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atw 2018-07


atw Vol. 63 (2018) | Issue 6/7 ı June/July 374 AMNT 2018 TESPA-ROD Code Prediction of the Fuel Rod Behaviour During Long-term Storage Heinz G. Sonnenburg | | AMNT 2018: Best Paper Award, awarded by Dr. Erwin Fischer (left) to Dr. Heinz G. Sonnenburg (right). | | Fig. 1. Crystallographic length change of UO 2 /PuO 2 -fuel relative to displacement per atom (dpa) [RAY 15]. The paper “TESPA- ROD Code Prediction of the Fuel Rod Behaviour During Long-term Storage” by Heinz G. Sonnenburg has been awarded as Best Paper of the 49 th Annual Meeting on Nuclear Technology (AMNT 2018), Berlin, 29 and 30 May 2018. Introduction The TESPA-ROD code is applicable to both LOCA and RIA transients. Recently, the code’s models have been extended in order to predict the transitional fuel rod behaviour during long-term storage [SON 17]. Due to permanent α-decay of actinides in the fuel during long-term storage, both fuel swelling and helium release continue and generate an impact on the fuel rod behaviour. Therefore, the TESPA-ROD code extension requires particular modelling of fuel swelling and modelling of the associated helium gas release. These processes and their modelling have significant impact on the prediction of cladding’s stress level. Continued fuel swelling reduces the gap between fuel and cladding which reduces the fuel rod fission gas volume and might increase the fuel rod inner pressure by that. Simultaneously, the release of helium tends to keep the rod internal pressure high, thus the gap between fuel and cladding could be enlarged. If fuel swelling is the dominating process, as in case of MOX fuel, even gap closure might occur which leads to pellet- cladding interaction which finally enhances significantly the stress level in the cladding. A priori, which effect dominates cannot be estimated with simple engineering judgment. Therefore, a code prediction is inevitable in order to get reliable estimates about dominating processes. Fuel swelling Fuel in a fuel rod accumulates fission gases in the fuel matrix during normal operation. E.g., small gas bubbles of micrometer size appear within the fuel grain at higher burn-up levels. Because the accumulation of fission gas in the fuel matrix is limited, some quantity of fission gas will get released from fuel. There is a well-known interlinkage between the accumulation of fission gas and swelling of the pellet. The more the fuel accumulates fission gas, the more the fuel swells. The same mechanism is true for the long-term storage, but here helium is accumulated instead of fission gases. This helium stems from the decay of α-emitting actinides. Patrick Raynaud [RAY 15] from US.NRC has compiled fuel swelling correlations for UO 2 fuel and PuO 2 fuel which refer to the α-decay in the fuel (Figure 1). Correlating parameter is dpa (displacement per atom). This compilation reveals a swelling mechanism which saturates at a certain maximal swelling level. Consequently, the swelling can be expressed as exponential function: upper bounding values (1) and mean values (2) where ∆a is the change of lattice parameter, a 0 is the undeformed lattice parameter. The parameter dpa correlates with time. Raynaud /RAY 15/ provides for 60 GWd/t UO 2 fuel the relation dpa(t) =0.01172 t 0.72246 , where t is measured in years. In case of MOX fuel, this relation can be multiplied by 3, because MOX fuel has 3-times more α-decays, see figure 5.3 on page 54 in [SON 17]. The swelling mechanism, as correlated above, refers mainly to the production of Frenkel pairs and helium atoms at interstitial positions in the crystal structure of UO 2 . The effect AMNT 2018 TESPA-ROD Code Prediction of the Fuel Rod Behaviour During Long-term Storage ı Heinz G. Sonnenburg

atw Vol. 63 (2018) | Issue 6/7 ı June/July AMNT 2018 375 | | Fig. 2. Evolution of fuel rod temperature (all curves FGR=10 %…40 % overlap). | | Fig. 3. Evolution of internal gas pressure. | | Fig. 4. Evolution of gap size. | | Fig. 5. Evolution of hoop stress. of helium bubble formation due to accumulation of α-particles in bubbles is presumably underrepresented in the experimental investigations mentioned in figure 1. According to Wiss et al. [WIS 14] this accumulation effect would dominate in case of very high alpha doses. Thus fuel swelling saturation could also be observed at 0.6 % instead of 0.4642 % as mentioned in Figure 1. Therefore, the characterization “upper bounding values” in Figure 1 has to be taken with caution. Helium gas release The characteristic saturation of the fuel length change ∆a/a 0 is an indication that the accumulation of helium within the fuel matrix is limited. Therefore, the shape of the curve above is taken in order to quantify the fraction of produced helium which gets released from fuel matrix. The more the saturation in Figure 1 is reached the larger the fraction of produced helium atoms is. E.g., this interlinkage can be expressed with: (3) where HeF released (t) is considered as fraction of produced helium mol rate at time t that gets released from fuel matrix. Equation (3) refers to the upper bounding swelling correlation given with equation (1). Consequently, the helium mol rate produced and retained in fuel matrix is: (4) Whereas the helium mol rate released from fuel matrix is: (5) The helium production rate from α-decay Ḣe produced (t) within the UO 2 fuel matrix can be approximated with: (6) Therefore, the TESPA-ROD modelling approach for helium release follows the concept of an athermal helium release, because a) annealing tests reveal a minimum temperature of 800 K for the start of thermal helium release [WIS 14] and b) high burn-up structure with fuel grain size below 1 µm offers a huge intergranular surface thus α-particles can easily escape from these grains without thermal controlled diffusion. TESPA-ROD predictions for long-term storage The TESPA-ROD prediction for longterm storage significantly depends on the normal operating condition of the fuel rod just before reactor shut down. If the fuel rod burn-up is e.g. at 70 GWd/t and the fission gas release (FGR) due to normal operation is low (e.g. 10 %) due to not demanding normal operation, the pellet-cladding gap would be closed and simultaneously the hoop stress in cladding would be rather low, which is a consequence of cladding irradiation creep during normal operation (stretch-out operation). Under these conditions (70 GWd/t, 10 % FGR, low cladding stress and gap closure at normal operation), the reactor shut down would consequently lead to an opening of the gap between fuel and cladding. This gap AMNT 2018 TESPA-ROD Code Prediction of the Fuel Rod Behaviour During Long-term Storage ı Heinz G. Sonnenburg