Research And Application Of Monte Carlo Variance Reduction Method

In the field of nuclear energy,nuclear analysis such as the simulation of global radiation field and dose field plays a very important role in the design and operation of reactors.Currently,Monte Carlo(MC)simulation is one of the most accurate methods.However,the slow convergence rate and computationally intensive of MC method render analogue simulation prohibitively for deep-penetration shielding calculation of fission and fusion reactors.Thus,developing a new global variance reduction technique(GVR)with high efficiency,high stability and high applicability is very necessary,although with great difficulty.Unfortunately,there still are some problems remind in the existing GVR method.To this end,this paper aims to develop a new GVR method to break through the bottleneck of the Monte Carlo method,and to provide high-precision and high-efficiency Monte Carlo global simulation.With this new GVR method,substantial computational time for Monte Carlo global calculation can be reduced,which has great theoretical and engineering significance.Firstly,this paper conducts an in-depth investigation and analysis of GVR method at home and abroad,and compares their advantages and disadvantages.On this basis,the forward MC calculation is selected to generate WWM in this research.To overcome the low efficiency in the process of iterative WWM generation in the traditional method,this paper proposes a new method called on-the-fly GVR(OTF),which means the WWM is re-set on-the-fly timely during the transport calculation rather than at the end of Monte Carlo simulation.Thus,only on single Monte Carlo calculation is needed.By this way,the WWM can be updated in time which helps to obtain more reasonable WWM faster.On the other hand,the shape of WWM can be obtained much more quickly due to the steep gradient of WWM.Updating the WWM on the fly automatically along the transport calculation can successfully eliminate the extra time and effort from the users in preparing WWM.Having completed the WWM generation,the MC calculation was automatically restarted with the final optimized WW,thus no laborious process of generating WWM is needed any more.To solve the problem of Long History which occurs in the process of using WWM,this paper explains the causes of long history problem in detail,and proposes a new solution to solve over-splitting problem called "out-flow controlled method".The use of out-flow controlled method is very important to the controlling of over-splitting problem,and the combination of on-the-fly updating the WWM and out-flow controlled method improve the efficiency of WWM generation of OTF method greatly.Secondly,based on the in-depth study of OTF GVR method,OTF method has been implemented and automated into MCNP5 v 1.6.In order to access the efficiency reliability,validity and applicability of OTF method,a series of thorough tests have been conducted.The test cases include:self-designed simple cubic with heavy shielding model,JPDR fission reactor,fusion reactor including ITER benchmark,ITER-C model,and IFMIF-DONES(International Fusion Material Irradiation Facility-DEMO Oriented NEutron Source).Results calculated with analogue(without any variance reduction technique),OTF and FW-CADIS have bee obtained and compared.Compared with analogue,OTF method has shown cheering results in speeding up global shielding calculation,and an acceleration factor of about ten to hundreds was achieved.For all the application scenarios,the OTF method has shown better acceleration effects than the FW-CADIS method.