Michio Ichikawa

FEMAXI-III, a computer code for fuel rod performance analysis

Abstract This paper presents a method of fuel rod thermal-mechanical performance analysis used in the FEMAXI-III code. The code incorporates the models describing thermal-mechanical processes such as pellet-cladding thermal expansion, pellet irradiation swelling, densification, relocation and fission gas release as they affect pellet-cladding gap thermal conductance. The code performs the thermal behavior analysis of a full-length fuel rod within the framework of one-dimensional multi-zone modeling. The mechanical effects including ridge deformation is rigorously analyzed by applying the axisymmetric finite element method. The finite element geometrical model is confined to a half-pellet-height region with the assumption that pellet-pellet interaction is symmetrical. The 8-node quadratic isoparametric ring elements are adopted for obtaining accurate finite element solutions. The Newton-Raphson iteration with an implicit algorithm is applied to perform the analysis of non-linear material behaviors accurately and stably. The pellet-cladding interaction mechanism is exactly treated using the nodal continuity conditions. The code is applicable to the thermal-mechanical analysis of water reactor fuel rods experiencing variable power histories.

In-Pile Diameter Measurement of Light Water Reactor Test Fuel Rods for Assessment of Pellet-Cladding Mechanical Interaction

Three Zircaloy-clad UO2 test fuel rods were irradiated at 50 kW/m to a burnup of 15 000 MWd/MTU with an in-pile rod diameter scanning device, and cladding deformations were measured at various powe...

Fuel rod deformation code FEMAXI-II and its application

Abstract This paper describes an axisymmetric, two-dimensional finite element method programme, FEMAXI-II, for fuel rod deformation analysis, and its application to the instrumented fuel assembly, IFA-508, in an HBWR. FEMAXI, the old version, was developed from Yamadas finite element programme by applying a pellet-cladding contact model and a pellet-cracking model and temperature distribution subroutine. Using the “softened” plastic-creep stress-strain matrix and the pseudo-stresses introduced by Cyr and Teter, we improved FEMAXI to FEMAXI-II in order to follow the changes of creep rate and yielding stress affected by the temperature increase. In the application of FEMAXI-II to IFA-508 experiments, the prediction of this programme agrees with results of the in-pile diameter measurements. The Cyr creep model predicts well that the ridge deformation is smaller in the thick cladding than in the thin one.

Void Migration in Sodium Chloride Crystal under Temperature Gradient

A simulation experiment on columnar grain formation in UO2 was performed using NaCl single crystals, to examine the progress of void formation and migration. When two single crystals of NaCl were held together under a temperature gradient, plate-like or lenticular voids were observed to generate and migrate towards the hot side, forming columnar grains in the NaCl crystal. The crystallographic orientation of the colder crystal was found to affect the void shapes observed in the hotter crystal. This void behavior is explained by calculations concerning the evaporation and condensation of the NaCl molecule.

Irradiation of Natural UO2 Pellets

Three kinds of capsules containing natural UO2 pellets were irradiated in the EFTL-2 irradiation facility of JRR-2. EFTL-2 is a hydraulic rabbit designed by the authors and completed in 1963. The purpose of the present irradiation is:(1) To assemble basic data pertaining to EFTL-2 when carrying fuel capsules under irradiation.(2) To ascertain the correctness of capsule design and to compare calculations with experimental data.(3) To gather information on the effects of irradiation on natural UO2 pellets of the same dimensions as fuel used in the JPDR.This is the first experiment undertaken in Japan on UO2 pellet irradiation, and consequently the irradiation capsule has been designed very conservatively to incorporate every precaution to ensure safety precautions.