Complex sentence: The solution was far more concentrated than he thought, and the mixer not big enough for safety; too much uranium was now in one container it reached critical mass and reacted, knocking him to the floor, splashing him with radioactive liquid.
Split into simpler sentences: The solution was far more concentrated than he thought, and the mixer not big enough for safety; too much uranium was now in one container. It reached critical mass and reacted, knocking him to the floor, splashing him with radioactive liquid.