[en] For the purpose of upgrading the control and instrumentation system to meet new AEC requirements, to increase the available neutron flux for experimenters and to replace burned out fuel the Puerto Rico Nuclear Center started a modification program on its old MTR type, one megawatt reactor on March 1971. A TRIGA core utilizing the newly developed FLIP fuel, capable of operating at two megawatts with natural convection cooling and with pulsing capabilities was chosen. The major conversion tasks included: 1. Modification of the bridge, tower and grid plate structures, 2. Modification of the water cooling system (inside the reactor pool), 3. Installation of a larger heat exchanger and cooling tower, 4. Installation of a new instrumentation and control console (including neutron detectors and rod drive mechanisms). 5. Installation of a TRIGA FLIP core. Initial criticality was achieved on January 1972. For the chosen operating configuration the critical mass was 11,522 grams of uranium 235. Core excess reactivity was $7.12 and the total (5) rod worth was $12.06. During the early stages of the startup program to determine the basic core parameters and while conducting a stepwise increase in power to the design power level of two megawatts a power fluctuation on all neutron detectors was noticed. It was determined that the power fluctuations started at about 1.4 megawatts and sharply increased as power approached 2 megawatts. Experiments to determine the cause of the problem and to correct the condition were conducted on July and December 1972 and June 1973. Modifications to the core included changing fuel pin pitch and the addition of dummy elements in the central region of the core. Final acceptance by AEC Headquarters was requested on October 1973. (author)