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Radionuclides Study of Lead-bismuth Eutectic Spallation Target in CiADS

Shiyu SONG Junkui XU Yonggang YAN Yuxuan HUANG Zhiweng WEN Jianling RAN Jinhuang YAN Yiwei GONG Dapeng LI Sicheng WANG Peng LUO

宋时雨, 徐俊奎, 颜永刚, 黄郁旋, 温志文, 冉建玲, 严金煌, 龚艺伟, 李大蓬, 王思成, 骆鹏. CiADS中LBE散裂靶的放射性核素研究[J]. 原子核物理评论. doi: 10.11804/NuclPhysRev.38.2021009
引用本文: 宋时雨, 徐俊奎, 颜永刚, 黄郁旋, 温志文, 冉建玲, 严金煌, 龚艺伟, 李大蓬, 王思成, 骆鹏. CiADS中LBE散裂靶的放射性核素研究[J]. 原子核物理评论. doi: 10.11804/NuclPhysRev.38.2021009
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. doi: 10.11804/NuclPhysRev.38.2021009
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. doi: 10.11804/NuclPhysRev.38.2021009

CiADS中LBE散裂靶的放射性核素研究

doi: 10.11804/NuclPhysRev.38.2021009
详细信息
  • 中图分类号: TL75+2.2

Radionuclides Study of Lead-bismuth Eutectic Spallation Target in CiADS

More Information
  • 摘要: 铅铋合金(LBE)作为中国加速器驱动系统(CiADS)散裂靶的候选材料,长期辐照使其具有很强的放射性。散裂靶放射性核素的研究仅考虑了质子束的散裂反应,而忽略了反应堆裂变中子的活化作用。本文采用FLUKA和MCNP耦合计算LBE和及其结构件的放射性产物。比较了裂变中子和高能质子在放射性产物的活度、主要放射性核素、毒性和衰变光子等方面的贡献。裂变中子的活化作用对主壳、导管和射束管有显著影响。当反应堆趋于临界状态时,裂变中子对LBE的活化作用是高于质子束流的。在LBE中,96.66%的210Po是由裂变中子诱导的。这些结果表明,裂变中子在LBE及其结构部件的活化计算中是必不可少的。此外,本研究为CiADS的辐射防护提供了参考数据,也为ADS系统中散裂靶的放射性核素研究提供了更准确的方法。
  • Figure  1.  (color online) Calculation model diagram and neutron information from the reactor to spallation target. (a) Longitudinal section of the reactor-target coupling model, (b) spallation target model, and (c) neutron emissivity distribution on the outer surface of the target with the position, energy, and emission direction (The position refers to the height of the boundary between the subcritical reactor and the spallation target, and the 0cm corresponds to the center of the fuel in the reactor. The negative normal direction of the boundary is set as the reference direction of the neutron emission, 0° means that fission neutrons incident perpendicular to the cylindrical boundary into the spallation target.).

    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.

    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.

    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).

    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.

    Table  1.   Number of radionuclide types induced by radiation source.

    Radiation source Beam window Beam tube Guide tube Main shell LBE
    Neutron 15 23 33 42 20
    Proton 311 82 129 105 750
    Total 312 83 130 106 750
    下载: 导出CSV

    Table  2.   Contribution(%) of neutron activation in the total activity of the main radionuclides after irradiated 10 000 h.

    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
    下载: 导出CSV

    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.

    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)
    下载: 导出CSV
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  • 收稿日期:  2021-01-25
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Radionuclides Study of Lead-bismuth Eutectic Spallation Target in CiADS

doi: 10.11804/NuclPhysRev.38.2021009
    作者简介:

    (1995–), Chenzhou, Hunan Province, Postgraduate, working on radiation protection: E-mail: songshiyu2019@impcas.ac.cn

    通讯作者: E-mail: luopeng@impcas.ac.cn.
  • 中图分类号: TL75+2.2

摘要: 铅铋合金(LBE)作为中国加速器驱动系统(CiADS)散裂靶的候选材料,长期辐照使其具有很强的放射性。散裂靶放射性核素的研究仅考虑了质子束的散裂反应,而忽略了反应堆裂变中子的活化作用。本文采用FLUKA和MCNP耦合计算LBE和及其结构件的放射性产物。比较了裂变中子和高能质子在放射性产物的活度、主要放射性核素、毒性和衰变光子等方面的贡献。裂变中子的活化作用对主壳、导管和射束管有显著影响。当反应堆趋于临界状态时,裂变中子对LBE的活化作用是高于质子束流的。在LBE中,96.66%的210Po是由裂变中子诱导的。这些结果表明,裂变中子在LBE及其结构部件的活化计算中是必不可少的。此外,本研究为CiADS的辐射防护提供了参考数据,也为ADS系统中散裂靶的放射性核素研究提供了更准确的方法。

English Abstract

宋时雨, 徐俊奎, 颜永刚, 黄郁旋, 温志文, 冉建玲, 严金煌, 龚艺伟, 李大蓬, 王思成, 骆鹏. CiADS中LBE散裂靶的放射性核素研究[J]. 原子核物理评论. doi: 10.11804/NuclPhysRev.38.2021009
引用本文: 宋时雨, 徐俊奎, 颜永刚, 黄郁旋, 温志文, 冉建玲, 严金煌, 龚艺伟, 李大蓬, 王思成, 骆鹏. CiADS中LBE散裂靶的放射性核素研究[J]. 原子核物理评论. doi: 10.11804/NuclPhysRev.38.2021009
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. doi: 10.11804/NuclPhysRev.38.2021009
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. doi: 10.11804/NuclPhysRev.38.2021009
    • China initiative Accelerator Driven System (CiADS) is a research facility that will serve as a major platform to explore advanced technologies for nuclear waste disposal and nuclear resource shortage problems. The spallation target is a key part of CiADS as well as an interface between high-power accelerator and sub-critical reactor. Lead-bismuth eutectic (LBE) is selected as a candidate material for the spallation target due to its excellent neutronic performance and heat removal property[1-2].

      There will be a large number of radionuclides in the LBE spallation target after the CiADS is operated. Certain characteristics of those nuclides determine the means of radiation protection and the disposal methods of LBE waste. For example, the half-life of some nuclides determines the storage time of spallation target, gamma rays emitted from the radioactivity products are a critical issue for shielding of the LBE cooling circuit and LBE waste storage[3-4], the toxic products limit the average daily operating quantity of waste[5]. Contamination of instrument surface is one of the important radiation sources for the worker who is responsible for the spallation target maintenance and replacement. The radionuclide inventory information of the spallation target can be used in accident analysis to estimate the waste release[6]. The radioactive product in LBE and its structural parts are also important for waste management and facility decommissioning[7]. Therefore, the radionuclides information in LBE is very useful for the safe operation of CiADS. On the other hand, some optimization measures can be formulated according to the information of radionuclides, like optimize the process of waste management to reduce the cost of LBE waste disposal and improve the reprocessing efficiency of the target material. The effectiveness of optimization measures needs to be ensured by the accurate computational evaluation and predictions for radionuclides, which is also a precondition for the operation and maintenance of CiADS. So the accurate radionuclides information is also important to ensure the safe operation of CiADS.

      When a high energy proton bombards LBE, a series of radioactive spallation products are generated. In addition, spallation neutrons could induce further radioactivity in the target. Liang et al. pointed out that the neutron flux around the outer surface of the spallation target was largely increased by the fission reaction in the sub-critical core[8]. Polonium is the most important radionuclide to consider in safety analyses of the LBE spallation target[9], and the bismuth component in LBE is easy to capture low-energy neutrons and finally decay into polonium[10-11]. So the fission neutrons from the reactor may be an important source for the spallation target activation. However, to the best of our knowledge, the effect of fission neutrons for spallation target activation has not been thoroughly considered. For instance, Zhang et al. established the reactor-target coupling model in FLUKA, but the fission neutrons were not considered due to the lack of $ ^{ 235 } {\rm{U}}$ database in the FLUKA code[12]. Shinya et al. analyzed inventories of LBE spallation products caused only by high-energy protons[13]. Many experiments of spallation targets in ADS have been carried out on MEGAPIE, such as delay neutron measurements, gas production analysis, and target activation calculation, but these experiments are all without loading fuel, which means that the influence of fission neutron is still not considered[11, 14-16]. Therefore, the radionuclide study of the spallation target is still insufficient, whether in experimental study or simulation calculation, especially considering the reactor target coupling.

      As a potential technology for the transmutation of high-level radioactive wastes and power generation, ADS is still under development and faces many unknown challenges. It is necessary to consider the effect of fission neutrons on the LBE radionuclide analyses to provide more accurate data for the design of the CiADS radiation safety system.

    • In the sub-critical reactor, neutron flux depends on the neutron multiplicative properties of the reactor when the proton beam is determined[17]. Three main fuel loading models have been investigated in this work, that is, 30 fuel assemblies with its effective multiplication coefficient $ k _{\rm eff} $ of 0.77, 42 fuel assemblies with $ k _{\rm eff} $ of 0.86, and 72 fuel assemblies with $ k _{\rm eff} $ of 0.98. The schematic of the reactor-target coupling model and the spallation target model are shown in Fig. 1(a) and Fig. 1(b), respectively. The spallation target system consists of the LBE, beam tube, beam window, guide tube, and main shell. Fuel assemblies, auxiliary assemblies, reflectors, and the LBE coolant constitute the subcritical reactor system. The spallation target is inserted into the center of the subcritical reactor. All calculations are based on a 500 MeV proton beam with a 5 mA current in the vertical direction. It should be noted that the geometry model is constructed based on the current CiADS preliminary design and it is not the final design in real engineering applications.

      Figure 1.  (color online) Calculation model diagram and neutron information from the reactor to spallation target. (a) Longitudinal section of the reactor-target coupling model, (b) spallation target model, and (c) neutron emissivity distribution on the outer surface of the target with the position, energy, and emission direction (The position refers to the height of the boundary between the subcritical reactor and the spallation target, and the 0cm corresponds to the center of the fuel in the reactor. The negative normal direction of the boundary is set as the reference direction of the neutron emission, 0° means that fission neutrons incident perpendicular to the cylindrical boundary into the spallation target.).

      The Monte Carlo method is the most commonly used simulation method to describe particle transport processes. MCNP is a general-purpose Monte Carlo radiation transport code designed to track many particle types over broad ranges of energies[18], which is suitable for neutron transport simulation in the reactor with a strong ability of geometric construction and particle statistics, but MCNP does not give us information about radionuclides with excited state and their decay processes over time. FLUKA is also a Monte Carlo code for calculations of particle transport and interactions with matter, covering an extended range of applications spanning from proton and electron accelerator shielding to target design, activation, Accelerator Driven Systems, neutrino physics, $ etc $[19]. Importantly, FLUKA can perform the time evolution and tracking of emitted radiation from unstable residual nuclei on line based on the half-analytic/half-Monte Carlo method[8, 19-20], but it as an open-source software lacks some crucial nuclear data of $ ^{235} {\rm{U}}$. In this work, the two codes are coupled to perform radionuclide calculations of the spallation target when it is operated in the CiADS subcritical reactor core.

    • Numerous spallation neutrons are generated when high-energy protons bombard the LBE spallation target. Some spallation neutrons, as the external neutron driving source for the reactor, enter the subcritical reactor to maintain the $ k _{\rm eff} $ as a certain fixed value and will induce a large number of fission neutrons at the same time. In previous radionuclide studies of the spallation target in ADS, proton beams were mostly considered as the only factor to induce radioactive isotopes, without considering the activation of spallation target by fission neutron comes from the reactor. To calculate the source term of LBE more accurately, MCNP was used to record the neutron flux at the boundary between the subcritical reactor and the spallation target when the CiADS is running online, and then the neutron flux information is used as an input beam in FLUKA to simulate the radionuclide generation of the spallation target. The radionuclide calculation of LBE and its structural components were conducted in three steps.

      Firstly, Surface Source Write (SSW) card is usually used to implement stepwise computation in MCNP, which will produce the surface source file WSSA and as a radiation source in the subsequent MCNP run[21-22]. In this work, all of the neutron crossing information from the reactor to the spallation target was recorded in WSSA file, which is equivalent to the fission neutron radiation field at the reactor-target coupling area for radionuclides calculation of spallation target.

      Secondly, HTAPE3X code is modified from the LAHET code system, which could edit the WSSA file and provide some options that are not available with standard MCNP F1 and F2 tallies[18, 23]. The neutron crossing information in file WSSA can be decrypted to the information of the particle’s position, energy, and emission direction.

      Thirdly, FLUKA offers rich routines for the user to simplify its input file, one of which is the source routine. Users can use FORTRAN language to write or modify the source.f file in FLUKA to control source content by reading from the file, generated by some sampling algorithm, or just assigned. Therefore, it is feasible to make FLUKA recognize the decrypted information in WSSA file. Finally, taking neutron crossing information as an external source term in FLUKA to simulate the radionuclide calculation of the LBE spallation target at the reactor-target coupling area.

      To simplify the sampling process of the external source term in FLUKA, we assume that the neutron weight meet the Eq. (1). In other words, the position, energy, and emission direction of the neutron are independent.

      $$ W(r,E,\theta) = W(r)\times W(E)\times W(\theta). $$ (1)

      The $ W(r,E,\theta) $ is the weight function of the neutron at the position $ r $, energy $ E $, and emission direction $ \theta. $ $ W(r) $, $ W(E) $, and $ W(\theta) $ are the average weight of the neutron at $ r $, $ E $, and $ \theta $, respectively.

    • In this study, we need to record the neutron information from the reactor to the spallation target on the outer surface of the main shell first. Fig. 1(c) shows the neutron emissivity distribution with the position, energy, and emission direction for the three fuel loading models. The negative normal direction of the main shell surface is set as the reference direction of the neutron emission. Most of the neutrons enter the main shell at angles between 20° and 70°. In the subcritical nuclear reactor, the fuel $ {\rm{UO}}_{2} $ is located in the range from $ -50 $ to 50 cm in the vertical direction, so fission neutrons are highly concentrated in this region. The neutron energy is widely distributed, ranging from several eV to several hundred MeV. Most neutrons are concentrated in the range between 1 keV and 1 MeV, a few high-neutron comes from the spallation reaction and are reflected in the target by the reactor system. The neutron flux level of the reactor core is determined by the energy and intensity of the proton beam and the neutron multiplicative properties[24]. According to simulation analyses, the neutron's emissivity on the main shell surface is 53.32 neutrons/proton, 71.85 neutrons/proton, and 276.38 neutrons/proton for the 30, 42, and 72 fuel assemblies, respectively. Neutron distribution is extremely similar in position, energy, and emission direction under different fuel loading modes. It provides a possibility to rapidly estimate the spallation target activation induced by fission neutrons through the $ k _{\rm eff} $ value. To estimate the maximum effect of fission neutrons on the target system, the result shown below are based on the 72 fuel assemblies loading mode reactor with $ k _{\rm eff} $ of 0.98.

    • 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.

    • 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.

    • 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.

      Table 1.  Number of radionuclide types induced by radiation source.

      Radiation source Beam window Beam tube Guide tube Main shell LBE
      Neutron 15 23 33 42 20
      Proton 311 82 129 105 750
      Total 312 83 130 106 750

      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.

      Table 2.  Contribution(%) of neutron activation in the total activity of the main radionuclides after irradiated 10 000 h.

      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
    • 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 106 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} $=5.25×104 a), most of them are induced by the proton beam. Although most of $ ^{210{\rm m}} {\rm{Bi}}$($ T_{1/2} $=3.04×106 a) are generated by neutron activation, its activity only accounts for 2.43% of the total activity.

      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.

      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)

      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 4.14×1015 photons/s at the cooling time of 0 nbsp;s, among which 4.01×1015 photons/s is caused by $ ^{207{\rm m}} {\rm{Pb}}$($ T _{1/2} $=0.806 s), then the photon release rate rapidly drops to 3.84×1013 photons/s after cooling for 0.1 d. In the high-energy proton radiation field, the photon release rate can reach 1.08×1017 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 1015 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 3.27×1011 photons/s, 3.26×$ 10^{11} $ photons/s, 3.08×1011 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}= $3.68×105 a), whose photon release rates is 3.07×1011 photons/s and 3.60×108 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 9.16×1015 photons/s, 1.13×1014 photons/s, 7.86×1013 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 7.71×1013 photons/s and 1.42×1012 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 1012 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 1012 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 9.95×1013 photons/s, accounting for 24.27% of the total release rates.

    • In the present work, MCNP and FLUKA are coupled to accomplish the radionuclide calculation of LBE and structural parts in the reactor-target coupling system of CiADS. The neutrons emitted into the target system can reach 276.38 neutrons/proton for 72 fuel assemblies loading mode. Fission neutron activation effect could change the specific activity of LBE and structural parts in both the irradiation process and cooling process, especially for the beam tube, guide tube, and main shell. When the reactor approaches to be critical, the activation effect of fission neutrons on the LBE target is even greater than that of the proton beam. In terms of radionuclide types, fission neutron activation does not increase the types of radionuclide products but does enhance the activity of some nuclides. $ ^{210} {\rm{Bi}}$ and $ ^{210} {\rm{Po}} $ account for more than half of the activity in the early stage of cooling, and about 96.66% of them are induced by fission neutrons. The fission neutron plays an important role in the evaluation of toxicity of the radionuclide products for LBE. In the study of photons released by radionuclides in LBE, it is found that most of the decay photons are triggered by the high-energy proton radiation field, and the effect of fission neutrons could be ignored. According to radionuclides analyses in this work, it is necessary to take the effect of fission neutron into account for the design of the CiADS protection system.

      The radiation field in the reactor-target coupling area is divided into the fission neutron radiation field and the high-energy proton radiation field can provide more specific and reasonable technical support for the radiation protection of CiADS. To further analyze the LBE target activation in the target-reactor coupling area, more detailed factors, such as proton beam loss and shielding design, would be considered in future work.

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