engleski

The Design and Safety Features of the IRIS Reactor

The pressurized light water cooled, medium power (1000 MWt) IRIS (International Reactor Innovative and Secure) reactor plant has been under development for three years by an international consortium of over 20 organizations from nine countries. The plant conceptual design was completed in 2001 and the preliminary design is currently underway. The pre-application licensing process with NRC started in October 2002 and IRIS is one of the designs considered by US utilities as part of the ESP (Early Site Permit) process. Major characteristics of the IRIS design and supporting analyses have been previously reported. This paper focuses on the status of the design and licensing process. IRIS is a pressurized water reactor that utilizes an integral reactor coolant system layout. The IRIS reactor vessel houses not only the nuclear fuel and control rods, but also all the major reactor coolant system components including: eight, small, spool type, reactor coolant pumps ; eight, modular, helical coil, once through steam generators ; a steel reflector which surrounds the core to improve neutron economy and reduce the neutron fluence on the vessel ; and, a pressurizer located in the reactor vessel upper head. The design of the reactor internals is also quite different from typical loop reactors, since all the reactor coolant system components are located in the pressure vessel. Also because of this fact, the IRIS integral vessel is larger than a traditional RV, with an ID of 6.21 meters and an overall height of 21.3 meters including the closure head. However, the large integral RV enables the size of the IRIS containment to be a fraction of the size of corresponding loop reactors. This compact containment size will enable the IRIS to achieve a significant reduction in the overall size of the reactor plant. IRIS has been primarily focused on achieving a design with innovative safety characteristics. The first line of defense in IRIS is to eliminate event initiators that could convincibly lead to core damage. In IRIS, this concept is implemented through the “ safety by design” approach, which can be simply described as “ design the plant in such a way as to eliminate accidents from occurring, rather than coping with their consequences” . If it is not possible to eliminate certain accidents altogether, then the design inherently reduces their consequences and/or decrease their probability of occurring. The key difference in the IRIS “ safety by design” approach from previous practice is that the integral reactor design is conducive to eliminating accidents, to a degree impossible in conventional loop-type reactors. The elimination of the large LOCAs, since no large primary penetrations of the reactor vessel or large loop piping exist, is only the most easily visible of the safety potential characteristics of integral reactors. Many others are possible, but they must be carefully exploited through an appropriate design that is kept focused on selecting design characteristics that are most amenable to eliminate accident initiating events. The IRIS design builds on the proven technology provided by over 40 years of operating PWR experience, and on the established use of passive safety features pioneered by Westinghouse in the NRC certified AP600 plant design. The use of passive safety systems provides improvements in plant simplification, safety, reliability, and investment protection over conventional plant designs. Because of the safety by design approach, the number and complexity of these passive safety systems and required operator actions are further minimized in IRIS. The net result is a design with significantly reduced complexity and improved operability, and extensive plant simplifications to enhance construction. This paper will describe the accident sequence improvements resulting from the IRIS safety by design approach and the IRIS safety systems, and provide a short overview of the licensing and pre-application activities with the NRC.

Design; Safety Features; IRIS Reactor

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