358 Ⅳ. 材料研究 CNIC-01638/37 CNNC-0003 Radiation Effects in Stainless Steels and Tungsten Using as ADS Spallation Neutron Source System XU Yongjun WANG Zhiqiang ZHU Jiazheng FU Youlin ZHENG Yongnan ZHOU Dongmei XU Guoji DU Enpeng ZHU Shengyun China Institute of Atomic Energy, Beijing, 102413 Abstract: Radiation effects have been studied in the home-made modified 316L stainless steel and standard stainless steel and tungsten irradiated by 80 MeV 12C or 85 MeV 19F ions. The experimental results show that the radiation resistant property of stainless steels is much better than that of tungsten and the homemade modified 316L stainless steel has the best radiation resistant property among them. The stainless steels are a good choice for beam window material of the ADS spallation neutron source system, and the homemade modified 316L stainless steel is the best choice. Key words: ADS spallation neutron source, Radiation effect, Heavy ion simulation, Positron annihilation lifetime INTRODUCTION The accelerator driven radioactive clean nuclear power system (ADS) is a novel innovative idea for the sustainable development of nuclear power system [1, 2]. Intense attention has been paid to it both at home and abroad. The spallation neutron source system is one of the three key parts of ADS [1, 3], which provides source neutrons of ～1018 n/s for the burning-up of fuels. It is mainly composed of the target and beam window. Stainless steels and tungsten are important candidate materials of the beam window and the spallation neutron source target. They are irradiated by high-energy and intense protons and neutrons during operation. The accumulated dose could reach a couple of hundred dpa per year, and radiation damage is very severe Ⅳ. 材料研究 359 in them. The radiation damage study of the spallation target and beam window materials is of great importance for the understanding of the their lifetimes and the safe operation of the ADS. The present work was motivated to investigate the radiation damage and to compare the radiation resistance properties of the standard stainless steel and tungsten and the home-made modified 316L stainless steel. The modified 316L stainless steel, standard stainless steel and tungsten were irradiated by heavy ions up to 30.0 dpa, 22.8 dpa and 20.0 dpa, respectively. It takes several hundred days to reach such a high dose of 20～30 dpa by the proton and neutron irradiations using currently available proton and neutron sources. Radiation damage is caused mainly by atomic displacement in materials irradiated by neutrons, protons and heavy ions, but the displacement rate of heavy ions is much higher [4] than those of neutrons and protons. Therefore, the heavy ion irradiation was adopted to simulate the high dose proton and neutron induced radiation damage. The produced radiation damage was examined by a positron annihilation lifetime technique that is a very sensitive tool for investigating created radiation damage on an atomic scale. 1 EXPERIMENTAL The samples used in the experiment were the home-made modified 316 austenitic stainless steel (MSS) and the commercially available standard stainless steel (SS) and tungsten (W). The size of the samples was φ 23 mm× 0.5 mm. SS was 1Cr18Ni9Ti SS. The purity of W was 99.9%. MSS was made of Cr-15.05%, Ni-14.76%, Ti-0.32%, P-0.007%, S-0.007%, Mn-1.78%, Si-0.52%, C-0.048% and Fe balanced to 100%, and treated by 20% cold- working. The cold-working and Ti- addition can greatly improve radiation resistance property of stainless steels. Before irradiation the MSS samples were annealed at 800℃ to eliminate mono- and di-vacancies and dislocation. All the samples used in the experiments were mechanically polished to a mirror-like surface. The samples were irradiated at room temperature by 80 MeV 12C or 85 MeV 19F ions from the HI-13 tandem accelerator at China Institute of Atomic Energy. In order to ensure the room temperature irradiation, the irradiating beam current was controlled to be ～0.3 µA and the samples were firmly contacted with the metal wall of irradiation chamber 360 Ⅳ. 材料研究 that was cooled by an electric fan. The irradiation doses were 2.28 dpa and 22.8 dpa for the SS samples, 2.0 and 20.0 dpa for the W samples, 30.0 dpa for the MSS samples. The displacement per atom (dpa) created by the heavy ion irradiation in the sample was calculated using a TRIM program [5,6]. The radiation damage generated in the samples was examined by a positron annihilation lifetime technique. The positron lifetime measurements were performed at room temperature for both irradiated and un-irradiated samples. The positron annihilation lifetime measurements were carried out by means of a fast-fast coincidence positron lifetime spectrometer consisting of a pair of BaF2 scintillation detectors. The time resolution of the spectrometer is 210 ps to 60Co γ rays. Two identical samples irradiated to the same irradiation dose were arranged as a sandwich with a 40 µCi 22Na positron source in the center. Besides the source components, all measured positron lifetime spectra were well fitted by two lifetime components with a fitting variance of less than 1.3. 2 RESULTS AND DISCUSSION τ τ The extracted lifetimes 1 and 2 and their relative I1 and I2 for MSS, SS and W are shown in Figs. 1 and 2 as a function of irradiation dose. τ The annihilation lifetime f of free positrons is ~110 ps for iron and ~105 for tungsten. The annihilation lifetimes of positrons trapped at mono- τ τ τ τ vacancies and di-vacancies are 1v = 1.3 f and 2v = 1.5 f, respectively. Vacancy clusters or voids possess much longer annihilation lifetimes. Usually, the larger the vacancy cluster, the longer the annihilation lifetime. In the τ present work we assume that the short lifetime 1 is a weighted average of the annihilation lifetimes of free positrons and positrons trapped at the mono- and τ di-vacancies, and the long lifetime 2 is related to the positrons trapped at the τ vacancy clusters or voids. It can be seen from Figs.1 and 2 that only 1 was detected the value of which is in good accord with the free positron τ τ annihilation lifetime for the un-irradiated W, and both 1 and 2 were τ τ observed for the un-irradiated MSS and SS. 1 = 112.9 ps and 2 = 240.0 ps τ τ for the un-irradiated SS, and 1 = 115.0 ps and 2 = 256 ps for the un- τ irradiated MSS. The 1 values of 112.9 and 115.0 ps are close to the free Ⅳ. 材料研究 361 τ positron annihilation lifetime. 2 is connected to the small vacancy clusters, and vacancy cluster is a bit larger in MSS than in SS. One can say that before irradiation there is no vacancy in W, and small vacancy clusters with a low intensity appear in SS and MSS. MSS is treated by cold working which produces mono- and di-vacancies, dislocations and vacancy clusters in it[7]. After thermal annealing at 800 ℃ all the defects except vacancy clusters were annealed away . τ For the SS irradiated to a dose of 2.28 dpa, 1 increases to 122.9 ps, indicating the irradiation mainly creates the mono-vacancies in SS, and the size of the vacancy cluster becomes larger and its intensity increases by ~9%. τ In the case of W 1 = 136 ps reveals the production of the mono- and di- τ vacancies after 2 dpa irradiation. The rapid increase of 2 to 372.6 ps means the formation of rather large vacancy clusters by the irradiation. 450 τ τ22 W 400 350 SS 300 MSS 250 Lifetime/ps 200 Lifetime (ps) Lifetime τ W τ11 150 SS MSS 100 0 5 10 15 20 25 30 IrradiationIrradiation dose/dpadose (dpa) τ τ Fig. 1 Positron lifetimes 1 and 2 as a function of irradiation dose in MSS, SS and W τ τ In W irradiated to 20.0 dpa the lifetimes, 1 and 2, and relative intensity Ι τ 2 arrive at the values of 178.4 ps, 424.1 ps and 33%, respectively. 1 = 178.4 ps means that the irradiation generates tri-and quadri -vacancy clusters 362 Ⅳ. 材料研究 τ besides mono- and di-vacancies, otherwise, one cann’t obtain such a large 1. τ Here, we assume 1 is the average value of lifetimes of free positrons and positrons trapped at the mono-, di- , tri- and quadri -vacancies. In metals the positron lifetime reaches 500 ps, indicating the zero-density limit of electron τ density. In the irradiated W the fitted lifetime 2 is ～424 ps. This large value τ of 2 clearly demonstrates the formation of large size voids; the intensity of τ τ which is 33%. In SS irradiated to 22.8 dpa the lifetimes, 1 and 2, and relative τ intensity I 2 are 135.1 ps, 343.9 ps and 23.8%, respectively. 1 = 135.1 ps reveals the possible creation of mono- and di-vacancies, giving rise to this average lifetime. The 22.8 dpa irradiation generates the ～ 10-vacancy clusters. The trapping of positrons by this vacancy clusters yield a lifetime of 343.9 ps the intensity of which is rather high and reaches 23.8%. For the MSS τ irradiated to 30 dpa the lifetime of 1 increases to 125 ps indicating the τ production of mono-vacancies, and 2 to 286 ps which is corresponding to the lifetime of positrons trapped at 5-vacancy clusters[9], the intensity of which is 19.6%.