A study on the uncertainty of setpoint for reactor trip system of NPPs considering rectangular distributions

• Published in: Nuclear engineering and technology Vol. 56; no. 5; pp. 1845 - 1853
• Main Authors: Jin, Youngho, Lee, Jae-Yong, Zhu, Oon-Pyo
• Format: Journal Article
• Language: English
• Published: Elsevier B.V 01.05.2024; 한국원자력학회
• Subjects: Channel uncertainty; GUM approach; Modified IEC61888 method; Monte Carlo simulation; Rectangular distribution; Safety system setpoint; 원자력공학; Rectangular distribution; Modified IEC61888 method; Channel uncertainty; Safety system setpoint; Monte Carlo simulation; GUM approach
• ISSN: 1738-5733; 2234-358X
• DOI: 10.1016/j.net.2023.12.043

The setpoint of the reactor trip system shall be set to consider the measurement uncertainty of the instrument channel and provide a reasonable and sufficient margin between the analytical limit and the trip setpoint. A comparative analysis was conducted to find out an appropriate uncertainty combination method through an example problem. The four methods were evaluated; 1) ISA-67.04.01 method, 2) the GUM95 method, 3) the modified GUM method developed by Fotowicz, and 4) the modified IEC61888 method proposed by authors for the pressure instrument channel presented in ISA-RP67.04.02 example. The appropriateness of each method was validated by comparing it with the result of Monte Carlo simulation. As a result of the evaluation, all methods are appropriate when all measurement uncertainty elements are normally distributed as expected. But ISA-67.04 method and GUM95 method overestimated the channel uncertainty if there is a dominant input element with rectangular distribution among the uncertainty input elements. Modified GUM95 methods developed by Fotowicz and modified IEC61888 method by authors are able to produce almost the same level of channel uncertainty as the Monte Carlo method, even when there is a dominant rectangular distribution among the uncertainty components, without computer-assisted simulations.