Calculation of core safety parameters and uncertainty analyses during unprotected transients for the ALLEGRO and a sodium-cooled fast reactor

The results show that the thermal expansion of the core structural elements has a significant effect on the reactivity of these fast spectrum reactors. Moreover, it was found that during unprotected overpower transients the most important uncertain parameters are the Doppler coefficient and the fuel thermal expansion coefficient of reactivity. In addition, the large uncertainty of the positive coolant and cladding temperature coefficients of reactivity have a significant influence on the uncertainty of the calculated peak cladding temperature, especially in the case of unprotected loss of flow transients. The results imply that the uncertainty of some core safety parameters should be reduced. Lastly, the results adumbrate that additional safety shutdown systems are to be applied to mitigate the consequences of unprotected transients.