Spatial Effects and Uncertainty Analysis for Rod Ejection Accidents in a PWR (NUREG/IA-0215)

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Publication Information

Date Published: September 2007

Prepared by:
A. Avvakumov, V. Malofeev, V. Sidorov

Nuclear Safety Institute
Russian Research Centre
"Kurchatov Institute"
1, Kurchatov Sq., Moscow
123182 Russia

H. H. Scott, NRC Project Manager

Prepared for:
U.S. Nuclear Regulatory Commission, Institute for Radioprotection and Nuclear Safety (France),
and Ministry of Science and Technologies of the Russian Federation

Published by:
Office of Nuclear Regulatory Research
U.S. Nuclear Regulatory Commission
Washington, DC 20555-0001

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A rod ejection accident is a design-basis event for a pressurized water reactor. It is well known that spatial effects play a very important role in this accident. In this study four cases using a model of Three Mile Island Unit 1 are considered: ejection of the central or peripheral control rod at the end of cycle or the beginning of cycle. Calculations were carried out using the BARS neutronic code coupled with the RELAP5/MOD3.2 thermal hydraulic code. This coupled code allows three-dimensional pin-by-pin neutronics and assembly-by-assembly thermal-hydraulics simulation of a transient. The results showed that the major parameters of the accident (the peak power and core energy deposition) were a function of spatial effects. Analysis of the dependence of the peak local fuel enthalpy on spatial effects was performed. Uncertainty analysis was carried out for the central control rod ejection accident at hot zero power conditions. The analysis of uncertainties was performed for the following parameters: local fuel enthalpy, maximum core power, and power pulse width. Calculated results showed that the uncertainty in key safety parameters would be determined to a great extent by the uncertainty in the control rod worth. The effect of initial core power on the above parameters was analyzed using a rod ejection accident starting from 33% of rated power.

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