In Korea, PHWRs account for 17% of operating units and have taken an important role in providing national energy supply. The recent biggest issue in domestic PHWR community was the continued operation of the Wolsong-1 CANDU plant, which has recently been approved to operate for 10 more years after a 30 year design life. In relation to this issue, various actions from both post-Fukushima lessons and Wolsong-1 stress test results are being taken. In KAERI R&D, the following topics are studied to support the basis for these actions. First, PHWR severe accident issues such as (1) PHTS overpressure protection capability; (2) containment overpressure protection capability and (3) bypass source term are evaluated. Second, a computer tool (called ISAAC) has been modified and updated to support analyzing Wolsong severe accident issues. Third, a decision supporting tool, called SAMEX-CANDU, has been developed to aid emergency response experts under severe accident conditions.