3 MeV中子诱发裂变测定铀同位素丰度

铀同位素丰度分析是核燃料循环中重要的分析项目。本工作研究了以D(d,n)³He反应产生的能量约为3 MeV的中子为源诱发裂变测定铀同位素丰度,并与以T(d,n)⁴He反应产生的14 MeV快中子为源的方法进行了比较,结果表明两种方法分析结果基本一致。     

利用光致裂变中子响应曲线确定铀材料丰度

用无损分析方法确定铀材料同位素丰度是深度核裁军核查技术中的一个具有挑战性的问题。本文通过分析和数值模拟研究了利用高能光子确定铀材料丰度的可能性。通过建立简化模型研究了反演铀材料丰度问题的方法、物理基础和测量条件。通过利用Monte-Carlo粒子输运程序对光子和中子的输运过程进行模拟,得到不同测量条件下的中子强度与铀材料丰度的关系。中子强度和铀材料丰度之间的关系曲线表明:对于已知结构的铀部件,铀材料丰度可以通过对比中子注量率和已知的光子中子关系曲线确定;对于未知结构的铀部件,该部件中铀材料丰度可以通过测量中子对不同能量光子响应曲线的形状参数得到。     

生产堆元件铀同位素丰度比和含量的测定

文章用同位素质谱法测定了辐照堆元件铀的同位素丰度比和含量。讨论了测定技术、方法及误差来源和误差传递等有关问题。上述各量与切片的分布相符,与其他方法的有关测量结果相一致。     

非平衡态铀样品同位素丰度的测定

平衡态铀样品是指样品中238U与其子体核素234Pam达到了放射性平衡的样品。在使用 g 能谱法测定铀样品同位素丰度的工作中,由于238U的特征 g 射线能量很低且分支比很小,需通过测定其子体核素234Pam的特征 g 射线来确定样品中238U的含量。可采用MGA-U软件分析低能区(小于300 keV)     

裂变产额差异法测量~(235)U丰度

利用裂变产额差异法,从裂变产额质量分布曲线,选取位于双驼峰曲线两翼的裂变产物的产额随可裂变核素的不同有很大变化的核素88Rb、104Tc和92Sr作为测量对象,选择平均裂变产额比88Rb Y/104Tc Y、92Sr Y/104Tc Y为监测对象,研究了平均产额比与铀丰度之间的关系曲线,获得了平均产额比/i j Y Y随丰度H0变化的关系式Y1/Y2-f(H0)。利用两条关系曲线分别对72.2%的模拟样品测量5次,RSD均优于2%。将样品分析结果与无源γ射线法的结果相比较,两种方法在规定的误差范围内结果是一致的。     

γ能谱法测定铀、钚同位素丰度

简单介绍了特殊核材料铀、钚同位素丰度测定在核材料管理和衡算中的作用;着重描述了γ能谱法测定铀、钚同位素丰度的原理和实验方法.实验结果表明,使用高分辨率的高纯锗探测器,γ能谱法测定铀、钚同位素丰度的不确定度在2%以内.     

容器内铀同位素丰度的非破坏探测技术研究

研制了三维样品旋转装置,用PC/FRAM软件分析了通过同轴HPGe探测器采集的铀总量在2—600g、能区在120—1001keV的铀同位素γ射线。建立了高放废物容器内非均匀分布的铀同位素丰度的γ能谱测量方法。对同位素丰度在20%—93%的235U,测量不确定度<0.6%,对238U的测量不确定度<1.3%。     

γ能谱法测定铀、钚同位素丰度

简单介绍了特殊核材料铀、钚同位素丰度测定在核材料管理和衡算中的作用;着重描述了γ能谱法测定铀、钚同位素丰度的原理和实验方法。实验结果表明,使用高分辨率的高纯锗探测器,γ能谱法测定轴、钚同位素丰度的不确定度在2%以内。     

ISOCS在铀同位素丰度比测量中的应用

本文介绍了应用就地计数系统(ISOCS)测量235U和238U丰度比的方法。该方法使用基于蒙特卡罗原理的无源效率刻度软件对235U和238U子体234mPa进行效率刻度,采用平衡铀测定方法计算出235U和238U的原子比。使用该方法对某一铀样品进行了分析,其测量结果与质谱法测量结果的相对偏差为2.2%。     

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。     

Role of Effective Distance in the Fission Mechanism Study by the Double-energy Measurement for Uranium Isotopes

Fission product kinetic energies were measured by the double-energy method for thermal-neutron fission of ^235,233U and proton-induced fission of ²³⁸U at the 15.8-MeV excitation. From the obtained energy-mass correlation data, the kinetic-energy distribution was constructed from each mass bin to evaluate the first moment of the kinetic energy for a given fragment mass. The resulting kinetic energy was then converted to the effective distance between the charge centers at the moment of scission. The effective distances deduced for the proton-induced fission was concluded to be classified into two constant values, one for asymmetric and the other for symmetric mode, irrespective of the mass though an additional component was further extracted in the asymmetric mass region. This indicates that the fission takes place via two well-defined saddles, followed by the random neck rupture. On the contrary, the effective distances obtained for thermal-neutron induced fission turned out to lie along the contour line at the same level as the equilibrium deformation in the two-dimensional potential map. This strongly suggests that it is essentially a barrier-penetrating type of fission rather than the over-barrier fission.     

干馏法从铀的裂变产物中分离131I

¹³¹I是一种重要的医用放射性同位素,但因湿法分离技术上的缺陷,使得从铀裂变产物中获取¹³¹I的工艺具有环境污染严重、提取效率低的缺点。因铀裂变产物中¹³¹I的产额较高,为拓展¹³¹I的获取途径,提高铀裂变产物的利用效率,开展铀裂变产物中¹³¹I分离的新工艺研究十分必要。与传统湿法分离工艺不同,本工作采用了干馏法进行铀裂变产物中¹³¹I的分离。为了得到高的¹³¹I分离效率,将分离过程分为低温粉化、高温干馏和中低温保温三个阶段,并研究高温干馏阶段温度对131I分离效率的影响。实验发现：当干馏温度高于950 ℃时,¹³¹I的分离效率≥98%。此外,研究结果还表明,在该干馏温度下,碘和¹⁰³Ru 均可挥发出铀靶片,但产物收集液中却仅含有碘。为了解释这一现象,对碘的分离过程进行分析,结合实验结果和理论计算,推测挥发物中碘和¹⁰³Ru分离的原因为：¹⁰³Ru与氧反应生成挥发性RuO₄,从铀的裂变产物挥发出;因加热管内温度较高,RuO₄在迁移过程中发生了分解,生成RuO₂沉积在加热管内部。因此,利用干馏法从铀的裂变产物中分离¹³¹I时,为了得到放化纯度高的碘产品,不仅要合理规划分离过程,还需科学设计加热管的长度。     

Separation of Uranium Isotope by Plasma Centrifuge

A method of centrifugal separation of isotopes by electromagnetic means is presented. The principle proposed utilizes electromagnetic acceleration by the interaction between an electric current in a slightly ionized gas and an external magnetic field. The analysis shows that an azimuthal flow of 2.6 km/sec can be realized with a magnetic field of 200 gauss and an electric current of 1.5kA. The resulting centrifugal force is large enough to permit realization of a more compact concentration cascade than the conventional mechanical centrifuge.     

氘/氚化铀热解吸的动力学同位素效应

在高真空金属系统上测定了氘/氚化铀在恒容体系和225～400℃范围内热解吸的压力-时间等温线,应用反应速率分析方法计算了各自在不同温度下的速率常数。随着温度的升高,热解吸反应的速率增大,反应的平衡压随之增高。由速率常数计算得到氘/氚化铀热解吸的表观活化能分别为(26.3±0.4)kJ/mol和(27.7±0.6)kJ/mol。活化能数据显示,氘/氚化铀热解吸反应的动力学同位素效应不明显。     

不同235U快裂变缓发中子群参数比较和验证

分析了ENDF/B7、JEFF3.1、JENDL3.3、CENDL2.2及Keepin数据中的²³⁵U快裂变缓发中子群参数的差异,通过CFBR-Ⅱ堆超瞬发临界实验检验了这几套缓发中子数据的准确性。检验结果表明,用于计算反应堆反应性,Keepin缓发中子群参数仍比数据库数据准确;数据库数据中,JEFF3.1的8群结构数据好于其他数据库6群结构数据。     

Preliminary Study on Production of Xenon-133 from Neutron-Irradiated Uranium Metal and Oxides by Oxidation

A method was developed for recovering the fission product ¹³³Xe from several kinds of reactor-irradiated U targets, including Al-U alloy, metallic U, and uranium oxides.

In order to observe the release of ¹³³Xe from U compounds at high temperatures, irradiated targets were heated at 500°~1,000°C in vacuum or under gas flow. The liberated ¹³³Xe was trapped on charcoal beds, and the release rate of ¹³³Xe from the compounds was determined by measuring the activity accumulating in the traps.

More than 90% of the ¹³³Xe was liberated from the alloy upon melting and from metal and uranium oxide upon oxidation.

The isolated Xe was purified by a system embodying cold traps and cryogenic distillation.

The final products were sealed in ampoules. They proved to possess radiochemical purity exceeding 99.9%, and less than 1 μ/ampoule (1 ml) of non-radioactive gaseous contaminants.     

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.     

球形浓缩铀装置的中子价值和裂变率分布测量

为得到Rossi-α测量临界装置的瞬发中子衰减常数的空间修正因子，利用²⁵²Cf中子源测量带贫化铀反射层的球形浓缩铀临界装置(CFBR-Ⅱ)的中子价值空间分布，同时用浓缩铀裂变电离室测量该装置的裂变率空间分布，得到该装置的空间修正因子为1.096。