2 research outputs found
Risk-Informed design process of the IRIS reactor
Westinghouse is currently conducting the pre-application licensing of the International Reactor Innovative and
Secure (IRIS). The design philosophy of the IRIS has been based on the concept of Safety-by-DesignTM and within this
framework the PSA is being used as an integral part of the design process. The basis for the PSA contribution to the design
phase of the reactor is the close iteration between the PSA team and the design and safety analysis team. In this process the
design team is not only involved in the initial phase of providing system information to the PSA team, allowing in this way the
identification of the high risk scenarios, but it is also receiving feedback from the PSA team that suggests design modification
aimed at reaching risk-related goals.
During the first iteration of this process, the design modifications proposed by the PSA team allowed reducing the initial
estimate of Core Damage Frequency (CDF) due to internal events from 2E-6/ry to 2E-8/ry. Since the IRIS design is still in a
development phase, a number of assumptions have to be confirmed when the design is finalized.
Among key assumptions are the success criteria for both the accident sequences analyzed and the systems involved in the
mitigation strategies. The PSA team developed the initial accident sequence event trees according to the information from
the preliminary analysis and feasibility studies. A recent coupling between the RELAP and GOTHIC codes made possible the
actual simulation of all LOCA sequences identified in the first draft of the Event Trees. Working in close coordination, the
PSA and the safety analysis teams developed a matrix case of sequences not only with the purpose of testing the assumed
success criteria, but also with the perspective of identifying alternative sequences developed mainly by relaxing the extremely
conservative assumptions previously made.
The results of these simulations, bounded themselves with conservative assumptions on the Core Damage definition,
suggested two new versions of the LOCA Event Tree with two possible configurations of the Automatic Depressurization
System. The new CDF has been evaluated for both configurations and the design team has been provided with an additional
and risk-related perspective that will help choosing the design alternative to be implemented
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IRIS Simplified LERF Model
Westinghouse is currently conducting the pre-application licensing of the International Reactor Innovative and Secure (IRIS). One of the key aspects of the IRIS design is its safety-by-designTM philosophy and within this framework the PRA is being used as an integral part of the design process. The most ambitious risk-related goal for IRIS is to reduce the Emergency Planning Zone (EPZ) to within the exclusion area by demonstrating that the off-site doses are consistent with the US Protective Action Guidelines (PAGs) for initiation of emergency response so that the required protective actions would be limited to the exclusion area. As a first step, a model has been developed to provide a first order approximation of the Large Early Release Frequency (LERF) as a surrogate predictor of the off-site doses. A key-aspect of the LERF model development is the characterization of the possible paths of release. Four main categories have been historically pointed out: (1) Core Damage (CD ) sequences with containment bypass, (2) CD sequences with containment isolation failure, (3) CD sequences with containment failure at low pressure and (4) CD sequences with containment failure at high pressure. They have been reevaluated to account for the IRIS design features