反应堆快中子实验装置辐射场参数测量

利用多箔活化法测量了设计的反应堆快中子实验装置的中子能谱及中子注量,并采用Monte Carlo方法分析了能谱的不确定度.用热释光剂量片法测量了装置的γ剂量.装置各参数测量结果均达到了预期的设计指标.     

Auto-correlation and variance-to-mean measurements in a subcritical core obeying multiple alpha-modes

Neutron noise measurements based on the Rossi-α and Feynman-α methodologies have been performed in a heterogeneous subcritical system. It is shown that the traditional single alpha-mode formulations of the Rossi-α and Feynman-α methods are not applicable due to the presence of higher alpha-modes. Formalisms taking into account multiple alpha-modes are applied resulting in satisfactory results. Three alpha-modes could be identified using the Rossi-α method, whereas only two could be obtained using the Feynman-α method. In the Feynman-α case, the possibility to obtain the fastest decaying alpha-mode was diminished due to detector dead time effects. It was found that the slowest decaying alpha-mode does not exactly correspond to the prompt decay found in pulsed neutron source measurements, which confirms the results of previous studies. Strengths and weaknesses of the multiple alpha-mode Rossi-α and Feynman-α methods observed in this study are pointed out.     

Source reactivity as an extra kinetic characteristic of coupled-source subcritical systems

The objective of the present work is to study the kinetic (i.e., in absence of in-core feedbacks) response of the subcritical system to variation of the efficiency of the external neutron source. The particular class of the subcritical system with the intensity of the external source being intrinsically dependent on neutron production in the core (so-called coupled hybrid systems) is considered. The accelerator coupled system is taken as example, although this analysis may be expanded to other types of the coupled subcritical systems. Within the framework of a simple mathematical model of coupled system, an interpretation of the external coupled source as supplementary group of delayed neutrons is given. An auxiliary quantity – ‘source reactivity’ is introduced for convenience and a modified inhour equation for coupled systems is deduced. Analytical solution of the modified inhour equation is obtained in approximation of one group of delayed neutrons. The principal conclusion resulting from this analysis is as follows: the response of the coupled system to ‘source reactivity’ variation is intrinsically different from the response to core reactivity variation. Namely, there is no equivalent of prompt criticality (accompanied by drastic decrease of the reactor period) in the case of ‘source reactivity’ variation.     

Preliminary measurements of the prompt neutron decay constant in MASURCA

Pulse counting techniques have been used to measure the prompt decay constant = (β - ) / Λ in the MASURCA reactor of CEA at critical state. The data has been analyzed in time domain using Rossi- and Feynman- techniques, and in frequency domain using the cross power spectral density.

The Rossi- technique has been studied using one and two detectors. Due to the strong inherent spontaneous fission source, the one-detector variant gives a very strong white-noise signal, which is absent in the two-detector method. Because each neutron detected recorded not only a pulse, but also an echo after 120 ns, corrections had to be made to the theory applied.

The Feynman- technique is even more sensitive to the echo in the signals, and quite large corrections had to be made. Nevertheless the results obtained are in reasonable agreement with those of the correlation methods. For both measurement techniques, experiments of long duration are needed to get accurate results. The results obtained agree within 10% with calculations.

The prompt decay constant has also been measured with a continuous current technique. From the cross power spectral density thus obtained, the -value is in agreement with that of the pulse counting techniques.     

Polarization Bremsstrahlung from fast ions with the core in polycrystal

This paper is devoted to the theoretical investigation of polarization Bremsstrahlung (PB) from fast ions with electronic core scattering in a polycrystalline medium. There are two channels of radiation in this case, namely, projectile PB and target PB. Projectile PB is a new radiation mechanism due to the scattering of the own target electric field into a real photon on a bound electrons of the projectile. The main features of projectile PB from hydrogen-like multi-charged incident ions are studied, including spectral-angular distribution of PB photons and its dependence upon the projectile kinetic energy.     

Commercialized fast reactor cycle systems and reactor core performance of the promising fast reactors

The Feasibility Study on Commercialized Fast Reactor (FR) Cycle Systems is under progress in order to propose prominent FR cycle systems that will respond to the diverse needs of society in the future. The design studies on various FR system concepts have been achieved and then the evaluations of potential to achieve the development targets have been also carried out. Crucial development issues have been found out for each FR system concept and their development plans for the key technologies are summarized as the roadmap. As a result, it has been confirmed that the sodium-cooled FR concept is highly suited to the development targets and R&D issues are related enhancing the economy with certain perspectives for realization. A flexible and robust development program for the FR cycle system will be proposed taking account of the characteristics for each FR concept until the end of the Phase II study.     

Study on kinetics of fast subcritical assembly—Contribution of delayed neutrons to the transition after a reactivity insertion in a subcritical system—

This study is an investigation of the effect of the delay neutron on the kinetics in the subcritical system. And, it proposes a method necessary for the kinetics code development that uses the Monte Carlo (MC) computation.

It is generally difficult to analyze three dimensional space and time dependent kinetics by using a MC method. It is because the sampling of the neutron in a region becomes difficult when conditions of the region changes with time. In this study, we consider about the effect of delayed neutron in the kinetics of ADS. The behavior of neutrons is considered spontaneous in this system. It means a neutron is absorbed or leaks in a short period, while the conditions of region do not change. Therefore they are treated by steady state calculation. On the other hand the densities of delayed neutron precursors changes slowly, and the conditions of region change. In the concept of developed MC method, the neutrons are calculated by using steady state equation at each time point, and the delayed neutron precursors are calculated by using time dependent equation. We tried to inspect the accuracy of this method by using a point equation. We obtained strict solution Φ* as a reference solution, Φ1 as a solution by the present method, and Φ2 as the solution where both neutrons and delayed neutron precursors are treated by using static equations. The obtained results show a good agreement between Φ1 and Φ*, though the Φ2 agrees with Φ* poorly in all cases. Especially, we showed that this technique was effective from the reactivity change by ADS, and the relation of a delayed neutron. Finally, the effect of the delay neutron on the beam trip in the neutron source for the drive was examined by using the technique of Φ2.     

Methods for the seismic verification of a fast reactor core

This paper presents the main features of the methods applied by the Fast Reactor Department of the Italian Energy Authority (ENEA) to the seismic verification of the core of the Italian PEC fast reactor test facility, which is in advanced construction at the Brasimone site. It also points out the problems which in general remain open in the fast reactor core seismic analysis. The aim is to contribute towards a generally acceptable philosophy on core seismic verification techniques and suggest items of possible co-operation for future developments.     

Gas-cooled fast breeder core element fabrication technology

The status of gas-cooled fast breeder reactor (GCFR) core element fabrication technology and testing is described. Special GCFR requirements arise from use of a high-pressure helium coolant. Ribbing of the cladding is employed to improve heat transfer, and venting and pressure equalization are utilized to avoid creep collapse of the cladding. Fabrication development, including fabrication of a full-scale core element model, has revealed no difficult fabrication or inspection problems, and testing of components has not indicated any feasibility problems.     

Fuel modelling in fast reactor whole core accident calculations

Transient fuel pin behaviour and fuel motion models are of major importance for the analysis of the initiation phase of unprotected whole-core LOF and TOP accidents in LMFBRs. The role played by these models is highlighted by discussing LOF and TOP accident sequences. This is followed by an overview of present whole-core fuel pin behaviour and fuel motion models and also a critical evaluation of these models.     

Measurement of k_(eff) by delayed neutron multiplication in subcritical systems

In this paper, we build on the concept of equivalent fundamental-mode source to propose using delayed neutrons as a neutron source in multiplication experiments to acquire the effective multiplication factor keffof subcritical systems, which is difficult to acquire directly from conventional neutron source multiplication method. We analyzed the difference between a fundamental-mode fission source and delayed neutron source,then adopted a factor to convert delayed neutron distribution to an equivalent fundamental-mode source distribution, and employed Monte Carlo code to acquire this factor.The delayed neutron multiplication measurement method was established for the first time, and corresponding experiments were conducted in subcritical systems. The multiplication of delayed neutrons was measured based on Chinese Fast Burst Reactor-Ⅱ(CFBR-Ⅱ) at subcritical states, and keffwas acquired from delayed neutron multiplication successfully(0.9921 and 0.9969, respectively).The relative difference between k_(eff)obtained by the new method and previous values acquired by the positive period method is less than 1% for these two studied cases.