Contents lists available at ScienceDirect Nuclear Engineering and Design journal homepage: www.elsevier.com/locate/nucengdes Reactor dynamics of in-pin fuel motion in fast breeder reactors Anuj Dubey a , T. Sathiyasheela b , Anil Kumar Sharma c, ⁎ a Homi Bhabha National Institute, Mumbai, Kalpakkam Centre, Tamil Nadu, India b Reactor Design Group, Indira Gandhi Centre for Atomic Research, Kalpakkam, India c Fast Reactor Technology Group, Indira Gandhi Centre for Atomic Research, HBNI, Kalpakkam, India ARTICLE INFO Keywords: In-pin fuel motion Nuclear fuel melting UTOP Nuclear safety ABSTRACT In-pin fuel motion is a multi-phase hydrodynamic movement of molten fuel inside the fuel pins of a fast reactor during a core disruptive accident. This study focuses on the potential consequences of this phenomenon on fast breeder reactor dynamics during an unprotected transient overpower (UTOP) accident. An in-house developed solver is benchmarked with the CABRI-E9bis test to addresses the complex interplay of hydrodynamic forces. It is deduced that in slow overpower, molten fuel hydrodynamics is dominated by gravity and solidification blockage. Next, the solver is integrated with an in-house reactor dynamics code ‘PREDIS’ to simulate UTOP under the effect of in-pin fuel motion. A 500 MWe mixed oxide fuel based fast reactor is chosen for analyses. Results show that in-pin fuel motion generates a negative reactivity feedback of the order of -0.45 $. This results in a reduced peak power (182%) and stabilized state power (175%). Damage to the reactor core is contained more effectively. It is found that in a beginning of equilibrium core (BOEC), high internal pressure of the fuel pin does not permit molten fuel vaporization at high temperatures. Higher reactivity insertion rates of up to 20 pcm/s are also countered with in-pin fuel motion. From this study, it is inferred that in-pin fuel motion feedback enhances the inherent safety features of fast reactors during UTOP. It scales in magnitude with the fuel Doppler, and is therefore an important parameter for future safety analyses of oxide fuelled fast reactors. 1. Introduction Fast reactors are designed with two independent and highly reliable shutdown systems. These shutdown systems are activated when there is a change in reactor parameters beyond the permissible limits due to unexpected transients. Under an unprotected incident when these shutdown systems are not available, there are inherent safety feedbacks to protect the reactor. However, if the transient is fast or the shutdown systems are unavailable for any remotely possible reasons, then there is a probability of rise in power and fuel/clad melting. Under these cir- cumstances, the objectives are to contain melting and to prevent the degradation of the reactor core. The thermal/hydrodynamic phe- nomena that follow rise in power and melting can meet this objective by generating negative reactivity feedbacks. In-pin fuel motion is one such phenomenon. It is multi-phase hy- drodynamic motion of molten fuel that takes place upon melting inside the fuel pellet cavity. In the first experimental literature of in-pin fuel motion, it was termed as ‘fuel squirting’ due to the high speeds at which molten fuel was ejected into the fission gas plena (Porten et al., 1979; Smith, 1983). These experiments were focussed on fast reactivity in- sertion (∼5 $/s), which was expected in the late stages of an unprotected loss of flow accident. It is noteworthy that the test pins were specially fabricated to facilitate fuel squirting (annular fuel/an- nular blanket/annular reflector). Molten fuel was observed to enter the fission gas plena at high velocity (∼30 m/s). The resultant fuel squirting feedback was also reported to be very large. Computational efforts were made to validate the observed flow behaviour (Smith et al., 1982). Later, the experimental phenomenology was extended towards slow reactivity insertion due to the possibility of an unwanted control rod withdrawal. Experiments with slow reactivity insertion (∼5 pcm/s) followed (Ferrell et al., 1981; Tsai et al., 1993). It was now observed that in solid fuel pins (solid fuel/solid blanket), molten fuel extruded into the gap between the fuel and upper blanket columns. As a result, a smaller but valuable negative reactivity feedback was expected. Com- putational efforts were made to explain this flow behaviour in solid pins (Tentner and Hill, 1985). For slow transients with modern annular fuel pins (annular fuel/solid blanket), molten fuel was reported to travel at low speeds and remained confined within the fuel column (Charpenel et al., 2000). The resultant reactivity feedback was considered negli- gible (Papin, 2012). Recently, a computational effort was made to ex- plain the observed flow behaviour (Dubey and Sharma, 2018). It was discussed that a small but valuable in-pin fuel motion feedback would https://doi.org/10.1016/j.nucengdes.2018.10.010 Received 4 July 2018; Received in revised form 11 October 2018; Accepted 15 October 2018 ⁎ Corresponding author. E-mail address: aksharma@igcar.gov.in (A.K. Sharma). Nuclear Engineering and Design 340 (2018) 431–446 0029-5493/ © 2018 Published by Elsevier B.V. T