The IAEA organizes International Collaborative Standard Problems (ICSPs) to facilitate co-operation on advancing reactor technology related activities. These activities include the development, assessment and validation of computer codes for design and safety analysis of nuclear power plants. The objective of this ICSP was to develop and facilitate open access to a set of standardized numerical test problems for selected postulated transients in pressurized heavy water reactors (PHWRs). These manufactured test problems are different from typical benchmark problems in that the specification is designed for a stylized model of a generic CANDU-6 PHWR and a reference solution is not postulated. The set of standardized numerical test problems supports the development of simulation codes, methods, and coupling frameworks. This publication provides a description of four numerical test problems, the participants' individual multi-physics methodologies applied, and simulations carried out, an independent synthesis of the results, and the lessons learned from the exercise.