Axially Heterogeneous FBR Core Concept and Its Nuclear Characteristics

This work consists of conceptual design and experimental studies on an axially heterogeneous core (AHC) for large LMFBRs. Trade-off studies were performed to optimize the AHC, from which it was found out that the AHC has the following advantages compared to the conventional homogeneous core (HOC); lower peak linear power, lower peak fast fluence in fuel cladding materials, lower burnup reactivity, lower potential of energetic consequences in hypothetical core disruptive accidents and so on. Aiming at further improvement of the core performance, the AHC core configuration was studied taking control rod operating into account.

Critical experiments of the AHC were performed using the FCA facility at JAERI. The experiments demonstrated satisfactory fundamental nuclear characteristics, such as a flat power distribution. The calculational accuracies for the AHC were found to be nearly the same as those for the HOC.

Through our studies, the AHC with improved safety and economic characteristics was proposed prior to designs in other countries, and the characteristics were demonstrated by experiments. These have provided a technical basis for development of future large LMFBR cores.     

A Proposal of Benchmark Calculation on Reactor Physics for Metallic Fueled and MOX Fueled LMFBR Based upon Mock-up Experiment at FCA

In order to assess the validity of the cross section library for fast reactor physics, a set of benchmark calculation is proposed. The benchmark calculation is based upon mock-up experiments at three FCA cores with various compositions of central test regions, two of which were mock-ups of metallic fueled LMFBR's, and the other was a mock-up of a mixed oxide fueled LMFBR. One of the metallic cores included enriched uranium in the test region, while the others did not.

Physics parameters to be calculated are criticality, reaction rate ratios, plutonium and B₄C sample worth, sodium void reactivity worth, and Doppler reactivity worth of ²³⁸U. Homogenized atomic number densities and various correction factors are given so that anyone can easily perform diffusion calculation in two-dimensional RZ-model and compare the results with the experiments. The validity of the correction factors are proved by changing the calculation method and used nuclear data file.     

Comparison of Cell Models for Analysis of Heterogeneous Fast Critical Assembly

We have investigated cell calculation models to be used in the analysis of neutronic characteristics of a heterogeneous fast critical assembly. As cell models we have considered a single drawer model with a critical buckling, a single drawer model with group dependent bucklings and a multidrawer model which consists of some fuel and blanket drawers. We have compared the cell averaged cross sections obtained from these cell models with the results of a reference transport calculation and estimated the effects of the cell models on k _eff, reaction rate ratios, reaction rate distributions and sodium void worths in the heterogeneous fast critical assembly ZPPR-13A. The multidrawer model and the single drawer model with group dependent buckling give reasonable cell averaged cross sections and have large effects on k _eff and ²³⁸U fission rate distribution.     

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.