Current and advanced nuclear reactors: How safe is safe enough?

Special Reports | November 16, 2022

Researchers at North Carolina State University are embracing risk technologies to enable and ensure safe, secure, and cost-competitive operations for the current and advanced nuclear reactors.

A research group led by Assistant Professor Mihai A. Diaconeasa at the North Carolina State University is developing and reimagining the use of probabilistic risk assessment technology to support the continued operation of existing nuclear reactors and deployment of advanced nuclear reactors.

Risk-informing the design and licensing of advanced nuclear reactors

The philosophy of defence-in-depth calling for multiple layers of protection to prevent and mitigate accidents has contributed to the excellent safety record of the nuclear industry, even if at a higher cost compared to other energy sources. In the United States, conservative deterministic methods were initially used to analyse and license nuclear power plant designs, such as the 10 CFR Part 50 rules. Inconsistencies and subjectivity of modelling, the release of the Reactor Safety Study, and the nuclear accidents at Three Mile Island, Chernobyl, and Fukushima led to a gradual shift to probabilistic risk assessment currently used today. Eventually, 10 CFR Part 52 allowed the use of probabilistic risk assessment insights into the safety case based on design basis accidents and defence-in-depth assurance. Nowadays, 10 CFR Part 53 is envisioned to fully embrace probabilistic risk assessment in most licensing requirements targeting advanced reactors. At the same time, the use of probabilistic risk assessment information enabled by improved capability to evaluate the nuclear power plants as integrated systems is changing how we design advanced reactors moving towards a rationalist approach that hopefully will lead to as safe and enable cost-competitive operations.

Probabilistic risk assessment models are developed to answer three fundamental questions: 1) What can go wrong? 2) How likely is it to go wrong? and 3) What are the consequences? This idea is at the forefront of a project supported by the U.S. Department of Energy’s Advanced Research Projects Agency-Energy (ARPA-E) on the “Advanced Operation & Maintenance Techniques Implemented in the Xe-100 Plant Digital Twin to Reduce Fixed O&M Cost.” X-energy’s digital twin project aims to reduce the fixed operations and management (O&M) cost of its advanced nuclear reactor design to $2 per MWh.  X-energy’s Xe-100 probabilistic risk assessment model development is used to inform the design of systems, human factors engineering program, and regulatory engagement in which uncertainties in the quantitative risk insights are explicitly accounted for in the evaluation of necessary and sufficient defence-in-depth measures. For example, risk insights are used to define the appropriate control room staffing levels by evaluating the procedures and timing of events under normal and abnormal conditions obtained from computer simulations and physical control room simulator exercises.

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