A polymerisation reactor (1) comprising a circulation loop (2), an inlet for raw material (3), an outlet (5), driving means for (6) circulating a reactor charge within the circulation loop (2), and a pig station (7) for storing, launching, and receiving cleaning pigs. The pig station (7) comprises a lid to cover an opening enabling removal of the cleaning pig. The pig station (7) is constructed as a box with a cylindrical bore connected at both ends to the circulation loop, the bore having a central part with a diameter which is larger than the inner diameter of the circulation loop. At a transitional part at the outer ends of the bore the bore diameter narrows down to be substantially equal to the inner tube diameter of the circulation vloop. Over at least a part of the inner bore substantially half of the circular cross-section is formed in the removable lid.