By February 1944, the reactor was irradiating a ton of uranium every three days. Over the next five months, the efficiency of the separation process was improved, with the percentage of plutonium recovered increasing from 40 to 90 percent. Modifications over time raised the reactor's power to 4,000 kW in July 1944. The effect of the neutron poison xenon-135, one of many fission products produced from the uranium fuel, was not detected during the early operation of the X-10 Graphite Reactor. Xenon-135 subsequently caused problems with the startup of the Hanford B reactor that nearly halted the plutonium project.