Effects of neutron spectrum and external neutron source on neutron multiplication parameters in accelerator-driven system

The neutron multiplication parameters: neutron multiplication M, subcritical multiplication factor k_s, external source efficiency φ^*, play an important role for numerical assessment and reactor power evaluation of an accelerator-driven system (ADS). Those parameters can be evaluated by using the measured reaction rate distribution in the subcritical system. In this study, the experimental verification of this methodology is performed in various ADS cores; with high-energy (100 MeV) proton–tungsten source in hard and soft neutron spectra cores and 14 MeV D–T neutron source in soft spectrum core. The comparison between measured and calculated multiplication parameters reveals a maximum relative difference in the range of 6.6–13.7% that is attributed to the calculation nuclear libraries uncertainty and accuracy for energies higher than 20 MeV and also dependent on the reaction rate distribution position and count rates. The effects of different core neutron spectra and external neutron sources on the neutron multiplication parameters are discussed.     

Study of the neutron multiplication effect in an active neutron method

The neutron multiplication effect appears when an item contains large amounts of nuclear material. The neutron multiplication effect in this paper means the effect of subsequent fission reactions which are caused by fission neutrons produced by interrogation neutrons from a neutron generator. The previous active neutron method could not distinguish between first-fission and subsequent-fission neutrons and might overestimate the amount of nuclear material. However, the neutron multiplication effect in the active neutron method has not been adequately investigated. We discuss the evaluation method of the multiplication effect in the fast neutron direct interrogation method, one of the active neutron methods, using simulations with the Monte Carlo code MVP and experiments involving uranium waste drums. The first-generation neutrons from an external neutron source generate fission neutrons called second-generation neutrons, the second-generation neutrons generate third-generation neutrons, and so on. This study supposes that the neutron multiplication effect is mainly caused by the third-generation neutrons under the condition that the fourth-generation neutrons are much fewer. This paper proposes a correction method for the neutron multiplication effect in the measured data.     

Measurement of k_(eff) with an improved neutron source multiplication method based on numerical analysis

In this work,we developed a numerical analysis-associated experiment method to determine the effective multiplication factor k_(eff),which is difficult to obtain directly from conventional neutron source multiplication(NSM)method.The method is based on the relationship between k_(eff),subcritical multiplication factor k_s and external neutron source efficiency Φ~* in the subcritical system.On basis of the theoretical analysis,the dependence of k_s and Φ~* on subcriticality and source position was investigated at the Chinese Fast Burst Reactor-Ⅱ(CFBR-Ⅱ).A series of k_s were measured by NSM experiments at four subcritical states(k_(eff) = 0.996,0.994,0.991 and0.986) with the ~(252)Cf neutron source located at different positions(from the system center to outside) at each subcritical states.The Φ~* was obtained by Monte-Carlo simulation for each condition.With the measured k_s and calculated Φ~*,k_(eff) of the subcritical system was evaluated with a relative difference of 1%between values obtained by the improved method and by positive period method.Especially,the relative difference of 0.18%with the source located at the system center.     

Subcritical multiplication factor and source efficiency in accelerator-driven system

The subcritical multiplication factor k_s   and the external neutron source efficiency φ^∗${\phi }^{\ast }$ are important parameters in the accelerator-driven system (ADS) performance assessment. The theoretical relation between k_s and the effective multiplication factor k_eff in a subcritical system is discussed in different cases of subcritical system. On the basis of the theoretical background, the dependence of k_s   and φ^∗${\phi }^{\ast }$ on subcriticality, source position, and energy is numerically investigated using a simple thermal subcritical model. For the sake of experimental evaluation of k_s   and φ^∗${\phi }^{\ast }$, the ADS experiments have been carried out in the subcritical systems combined with 14 MeV pulsed neutrons of the Kyoto University Critical Assembly (KUCA). The k_s   and φ^∗${\phi }^{\ast }$ parameters are successfully measured by utilizing the reaction rate distribution obtained by the optical fiber detectors in the subcritical system, within a relative difference of less than 7% and 12% for k_s   and φ^∗${\phi }^{\ast }$, respectively, between measured and calculated values for most studied cases.     

反应堆物理实验中的源倍增法研究

给出了反应堆物理实验中临界测量和次临界度测量通常所采用的源倍增方法研究。首先从有源的扩散理论出发,导出了与以前不同的源倍增方法的公式。源倍增方法测量的参数实际是次临界系统在外源作用下的有源次临界中子倍增因子ks,而不是在这之前的中子有效倍增因子keff,然后研究了实验装置的临界质量,研究了ks与外源位置和能谱的关系,证明了导出的源倍增方法的理论是正确的。该方法可像过去那样用于反应堆物理实验中的临界外推测量,但不能用于次临界度测量。解决了长期困扰人们有关源倍增方法测量的参数问题。最后讨论了ks和keff的差别和关系以及对临界外推测量和核临界安全的影响。     

On the neutron spectrum and multiplication factor in the infinite homogeneous reactor

The analysis of the infinite homogeneous reactor can be useful to have a quick understanding of nuclear reactor phenomena. Thanks to the fact that the neutron emission spectrum from fission is modeled in order to depend neither on the energy of the incident neutron nor on the fissioning nuclide, the critical neutron balance equation can be transformed from an eigenvalue problem to a simple inhomogeneous system. Similar considerations can be addressed to the adjoint problem and to the generalized importance, which can be computed with direct expressions. After having derived the equations relative to the above quantities, we apply them to the investigation of some features of the neutron spectrum in UO₂ and MOX fuel. We deduce some relationships between the properties of the medium and the neutron spectrum, such as the independence of the neutron spectrum from the fission cross section and the neutron multiplicity. We show also in which extent these conclusions can be applied to real reactors.     

Determination of the effective multiplication factor in a slab by the LTSN method

The aim of this work is to present a new approach based upon the application of the LTS_N method to solve the eigenvalue problem in a slab, namely the problem of criticality. This procedure leads to the solution of a transcendental equation for the effective multiplication factor. We report numerical simulations considering the S_N multigroup problem in heterogeneous slab.     

外中子源驱动式核电源

外中子源驱动式核电源是新型核电源系统,能满足深空探测任务的需求.对该核电源的简化模型做了MCNP模拟,其比功率和功率分别为4.45 W/g和9.88 kW,比功率是通用型热源放射性同位素温差核电源(GPHS-RTG)的8倍,名义功率是GPHS-RTG的3.54倍.由GPHS-RTG模块的细致模型计算,该核电源的比功率和...     

裂变-聚变中子源

为适应聚变堆的发展和处理高放废物的需要,提出裂变-聚变中子源的概念,它是采用LiD组件放在高通量反应堆中或中国先进研究堆(CARR)重水区中,通过慢中子与6Li(n,a)反应产生2.739 MeV氚离子,它与LiD中的D发生聚变反应,产生聚变中子;随着LiD中氚的快速积累,14 MeV 中子产生的D反冲粒子流与氚发生聚变反应,增长聚变中子产额,使 14 MeV 中子注量率逐渐升高.当氚浓度接近0.5×1022时,D反冲粒子流与氚的聚变反应率的产额接近于1,聚变中子将成倍的增长,类似于连锁反应,使聚变中子产额达到饱和,即t时刻产生氚,都被用于产生聚变反应,形成裂变-聚变中子源.这时的通量非常高,必须在接近饱和前对设定的通量(如3.5×1014n/cm2·s)下逐步降低反应堆功率,如降低CARR 中子注量率,使其在设定的通量下达到饱和,适应聚变堆中子注量率的需求.论述了裂变-聚变中子源的原理,聚变中子产生率,氚的积累速率和浓度,D反冲粒子流和与氚的聚变反应速率,以及其影响因素.在均匀中子场下(即不考虑中子降抑的情况下)计算了外径180 mm、内径100 mm的LiD管道中聚变中子注量率.     

Experimental study of neutron multiplication in BeO by the shift-register coincidence technique

The multiplication of 14 MeV neutrons by Be, which is used as a neutron multiplier in the design of fusion reactor blankets, is of topical interest as T-breeding depends on this. Earlier investigations have shown that the experimental multiplication of 14 MeV neutrons in Be and BeO is 20–25% lower than the calculated values. Recently, a different method has been proposed to measure the leakage multiplication based on a statistical correlation technique. This note explores the applicability of such a technique to determine experimentally the leakage multiplication of Am-Be source neutrons of average energy 4.5 MeV in a BeO assembly. The experimental method uses a high-level neutron coincidence counter (HLNCC) equipped with a shift-register coincidence logic unit. The deduced value of neutron leakage multiplication for 5 and 10 cm thick BeO in near rectangular geometry is 1.12 ± 0.03 and 1.15 ± 0.03, respectively.     

脉冲中子在煤中所形成中子场的研究

本文采用蒙特卡罗计算程序MCNP(Monte Carlo N-Particle Transport code),对煤炭中Ti、Si元素量的变化前后14MeV脉冲中子源在煤中形成的中子场进行了计算.其中Ti元素量的变化对中子场影响程度较大,Si元素则较小.实验事实验证了分析结果.     

Accelerator driven sub-critical systems with enhanced neutron multiplication

Utilisation of thorium, by way of the U-Th cycle, is of particular interest to the Indian Nuclear Power Programme because of large thorium deposits and limited Uranium reserves. Several schemes, such as fast and advanced heavy water reactors, leading to thorium utilisation, are under study at this centre. The present paper discusses a scheme for evolving a practical accelerator driven sub-critical U-Th system with increased neutron multiplication and consequentually a reduced requirement of the accelerator current. It is shown that the requirement of the accelerator current is considerably reduced if a sub-critical assembly with a given K_eff is composed of two partially coupled regions.     

Determination of the effective multiplication (breeding) factor of neutrons from the measured differential reactivity

A relation is established between the effective multiplication factor of neutrons in a reactor and the experimental value of the reserve of reactivity, determined from measurements of differential reactivity. Correction terms are determIned in integral form.Translated from Atomnaya Énergiya, Vol. 22, No. 2, pp. 113–117, February, 1967.