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Activation of high flux test module sample holder after IFMIF-DONES operation

Nuclear safety assessment in nuclear fusion devices relies on the Monte Carlo method based neutron transport calculations. This paper presents information about the calculation results of the activities and dose rates caused by neuron irradiation for the structural materials of the high flux test module sample holder of IFMIF-DONES. The neutron induced activities and dose rates at shutdown were calculated by means of the FISPACT-2010 code with data from the EAF-2010 nuclear data library. Neutron fluxes and spectra were obtained with MCNP neutron transport calculations. The activities and dose rates were calculated at the end of irradiation of the assumed device operation scenario for cooling times of 0 s – 1000 year. In addition, radionuclides with contribution of at least 0.5% to the total value of activation characteristics at the previously mentioned cooling times were identified. After the operation, the most active radionuclide is 55Fe, with an activity share ranging from 30% (M200) to 63% (M8), and at the end of the prediction it accounts for 86% of the total activity. The highest dose rates at the end of irradiation are attributed to 56Mn radionuclide. 54Mn and 60Co are the most dominant radionuclides during intermediate and long cool-down periods.


Introduction
Nuclear energy is a relatively clean energy source in terms of carbon footprint in comparison to widely used fossil fuels. There are two main nuclear processes that can result in net energy gain: fusion and fission. Although nuclear fission based technologies currently play an important role in the energy market, they have issues with radiation protection, nuclear waste storage and limited fuel resources. Nuclear fusion offers the alternative that mitigates the formerly mentioned issues to more manageable levels [1]. On the other hand, nuclear fusion technologies face their own challenges, for example: fusion devices are very complicated and achieving sustainable fusion on earth is not an easy task [2]. In order to ensure safety of fusion devices, most times Monte Carlo method based neutron transport calculations are performed. The Monte Carlo method is based on simulation of individual particle histories in complex environment. Particle histories follow the established laws of physics; however, the outcome is decided by random events that are described by probability distributions [3].
There are many possible fusion reactions that result in energy release. At the moment, the most promising and feasible for the controlled thermonuclear process are reactions between hydrogen isotopes (deuterium and tritium) and helium [4]. Most of the energy released during D-T fusion reactions is carried by neutrons. Self-sustaining nuclear fusion can only be achieved if certain particle densities, temperatures and confinement times are present. The cross-section of the D-T reaction is the highest at the selected plasma temperature (around 100 million degrees) [5].
Based on the preliminary engineering project of IFMIF (International Fusion Materials Irradiation Facility), the European Union has proposed a plan for the development of the DONES (DE-MO-Oriented Neutron Source) nuclear fusion facility [6]. IFMIF-DONES neutrons will be generated during the Li (d, xn) reaction by liquid lithium target bombardment with deuterium ions (up to 40 MeV). The device will be capable to produce energetic neutrons and will employ acceleration current (125 mA) in order to procure high neutron flux [>10 18 m −2 s −1 ] in order to achieve exposure conditions relevant to nuclear fusion for material testing and qualification. Neutron exposure in material samples will result in structural damage and activation. Activation will not be limited to samples as a large part of the facility will be exposed to neutron irradiation as well. The highest neutron intensity will be at the Test Cell (TC) (Fig. 1) [7] and its High Flux Test Module (HFTM) [8] where material samples are held (for more details see Figs. 2, 3) [9]. Estimation of activation in TC and HFTM is critical for the operation and maintenance of the device as it could lead to radiological hazards for staff and sensitive electronics.

Computational methods
The MCNP (Monte Carlo n-particle) particle transfer code was developed by the Los Alamos National Laboratory in the United States. It was programmatically written using the FORTRAN and C programming languages.
For neutron spectrum calculations the neutron transport equation must be solved: The MCNP uses three processes to solve neutron transport equations: (1) using the probability distribution determines the source parameters; (2) neutron location, energy and direction tracking; (3) data recording and analysis of results [10]. The FISPACT-2010 is describing the amounts of various nuclides in materials after given irradiation  steps. It can be achieved by solving the Bateman differential equations where N i is the amount of nuclide i at time t, λ i is the decay constant of nuclide i, λ ij is the decay constant of nuclide j producing i, σ i is the total cross section for reaction i, σ ij is the reaction cross-section for reaction j producing on i, σ k f is the fission crosssection for reactions on actinide k, ϕ is the neutron flux, S i is the source of nuclide i from fission, and Y ik is the yield of nuclide i from the fission of nuclide k. The set of the previously mentioned differential equations is solved using the Sidell method. Due to the dependence on the duration of irradiation and cooling sequences only nuclides with sufficiently long half-lives are calculated by this method, and nuclides with shorter half-lives are considered to be in equilibrium [11].

Calculation results
Neutron spectra (Fig. 4) were determined by using the IFMIF-DONES neutron source and geometry model. Neutron fluxes and energy distribution were obtained for the structure of the HFTM sample holder. All neutron transport calculations were done by using MCNP5 with the JEFF-3.1.2 [12] nuclear library.
Activation analysis and radionuclide identification were performed with the FISPACT-2010 code with the EAF-2010 nuclear data library [13].
JEFF-3.1.2 is the general purpose fusion and fission nuclear data library, while EAF-2010 is the nuclear data library optimized for fusion applications. A concise structure of EAF-2010 enables a shorter computation duration.
For activation calculations the IFMIF-DONES irradiation scenario was employed that assumes facility operation for 10 years. Each year consists of 345 days of continuous irradiation and 20 days of downtime representing the maintenance work. Specific activities and dose rates were obtained and dominant radionuclides were identified for HFTM. Only radionuclides contributing more than 0.5% to the total value after any of the considered time interval are presented.
The specific activity dependence on time is shown in Fig. 5. At the end of irradiation, 55 Fe is the most active radionuclide and represents 30%   (M200) to 63% (M8) of the total activity. In addition to this, the contribution of 54 Mn and 56 Mn is 7% (M200) to 13% (M8) of total activities. Although the half-life of the 55 Fe iron isotope is relatively long (half-life is 2.734 yr), it remains the most relevant isotope for almost all prediction time period. A more detailed analysis of the obtained results shows that 55 Fe is produced from the 54 Fe neutron capture reaction with gamma radiation as a by-product. The neutron capture reaction is responsible for 56 Mn production from 55 Mn.
The main radionuclides that are gamma radiation sources are presented in Fig. 6. As we can see, the largest contribution to the equivalent dose rate   Co emits almost 100% of the total dose.
In Fig. 7, we can see that the main dominant radionuclides are the same as in the material in Fig. 6; however, in Fig. 7 there are some differences in radionuclides responsible for the total dose rate. The main contributors are 56 Mn and 54 Mn, while 60 Co is insignificant and amounts to less than 0.5% of the total dose rate.
As seen in Fig. 8, in the first second after the end of irradiation, the increase in the iron content increases the activity of the 55 Fe isotope  fraction in the material. The highest activity (272.8 TBq/kg) is reached in the AISI SS316L material with 65% iron content. 51 Cr has the greatest influence on SS316L. For the activity of M40 (concentration of Fe in material is ~75%), in addition to 51 Cr, 56 Mn and 54 Mn are also significant sources. Figure 9 depicts the iron percentage and related specific activity values present for 181 days of the observation time. During this time period, the influence of iron activation products (mainly 55 Fe) increases with regard to the total specific activities. It should also be noted that the activity of iron isotopes ( 55 Fe and 53 Fe) ranges from 65% (M200) to 86% (M8). The most active substance is M8, containing 87.97% of iron, and has a total activity of 159.1 TBq/kg after 181 d.

Summary
The objective of this study was to calculate the activities and dose rates in the HFTM sample holder structure at the IFMIF-DONES induced by neutron irradiation. Subsequent activities and dose rates at shutdown were calculated by means of the FISPACT-2010 code using the irradiation scenario specified for the IFMIF-DONES. After the end of irradiation, the activities and dose rates were calculated at the cooling time of 0 and 1 s, 5 and 30 min, 1, 3, 5 and 10 h, 1 and 3 days, 2, 4 and 8 weeks, 181 days, 1, 10, 100, 300 and 1000 years.
The investigation of the iron based materials shows that the total dose rate values range from 2.5 • 10 4 to 4 • 10 4 Sv/h within the first second after the end of irradiation and drops to 1.05 · 10 -3 -5.92 · 10 -2 Sv/h, respectively, after 1000 years of cooling.
The specific activity of 55 Fe in the metal samples ranges from 4.92 · 10 13 to 1.57 · 10 14 Bq/kg at the first second of the cooling time and from 1.54 · 10 8 to 4.95 · 10 8 Bq/kg after 50 years of the cooling time. The highest activity value belongs to M8. After 50 years, no activity of 55 Fe was observed.
The total specific activity in the metal samples ranges from 1.63 · 10 14 to 2.74 · 10 14 Bq/kg at the first second of the cooling time and from 1.72 · 10 8 to 5.21 · 10 10 Bq/kg after 50 years of the cooling time. The highest activity values belong to M8.
At 0 second of the cooling time, the most active radionuclide is 55 Fe: its activity ranges from 30% (M200) and 63% (M8) of the total activity up to 86% at the end of the observation. 56 Mn is the radionuclide emitting the highest doses, but it decays fast and due to the longer half-life the dominant nuclides at the end of the cooling time are 54 Mn and 60 Co.