## 6 Conclusions  
This paper solves the problem of formulating the optimal allocation scheme of hazardous materials and realizing the sustainable management of hazardous materials. Firstly, it abstractly models the realistic situation that needs to be considered for the multi-period allocation scheme of hazardous materials under uncertainty, and gives a Bilevel chance constrained programming model for formulating a double-objective multi-period allocation scheme of hazardous materials that considers the minimum cumulative number of reserve points participating in the allocation and the minimum cumulative allocation risk. Then, we improved the differential evolution algorithm and established a hierarchical difference evolution algorithm considering multi-dimensional problems to solve the model and find the optimal allocation scheme of hazardous materials.  
The experience results provide the following conclusions: (1) In solving the multi-period allocation model, the hierarchical differential evolution algorithm converges faster than the other multi-objective evolution algorithm, and can search more Pareto solutions, which means there is more space for selecting the optimal allocation scheme. (2) The Pareto solution set obtained by the hierarchical difference algorithm is densely and evenly distributed between the cumulative allocation risk and the cumulative number of reserve points participating in the allocation, with high similarity, and better solution quality (the quality of the searched allocation scheme set is better). (3) Compared with the single-period decision making method, the method proposed in this paper can obtain a safer and  
Figure 10: Comparison of allocation schemes with different decision making methods.  
more efficient hazardous material allocation scheme, which is conducive to the sustainable management of hazardous materials.  
The practical implications of this study include the following: (1) Type-II fuzzy numbers are used in the hazardous materials allocation model proposed in Section 3.3 to measure the transportation risk in the hazardous materials allocation process, which provides a new measurement method for the subsequent risk assessment of nuclear materials, explosives, hazardous chemicals, and other hazardous materials. (2) This study can solve the long-term allocation scheme of hazardous materials from a global perspective, and realizes that multiple storage points can allocate multiple hazardous material resources for multiple distribution points in a coordinated manner without causing resource conflicts, which provides a certain method reference for the subsequent solution to the long-term optimal allocation of large-scale resources in industrial production. (3) This paper designs a new hierarchical differential evolution algorithm, which provides an alternative global optimization algorithm for subsequent solution of multi-dimensional and multi-objective task allocation problems.  
It is worth noting that the method is proposed to make a multi-period scheduling plan for hazardous materials on the basis of inherent inventory without considering the replenishment of inventory, the uncertainty of demand at each consumption point in each period, and the inherent attributes of each hazardous material, such as residual life, economic benefits, and other factors, which cannot be ignored when making a long-term allocation scheme for hazardous materials, and further research is still needed in the follow-up work.  
Methodology, X.Z. and Q.M.; Writing--review & editing, X.L. and W.W. All authors have read and agreed to the published version of the manuscript.  
This research received no external funding.  
The authors declare no conflict of interest.