After the Fukushima accident, the public has expressed concern regarding the safety of nuclear power
plants. This accident has strengthened the necessity for further improvement of safety in the design of
existing and future nuclear power plants. Pressurized heavy water reactors (PHWRs) have a high level of
defence in depth (DiD) philosophy to achieve the safety goal. It is necessary for designers to demonstrate
the capability of decay heat removal and integrity of containment in a PHWR reactor for prolonged
station blackout to avoid any release of radioactivity in public domain. As the design of PHWRs is
distinct, its calandria vessel and vault cooling water offer passive heat sinks for such accident scenarios
and submerged calandria vessel offers inherent in-calandria retention features. Study shows that, in case
of severe accident in PHWR, in-calandria retention (ICR) is only option is to contain the corium inside
the calandria vessel by cooling it from outside using the calandria vault water to avoid release of
radioactivity to public domain. There are critical issues on ICR of corium that have to be resolved for
successful demonstration of ICR strategy and regulatory acceptance.
This paper tries to investigate some of the critical issues of ICR of corium. The present study focuses on
experimental investigation of the coolability of molten corium with and without simulated decay heat and
thermal behaviour of calandria vessel performed in scaled facilities of an Indian PHWR.