# Nuclear Science and Techniques

《核技术》(英文版) ISSN 1001-8042 CN 31-1559/TL     2019 Impact factor 1.556

Nuclear Science and Techniques ›› 2019, Vol. 30 ›› Issue (4): 63

• NUCLEAR ENERGY SCIENCE AND ENGINEERING •

### Improvements in methodology to determine feedback reactivity coefficients

Faisal Qayyum1 • Muhammad Rizwan Ali1 • Awais Zahur1 • R. Khan1

1. 1 Department of Nuclear Engineering, Pakistan Institute of Engineering and Applied Sciences (PIEAS), Nilore, Islamabad 45650, Pakistan
• Received:2018-05-11 Revised:2018-08-28 Accepted:2018-09-09
• Contact: R. Khan E-mail:rustam_pieas@yahoo.com
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Faisal Qayyum, Muhammad Rizwan Ali, Awais Zahur, R. Khan. Improvements in methodology to determine feedback reactivity coefficients.Nuclear Science and Techniques, 2019, 30(4): 63
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Abstract: The reactivity of a nuclear reactor is the most important safety and operating parameter. Due to short reactor period, the Light Water Reactor (LWR) designs require the compensations of rapid unfavorable reactivity increases. The increase in fuel or moderator temperature leads to compensate the reactivity jumps as inherent safety characteristics. The safe and reliable reactor operation requires the accurate assessment of these reactivity changes. This paper highlights the improvements in the methodology to determine the feedback reactivity changes in IAEA MTR benchmark. This method incorporates the reactivity effects of fuel temperature in moderator regions and vice versa. For this purpose, a detailed 3D model of the IAEA 10 MW MTR benchmark reactor is developed employing OpenMC computer code. OpenMC is a probabilistic computer code for neutronic calculations. This work uses temperature-dependent JEFF 3.2 cross-sectional library. The model is validated against the reference results of eigenvalues for control rods (inserted and in fully withdrawn position), control rod reactivity worth, averaged thermal flux in the central flux trap, and power fraction for each fuel element at beginning of life. The validated model is applied to simulate the feedback reactivity coefficients against the conventional reference results. In order to improve the methodology, the effect of the moderator temperature and void on fuel is incorporated to obtain a more realistic value of the fuel temperature coefficient. Similarly, the moderator temperature coefficient and void coefficient are improved by incorporating the coupling effects of fuel temperature on moderator. This methodology can be applied to improve the LWR designs.