14 MeV中子穿过组合材料引发238U绝对裂变率的测量

为检验计算方法与参数，用小型平板铀裂变室测量了铁球壳表面沿赤道方向不同位置14 MeV中子穿透实验模型后引起的。^238U绝对裂变反应率，并与MCNP/4A和ENDF/B-V库数据计算结果相比较。实验总误差为6．1％。     

快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。本工作在中子活化基础上提出用T(d,n)⁴He反应产生的快中子诱发裂变铀样品来测定铀同位素丰度的分析方法。该方法利用铀同位素丰度与特定裂变产物的平均产额比呈指数关系的特点,选择〖AKY-〗(⁹²Sr)/〖AKY-〗(¹⁰⁵Ru)、〖AKY-〗(⁹²Sr)/〖AKY-〗(¹³⁵I)与铀同位素丰度间的关系曲线作为工作曲线,并对模拟样品进行分析。结果表明,〖AKY-〗(⁹²Sr)/〖AKY-〗(¹⁰⁵Ru)、〖AKY-〗(⁹²Sr)/〖AKY-〗(¹³⁵I)计算结果与模拟样品的相对偏差分别为0.8%和1.8%,RSD优于3%,与γ能谱法的分析结果相符合。     

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%。     

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。     

Multi-parametric Measurement of Prompt Neutrons and Fission Fragments for 233U(n th,f)

The multiplicity and the energy of prompt neutrons from the fragments for ²³³U(n _th, f)were measured as functions of fragment mass and total kinetic energy. Average neutron energy against the fragment mass showed a nearly symmetric distribution about the half mass division with two valleys at 98 and 145 U. This shape formed a contrast with a saw-tooth distribution of the average neutron multiplicity. It indicates that the shell-effects, which are pronounced for the fragments having the proton number or neutron number close to the magic-number of 50 or 82, affected the neutron emission process. The slope of the neutron multiplicity with total kinetic energy depended on the fragment mass and showed the minimum at about 130 U. The obtained neutron data were applied to determine the total excitation energy of the system, and the resulting value in the typical asymmetric fission lied between 22 and 25 MeV. The excitation energy agreed with that determined by subtracting the total kinetic energy from the Q-value within 1MeV, thus satisfied the energy conservation. In the symmetric fission, where the mass yield was drastically suppressed, the total excitation energy is significantly large and reaches to about 40MeV: suggesting that fragment pairs are preferentially formed in a compact configuration at the scission point.     

加速器质谱法测量含铀微粒中铀同位素比的方法研究

准确测定含铀微粒同位素比在核保障中有重要的应用价值。本文采用将含铀微粒溶解并加入高纯Fe粉烘干的方法制样,采用中国原子能科学研究院的HI-13串列加速器质谱测量靶样中的同位素比。通过对CRM铀系列同位素标准样品的分析表明,该方法可测定高于10^-5的²³⁶U/²³⁸U同位素比;对于²³⁵U/²³⁸U同位素比在10^-4～10^-1范围内的含铀微粒,²³⁵U/²³⁸U同位素比相对扩展不确定度均小于10%。     

Statistical Estimation of Distributions of Physical Quantities in Nuclear Fission

Making use of a model based on the statistical theory, calculations were performed to obtain the mass yields, the most probable charges, the kinetic energies and the prompt neutron yields of fission fragments, and the mass yields of fission products from thermal-neutron-induced fission of ²³³U, ²³⁵U, ²³⁹Pu, ²⁴¹Pu and from the spontaneous fission of ²⁵²Cf. The calculations are further extended to fast-neutron-induced fission. The scission-point distance is treated as a parameter varying with the mass number of the heavy fragments of fission. This proved successful in approaching the calculated curves closer to the observed values.

It is possible to predict unknown physical quantities in nuclear fission with use made of the method developed in the present work.     

Theoretical Investigation on Shell and Deformation Effects on Most Probable Charge in Nuclear Fission

The most probable charge Z_P is obtained from Myers & Swiatecki mass formula by analytical method and the Z_P is expressed in terms of the part of simple liquid drop model, the shell and deformation effects. The deviation of the predicted Z_P- value from the Z_P ^UCD based on the unchanged charge distribution can be well interpreted by these terms. The shell effect shows the saw-tooth curve and the deformation effect shows a hump on the range of mass numbers where the shell function S(N, Z) exceeds the critical value Scrit introduced by Myers & Swiatecki. The predicted Z_p- values fairly well agree with the semi-empirical result obtained by Wahl and the fine structure is well reproduced. The magnitudes of the shell and deformation effects in the thermal neutron fission of ²³⁵U are about 0.7 and 0.25 charge units, respectively and both effects act so as to cancell each other.     

长寿命裂变产物核素核数据测量进展

文章对与高放废物深地层处置以及分离嬗变相关的半衰期大于10a、裂变产额高于0.01%的13种长寿命裂变产物核素的半衰期、裂变产额和热中子反应截面的测量研究、数据现状及其进展进行概要评述。就长寿命核素的分离纯化、原子数测定及放射性活度测量方法和技术进行了分析和论述。     

缓发中子法测量脉冲堆裂变总数

用长计数器测量已知裂变率的缓发中子计数率，然后用同一探测系统测量脉冲后的缓发中子计数率，探测系统的探测效率、缓发中子份额和缓发中子先驱核半衰期等参数均不变，从而建立起利用脉冲堆脉冲后的缓发中子衰减曲线确定脉冲裂变总数的方法。该方法的相对标准不确定度（不包含稳定功率运行裂变率的不确定度）为4%。测量的30次脉冲的裂变总数与其它方法得到的结果一致。     

Estimation of Neutron Yield from Individual Fragment in Medium-Excitation Fission

A method of calculation is described to estimate the average number of neutrons emitted per fragment in medium-excitation fission from published experimental data on neutron emission in thermal-neutron induced fission, average total kinetic energy as a function of fragment mass and mass yield in low- and medium-excitation fission reactions. Use is made of a relation of fragment excitation energy with internal excitation and deformation energies, and the difference in kinetic energy between the fission reactions at two-excitation energies. A tentative calculation is made for the fission of ²³⁸U induced by 12 MeV protons. The results are in good agreement with experimental data.

The method developed in the present work may make it possible to predict the average number of neutrons emitted from individual fragment in medium-excitation fission which has not yet been measured experimentally.     

固体径迹探测器测量重水零功率反应堆快裂变因子

介绍了利用固体径迹探测器测量快裂变因子的原理、方法和实验结果。并将实验结果与理论计算结果进行了比较,两者在误差范围内相符合。     

Measurement and Analysis of241 Am Fission Rate Ratio Relative to235 U Fission Rate in Thermal Neutron Systems Using Kyoto University Critical Assembly

Integral measurements of²⁴¹ Am fission rate ratio relative to²³⁵ U fission rate were performed at Kyoto University Critical Assembly. The fission rates were measured using the back-to-back type double fission chamber at five thermal cores with different H/²³⁵ U ratio so that the neutron spectra of the cores were systematically varied. The measured fission rate ratios, normalized to number of atoms, were 0. 0144 to 0. 0522, with a typical uncertainty of 2%. The measured data were compared with the calculated results using MVP based on JENDL-3.2, which gave the averaged calculated-to-experimental ratio (C/E) of 0.88. Obtained results of C/E using ²⁴¹Am fission cross sections from JENDL-3/2, ENDF/B-VI and JEF2.2 showed that the latter two gave larger C/E values than those by JENDL-3.2 by about 2% and 7 to 9%, respectively. It has been found that the large difference between JENDL-3.2 and JEF2.2 arises mainly from the significant cross section difference at the vicinity of resonance at 0.576 eV, whereas the difference of thermal cross sections, especially in the range of 0.01 eV to 0.2 eV also has significant contribution for well-thermalized cores.     

Neutron Emission,Mass Distribution and Energetics in 14.5 MeV-Neutron Induced Fission of Uranium-238

A study is performed on 14.5 MeV-neutron induced fission of ²³⁸U by means of three-parameter experiment in which the energies of both fragments and the time-of-flight of one fragment are measured. A mosaic-arrayed surface barrier detector of large sensitive area is used at the remote end of a flight tube. The pre- and post-neutron-emission fragment mass distributions are obtained, together with the average total kinetic energy of fragment as a function of its mass. The average number of neutrons emitted from an individual fragment and the average total number of emitted neutrons are also derived as a function of fragment mass. The results agree well with those calculated by the method developed in our laboratory for medium-excitation fission. The average number of emitted neutrons and the mass distribution of fission fragment are derived for the respective reactions of first-, second- and third-chance fission.     

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.