The kinetics and mechanism of the uranium-water vapour reaction — an evaluation of some published work

The published results of Grimes and Morris on the rate of the uranium-water vapour reaction which were obtained using interferometry have been recalculated using the best values derived from the literature for the complex refractive indices of uranium and uranium dioxide (3.1–3.91 for uranium and 2.2-0.51 for uranium dioxide). The kinetics have been described by Haycock's model and the linear rate constant is given by K₁ = 1.3 × 10⁴P^1/2_H2O exp( − 9.0 kcal/RT )mg U/cm² h, where P_H2O is the water vapour pressure in torr or K₁ = 3.48 × 10⁸r^1/2 exp( −14.1 kcal/RT)mg U/cm² h, where r is the fractional relative humidity, R is the gas constant and T is the absolute temperature.

A mechanism is described which accounts for the observed dependence of the rate of uranium-water vapour reaction on the square root of the water vapour pressure.     

Measurement of neutron spectra in uranium-water and uranium-monoisoprolyldiphenyl lattices

The neutron spectra in a cell of a heterogeneous subcritical system are investigated in detail. The neutron spectra were measured by means of a mechanical velocity selector and a time analyzer. A water-cooled water-moderated reactor (WWR) was used as the neutron source for the subcritical system. The results indicate that the neutron spectrum differs from the Maxwell equilibrium distribution both in the uranium and in the moderator. The neutron spectrum changes sharply at the boundary between the uranium block and the moderator. The spectrum varies only slightly within the block and within the moderator. A simple semiempirical functional relationship is found to exist between the thermal neutron spectra in the moderator and in the block.Translated from Atomnaya Énergiya, Vol. 13, No. 6, pp. 547–555, December, 1962     

Simulation of control and temperature variation of water density in intermediate-neutron uranium-water reactors

This article deals with the results of experiments on the physical simulation of control and of the temperature effect due to changes in water density in intermediate-neutron uranium-water reactors. The values of control rods of different types were determined, and the dependence of the critical size of uranium-water reactors on the density of the water filling the reactor core was found.In conclusion, the authors would like to take this opportunity to express their heartfelt gratitude to S. M. Feinberg who was responsible for the general supervision of the work, to V. A. Tsykanov for his participation in discussion of the work, and to A. D. Merkulov, N. N. Kolpakov, B. A. Pryadekhin, and other workers who took; part in the experimental work.     

Modeling of critical experiments on the ZR-6 assembly with VVÉR-type uranium-water lattices

Conclusion An electronic atlas for describing the 2D and 3D mathematical models of critical experiments carried out on the ZR-6 assembly has been developed. A program-generator of the initial data for modeling assemblies using the MCU-RFFI/A program has been produced, by means of which mathematical modeling of 154 configurations can be carried out. The results have been compared with experimental data. For the two-dimensional model the average value of k_eff over all the configurations amounts to 0.9989±0.0034 (for the KENO-Va program it is 0.9939±0.0048). For the three-dimensional model of 13 configurations an average value of k_eff of 0.9994±0.0023 has been obtained (for the same assembly in the two-dimensional model the value obtained is 0.99871±0.0030). This enables us to recommend the two-dimensional models investigated for verifying programs of neutron-physical calculations on VVéR reactors. “Kurchatov Institute” Russian Scientific Center. Translated from Atomnaya énergiya, Vol. 84, No. 6, pp. 501–507, June, 1998.     

The ratio of the average fission cross sections of Pu239 and U235 in uranium-water lattices

The ratio of the average fission cross sections of Pu²³⁹ and U²³⁵ is determined for lattices consisting of natural uranium and ordinary water. For comparison the same ratio was measured for a uranium-graphite reactor. It was found that the ratio _Pu ^–f / _U ^–f for uranium-water lattices with spacings of 45, 50, 55 and 60 mm and for a uranium-graphite reactor with 200 mm lattice spacing are 2.24, 1.99, 1.88 and 1.79, respectively.In conclusion the authors wish to thank E. S. Antsiferov for much valuable assistance.     

Measurement of niobium reaction rate formaterial surveillance tests in fast reactors

A highly accurate and precise technique for measurement of the ⁹³Nb(n,n’)^93mNb reaction rate was established for the material surveillance tests,etc.in fast reactors.The self-absorption effect on the measurement of the characteristic X-rays emitted by^93mNb was decreased by the dissolution and evaporation to dryness of niobium dosimeter.A highly precise count of the number of ⁹³Nb atoms was obtained by measuring the niobium solution concentration using ...     

Comparison of measured and calculated reaction rate distributions in an scwr-like test lattice

High resolution gamma-ray spectroscopy measurements were performed on 61 rods of an SCWR-like fuel lattice, after irradiation in the central test zone of the PROTEUS zero-power research reactor at the Paul Scherrer Institute in Switzerland. The derived reaction rates are the capture rate in ²³⁸U (C₈) and the total fission rate (F_tot), and also the reaction rate ratio C₈/F_tot. Each of these has been mapped rod-wise on the lattice and compared to calculated results from whole-reactor Monte Carlo simulations with MCNPX. Ratios of calculated to experimental values (C/E’s) have been assessed for the C₈, F_tot and C₈/F_tot distributions across the lattice. These C/E’s show excellent agreement between the calculations and the measurements. For the ²³⁸U capture rate distribution, the 1σ level in the comparisons corresponds to an uncertainty of ±0.8%, while for the total fission rate the corresponding value is ±0.4%. The uncertainty for C₈/F_tot, assessed as a reaction rate ratio characterizing each individual rod position in the test lattice, is significantly higher at ±2.2%. To determine the reproducibility of these results, the measurements were performed twice, once in 2006 and again in 2009. The agreement between these two measurement sets is within the respective statistical uncertainties.     

Characterisation of radial reaction rate distributions across the 92-pin section of a SVEA-96 Optima2 assembly

Characterisation of the SVEA-96 Optima2 boiling water reactor assembly, in terms of the radial distributions of normalised total fission and ²³⁸U capture rates, is reported at its central elevation, i.e. at the 92-pin section, where the one-third part-length pins are replaced by water. Measurements performed in the PROTEUS facility are compared with MCNPX predictions. The calculation model included the measured locations of the SVEA-96 Optima2 assemblies and sub-assemblies, within the PROTEUS test zone. Predicted and experimental fission and ²³⁸U capture rates are found to agree, respectively, within 3.5% and 4% for all pins. Fission rates in the burnable-absorber UO₂–Gd₂O₃ fuel pins have been predicted without bias using the ENDF/B-VI data library but show an average 1.4% under-prediction with the JEFF-3.1 data library. A slight overestimation of the total fission rate in the pins located at the periphery of the assemblies was observed and has been attributed to an inaccurate modelling of the pin positions. However, there was no systematic bias observed due to the absence of the one-third pins at the corners of the assembly.     

Experimental analysis for neutron multiplication by using reaction rate distribution in accelerator-driven system

Basic experiments are carried out in the Kyoto University Critical Assembly (KUCA) to study the nuclear characteristics of the accelerator-driven systems (ADS) and to establish a new neutron source for research. A method for measuring the neutron multiplication in the subcritical system is proposed by utilizing the reaction rate distribution obtained by the optical fiber detector. A comparison between the measured and calculated neutron multiplication shows agreement within a relative difference of 5% in most cases, through introducing certain correction factors.     

On the velocity profile of vapour bubbles at critical heat flux

A theoretical model is put forward to describe the flow velocity of the vapour phase near a heated wall when departure from nucleate boiling (DNB) is approached. The model refers to experimental observations which have been put into correlations and presented in an earlier article. It consists in a possible extrapolation of these previously proposed DNB-correlations towards the determination of the maximum critical heat flux in cooling water under forced convection. Comparison is made with the well-known Kutateladze/Zuber correlation, which usually applies to CHF in saturated pool boiling. It is suggested that the present hydrodynamic model could generate analogous results by using a different approach. Moreover, it is shown that the model could equally be applied to determine the DNB limit in both free and forced convection, constituting a link between them.     

The plutonium/hydrogen reaction: The pressure dependence of reaction initiation time and nucleation rate controlled by a plutonium dioxide over-layer

As part of an ongoing programme to quantify those parameters which influence the early stages of the plutonium hydriding reaction, the hydrogen pressure dependence of both plutonium hydriding initiation time (I_t) and hydriding nucleation rate (N_r) have been determined for plutonium covered in a reproducible dioxide over-layer. The data show that initiation time is inversely proportional to hydrogen pressure, while nucleation rate is proportional to hydrogen pressure. Both observations are consistent with a model of hydriding attack in which the dioxide over-layer acts as a diffusion barrier, controlling the flow of hydrogen from the gas phase to the oxide/metal interface. The low scatter and reproducibility of the experimental data set demonstrate the importance of synthesising well controlled and characterised oxide layers prior to determining these experimental parameters.     

锰结核中放射性含量的测量

锰结核是海洋底部的含多种金属的结核体,主要由锰和铁的氧化物、氢氧化物层所组成。最近我们用反符合屏蔽低本底γ谱仪测量了太平洋底锰结核样品的放射性。样品平均直径约4cm,重量75.2g。对测得的γ能谱进行分析,发现其中含有铀、钍系放射性。 测量装置和安排如图1所示。样品贴近主