Abstract
The fission gas release microscopic model of the mechanistic code MFPR is further developed for modelling of enhanced release from irradiated UO2 fuel under transient conditions of the power ramp tests, along with the microstructure evolution characterised by the formation of a new population of large intragranular bubbles with a rather wide size distribution (from 30 to 500 nm), observed in transient-tested UO2 fuel samples. Implementation of the additional microscopic mechanisms results in a notable improvement of the code predictions (in comparison with the previous code version) for the fractional gas release in the Riso ramp tests with three different hold times of 3, 40 and 62 h at the terminal linear power of ≈40 kW/m.
Highlights
For realistic description of fission-gas release and fuel swelling as a function of fuel-fabrication variables and in a wide range of reactor operating conditions, mechanistic models must treat them as coupled phenomena and must include various microscopic mechanisms influencing fission gas behaviour
The maximum bubble coalescence effect and the broadest size distribution function (SDF) was found at a distance of x ∼ 0.2 from the pellet centre where some decrease of the temperature in comparison with that in the centreline is compensated with an excess induced by the biased migration in the temperature gradient
The modified Nelson model for gas atom resolution from bubbles under irradiation, taking into consideration a tendency of gas atoms ejected from a bubble into surrounding matrix to return back to this bubble by diffusion, results in significant microstructure changes characterised by the formation of a new population of large intragranular bubbles with a rather wide size distribution, observed in transient-tested fuel samples [5]
Summary
For realistic description of fission-gas release and fuel swelling as a function of fuel-fabrication variables and in a wide range of reactor operating conditions, mechanistic models must treat them as coupled phenomena and must include various microscopic mechanisms influencing fission gas behaviour. The improved model for the irradiation induced resolution of gas atoms from bubbles, which allows a reasonable interpretation of a broad “trimodal” bubble size distribution (including gas atoms and two populations of bubbles), observed in the transient-tested fuel pellets [1], and the additional gas transport mechanisms necessary for correct description of the enhanced gas release in these tests, were recently developed by the authors [2]. In the current paper the main microscopic mechanisms of the intragranular bubble system evolution and fission gas release considered in [2] are briefly overviewed. Additional mechanisms of bubbles transport to grain boundaries, considered earlier in MFPR in application to post-irradiation annealing regimes [7], were adapted to transient conditions and applied to analysis of the fission gas release in the tests [6]
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