Neutronics effects of homogeneous model on solid breeder blanket of CFETR

Highlights

• The homogeneous model has small impacts on the neutronics performance for helium cooled blanket concept and the TBR (tritium breeding ratio) is overestimated by ∼2.5% for water-cooled blanket concept.

• The space self-shielding effect caused by pebble beds has little influence on the neutronics calculation for helium-cooled concept, but ∼1.3% TBR overestimation for water-cooled concept.

• The homogeneous model is rational for neutronic analyses with helium coolant. However, the high-fidelity model should be adopted in the neutronics transport calculation because the homogeneous model overestimates the neutronics performance for water-cooled blanket concept.

Abstract

In the design of solid breeder blanket, normally the homogenous model is used for the neutronics analysis to assess the performance of blanket, but the adaptability of homogenous model is neglected. Referring to the design of CFETR (China Fusion Engineering Test Reactor) blanket, impact assessments of homogeneous model with different coolants, helium and water, are investigated with 3D high-fidelity neutronics models in this paper. The neutronics effects of the homogeneous structure and space self-shielding are analyzed separately. The results show that the homogeneous model has small impacts on the neutronics performance for helium cooled blanket concept and the TBR (tritium breeding ratio) is overestimated by ∼2.5% for water-cooled blanket concept; the space self-shielding effect caused by pebble beds has little influence on the neutronics calculation for helium-cooled concept, but ∼1.3% TBR overestimation for water-cooled concept. Finally, the results verify that the homogeneous model is rational for neutronic analyses with helium coolant. However, the high-fidelity model should be adopted in the neutronics transport calculation because the homogeneous model overestimates the neutronics performance for water-cooled blanket concept.

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:

$$TBR=\frac{\int {{\displaystyle N}}_{{{\displaystyle 6}}_{Li}}{{\displaystyle \sigma }}_{{{\displaystyle 6}}_{Li}}\varphi dV+\int {{\displaystyle N}}_{{{\displaystyle 7}}_{Li}}{{\displaystyle \sigma }}_{{{\displaystyle 7}}_{Li}}\varphi dV}{{{\displaystyle N}}_{\varphi }}$$

Where, ${{\displaystyle N}}_{{{\displaystyle 6}}_{Li}}$ and ${{\displaystyle N}}_{{{\displaystyle 7}}_{Li}}$ are the atom densities of ⁶Li and ⁷Li separately, ${{\displaystyle \sigma }}_{{{\displaystyle 6}}_{Li}}$ and ${{\displaystyle \sigma }}_{{{\displaystyle 7}}_{Li}}$ are cross sections of the (n, t) reaction of ⁶Li and ⁷Li separately, $\varphi$ is the neutron flux and $N_{\varphi }$ 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:

• 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;

• 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;

• 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].