrTi), and high manganese-nitrogen (3.84%Mn-0.235%N)freenickelaustenitic stainless steel (SSMn4N) were also prepared.The chemical compositions of the investigated steels are given in Table (1).The different produced steels were hot forged from the cast ingots of diameter 70 mm to bars with final cross section of 30x30 mm 2 .Samples from all forged steels, except Maraging steel(MRG-MoCrTi)were initially solution annealed at 1050°C for 30 min, followed by water quenching.WhileMaraging steel(MRG-MoCrTi)was initially solution treated at 820°C for 60 min, and air cooled.For martensite aging, the solution treated maraging steel samples were subjected to age-hardening at 480 o C for 120 min and air cooled [12] .After etching of SS304, SS304L, SS316L andSS430with a marble reagent, Maraging steel with copper sulphate, and high manganes-nitrogen stainless steel with Picral+Viellathe, microstructure was observed using an optical microscope.Vickers hardness test was carried out on polished steel samples.The attenuation properties of neutrons and gamma rays were carried out using 241 Am-Be neutron source; with activity 5Ci and neutron yield = (1.1-1.4)x 10 7 n/sec,in addition to 60 Co and 232 Th gamma ray sources.The 3 He neutron detector was used to counter the transmitted beam of total and slow neutrons, figure (1).Whilethe Hyper Pure Germanium detector (HPGe) was used to measure the gamma ray intensities for the studied energy lines, figure (2).Neutron and gamma rays attenuation parameters such as removal cross section of both total and slow neutrons, linear and mass attenuation coefficients of gamma rays were deduced from the experimentally attenuation curves.Additionally, theoretical calculations of gamma ray mass attenuation coefficients were carried out using WinXCom computer program (Version 3.1) [13] and compared with the experimental ones.

Results and Discussion
The observed microstructures of the different investigated steels were given in figure (3).The examination of these photos reveals that SS304, SS304L, SS316Land high manganese-nitrogen stainless steels have microstructure composed of mainly austenite.On the other hand, SS430 showed ferritic phase while maraging steel revealed martensite microstructure.

Steel SS 304
Steel SS 430

Steel SSMn4N
Steel MRG-MoCrTi Hardness variation of the investigated steelswas shown in figure (4).A higher Vickers hardness value was observed for thecobalt free Maraging and high manganese-nitrogen steel alloys,respectively.The higher hardness of high manganese-nitrogen free-nickel austenitic stainless steel (SSMn4N) comparing with the other stainless steels SS304, SS304L, SS316L and SS430could be attributed to the its higher manganese and nitrogen contents resulting in higher solid solution hardening.The highest hardness of cobalt-free Maraging steel (MRG-MoCrTi) could be attributed to the martensite microstructure and age hardening [14][15][16][17] .

Fig 4:Hardness variation of the investigated steels
Total and slow neutron removal cross section of the studied steelalloyswere carried out and listed in table (2).It is observed that, the highest value of the effective removal cross sections of total neutrons for the high manganese-nitrogen stainless steel (SSMn4N).On the other hand, thehighestvalues of effective removal cross section of slow neutron for cobalt-free Maraging steel.The small values of slow neutron removal cross-section may be attributed to the absence of high slow neutrons cross section element in the studied steel samples.The linear attenuation coefficients (μ) and half value layer(HVL) of the selected steel alloyswere investigatedand listed in table ( 3) and ( 4) respectively.From table (3), it was found that the high manganese-nitrogen stainless steel (SSMn4N) and cobalt free Maraging steel (MRG-MoCrTi) have high linear attenuation coefficient and hence a lower HVL respectively.The experimental mass attenuation coefficients σ Exp and those compute theoretically σ Theo .using WinXCom computer programofthe selected steel alloys wereshown in figure (5).It is shown that the values of mass attenuation coefficient of the alloys arehigh in photo-absorption region, reduces gradually and become lowest in Compton scattering region.These variationscan be explained by photon energy and Z-dependency ofinteraction cross section of the elements.Additionally, a good agreement was observed between both measured and calculated data of mass attenuation coefficients.

Conclusion
From the previous discussions we conclude that,CobaltFree Maraging Steel (MRG-MoCrTi)and Modified High Manganese-Nitrogen austenitic stainless steel (SSMn4N) have a high hardness in comparison with the standard stainless steels SS304, SS304L, SS316L and SS430.The microstructure of the SS304, SS304L, SS316L, SS430 and SSMn4Nshows an austenite phase while the steel (MRG-MoCrTi)has a matensite phase.The highest value of total neutron macroscopic cross-sectionwas found for the high manganese-nitrogen austenitic stainless steel,while,the cobalt free Maraging steel has the highest value of effective removal cross section of slow neutron.The high manganese-nitrogen austenitic stainless steel and cobalt free Maraging steel have high linear attenuation coefficients and hence a lower HVL, respectively.Good agreement was achieved between the experimental data of gamma ray mass attenuation coefficients and the corresponding theoretical prediction.The present study are useful for potential applications of these materials in nuclear reactor design as an effective gamma ray and neutron attenuator materials with high hardness.