Activation and shutdown dose-rate analyses for the EAST NBI test facility

Activation and shutdown dose-rate analyses for the EAST NBI test facility 1 corresp first-author Email: chenyq205@ipp.ac.cn Email: chenyq205@ipp.ac.cn chenyq205@ipp.ac.cn 1 1 corresp Corresponding author: WANG Ji; Email: wangji@ipp.aac.cn Corresponding author: WANG Ji; Email: wangji@ipp.aac.cn wangji@ipp.aac.cn 1 1 1 1 Institute of Plasma Physics, Chinese Academy of Sciences, Hefei 230031, China Institute of Plasma Physics, Chinese Academy of Sciences, Hefei 230031, China An analysis of the neutron activation and shutdown dose rates was performed for the EAST high-power deuterium neutral beam injector test facility (EAST NBITF) by using the multiparticle transport code FLUKA 2011.2. Also, the neutron and neutron-induced gamma spectra of the facility’s vacuum vessel were evaluated. The results identified the major radionuclides 99 Mo, 65 Ni, 58 Co, 56 Mn, and 51 Cr for the vacuum vessel and the primary nuclide 64 Cu for both the ion dump and the calorimeter. The simulated results indicated that, when the EAST NBITF operates within the design parameters of deuterium energy 80 keV, current 50 A, and pulse 100 s, the neutron intensity will achieve 10 11 n/s with no radiation safety problems after the EAST NBITF shuts down for 1 h. Shutdown dose rate Neutron activation Radionuclides 1 Introduction 1 To raise plasma temperature and initiate sustained fusion, high-power deuterium neutral beam injectors (NBIs), including JET, TFTR, and DIIID, have become an important auxiliary heating method. The Experimental Advanced Superconducting Tokomak has also developed the first neutral beam injection system (NBIS). To test the performance of the EAST NBI, a test facility (EAST NBITF) was constructed. When the EAST NBITF operates, large sections of the energetic beam ion are stopped by both the ion dump and the calorimeter (approximately 80%) along the beam line, and neutrons are produced via D (d,n)3He reaction [ 1 1 – 5 5 1–5 ]. Calculations of neutron production demonstrate that the calorimeter is the main neutron source [ 6 6 – 8 8 6–8 ]. These neutrons activate the components for the EAST NBITF and cause the material deterioration of the properties [ 2 2 , 8 8 – 11 11 8–11 ]. After NBITF shutdown, these neutron-induced photons become a physical health concern for maintenance and repair workers. However, the neutron activation and shutdown rates of EAST NBITF have not been analyzed. This paper focuses on neutron activation and dose rates simulated by FLUKA 2011.2 code, a multiparticle transport code widely applied in radiation-protection studies [ 12 12 – 14 14 12–14 ]. 2 Geometrical Model 1 To evaluate neutron transport in the EAST NBITF structure, a geometric model, the 2D EAST NBITF, was implemented into FLUKA code. The model, shown in Fig. 1 Fig. 1 , contains all the critical components: beam source, neutralizer, residual ion dump (RID), calorimeter, cryo-panel, and vacuum vessel (VV). The ion source consists of rectangular parallelepipeds 125 cm long, 66 cm high, and 45 cm wide. For the beamline component, the VV had a cylindrical shape, approximately 1.5 cm thick. The neutralizer is a box [ 15 15 ]. The calorimeter consisted of two slab vertical parallelepiped panels, which formed an 11° angle from the top view. Each panel was 240 cm long, 200 cm wide, and 5 cm thick, and the RID had the same structure. The EAST NBITF had a total length of 5.25 m. The components of the geometrical model and the material compositions are listed in Table 1 Table 1 . 1 1 Regions 1 1 Components 1 1 Compositions (weight %) 1 1 Density (g/cm3) 1 1 1. Arc chamber 1 1 Cu 1 1 100 1 1 8.96 1 1 2. Vacuum 1 1 Vacuum 1 1   1 1   1 1 3. Vacuum vessel   1 1 SS316 1 1 C, 0.041; Si, 0.507; P, 0.023; S, 0.015; Cr, 17; Mn, 1.014; Fe, 66.9; Ni, 12; Mo, 2.5 1 1 8.0 1 1 4. Beamline vessel 1 1 SS316 1 1 C, 0.041; Si, 0.507; P, 0.023; S, 0.015; Cr, 17; Mn, 1.014; Fe, 66.9; Ni,12; Mo, 2.5 1 1   1 1 5. Neutralizer 1 1 Cu 1 1 100 1 1 8.96 1 1 6. Ion dump 1 1 Cu 1 1 100 1 1 8.96 1 1 7. Bending magnet 1 1 Cu Fe 1 1 Cu, 50; Fe, 50 1 1 8.96 1 1 8. Calorimeter 1 1 Cu 1 1 100 1 1 8.96 1 1 9. Front cryo-panel; 1 1 Al 1 1 100 1 1 2.70 1 1 10. Rear cryo-panel 1 1 Al 1 1 100 1 1 2.70 1 1 11. Environment  1 1 Air 1 1 C, 0.0124; N, 75.5268, 023.1781; Ar, 1.2827 1 1 1.205 × 10 −3 3 Mathematical Method Theory 1 3.1 Neutron-Intensity Calculations 2 During deuterium operation of EAST NBITF, the deuterium ions were produced and accelerated by the ion source, and the largest part of the beam power was impinged onto the ion dump and the calorimeter (approximately 80%). The neutrons were produced in the same manner as with the drive-in solid-target neutron sources. Therefore, the calorimeter serves as the main neutron source. To calculate neutron intensity, the adopted model assumes that the deuteron beam, with initial energy, E o, and the current density, I, normally occur on the planar surface of the calorimeter target (a similar method is used for the RID). A theoretical method of neutron intensity for high-energy deuterons stopped by a metal target was given by J. Kim [ 3 3 – 6 6 3–6 ]: <math display="block" id="M1"><mrow><mi>y</mi><mo>=</mo><mfrac><mrow><msub><mi>I</mi><mi>b</mi></msub></mrow><mi>e</mi></mfrac><mstyle displaystyle="true"><munderover><mo>∑</mo><mrow><mi>i</mi><mo>=</mo><mn>1</mn></mrow><mtext>3</mtext></munderover><mrow><mi>i</mi><mi>f</mi><msub><mtext> </mtext><mi>i</mi></msub></mrow></mstyle><mstyle displaystyle="true"><mrow><munderover><mo>∫</mo><mn>0</mn><mtext>R</mtext></munderover><mrow><mi>σ</mi><mrow><mo>(</mo><mrow><mi>E</mi><mo stretchy="false">(</mo><mi>x</mi><mo stretchy="false">)</mo><mo>/</mo><mi>i</mi></mrow><mo>)</mo></mrow></mrow></mrow></mstyle><msub><mi>C</mi><mrow><mtext>iD</mtext></mrow></msub><mrow><mo>(</mo><mrow><mi>x</mi><mo>,</mo><mi>θ</mi></mrow><mo>)</mo></mrow><mi>d</mi><mi>x</mi></mrow></math> [Please check revisions to equations, “x” as variables “ x ”.] I b is the total beam current; i is the number of nuclei per ion; f i is the fraction of each ion species; and <math id="M2"><mrow><mi>E</mi><mo stretchy="false">(</mo><mi>x</mi><mo stretchy="false">)</mo></mrow></math> is the particle energy at depth x . <math id="M3"><mrow><mi>σ</mi><mo stretchy="false">[</mo><mtext>E</mtext><mo stretchy="false">(</mo><mi>x</mi><mo stretchy="false">)</mo><mo stretchy="false">]</mo></mrow></math> is the cross section; and <math id="M4"><mrow><msub><mi>C</mi><mtext>D</mtext></msub><mrow><mo>(</mo><mrow><mi>x</mi><mo>,</mo><mi>θ</mi></mrow><mo>)</mo></mrow></mrow></math> is the saturation deuterium concentration (cm −3 ) in the target. <math id="M5"><mrow><mi>θ</mi><mo>=</mo><msub><mi>D</mi><mrow><mtext>diff</mtext></mrow></msub><mi>t</mi><mo>/</mo><msup><mi>R</mi><mn>2</mn></msup></mrow></math> is calculated with the diffusion coefficient <math id="M6"><mrow><msub><mi>D</mi><mrow><mtext>diff</mtext></mrow></msub></mrow></math> , the pulse time t , and the beam ion stopping range R . Through the use of Equation (1), the calculated neutron intensity for the calorimeter was 1.64 × 10 10 n/s/A during the EAST NBITF running with the parameters 80 KeV and 50 A and a pulse of 10–100 s [ 16 16 ]. The simulations indicate that the neutron intensity from D (T,p)4He was lower than that from D(d,n)3He by approximately five orders of magnitude [ 6 6 , 15 15 ]. Therefore, the D T neutron was omitted from neutron-activation analysis for this paper. [AU: Edits correct? Please verify. Thanks.] 3.2 Activation Calculations 2 The process of 2.45 MeV neutron activation is (n, γ ) reaction. The cross sections of activation greatly heighten at low neutron energies (at most, only a few electronvolts). With the distribution of neutron spectra, the number of radionuclides in a given material can be acquired by Equation (2) [ 14 14 ]: <math display="block" id="M7"><mrow><msub><mi>A</mi><mi>i</mi></msub><mo>=</mo><msub><mi>ϕ</mi><mi>n</mi></msub><mstyle displaystyle="true"><munder><mo>∑</mo><mrow><mi>j</mi><mo>≠</mo><mi>i</mi></mrow></munder><mrow><msub><mi>σ</mi><mrow><mi>i</mi><mi>j</mi></mrow></msub><msub><mi>n</mi><mrow><mi>j</mi><mi>t</mi></mrow></msub><msub><mi>θ</mi><mrow><mi>j</mi><mi>t</mi></mrow></msub></mrow></mstyle><mrow><mo>(</mo><mrow><mn>1</mn><mo>−</mo><msup><mi>e</mi><mrow><mo>−</mo><msub><mi>λ</mi><mi>i</mi></msub><msub><mi>t</mi><mrow><mtext>irra</mtext></mrow></msub></mrow></msup></mrow><mo>)</mo></mrow><msup><mi>e</mi><mrow><mo>−</mo><msub><mi>λ</mi><mi>i</mi></msub><msub><mi>t</mi><mi>c</mi></msub></mrow></msup><mo>.</mo></mrow></math> [Please review edits to Eq. 2: “e” as a variable was changed to “ e ”; “ irra ” as an abbreviation was changed to “irra”.] <math id="M8"><mrow><msub><mi>λ</mi><mi>i</mi></msub></mrow></math> is the decay constant of the nuclide i ; <math id="M9"><mrow><msub><mi>ϕ</mi><mi>n</mi></msub></mrow></math> is the neutron flux (n/cm 2 /s); <math id="M10"><mrow><msub><mi>σ</mi><mrow><mi>i</mi><mi>j</mi></mrow></msub></mrow></math> is the reaction cross section for reaction on producing i (cm 2 ); n jt is the density of nuclide j in the target material (n/cm 3 ); t c is the cooling time; t irra is the irradiation time (can be considered effective time of EAST NBITF operating); and <math id="M11"><mrow><msub><mi>θ</mi><mrow><mi>j</mi><mi>t</mi></mrow></msub></mrow></math> is abundance of the i nuclide isotope in nature. 4 Results 1 4.1 Neutron and Gamma Spectra 2 To obtain the material activation and shutdown dose rate, it was necessary to analyze the neutron and photon spectra. Workers are first to exposed to the VV after a facility shutdown. Also, the material (SS316) is easily activated. The energy spectra of the VV was simulated through FLUKA 2011.2 in Figs. 2a and b Figs. 2a and b . Because the effect of neutron scattering is significant [ 17 17 – 19 19 17–19 ], it can be included with the low-bias card in FLUKA. Fig. 2a Fig. 2a reveals that the 2.45 MeV neutron intensity is still high after penetrating the calorimeter and cyro-panel. Fig. 2b Fig. 2b shows the neutron-induced photon flux. During input of the neutron and photon spectra by the source card, the material activation can be simulated in the VV. 4.2 Activity Analysis for NBI Material 2 Through the use of Equation (2), neutron activation can be evaluated for different irradiation times. To verify the reliability of the FLUKA code program, the values from Equation (2) and FLUKA are were compared with both the operating time (100 s) and the cooling time (1 h). Table 2 Table 2 lists the activities of the material 1 h after shutdown. Deviations between Equation (2) and FLUKA are 10× or fewer, which indicates the reliability of the values derived from FLUKA 2011.2. 1 1 Nuclide 1 1 Reaction 1 1 Theoretical value (Bq/kg) 1 1 FLUKA (Bq/kg) 1 1 99 Mo 1 1 98 Mo(n,γ) 99 Mo 1 1 5.44E+06 1 1 7.88E+06 1 1 93 Mo 1 1 92 Mo(n,γ) 93 Mo 1 1 5.02E+00 1 1 4.49E+00 1 1 65 Ni 1 1 64 Ni(n,γ) 65 Ni 1 1 6.52E+06 1 1 1.17E+06 1 1 58 Co 1 1 58 Ni(n,p) 58 Co 1 1 4.14E+05 1 1 1.36E+06 1 1 55 Fe 1 1 54 Fe(n,γ) 55 Fe 1 1 7.52E+04 1 1 4.01E+04 1 1 56 Mn 1 1 55 Mn(n,γ) 56 Mn 1 1 1.41E+08 1 1 7.28E+07 1 1 54 Mn 1 1 54 Fe(n,p) 54 Mn 1 1 2.40E+05 1 1 1.37E+05 1 1 51 Cr 1 1 50 Cr(n,γ) 51 Cr 1 1 5.52E+05 1 1 4.37E+05 The dominant radionuclides are 58 Co, 55 Fe, 54 Mn, 58 Co, and 51 Cr for vacuum vessels [AU: This text was replaced from where it originally appears, between Table 2 Table 2 and Fig. 3 Fig. 3 . Please delete the original text.] 1 h after shutdown. Fig. 3 Fig. 3 gives the functions between the activities of the NBI key components and the cooling times. In the VV, 99 Mo, 58 Co, 55 Fe, and 54 Mn are the dominant nuclei after 1 d of cooling time. The activities of the VV subside slowly after 10 d of cooling because the primary activities were the long half-lives of the decaying nuclei 55 Fe (T 1/2 , 2.73 y); 99 Mo (T 1/2 , 65.94 h); 51 Cr (T 1/2 , 27.25 d); and 58 Co (T 1/2 , 70.86 d). For the main radiation source calorimeter, 64 Cu (T 1/2 12.7h) had the shortest decaying half-life, and the activities of the ion calorimeter declined by six orders of magnitude after 10 d of cooling. However, the activity of calorimeter was higher than that of the VV during the initial cooling time. Figure 4 Figure 4 shows the activities of the NBITF key components. Because the material and the neutron flux differ, the variation rules between total activity and cooling time are not same. Fig. 4 Fig. 4 indicates that neutron spectrum, neutron flux, target material, and cooling time affect the activities of the different components. 4.3 Shutdown Dose Rate of NBI 2 Neutron-induced photons are a major source of radiation after a NBITF shutdown. The dose rates can be obtained by multiplying the fluxes of the neutron/photon by the conversion factors of the flux-to-dose rate. The shutdown dose rates were analyzed in detail, and Figs. 5a and b Figs. 5a and b demonstrate that the dose rates vary with distance after one pulse of discharge at 50–1000 s and after 1 min of cooling, with parameters of energy 80 keV and current 50A. The shutdown dose rates rise as the length of the discharge pulse increases. Along the radial direction, the dose rates fell from the center of the deuterium beam. [AU: Please review edits.] When the beamline direction was followed, the highest dose rates were 5.5μSv/h at the calorimeter ( R = 420 cm) with a pulse of 100 s because the calorimeter, situated there, was the main neutron source. Fig. 5 Fig. 5 demonstrates that the dose rates for pulses between 50–200 s (10μSv/h) fall below the safety standards of IAEA [ 20 20 ]. [AU: Please verify edits, or revise further if necessary.] The dose-rate limits for spot staff, occupational radiation workers, and the natural background are 10μSv/h, 100μSv/h, and 0.1μSv/h, respectively. The place away from the VV, approximately 50 cm, is the passageway for NBITF workers.[AU: Please verify edits. Would this be the passageway for the workers?] Therefore, Figs. 6a and b Figs. 6a and b show the shutdown dose rates with different pulses. These simulations indicate that maintenance or repair staff can enter the hall at least 10 min after shutdown, when the pulse length extends 500 s after one discharge. In many cases, multipulse discharge is a necessary attribute of the diagnostics schemes. The condition was simulated for pulse lengths of 100 s (the design pulse) with 10 pulses. The interval time between two pulses was 10 min. Compared with single-pulse discharge, the results (described in Fig. 7 Fig. 7 .) were one magnitude-of-order lower because the multiple pulse intervals can cool the target. Therefore, the maximum shutdown dose rate is associated with pulses, cooling times, distance from the main neutron source, and material property. 5 Conclusion 1 FLUKA code was used to assess neutron activation for the key components of the EAST NBITF system, with operating the parameters of energy 80 keV, current 50A, and pulses of 50–1000 s, respectively. In addition, the distributions of the neutron and the photon spectra of VV were simulated. Finally, the shutdown dose rates were analyzed after differential pulse discharges. The results indicate the following. The major radionuclides were 99 Mo, 65 Ni, 58 Co, 56 Mn, and 51 Cr for the VV, 1 h after shutdown of the NBITF. Total activity dropped slowly because of the dominant nuclide mediate lives: 55 Fe (T 1/2 , 2.73 y); 51 Cr (T 1/2 , 27.25 d); 58 Co (T 1/2 , 70.86 d); and 54 Mn (T 1/2 , 312.3 d). For the neutron source calorimeter, the primary nuclides were 64 Cu, with a half-life of only 12.7 h. After one day of calorimeter, the total activity declined by three orders of magnitude. However, the long-lived nuclides were the main contribution to the shutdown dose rates, and the shutdown dose rates were calculated for different pulse lengths. When the NBITF operated within the design parameters, the maximum value was 4μSv/h in the perpendicular direction. Following the beamline direction, the highest dose rate was 5.5μSv/h, which was below the dose-rate limit of IAEA’s safety level of 10μSv/h. As such, maintenance and repair staff can access NBITF after shutdown at a pulse length under 100 s. 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International journal of cardiology , 2007 , 114 : 256 - 260 . Training of interventional cardiologists in radiation protection—the iaea's initiatives The authors would like to thank the NBI staff for their helpful suggestions on this manuscript. 10.1007/s41365-016-0079-5 This work was supported by the International S&T Cooperation Program of China (No 2014DFG61950), National Magnetic Confinement Fusion Science Program (No. 2013GB101001), and the National Natural Science Foundation of China (No.11405207) 2015-03-15 2015-06-02 2015-06-02 2016-07-08 2016-08-20 2021-04-11 © China Science Publishing & Media Ltd. (Science Press), Shanghai Institute of Applied Physics, the Chinese Academy of Sciences, ChineseNuclear Society and Springer Nature Singapore Pte Ltd. 2016 This is a post-peer-review, pre-copyedit version of an article published in Nuclear Science and Techniques. The final authenticated version is available online at: http://dx.doi.org/10.1007/s41365-016-0079-5 http://dx.doi.org/10.1007/s41365-016-0079-5 . 2016 Nuclear Science and Techniques 04 27