of current PWR/BWRs). This higher efficiency would lead to better fuel economy and a lighter fuel load, lessening residual (decay) heat. SCWR is typically designed as a direct cycle, whereby steam or hot supercritical water from the core is used directly in a steam turbine. This makes the design simple. As a BWR is simpler than a PWR, a SCWR is a lot simpler and more compact than a less-efficient BWR having the same electrical output. There are no steam separators, steam dryers, internal recirculation pumps, or recirculation flow inside the pressure vessel. The design is a once-through, direct-cycle, the simplest type of cycle possible. The stored thermal and radiologic energy in the smaller core and its (primary) cooling circuit would also be less than that of either a BWR's or a PWR's. Water is liquid at room temperature, cheap, non-toxic and transparent, simplifying inspection and repair (compared to liquid metal cooled reactors). A fast SCWR could be a breeder reactor, like the proposed Clean and Environmentally Safe Advanced Reactor and could burn the long-lived actinide isotopes. A heavy-water SCWR could breed fuel from thorium (4x more abundant than uranium). Similar to a CANDU it could also use unenriched natural uranium if enough moderation is provided Process heat can be delivered at higher temperatures than other water-cooled reactors allow Disadvantages Lower water inventory (due to compact primary loop) means less heat capacity to buffer transients and accidents (e.g., loss of feedwater flow or large break loss-of-coolant accident) resulting in accident and transient temperatures that are too high for conventional metallic cladding. However, it is not too high for stainless steel cladding. Safety analysis of LWR type SCWR showed that safety criteria are met with margins at accidents and abnormal transients including total loss of flow and loss of coolant accident. No double ended break occurs because of the once-through coolant cycle. Core is cooled by the induced flow at the loss of coolant accident. The water inventory in the top dome of the reactor vessel serves as an in-vessel accumulator. The SCWR safety principle is not to maintain coolant inventory, but to maintain core coolant flow rate. It is easy to monitor than water level at accidents. There was an error in the water level signal in the Three Mile Island accident and the operators shut down the ECCS. Higher pressure combined with higher temperature and also a higher temperature