[en] The JOYO FOSTER has been used since 1983 for over 15 years. FOSTER consists of simulated control panels, a computer system, an instructor console, etc.. FOSTER is able to offer operators real reactor plant behaviors under normal or abnormal conditions. As the computer system was outdated, FOSTER was replaced from March to November 1998. Together with replacing the computer system, the simulation accuracy and the training function were improved in order to upgrade the simulations performance. The purpose for improving the simulation accuracy was two-fold: to enable utilization as a plant simulator and to provide for Emergency Procedure Guidance (EPG) training. To improve the simulation accuracy, the reactor model and the Intermediate Heat Exchanger (IHX) model were redone. Since the former computer system's memory size was limited, the reactor core was modeled with a single analysis channel. Consequently, the previous simulator could not calculate the thermo-hydraulic behaviors in the reactor vessel accurately. To obtain a more realistic simulation, the reactor model was remodeled to 16 channels and calculating meshes in the axial direction of each channel were increased from 10 to 17 nodes. Useful technical information on improving the calculating accuracy obtained from post analysis of the natural circulation testing in JOYO was reflected in remodeling. Additionally, the nuclear characteristic calculating model in the reactor model was also improved so as to calculate individual control rod worth for each of the six control rods; the previous model calculated the reactivity with one value for all control rods. Regarding the IHX model, the following three improvements were made: modifying a heat transfer coefficient based on the reactor power, calculating the bypass flow in IHX(A) depending on temperature distribution, increasing the calculating meshes in the axial direction. Concerning the training function, the former simulator did not simulate local control panel functions. So, local panel function simulations were developed for training function. It offers operators 76 display images of local control panels. (author)