A polymerization loop reactor including a loop reaction zone, a continuous takeoff, and a fluid slurry disposed in the reaction zone. A generally cylindrical wall defines the loop reaction zone. The length of the loop reaction zone and the nominal outside diameter of the generally cylindrical wall define a length/diameter ratio greater than 250. The reactor can be charged with a fluid slurry including an olefin monomer reactant, solid olefin polymer particles, and a liquid diluent. The concentration of the solid olefin polymer particles in the slurry can be greater than 40 weight percent based on the weight of polymer particles and the weight of liquid diluent. Also disclosed is a polymerization process carried out by polymerizing, in the loop reaction zone of a reactor as defined above, at least one olefin monomer in a liquid diluent to produce a fluid slurry as defined above.