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

NUCLEAR ENERGY SCIENCE AND ENGINEERING

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

Yu-Qing Chen，

Bin Wu，

Jin-Fang Wang，

Jun Li，

Ya-Hong Xie，

Guo-Qiang Zhong，

Chun-Dong Hu

Nuclear Science and Techniques

Vol.27, No.4

Article number 85

Published in print 20 Aug 2016

Available online 08 Jul 2016

DOI：10.1007/s41365-016-0079-5

37600

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 ⁹⁹Mo, ⁶⁵Ni, ⁵⁸Co, ⁵⁶Mn, and⁵¹Cr for the vacuum vessel and the primary nuclide ⁶⁴Cu 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 10¹¹n/s with no radiation safety problems after the EAST NBITF shuts down for 1 h.

Shutdown dose rateNeutron activationRadionuclides

1 Introduction

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 [1–5]. Calculations of neutron production demonstrate that the calorimeter is the main neutron source [6–8].

These neutrons activate the components for the EAST NBITF and cause the material deterioration of the properties [2, 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 [12–14].

2 Geometrical Model

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, 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].

Fig. 1

NBI geometric model (a) side view of NBI (b) top view of NBI

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.

Regions and materials of the components

Regions	Components	Compositions (weight %)	Density (g/cm3)
1. Arc chamber	Cu	100	8.96
2. Vacuum	Vacuum
3. Vacuum vessel	SS316	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	8.0
4. Beamline vessel	SS316	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
5. Neutralizer	Cu	100	8.96
6. Ion dump	Cu	100	8.96
7. Bending magnet	Cu Fe	Cu, 50; Fe, 50	8.96
8. Calorimeter	Cu	100	8.96
9. Front cryo-panel;	Al	100	2.70
10. Rear cryo-panel	Al	100	2.70
11. Environment	Air	C, 0.0124; N, 75.5268, 023.1781; Ar, 1.2827	1.205 × 10⁻³

3 Mathematical Method Theory

3.1 Neutron-Intensity Calculations

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 [3–6]:

y = \frac{I_{b}}{e} \sum_{i = 1}^{3} i f_{i} \int_{0}^{R} σ (E (x) / i) C_{iD} (x, θ) d x

(1)

[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 $E (x)$ is the particle energy at depth x. $σ [E (x)]$ is the cross section; and $C_{D} (x, θ)$ is the saturation deuterium concentration (cm⁻³) in the target. $θ = D_{diff} t / R^{2}$ is calculated with the diffusion coefficient $D_{diff}$ , 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 × 10¹⁰ n/s/A during the EAST NBITF running with the parameters 80 KeV and 50 A and a pulse of 10–100 s [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, 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

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]:

A_{i} = ϕ_{n} \sum_{j \neq i} σ_{i j} n_{j t} θ_{j t} (1 - e^{- λ_{i} t_{irra}}) e^{- λ_{i} t_{c}} .

(2)

[Please review edits to Eq. 2: “e” as a variable was changed to “e”; “irra” as an abbreviation was changed to “irra”.]

$λ_{i}$ is the decay constant of the nuclide i; $ϕ_{n}$ is the neutron flux (n/cm²/s); $σ_{i j}$ is the reaction cross section for reaction on producing i(cm²); n_jt is the density of nuclide j in the target material (n/cm³); t_c is the cooling time; t_irra is the irradiation time (can be considered effective time of EAST NBITF operating); and $θ_{j t}$ is abundance of the i nuclide isotope in nature.

4 Results

4.1 Neutron and Gamma Spectra

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. Because the effect of neutron scattering is significant [17–19], it can be included with the low-bias card in FLUKA.

Fig. 2.

Energy spectrum distribution: (a) neutron; (b) Gamma

Fig. 2a reveals that the 2.45 MeV neutron intensity is still high after penetrating the calorimeter and cyro-panel. 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

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 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.

Activities of vacuum vessels through different methods

Nuclide	Reaction	Theoretical value (Bq/kg)	FLUKA (Bq/kg)
⁹⁹Mo	⁹⁸Mo(n,γ)⁹⁹Mo	5.44E+06	7.88E+06
⁹³Mo	⁹²Mo(n,γ)⁹³Mo	5.02E+00	4.49E+00
⁶⁵Ni	⁶⁴Ni(n,γ)⁶⁵Ni	6.52E+06	1.17E+06
⁵⁸Co	⁵⁸Ni(n,p)⁵⁸Co	4.14E+05	1.36E+06
⁵⁵Fe	⁵⁴Fe(n,γ)⁵⁵Fe	7.52E+04	4.01E+04
⁵⁶Mn	⁵⁵Mn(n,γ)⁵⁶Mn	1.41E+08	7.28E+07
⁵⁴Mn	⁵⁴Fe(n,p)⁵⁴Mn	2.40E+05	1.37E+05
⁵¹Cr	⁵⁰Cr(n,γ)⁵¹Cr	5.52E+05	4.37E+05

The dominant radionuclides are ⁵⁸Co, ⁵⁵Fe, ⁵⁴Mn, ⁵⁸Co, and ⁵¹Cr for vacuum vessels [AU: This text was replaced from where it originally appears, between Table 2 and Fig. 3. Please delete the original text.] 1 h after shutdown. Fig. 3 gives the functions between the activities of the NBI key components and the cooling times. In the VV, ⁹⁹Mo, ⁵⁸Co, ⁵⁵Fe, and ⁵⁴Mn 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 ⁵⁵Fe (T_1/2, 2.73 y); ⁹⁹Mo (T_1/2, 65.94 h); ⁵¹Cr (T_1/2, 27.25 d); and ⁵⁸Co (T_1/2, 70.86 d). For the main radiation source calorimeter, ⁶⁴Cu (T_1/212.7h) 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.

Fig. 3.

Activities vs. cooling times: (a) vacuum vessel activity; (b) calorimeter activity

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 indicates that neutron spectrum, neutron flux, target material, and cooling time affect the activities of the different components.

Fig 4.

Total activity vs. cooling time

4.3 Shutdown Dose Rate of NBI

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 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 demonstrates that the dose rates for pulses between 50–200 s (10μSv/h) fall below the safety standards of IAEA [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.

Fig. 5

Dose rates vs. distance: (a) shutdown dose-rate distribution in the vertical direction; (b) shutdown dose-rate distribution along the beamline

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 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.

Fig. 6

Dose rate vs. cooling time: (a) Point A dose-rate distribution away from the vacuum vessel, 50 cm in the vertical direction; (b) Point B dose-rate distribution away from vacuum vessel, 50 cm in the beam line direction.

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.) 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.

Fig.7.

Dose rate vs. distance at different cooling time after multiple discharges for pulses of 100 s: (a) dose-rate distribution along the radial direction; (b) dose-rate distribution along the beam line direction

5 Conclusion

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 ⁹⁹Mo, ⁶⁵Ni, ⁵⁸Co, ⁵⁶Mn, and ⁵¹Cr for the VV, 1 h after shutdown of the NBITF. Total activity dropped slowly because of the dominant nuclide mediate lives: ⁵⁵Fe (T_1/2, 2.73 y); ⁵¹Cr (T_1/2, 27.25 d); ⁵⁸Co (T_1/2, 70.86 d); and ⁵⁴Mn (T_1/2, 312.3 d). For the neutron source calorimeter, the primary nuclides were ⁶⁴Cu, 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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