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The radioactive products of the spallation target are the result of the fission neutron radiation field and high-energy proton radiation field, where the fission neutron radiation field includes the fission neutrons from the reactor to the spallation target and a small number of spallation neutrons reflected by the reactor. The high-energy proton radiation field includes the proton and its induced secondary particles in the spallation target. In this section, the results of the activity, main radionuclides, the toxicity of radionuclide products, and the photon emitted from radionuclide products are analyzed for the spallation target. The contributions of the fission neutron radiation field and the high-energy proton radiation field to those results are compared to demonstrate that the effect of fission neutron is an indispensable factor for the radionuclides analyses of the target system.
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The spallation target structural parts consist of the beam tube, bean window, main shell, and guide tube, all of which are made up of 316L steel. The specific activity of the LBE spallation target and its structural parts is related to the production rate and decay rate of radioactive products. In general, the specific activity of the irradiated materials is increasing until it approaches saturation during the irradiation period, and be reducing continuously during the cooling period. The radioactive products are determined by the radiation field and the irradiated materials, so the specific activity of the same material under the fission neutron radiation field is different from that under the high-energy proton radiation field, and the specific activity of different materials varies under the same radiation field, such as LBE material and 316L material, as shown in Fig. 2.
Figure 2. (color online) Specific activity of spallation target on (a) the fission neutron radiation field, (b) high-energy proton radiation field, and (c) the mixed radiation field. (d) Activity ratio,
$ A_{\rm n} $ and$ A_{\rm p} $ are activities induced by the fission neutron radiation field and the high-energy proton radiation field, respectively.Fig. 2 shows the time evolution of the specific activity of the LBE spallation target and its structural parts. Fig. 2(a), Fig. 2(b), and Fig. 2(c) are the evolution of the spallation target’s specific activity with time under the fission neutron radiation field, the high-energy proton radiation field, and the mixed radiation field respectively. Fig. 2(d) is the time evolution of the ratio of the activity induced by the fission neutron radiation field to the activity induced by the high-energy proton radiation field.
In the irradiated period, the specific activity of the beam window is 1~2 orders of magnitude higher than other structural parts because the beam window is the main area for proton energy deposition [ 9] , as shown in Fig. 2(b). The activation of material will reach equilibrium after a long irradiation time [ 25] . The saturation point of radioactivity is about 10 h for the beam tube, guide tube, and main shell as shown in Fig. 2(c). The radioactivity saturation point of LBE is about 10 000 h. The beam window is the most radioactive structural part, needs to be replaced frequently, it is reasonable to choose 10 000 h as the irradiation time for the activation analyses in this paper. The specific activity of LBE spallation target can be on the magnitude of
$ 10^{13} $ Bq/kg or higher when saturation irradiation is reached. When the irradiation is terminated, the specific activity decreases with cooling time. In the Fig. 2(c), it takes about 1 000 d cooling time for the specific activity of beam window to go down an order of magnitude, 200 d for others structural part, and 500 d for LBE.In Fig. 2(d), the activity ratio of the beam tube, guide tube, and main shell is always higher than 10 after 10 000 cooling days, which means that long-lived nuclides are mainly induced by the fission neutron. The activity ratio of the beam window becomes greater than 1 after 100 000 cooling days, which means that the total amount of long-lived nuclides produced by the neutrons are more than those produced by protons. For the LBE, the activity ratio increases first and then decreases, and it is always less than 1 after cooling 1 000 d. It means that the radionuclide caused by protons will rapidly decay in the initial stage of cooling, and the long-lived nuclides in LBE are mainly caused by the protons. It is worth noting that the activity ratio of LBE tends to be 1 after the target is irradiated for 10 000 h, which means that the neutron and the proton play a similar role in the LBE activation. So the final disposal of the spallation target needs to take the fission neutron into account, especially for its structural part.
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After 10 000 h irradiation, the radionuclides whose activity contributed to more than
$ 10^{-6} $ of the total activity are recorded and analyzed. Table 1 shows the number of radionuclide types induced by neutrons and protons.Radiation
sourceBeam
windowBeam
tubeGuide
tubeMain
shellLBE Neutron 15 23 33 42 20 Proton 311 82 129 105 750 Total 312 83 130 106 750 Table 1. Number of radionuclide types induced by radiation source.
The types of radionuclides induced by proton beam include that induced by fission neutrons except for
$ ^{59} {\rm{Co}}$ in structural parts. In the fission neutron radiation field, neutrons entering into the spallation target include the fission neutrons and a small number of secondary neutrons are produced in the spallation target. These neutrons are moderated and scattered by nuclear fuel, LBE coolant, and structural materials before they get into the target system. Therefore, their energy is reduced, and could not induce as many nuclear reactions as high energy protons. As a result, the types of radionuclides reduces in turn in the main shell, guide tube, beam tube, and beam window due to their position far away from fuel.In the high-energy proton radiation field, the spallation reaction occurs basically on the beam window and LBE, the atomic number of spallation products is range from 1 to the maximum atomic number of irradiated material chemical composition [ 13] . The maximum atomic number of the nuclide in 316L and LBE is 42 and 83, respectively. Therefore, the radionuclide types of LBE can be up to 750, which is higher than the 316L radionuclide types 311.
Fig. 3 describes the radionuclides with main contributions in the total activity during the cooling time from 0 to
$ 1\times 10 ^{6} $ d. Since the beam tube, guide tube, and main shell are all made of 316L and mainly irradiated by neutrons, so the main radionuclides are isotopes of 316L material elements or their adjacent nuclides. The radionuclides with high activity in the cooling process are$ ^{56} {\rm{Mn}}$ ,$ ^{99} {\rm{Mo}}$ ,$ ^{51} {\rm{Cr}}$ ,$ ^{55} {\rm{Fe}}$ ,$ ^{63} {\rm{Ni}}$ ,$ ^{59} {\rm{Ni}}$ , and$ ^{93} {\rm{Mo}}$ , as shown in Fig. 3(a). The beam window is mainly irradiated by protons, so some main nuclides are not present in other structural parts, like$ ^{39} {\rm{Ar}}$ ,$ ^{3} {\rm{H}}$ . As shown in Fig. 3(b),$ ^{56} {\rm{Mn}}$ ,$ ^{51} {\rm{Cr}}$ ,$ ^{55} {\rm{Fe}}$ ,$ ^{3} {\rm{H}}$ ,$ ^{63} {\rm{Ni}}$ ,$ ^{59} {\rm{Ni}}$ , and$ ^{93} {\rm{Mo}}$ are the major radionuclides with high activity in the cooling process. In Fig. 3(c),$ ^{210} {\rm{Bi}}$ ,$ ^{210}{\rm{Po}} $ ,$ ^{207} {\rm{Bi}}$ ,$ ^{194} {\rm{Hg}}$ , and$ ^{202} {\rm{Pb}}$ are the main radionuclides in LBE.$ ^{210} {\rm{Bi}}$ and$ ^{210}{\rm{Po}} $ account for the majority of the total activity before cooling for 1 000 d.Figure 3. (color online) The relative contribution of the main radionuclides in the total activity of (a) main shell, (b) beam window, and (c) LBE after the irradiation.
Table 2 shows the contribution from fission neutron activation in the total activity of the main radionuclides after irradiated 10 000 h. In the structural parts of the spallation target,
$ ^{3} {\rm{H}}$ ,$ ^{39} {\rm{Ar}}$ ,$ ^{49} V$ , and$ ^{91} {\rm{Nb}}$ are mainly produced by the proton beam.$ ^{56} {\rm{Mn}}$ ,$ ^{59} {\rm{Ni}}$ ,$ ^{63} {\rm{Ni}}$ ,$ ^{93} {\rm{Mo}}$ ,$ ^{99} {\rm{Mo}}$ , and$ ^{99{\rm m}} {\rm{Tc}}$ are mainly produced by fission neutron activation. Neutrons and protons work together to generate$ ^{51} {\rm{Cr}}$ ,$ ^{54} {\rm{Mn}}$ ,$ ^{55} {\rm{Fe}}$ , and$ ^{58} {\rm{Co}}$ . As a result, the relative contribution of radionuclides in the total activity differs between Fig. 3(a) and Fig. 3(b), although the maximum activity radionuclide is similar during the cooling process.$ ^{63} {\rm{Ni}}$ ($ T_{1/2} $ =101.1 a),$ ^{91} {\rm{Nb}}$ ($ T _{1/2} $ =6.8×$10 ^{2} $ a),$ ^{93} {\rm{Mo}}$ ($ T_{1/2} $ =4.0×$ 10^{3} $ a), and$ ^{59} {\rm{Ni}}$ ($ T_{1/2} $ =7.6×$ 10^{4} $ a) are the long-life radionuclides in 316L, most of them are induced by the fission neutrons. In the LBE, 28.76% of$ ^{207} {\rm{Bi}}$ ($ T_{1/2} $ =31.55 a) is induced by neutrons.$ ^{209} {\rm{Bi}}$ is activated by neutron to produce$ ^{210} {\rm{Bi}}$ ($ T_{1/2} $ =5.01 d), which decays to highly toxic progeny$ ^{210}{\rm{Po}} $ ($ T_{1/2} $ =138.38 d). So only taking a proton beam as an irradiation source may not bring much error to the type of radioactive products, but it could significantly underestimate the activity of some nuclides.Radionuclides Beam window Beam tube Guide tube Main shell Radionuclides LBE $ ^{3} {\rm{H}}$ 0.00 0.00 0.00 0.00 $ ^{193} $Pt 0.09 $ ^{39} {\rm{Ar}}$ 0.00 0.00 0.00 0.00 $ ^{194} {\rm{Au}}$ 0.00 $^{49}{\rm V}$ 0.00 8.07 0.73 2.87 $ ^{202} {\rm{Pb}}$ 0.00 $ ^{51} {\rm{Cr}}$ 25.09 94.48 88.77 94.85 $ ^{194} {\rm{Hg}}$ 0.03 $ ^{54} {\rm{Mn}}$ 1.38 32.62 26.15 69.96 $ ^{202} {\rm{Tl}}$ 0.09 $ ^{56} {\rm{Mn}}$ 86.97 98.31 97.77 98.95 $ ^{204} {\rm{Tl}}$ 0.09 $ ^{55} {\rm{Fe}}$ 23.04 88.74 82.66 92.68 $ ^{207} {\rm{Bi}}$ 28.76 $ ^{58} {\rm{Co}}$ 7.02 47.95 48.72 87.19 $^{207{\rm m}} {\rm{Pb} }$ 0.40 $ ^{59} {\rm{Ni}}$ 86.52 96.82 95.59 98.24 $ ^{208} {\rm{Bi}}$ 1.89 $ ^{63} {\rm{Ni}}$ 96.09 99.00 98.62 99.35 $ ^{210} {\rm{Bi}}$ 96.66 $ ^{91} {\rm{Nb}}$ 0.00 0.00 2.21 13.37 $ ^{210}{\rm{Po}} $ 96.66 $ ^{93} {\rm{Mo}}$ 92.71 96.76 95.42 98.05 $ ^{99} {\rm{Mo}}$ 94.99 98.48 97.99 99.08 $^{99{\rm m}} {\rm{Tc} }$ 94.99 98.48 97.99 99.08 Table 2. Contribution(%) of neutron activation in the total activity of the main radionuclides after irradiated 10 000 h.
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The damage of radionuclides to the human is affected by many factors, such as radiation characteristics of nuclides, physical and chemical characteristics of nuclides, metabolic characteristics of the organism, and functional state of the organism,
$ etc $ . So some high activity nuclides may not harm to organs, but some low activity nuclides may cause serious damage to humans. Evaluating the risk of radionuclides is an important part of radiation safety work. Dividing the nuclides into four categories, extreme toxicity group, high toxicity group, medium toxicity group, and low toxicity group, depending on the dose limit and dose conversion coefficient of the radioactive material [ 5] . The toxicity group correction factor, is 10, 1, 0.1, 0.01 for extreme toxicity group, high toxicity group, medium toxicity group, and low toxicity group. These factors can roughly calibrate the hazards of each toxicity group nuclides to an organism and could be used to estimate the average daily operating quantity of nuclides.LBE acts as the spallation target material and the coolant in the spallation target system, there are many radionuclides in LBE after irradiation. It is necessary to study the toxic nuclides for evaluating the hazard of LBE waste in the disposal process. Fig. 4(a) describes the activity of the toxicity group during the cooling period, Fig. 4(b) is the activity of the toxicity group that is modified by the correction factor during the cooling period. The total activity decreased by 4 orders of magnitude at the 2000 cooling days as shown in Fig. 4(b). The extremely toxic group nuclides contain only two nuclides
$ ^{210}{\rm{Po}} $ and$ ^{148} {\rm{Gd}}$ , and the activity of$ ^{210} {\rm{Po}} $ is 10 6 times more than that of$ ^{148} {\rm{Gd}}$ at the end of irradiation. Before 2 000 cooling days, the activity was dominated by the extremely toxic group nuclides, followed by the medium toxicity group nuclides. Using a$ ^{210}{\rm{Po}} $ removal device or method in the LBE waste disposal process will effectively reduce the storage time of LBE waste or increase its average daily operating quantity. For the medium toxicity group nuclide and the low toxicity group nuclide, the radioactive product's activity induced in the fission neutron radiation field is nearly 1 000 times higher than that induced in the high-energy proton radiation field. For extreme toxicity group nuclides and the high toxicity group nuclides, the radioactive product's activity induced in the fission neutron radiation field is as much as 30 times that induced in the high-energy proton radiation field at the beginning of the cooling.Figure 4. (color online) The activity of each group of toxic nuclides during the cooling period, the toxicity group correction factor was not modified (a) and modified (b).
Table 3 shows the activity information for the top five radionuclides in each toxic group at different cooling times. On the first day of cooling, the radionuclides mainly distribute in the extreme toxic group and the highly toxic group, among which
$ ^{210}{\rm{Po}} $ and$ ^{210} {\rm{Bi}}$ are the most radioactive nuclides, respectively. About 96.66% of these two nuclides are induced by neutron activation. About 67% of the medium toxicity group nuclides is distributed in$ ^{206} {\rm{Bi}}$ ,$ ^{205} {\rm{Bi}}$ , and$ ^{197} {\rm{Hg}}$ . Nearly 61% of the low toxicity group nuclides are distributed in$ ^{203} {\rm{Pb}}$ ,$ ^{201} {\rm{Tl}}$ , and$ ^{200} {\rm{Tl}}$ . At 1 000 d of cooling, the main distribution of radionuclides shifts from the highly toxic group to the medium toxic group, and 92% of medium toxic group nuclides are distributed in$ ^{207} {\rm{Bi}}$ ,$ ^{204} {\rm{Tl}}$ , and$ ^{195} {\rm{Au}}$ . The activity of$ ^{210}{\rm{Po}} $ accounts for 39.4% of the total activity and it is mostly induced by neutron activation. At 1 000 000 d of cooling, long-lived radionuclides concentrate in the medium toxic group and the low toxic group, such as$ ^{194} {\rm{Hg}}$ ($ T_{1/2} $ =444 a) and$ ^{202} {\rm{Pb}}$ ($ T_{1/2} $ =5.25×10 4 a), most of them are induced by the proton beam. Although most of$ ^{210{\rm m}} {\rm{Bi}}$ ($ T_{1/2} $ =3.04×10 6 a) are generated by neutron activation, its activity only accounts for 2.43% of the total activity.Toxicity group $ X(R _{1} \%,\; R _{2} \%,\; R _{3} \% $) 1 d 1 000 d 1 000 000 d extreme toxicity group $ ^{210} {\rm{Po}} $(96.66,100.00, 35.02) $ ^{210} {\rm{Po}} $(96.66,100.00, 39.40) $ ^{148} {\rm{Gd}}$(0.00,100.00, 0.00) highly toxicity group $ ^{210} {\rm{Bi}}$(96.66, 99.91, 34.99) $ ^{106} $Ru(0.00, 58.66, 0.71) $^{210{\rm m}} {\rm{Bi} }$(96.79, 75.80, 2.43) $ ^{106} $Ru(0.00, 0.08, 0.03) $ ^{ 90} $Sr(0.00, 25.01, 0.30) $ ^{94} {\rm{Nb}}$(0.00, 22.39, 0.72) $ ^{172} {\rm{H}}$f(0.00, 0.01, 0.00) $ ^{172} {\rm{H}}$f(0.00, 11.19, 0.14) $^{108{\rm m}}$Ag(0.00, 1.72, 0.06) $ ^{90} $Sr(0.00, 0.01, 0.00) $ ^{60} {\rm{Co}}$(0.00, 4.73, 0.06) $ ^{10} $B(0.00, 1.23, 0.30) $ ^{108} $Ag(0.00, 0.11, 0.00) $ ^{60} {\rm{Co}}$(0.00, 0.03, 0.00) medium toxicity group $ ^{206} {\rm{Bi}}$(0.17, 30.32, 3.75) $ ^{207} {\rm{Bi}}$(0.40, 60.76, 20.32) $ ^{194} {\rm{Au}}$(0.00, 48.71, 11.97) $ ^{205} {\rm{Bi}}$(0.14, 26.70, 3.30) $ ^{204} {\rm{Tl}}$(0.09, 21.93, 7.33) $ ^{194} {\rm{Hg}}$(0.00, 48.71, 11.97) $ ^{197} {\rm{Hg}}$(0.00, 10.36, 1.28) $ ^{195} {\rm{Au}}$(0.00, 9.91, 3.31) $ ^{94} {\rm{Mo}}$(0.00, 1.29, 0.32) $ ^{195} {\rm{Au}}$(0.00, 6.85, 0.85) $ ^{197} $W(0.00, 1.85, 0.62) $^{94{\rm m}} {\rm{Nb} }$(0.00, 1.23, 0.30) $ ^{203} {\rm{Bi}}$(0.10, 4.25, 0.53) $ ^{90} $Y(0.00, 0.90, 0.30) $ ^{137}{\rm{I}} $(0.00, 0.06, 0.01) low toxicity group $ ^{203} {\rm{Pb}}$(0.15, 25.15, 3.71) $ ^{193} $Pt(0.00, 96.81, 1.79) $ ^{202} {\rm{Pb}}$(0.09, 49.63, 28.21) $ ^{201} {\rm{Tl}}$(0.03, 20.04, 2.95) $ ^{181} $W(0.00, 1.21, 0.02) $ ^{202} {\rm{Tl}}$(0.09, 49.14, 27.94) $ ^{200} {\rm{Tl}}$(0.01, 16.33, 2.41) $ ^{202} {\rm{Pb}}$(0.09, 0.53, 0.01) $ ^{205} {\rm{Pb}}$(0.00, 0.65, 0.37) $ ^{197} {\rm{Hg}}$(0.00, 8.69, 1.28) $ ^{202} {\rm{Tl}}$(0.09, 0.53, 0.01) $ ^{99} {\rm{Tc}}$(0.00, 0.44, 0.25) $ ^{200} {\rm{Pb}}$(0.01, 8.21, 1.21) $ ^{121} {\rm{Sn}}$(0.00, 0.48, 0.01) $ ^{79} {\rm{Se}}$(0.00, 0.08, 0.05) Table 3. The top five toxic nuclides
$ X(R _{1} $ %,$ R _{2} $ %,$ R _{3} $ %) in each toxic group at three cooling times. Where$ R _{1} $ is the contribution of neutron activation in the total activity of radionuclides$ X $ ,$ R _{2} $ is the ratio of the$ X $ activity to toxicity group activity,$ R _{3} $ is the ratio of the$ X $ activity to total activity in LBE.Only a small portion of
$ ^{210} {\rm{Bi}}$ is produced by proton irradiation since only fast neutrons are generated during the irradiation processes and the generation of thermal neutrons is negligible [ 26] . But$ ^{210} {\rm{Bi}}$ can be mainly produced via the radiative neutron capture in LBE,$ ^{209} {\rm{Bi}}$ (n, γ)$ ^{210} {\rm{Bi}}$ →$ ^{210}{\rm{Po}} $ [ 10] .$ ^{210} {\rm{Bi}}$ ($ T_{1/2} $ =5.013 d) and$ ^{210}{\rm{Po}} $ ($ T_{1/2} $ =138.376 d) account for more than half of the activity on the first day of cooling. So the neutron activation is an important factor to estimate the toxicity of LBE radioactive products in the early stage of cooling. For long-lived radionuclides, most of$ ^{210{\rm m}} {\rm{Bi}}$ are induced by neutron capture via$ ^{209} {\rm{Bi}}$ (n, γ)$ ^{210{\rm m}} {\rm{Bi}}$ [ 10] , but it is not a major radionuclide in the LBE target. -
Decay photons emitted from radioactive products are important in the process of transport, storage, and reprocessing of LBE, which directly affects the formulation of radiation protection strategies to protect staff from unnecessary external exposure, examples include shielding design of the LBE cooling circuit, LBE storage time, means of maintenance, and treatment of irradiated products,
$ etc $ . This section focuses on the decay photon released by LBE radioactive products, including the photons release rate, energy spectrum distribution, main nuclides releasing photons, and comparing the decay photon emitted from the LBE radioactive products induced by the fission neutron radiation field and the high-energy proton radiation field.Radionuclides information can be extracted from FULKA and imported into ORIGEN2.1 to obtain the decay photon information of radionuclides. After CiADS operates for 10 000 h, the photon release rate of the radioactive products in the LBE spallation target is shown in Fig. 5(a). In the fission neutron radiation field, the photon release rate reaches 4.14×10 15 photons/s at the cooling time of 0 nbsp;s, among which 4.01×10 15 photons/s is caused by
$ ^{207{\rm m}} {\rm{Pb}}$ ($ T _{1/2} $ =0.806 s), then the photon release rate rapidly drops to 3.84×10 13 photons/s after cooling for 0.1 d. In the high-energy proton radiation field, the photon release rate can reach 1.08×10 17 photons/s at the cooling time of 0 s, and it drops by 1 order of magnitude after 10 d of cooling. Nuclides with the photon release rate higher than 10 15 photons/s and half-time less than 1 h include$ ^{207{\rm m}} {\rm{Pb}}$ ,$ ^{200} {\rm{Bi}}$ ($ T _{1/2} $ =36.4 min),$^{205{\rm m}} {\rm{Pb}}$ ($ T _{1/2} $ =5.55 ms),$ ^{196} {\rm{Pb}}$ ($ T _{1/2} $ =37 min),$ ^{197} {\rm{Pb}}$ ($ T _{1/2} $ =8.1min),$ ^{194} {\rm{Tl}}$ ($ T _{1/2} $ =33 ms). After cooling for 0.1 d, the decay photons induced by the fission neutron radiation field are less than 1% of that of the high-energy proton radiation field, which is negligible.Figure 5. (color online) The information of decay photon in LBE radioactive products (a) photon release rate, (b) photon energies spectrum, and (c) main nuclides.
The photon energies in ORIGEN2.1 are divided into 18 groups, average energy is from 0.01 to 9.5 MeV. Fig. 5(b) shows the photon spectrum on the cooling 0 s and cooling 10 d, most photons emitted from spallation products are concentrated in the range of 0~1 MeV. In the fission neutron radiation field, the photon release rates with energy higher than 1 MeV at the cooling time of 0 s, 10 d, and 1 000 d are 3.27×10 11 photons/s, 3.26×
$ 10^{11} $ photons/s, 3.08×10 11 photons/s, accounting for 0.01%, 3.87%, and 26.32% of the total release rates, respectively. After a cooling time of 1 000 d, the nuclides with energy higher than 1 MeV are mainly$ ^{207} {\rm{Bi}}$ ($ T _{1/2} $ =31.55 a) and$ ^{208} {\rm{Bi}}$ ($ T _{1/2}= $ 3.68×10 5 a), whose photon release rates is 3.07×10 11 photons/s and 3.60×10 8 photons/s, respectively. In the proton radiation field, the photon release rates with energy higher than 1 MeV at the cooling time of 0 s, 10 d, and 1 000 d are 9.16×10 15 photons/s, 1.13×10 14 photons/s, 7.86×10 13 photons/s, accounting for 8.50%, 1.36%, and 18.95% of the total release rates, respectively. After a cooling time of 1 000 d, the nuclides with energy higher than 1 MeV are mainly$ ^{207} {\rm{Bi}}$ and$ ^{90{\rm m}} {\rm{Zr}}$ ($ T _{1/2} $ =0.809 s), whose photon release rates are 7.71×10 13 photons/s and 1.42×10 12 photons/s, respectively.$ ^{90{\rm m}} {\rm{Zr}}$ is decayed from its nearby nuclides, such as 90Sr($ T _{1/2} $ =28.90 a).In the fission neutron radiation field, there are 22 nuclides emitting photons, among which 13 nuclides of photon release rate higher than 10 12 photons/s, and the half-life of 2 nuclides is more than 1 year. In the proton radiation field, there are 185 nuclides emitting photons, among which 146 nuclides of photon release rate higher than 10 12 photons/second, and the half-life of 13 nuclides is more than 1 year. Fig. 5(c) shows the cooling time evolution of the main nuclides of the emitting photon, the main nuclides are isotopes of Pb and Bi. During the cooling time from 0 s to 1 000 d, the nuclides with the maximum photon release rates are
$ ^{203} {\rm{Pb}}$ ,$ ^{206} {\rm{Bi}}$ ,$ ^{201} {\rm{Pb}}$ ,$ ^{205} {\rm{Bi}}$ ,$ ^{195} {\rm{Au}}$ ,$ ^{207} {\rm{Bi}}$ . Some short-life nuclides also need to be paid attention, such as$ ^{204{\rm m}} {\rm{Pb}}$ ($ T _{1/2} $ =66.93 min) which α decays through$ ^{208} {\rm{Pb}}$ ($ T _{1/2} $ =2.90 a). After a cooling time of 1000 d, the photon release rates of$ ^{204{\rm m}} {\rm{Pb}}$ reach 9.95×10 13 photons/s, accounting for 24.27% of the total release rates.
Radionuclides Study of Lead-bismuth Eutectic Spallation Target in CiADS
doi: 10.11804/NuclPhysRev.38.2021009
- Received Date: 2021-01-25
- Rev Recd Date: 2021-05-14
- Available Online: 2021-09-27
- Publish Date: 2021-09-20
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Key words:
- radionuclides /
- fission neutrons /
- lead-bismuth eutectic /
- CiADS /
- monte carlo
Abstract: Lead-bismuth eutectic(LBE) is the candidate material of spallation target for China initiative Accelerator Driven System(CiADS). Long-term irradiation makes the LBE highly radioactive. Most of the spallation target radionuclide studies focused on the effects of the proton beam and ignored fission neutrons from the reactor. In this paper, both the fission neutrons and the high-energy protons have been taken into account in the radionuclide calculations of LBE and spallation target structural parts by coupling codes FLUKA and MCNP. Contributions from the fission neutron and the high-energy proton have been compared in the aspects of the activity, main radionuclides, toxicity, and the decay photon of radioactive products. The main shell, guide tube, and beam tube are significantly affected by the fission neutron activation. When the reactor tends to be critical, the fission neutron-induced LBE target activations are even greater than that induced by the proton beam. In the LBE itself, 96.66% of 210Po is induced by the fission neutrons. These results illustrate that fission neutrons are also essential for the radionuclides calculation of LBE and its structural parts. In addition, this study provides reference data for the radiation protection of CiADS and a more accurate method for the radionuclides study of the spallation target in ADS systems.
Citation: | Shiyu SONG, Junkui XU, Yonggang YAN, Yuxuan HUANG, Zhiweng WEN, Jianling RAN, Jinhuang YAN, Yiwei GONG, Dapeng LI, Sicheng WANG, Peng LUO. Radionuclides Study of Lead-bismuth Eutectic Spallation Target in CiADS[J]. Nuclear Physics Review, 2021, 38(3): 345-354. doi: 10.11804/NuclPhysRev.38.2021009 |