Reactor physics tests of TRIGA Mark-II Reactor in Ljubljana

TRIGA Mark-II Reactor in Ljubljana was recently reconstructed. The reconstruction consisted mainly of replacing the grid plates, the control rod mechanisms and the control unit. The standard type control rods were replaced by the fuelled follower type, the central grid location (A ring) was adapted for fuel element insertion, the triangular cutouts were introduced in the upper plate design. However, the main novelty in reactor physics and operational features of the reactor was the installation of a pulse rod. Having no previous operational experience in pulsing, a detailed and systematic sequence of tests was defined in order to check the predicted design parameters of the reactor with measurements. The following experiments are treated in this paper: initial criticality, excess reactivity measurements, control rod worth measurement, fuel temperature distribution, fuel temperature reactivity coefficient, pulse parameters measurement (peak power, prompt energy, peak temperature). Flux distributions in steady state and pulse mode were measured as well, however, they are treated only briefly due to the volume of the results. The experiments were performed with completely fresh fuel of 12 w% enriched Standard Stainless Steel type. The core configuration was uniform (one fuel element type, including fuelled followers) and compact (no irradiation channels or gaps), as such being particularly convenient for testing the computer codes for TRIGA reactor calculations. Comparison of analytical predictions, obtained with WIMS, SLXTUS, TRIGAP and PULSTRI codes to measured values showed agreement within the error of the measurement and calculation. The paper has the following contents: 1. Introduction; 2. Steady State Experiments; 2.1. Core loading and critical experiment; 2.2. Flux range determination for tests at zero power; 2.3. Digital reactivity meter checkout; 2.4. Control rod worth measurements; 2.5. Excess reactivity measurement; 2.6. Thermal power calibration; 2.7. Flux distribution measurements, core no. 133 A; 2.8. Increasing of excess reactivity; 2.9; Control rod worth measurement; 2.10. Fuel element reactivity worth in different rings; 2.11. Thermal power calibration; 2.12. Void coefficient measurement; 2.13. Fuel temperature coefficient measurement; 2.14. Fuel temperature measurement in different rings; 3. Pulse Experiments; 4. Conclusions. To summarize, the main purpose of the reactor physics tests was to verify the predicted and calculated values for the most important reactor parameters which had been used in safety evaluation of pulse operation. Comparison of measured results to the calculations show satisfactory agreement so as to qualify the computer codes used and calculational procedures as analytical tools for evaluating the pulse experiments in the future