AMS直接测量铀微粒次同位素比的方法

铀微粒同位素比测定在核保障环境取样中发挥着重要作用,目前铀微粒中次同位素比的准确测定方法尚未完善。本工作使用小型加速器质谱研究了一种直接测量铀微粒中次同位素比的分析方法,采用CRM铀系列同位素标准样品,选取不同丰度、不同粒径的铀微粒进行测量分析,CRM-U200、CRM-U970微粒²³⁴U/²³⁵U和²³⁴U/²³⁶U同位素比的测量值与标称值之间的相对误差分别小于10%和20%,该法可实现微米级铀微粒的高灵敏测定,为单铀微粒的次同位素比分析提供了新的技术路线。     

FT—TIMS测量含铀微粒铀同位素比的分析方法研究

环境取样是国际核保障的重要手段。含铀微粒同位素比是铀浓缩设施核保障必须分析的项目。微粒铀同位素分析有许多技术途径，FI-TIMS是其中一种。     

用产额比测量~(235)U/~(238)U同位素丰度比

用裂变产额比法测量了样品中235 U/238 U同位素丰度比。样品受14.8MeV中子短时间辐照后,用HPGe谱仪系统跟踪测量其γ能谱,从各自的特征峰分析得到不同裂变产物的加权平均产额,得到了若干对产物核素的产额比与丰度比的相关曲线。     

通过测量模拟样品中236U与238U原子个数比估算燃耗的探索研究

自然界中²³⁶U与²³⁸U原子个数比约10^-14,不同反应堆类型及核燃料辐照情况辐照后的核材料中²³⁶U与²³⁸U原子个数比不同,一般为天然²³⁶U与²³⁸U原子个数比的10⁷～10¹¹倍。通过测量环境样品中的²³⁶U与²³⁸U原子个数比可探知取样点附近进行过的辐照活动、环境污染的来源及对应核燃料的燃耗。本研究使用配制的模拟样品,建立了多接收电感耦合等离子质谱（MC-ICP-MS）技术测定²³⁶U与²³⁸U原子个数比的方法以及估算核燃料燃耗的工作方案,并与其他燃耗计算方法比较,燃耗的相对偏差约10%。     

238U/235U裂变截面比与俘获裂变截面比测量分析

在中国实验快堆（CEFR）中直接测量²³⁸U的截面数据较困难且误差较大,但可通过测量其与²³⁵U的截面比值来获取²³⁸U的相关数据。本工作采用活化法测量²³⁸U与²³⁵U的裂变截面比及俘获裂变截面比(即σ⁸_f/σ⁵_f与σ⁸_c/σ⁵_f),获取²³⁸U的截面数据并与MCNP计算结果进行比较。结果表明,CEFR的轴向转换区或反射层位置为最佳增殖区域。     

贫铀/聚乙烯交替球壳中裂变反应率的测量与分析

为校验次临界能源堆的概念设计,采用活化法在贫铀/聚乙烯球壳交替装置上开展14 MeV中子学积分实验。用HPGe探测器测量²³⁸U(n,f)及²³⁵U(n,f)反应的裂变碎片¹⁴³Ce衰变产生的293.3 keV特征γ射线,得到装置中与入射D粒子束成90°方向上的²³⁸U(n,f)及²³⁵U(n,f)反应率分布,相对不确定度为5.1%~6.9%。采用MCNP5程序在ENDF/B-Ⅵ库下进行模拟计算,计算结果较实验结果高约5%。     

系列铀同位素基准物质的研制

本工作利用高纯^235U和^238U研制出系列铀同位素基准物质。该基准物质的制备工作流程图示于图1。     

SIMS含铀微粒同位素比分析中多原子离子影响及消除方法研究

实际环境样品基体成分十分复杂,多原子离子对二次离子质谱(SIMS)单微粒铀同位素比分析的影响不可忽略。本文实验分析了Pb、Ni、Zn、Si的多原子离子在SIMS单微粒铀同位素比分析中的干扰,并分别采用提高质量分辨率以及根据核素离子强度扣除其多原子离子的方法对结果进行校正。结果表明：Pb和Ni的多原子离子会影响含铀微粒次同位素比的测量,对铀主同位素比的影响可忽略;Zn和Si的多原子离子对铀主、次同位素比测量均基本无影响。将质量分辨率提高至800,能完全消除Ni多原子离子的影响,Ni-CRM U030混合（Ni粉混合CRM U030）微粒²³⁴U/²³⁸U同位素比测量值与参考值之间的相对偏差基本好于5%,²³⁶U/²³⁸U同位素比测量值与参考值之间的相对偏差基本好于15%;Pb多原子离子干扰无法通过提高质量分辨率进行消除,根据Pb离子强度扣除其多原子离子后,Pb-CRM U030混合微粒的²³⁴U/²³⁸U同位素比测量值与参考值之间的相对偏差基本好于10%,²³⁶U/²³⁸U同位素比测量值与参考值之间的相对偏差基本好于50%。将以上消除干扰的方法应用于真实样品分析,结果表明,其有效消除了多原子离子带来的干扰。     

扫描电子显微镜结合热电离质谱测定单微粒中铀同位素比值

单微粒铀同位素分析是核保障环境监测技术的重要手段。作为现阶段应用最可靠且广泛的微粒分析技术之一,裂变径迹-热电离质谱(FT-TIMS)技术需依赖反应堆辐照,分析步骤繁琐,效率较低。扫描电子显微镜结合热电离质谱(SEM-TIMS)在保持原有TIMS的高测量精密度的同时,由扫描电子显微镜结合X射线能量色散谱仪(SEM-EDX)完成含铀微粒的寻找和鉴别,由微操作系统进行微粒转移,缩短了分析流程,提高了分析效率。本文应用建立的SEM-TIMS分析方法对已知同位素组成的单分散铀氧化物标准微粒进行了测量,测量结果与其标称值一致。     

~(236)U灵敏度的AMS测量

长寿命放射性核素236U的半衰期为2.34×107a,主要来源是235U的中子俘获反应。加速器质谱法是目前测量236U灵敏度最高的方法。本工作利用意大利那不勒斯第二大学同位素研究环境与文化遗产中心的AMS装置测量236U的灵敏度,并分析了加速器质谱测量236U时本底的可能来源。初步结果表明,236U的灵敏度(236U/238U原子比)为(2.29±0.13)×10-10。     

Anomalous Isotope Fractionation in Uranium Enrichment Process

A benchmark calculation for a deep penetration problem of 14 MeV neutrons through a 3m thick iron slab was carried out by using a vectorized continuous energy Monte Carlo code MVP with the JENDL-3 and ENDF/B-IV cross sections. Reference solutions for neutron spectra and averaged cross sections were obtained at various locations through the iron slab with good statistics owing to a high computation speed of the code. The accuracy of multigroup calculations with the JSSTDL/J3 library was investigated by comparison with the obtained reference solutions.

Both calculations with JENDL-3 and ENDF/B-IV showed a similar attenuation of total fluxes from thermal to 14 MeV through the slab, while differences of one order at the maximum were observed in the calculated fluxes in the resonance energy region. The multigroup calculations with the JSSTDL/J3 295- and 125-group libraries underestimate the streaming effect through the cross section minima above the well-known 24 keV window, which resulted in the underestimation of fluxes above this window by more than two decades at 3 m penetration compared with the continuous energy method. Taking into account the spatial dependence of averaged cross sections, the underestimation was reduced to about one decade. It was found, however, that an accurate prediction of streaming effect is fairly difficult by the multigroup method.     

Uranium Isotope Exchange between Uranium Hexafluoride and Uranium Pentafluoride

To aim at a better understanding of the uranium isotope exchange reaction between gaseous UF₆ and solid UF₅ experiments were done with natural UF₆ gas and solid UF₅ containing 3% ²³⁵U under different pressures of UF₆. The experimental results suggest a two-process reaction with an initial rapid increase of ²³⁵UF₆ in the gas phase followed by its slight and gradual increase. A rate equation based on a collision model is given for the two-process reaction which includes a primary exchange reaction on the solid surface and a secondary reaction participated by underlying UF₅ molecules. An analytical solution is provided for both of ²³⁵UF₆ concentration in the gas phase and ²³⁵UF₅ concentration on the solid surface, which is useful for determining the parameters characterizing the exchange reaction. A numerical analysis is also made to evaluate the influence of gas samplings. A remarkable agreement is found between the particle sizes of UF₅ estimated from the reaction parameter and from the direct observation with an electron microscope. The depletion of ²³⁵UF₅ concentration by the exchange reaction is very small when averaged over the whole solid UF₅, because the depletion is virtually limited to the solid surface due to the small reaction probability of underlying UF₅ molecules.     

Nd同位素溯源铀矿石浓缩物产地研究

铀矿石浓缩物(UOC)是核法证学中溯源研究的核材料之一,通过测量其中Nd同位素丰度比能提供地理指示信息。本文基于TRU树脂对铀和稀土元素的吸附特性,通过条件实验获得了最佳淋洗酸度和体积,分离稀土元素和大量铀基体;利用LN树脂分离镧系元素的特性,分离具有同量异位素干扰的Sm,使用TRU和LN树脂联用方式有效分离了Nd,建立了一种能分离UOC中U、Sm和Nd的方法。应用该方法对UOC样品进行多次测量,洗脱液中U含量低于5 ng/g,有效实现了UOC中大量U与微量Nd的分离。采用多接收电感耦合等离子质谱对Nd同位素进行测量,测量精度达0.002%,满足Nd同位素测量需求。该方法已用于实际UOC样品中的Nd分离,并进行了初步溯源研究。     

An Apparatus for Isotopic Ratio Measurement in UF5 Formed by Infrared Multiphoton Dissociation of UF6 in Molecular Beam

A Separation factor was measured in isotopically selective infrared multiphoton dissociation (IRMPD) of supercooled UF₆ in a supersonic expansion by multiphoton ionization (MPI) and time-of-flight mass spectrometry (TOFMS). A pulsed free-jet nozzle was used to generate a UF₆-molecular beam seeded in Ar (–10^?7 Torr in UF₆ partial pressure). Two-frequency ρ-H₂ Raman laser beams around 16μm were used for the dissociation of UF₆ under collisionless conditions in the molecular beam where the flow velocity for UF₆ is about 500m/s. The ²³⁵U/²³⁸U isotopic ratios in nascent UF₅ photoproducts were determined by selective MPI of UF₅ at 532 nm followed by TOFMS with a mass resolution as high as 1200. A separation factor of about 2 was observed under the experimental conditions chosen for the demonstration of this method.