Neutronics effects of homogeneous model on solid breeder blanket of CFETR
Introduction
CFETR (China Fusion Engineering Test Reactor) is a new tokamak test reactor being designed in China aimed to bridge gaps between ITER (International Thermonuclear Experimental Reactor) and DEMO [1,2]. Based on the experience of China Helium Cooled Ceramic Breeder Test Blanket System (HCCB TBS) for ITER [3], the solid breeder blanket concept is considered for CFETR, which shall satisfy the requirements of tritium breeding, energy extraction, neutronics shielding and etc. Nuclear performance evaluation of the tritium breeding blanket, including the TBR (tritium breeding ratio), neutron flux, nuclear thermal distribution, etc., is a core basis of blanket design and also the input for thermo-hydraulic and thermo-mechanic numerical analysis. TBR is the short form of Tritium Breeding Ratio which can be calculated as follows:Where, and are the atom densities of 6Li and 7Li separately, and are cross sections of the (n, t) reaction of 6Li and 7Li separately, is the neutron flux and is the total neutron number produced in plasma.
Neutronics modeling and transport calculation are very challenging tasks for the fusion reactors, such as tokomak, spherical tokomak and stellarator, because the tokamak complex surrounds the high-temperature plasma with large scale of volumes and complex geometry, an intense heterogeneous neutron flux distribution and neutrons with a large energy span. The homogeneous blanket model has been used in worldwide for the neutronics calculation, but this model may cause some problems and concerns:
- 1
In the homogeneous model, different materials of the breeding blanket are mixed together according to their volume fractions in each functional region, but real neutron flux distribution in the real structure may be different, which may cause calculation error;
- 2
In neutronics calculations with the homogeneous model, the space self-shielding effect is neglected in the regions of Li4SiO4 pebble beds and Be pebble beds. As a result, there may be an impact on the reaction rate of the (n,T) and (n,2n) reactions;
- 3
The effects of neutron moderation, especially for water-cooled concept, are small in the homogenous model, which may lead to incorrect results of the neutron spectrum and the neutron flux distribution.
Two types of neutronics transport calculation methods are mainly adopted: Monte Carlo method and deterministic method [[4], [5], [6]], but codes based on the Monte Carlo method are widely used in neutronics analysis of fusion reactors.
In this paper, neutronics calculations with 3D high-fidelity neutronics models of a single blanket are performed based on the design of CFETR HCCB blanket [7]. The neutronics effect of the homogeneous structure and the space self-shielding effect of pebble beds are analyzed separately considering two types of coolants, helium and water [8].
Section snippets
High-fidelity model of solid breeder blanket
The MCNP-4C [9] is applied for the 3D neutronics transport calculation for the solid breeder blanket, and the nuclear data library FENDL-2.1 [10] is used. One million particles are taken into account in the simulation and the Monte Carlo relative deviation reduces to ∼0.1%. With the highest neutron wall load and radiation, a typical solid breeder blanket configuration is modelled by using the McCAD code [11] which realizes neutronics visual modeling for the MCNP based on the SALOME platform [12
Neutronics effects of homogeneous structure
To study the neutronics effect of the homogeneous structure on the solid breeder blanket, Models A and B are considered in this chapter (shown in Fig. 2(a) and (b)). Based on these two models, the neutronics transport calculations are performed and results are compared. A packing fraction of 52.36% in both the Li4SiO4 and Be pebble beds is assumed. A general neutron source of a Gaussian fusion energy spectrum is added in the front of FW shown in Fig. 3 and the red lines represent the reflecting
Neutronics analysis of space self-shielding effect
6Li has a large cross section of the (n,T) reaction for slow neutrons. Therefore, slow neutrons are absorbed and produce a tritium when they just get into the surfaces of Li4SiO4 pellets. Consequently, the rate of neutrons flying into the core of Li4SiO4 pellets is reduced. In this way, as for slow neutrons, there is a shielding effect from the outer sphere to the inner sphere of the Li4SiO4 pellets, which is so called the space self-shielding effect. Meanwhile, as for the fast neutrons, there
Verification for space self-shielding effect
The space self-shielding effect is verified in a single blanket module and a high-fidelity neutronics model (Model C). Pellets with a diameter of 1 cm is assumed in the high-fidelity model and the neutronics analysis is performed both for the helium-cooled and water-cooled concepts. The flow chart of the whole verification is shown in Fig. 11.
Where, , , and are the calculated local TBR in Model B, C, D2 and D1 with pellets with a diameter of 1 cm, and
Conclusions
Referring to the design of CFETR blanket, 3D high-fidelity neutronics models have been built considering the impact assessment of different coolants, helium and water. The neutronics effect of the homogeneous structure and space self-shielding caused by pebble beds are analyzed separately including the normalized neutron energy spectrum, the local TBR of each tritium breeder region and the radial neutron flux distribution. Then, the neutron mean-free path of Li in each tritium breeder region is
CRediT authorship contribution statement
Shen Qu: Methodology, Software, Writing - original draft. Qixiang Cao: Visualization, Supervision. Xuru Duan: Supervision. Xueren Wang: Supervision. Zaixin Li: . Xiaoyu Wang: Supervision.
Declaration of Competing Interest
On behalf of co-authors, I am submitting our manuscript entitled “Neutronics effect of homogeneous model on solid breeder blanket of CFETR” for possible publication in Fusion Engineering and Design.
Acknowledgments
The work at SWIP (Southwestern Institute of Physics) was supported under National Natural Science Foundation of China Number 11905046 and National Key R&D Program of China Number 2017YFE0300503 and 2017YFE0300601. Also acknowledge to the KIT (Karlsruhe Institute of Technology) for the development of McCAD code.
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