Abstract
Composite fuels such as UN-UO2 are being considered to address the lower oxidation resistance of the UN fuel from a safety perspective for use in light water reactors, whilst improving the in-reactor behaviour of the more ubiquitous UO2 fuel. An innovative UN-UO2 accident tolerant fuel has recently been fabricated and studied: UN microspheres embedded in UO2 matrix. In the present study, detailed oxidative thermogravimetric investigations (TGA/DSC) of high-density UN/U2N3-UO2 composite fuels (91-97 %TD), as well as post oxidised microstructures obtained by SEM, are reported and analysed. Triplicate TGA measurements of each specimen were carried out at 5 K/min up to 973 K in a synthetic air atmosphere to assess their oxidation kinetics. The mass variation due to the oxidation reactions (%), the oxidation onset temperatures (OOTs), and the maximum reaction temperatures (MRTs) are also presented and discussed. The results show that all composites have similar post oxidised microstructures with mostly intergranular cracking and spalling. The oxidation resistance of the pellet with initially 10 wt% of UN microspheres is surprisingly better than the UO2 reference. Moreover, there is no significant difference in the OOT (~557 K) and MRT (~615 K) when 30 wt% or 50 wt% of embedded UN microspheres are used. Therefore, the findings in this article demonstrate that the UO2 matrix acts as a barrier to improve the oxidation resistance of the nitride phases at the beginning of life conditions.
Highlights
After the Fukushima Daiichi disaster in 2011, the nuclear community has strived to engineer a successor to the standard uranium dioxide (UO2)-Zr fuel-cladding system within light water reactors (LWRs)
The results show that the oxidation resistance of the composite with initially 10 wt% of Uranium nitride (UN) microspheres is surprisingly better than the UO2 reference
The oxidation onset temperatures (OOTs) of UN(10)-UO2 (SC) (593 ± 6 K) is about 23 K higher than the reference UO2 (SC). This composite shows a slightly higher maximum reaction temperatures (MRTs) and lower reaction rate at the maximum rate, indicating that the oxidation reaction is smoother in the composite than in the UO2 (SC)
Summary
After the Fukushima Daiichi disaster in 2011, the nuclear community has strived to engineer a successor to the standard uranium dioxide (UO2)-Zr fuel-cladding system within light water reactors (LWRs) This severe accident scenario demonstrated that the standard fuel system degrades rapidly in such extreme conditions [1]. Uranium nitride (UN) has been considered a promising ATF candidate to substitute UO2 in LWRs mainly because of its higher uranium density, thermal conductivity, and similar melting point in comparison with UO2 [3]. These improved properties would allow operating the reactor with a lower fuel centreline temperature, which provides the benefit of a higher margin for melting. The nitride fuel readily reacts with the coolant and loses its structural integrity, resulting in fuel pellet oxidation, pulverisation, washout and relocation
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