Calculational Methods of Control Rod Worth for High Conversion Light Water Reactor

By making use of the isotope approximation; neglecting squares of relative mass differences among the isotopes, the authors derived analytically approximations to ordinary diffusion coefficients in a 3- and 4- component isotope mixture. Moreover, approximations to multi-component diffusion coefficients were given on the analogy of those to the 3- and 4-component coefficients, and these approximations were verified to satisfy constrains on the exact ordinary diffusion coefficients.

For 4-component mixture of uranium hexafluoride isotopes, ²³⁴UF_6- ²³⁵UF_6- ²³⁶UF_6- ²³⁸UF₆, composition dependences of the approximation were equal to those of the exact diffusion coefficients. In addition, relative errors between the exact and the approximations were less than 0.2% for 5-component mixture of krypton 80–82-83-84-86 isotopes.     

An Effective Method for Estimating Multiple Control Rod Worths in Large Fast Reactors

A method for estimating the multiple control rod worth in large fast reactors has been developed using the higher order perturbation technique. With this method, physical significance of the interaction effect between multiple control rods can be understood by decomposing it into independent components with different physical processes. Furthermore, a new technique is proposed, whereby the interaction effect between multiple control rods can be estimated from a combination of the interaction effects between two or three control rods depending on the strength of the effect. The validity of the method is examined with the experimental data of PHENIX and numerically for MONJU. The results show that the interaction effect between multiple control rods in prototype fast reactors can be estimated from a combination of the effects between 2 control rods with a good accuracy. The numerical examinations for a 1,000 MWe fast reactor showed that the interaction effect is too large to apply the present method for large fast reactors. To overcome this difficulty, a new concept of “quasi-control-rod” is introduced in the method. With this improved method, the worths of 19 and 18 control rods in the 1,000 MWe fast reactor are estimated with accuracies of ±2%.     

低价值控制棒中子吸收体材料燃耗相关数据的制作及验证研究

本文从燃耗方程出发给出了燃耗计算相关数据的内容,提出了使用蒙卡燃耗计算程序作为基准程序进行燃耗计算相关数据制作和验证的方法。应用该方法制作了低价值控制棒中子吸收体材料铽(Tb)和镝(Dy)同位素燃耗计算相关数据。数值计算结果表明,新制作的燃耗计算相关数据具有很高的计算精度。最终给出了满足低价值控制棒中子吸收价值要求的铽镝合金设计方案的计算结果。     

改进的源倍增方法测量控制棒价值

该文给出了改进的源倍增方法测量控制棒价值的原理,在高富集度235 U燃料元件转换为低富集度235 U的微型中子源零功率反应堆上进行研究,实验测量微型中子源零功率反应堆中心控制棒的价值,与周期方法相比在2%内符合,但减少了测量时间。该方法为今后加速器驱动次临界系统ADS的次临界在线监督提供一种可能的方法。     

Evaluation of Gap Heat Transfer between Boron Carbide Pellet and Cladding in Control Rod of FBR

Heat transfer across a gas gap between a boron carbide pellet and a cladding in FBR control rod has been experimentally investigated. The main purpose of this investigation is to present a calculational method for the gap heat transfer in order to improve accuracy of thermal design for the control rod with 0.5 mm gap width at a beginning of reactor operation.

Two types of tests have been carried out using simulated control rods. One is low temperature tests below 200°C. The test results indicated that the convective heat transfer has a negligible effect on the gap conductance when the Rayleigh number using the gap width as the characteristic length is below 0.1. The other is high temperature tests up to 600°C.

The results showed +10 to — 5% variations in the gap conductance data due to eccentricity between the pellet and the cladding. The prediction based on conduction and radiation heat transfers considering a thermal expansion and an eccentricity gave better results of gap conductance having a maximum difference of only 17% from the measured ones. Calculation in the radiation heat transfer used thermal emissivities. 0.85 for the boron carbide and 0.15 for the cladding, measured by infrared thermography.     

Effective Spatial Homogenization with Neutron Leakage Effect for FBR Control Rods

An effective homogenization of control rods is studied by considering neutron leakage as a fixed boundary source. Good numerical convergence is seen in the iterative calculation of effective homogenized cross sections that preserve integrated reaction rates. Usefulness of the present method is demonstrated by treating off-center control rods in a one-dimensional slab model of an FBR.     

Experimental Study on Reactivity Worth for Absorber Material in High Conversion Light Water Reactor Using FCA-HCLWR Core Fueled with Enriched Uranium

Experimental study on reactivity worth for absorber material in HCLWR core has been carried out in a series of experiments using the Fast Critical Assembly (FCA) in Japan Atomic Energy Research Institute (JAERI). The central reactivity worth as well as the simulated control rod worth of B₄C with different ¹⁰B content and of Hf was measured in FCA-HCLWR core fueled with enriched uranium. Both reactivity worths of B₄C increase with ¹⁰B content. These increasing trends do not saturate to 90% enriched B₄C. The Hf has the smaller reactivity worth than the 20% B₄C. The experimental values are compared with the calculated ones which obtained from JENDL-2 data and the SRAC system. The calculation predicts well the dependence of reactivity worth on ¹⁰B content and underestimates the reactivity worth ratios of the Hf to the 20% B₄C.     

Simple Method for Evaluating Mesh Size Effects in Control Rod Worth Analysis

It has been pointed out that the reactor coolant system piping could fail prior to the meltthrough of the reactor pressure vessel in a high pressure sequence of pressurized water reactor severe accidents. In order to apply to the evaluation of the piping failure which influences the subsequent accident progression, models for the strength of piping materials at high temperatures were examined. It was found that 0.2% proof stress and ultimate tensile strength above 1,073 K obtained from tensile tests was reproduced by a quadratic equation of the reciprocal absolute temperature. Short-term creep rupture time and minimum creep rate at high temperatures were well correlated by the modified Norton's Law as a function of stress and temperature, which implicitly expressed the effect of the precipitation and the resolution of precipitates on the creep strength. The modified Norton's Law gave better results than the conventional Larson-Miller method. Relating applied stress vs. minimum creep rate and tensile properties vs. applied strain rate obtained from the creep and tensile tests, a temperature range where the dynamic recrystallization significantly occurred was evaluated.     

Effect of Fast Neutron Irradiation on the Properties of Boron Carbide Pellet

Boron carbide pellets were irradiated in the experimental fast reactor “JOYO” to ¹⁰B burnup of up to 170x10²⁶cap/m³, fluences of 2x10²⁶/m²(E>0.1MeV), and maximum temperatures of about 1,200°C. Post irradiation examinations were made of microstructural changes, helium release, swelling, and thermal conductivity.

Boron carbide pellets irradiated to high burnups developed extensive cracking. Helium release from the pellets was initially low, but enhanced helium release was observed at high burnups and high temperatures. The swelling linearly increased with burnup, and when boron carbide was irradiated at high temperatures, the swelling rate began to decrease corresponding to the beginning of enhanced helium release. The correlation between swelling and the helium release was studied and the swelling was interpreted in terms of accumulation of helium in the boron carbide pellet. The thermal conductivity of the boron carbide pellets decreased rapidly by neutron irradiation accompanied with loss of temperature dependence.     

Investigation on Space Dependence of C/E Values for Control Rod Worths and Reaction Rate Distributions in ZPPR-10A and -10D Assemblies

In order to investigate the redox equilibrium of uranium ions in molten NaCL-2CsCL, UV-Vis absorption spectrophotometry measurements were performed for U⁴⁺ and U³⁺ in molten NaCL-2CsCL at 923 K under simultaneous electrolytic control of their ratio. Prominent absorption bands at 480 and 570 nm were assigned to U³⁺, and their molar absorptivities were determined to be 1,260±42 and 963±32 mol^?1.l.cm^?1 respectively. From the dependence of the rest potential of the melt on the spectrophotometrically determined ratio of [U⁴⁺]/[U³⁺], the standard redox potential of the couple U⁴⁺/U³⁺ at 923K was determined to be ?1.481±0.004 V vs. Cl₂/Cl^?. Cyclic voltammetry measurements were carried out for the couple U⁴⁺/U³⁺, and the results agreed well with this standard redox potential value. By the results of cyclic voltammetry, a temperature dependence of the standard redox potential was found to be ?2.094+6.639×10^?4 T (T=823-923 K).