# Comparative Theoretical Analysis of Halden Reactor Creep Tests on M5 Cladding Material
## 6 The TRANSURANUS Simulations of the Tests: Preliminary Results  
The M5 creep tests mentioned above were reproduced with the TRANSURANUS code in order to test its capability and accuracy to reproduce experimental results. The adopted input data were the following:  
Figure 5: Comparison of the M5 IFA-663 secondary creep results discussed earlier in this paper, with the Franklin model for Zr-4 operated at same conditions as in IFA-663. The M5 secondary creep rates obtained in Figure 3 with both \(K=1\) and \(K=1.3\) are shown. The M5 secondary creep is linear and starts at about 500 h after loading. At this point in time, the secondary creep rate obtained here equals the slope of the Franklin model.  
* a PWR-type reactor, with a thermal spectrum, oxide fuel, and an M5 cladding (PINCHA card);
* the rod was subdivided in 10 axial slices;
* small-strain approximation concerning the cladding mechanical treatment was applied (ITEOMEC = 1);
* fission gas release was not considered (i.e., FGDIFF = 0);
* the empirical model for LWRs was selected for the fuel densification model (IDENSI = 2);
* Modified FRAPCON-3 model relocation was applied (IRELOC = 8);
* the analysis included both thermal and mechanical analysis (ITEMTE = 0);
* the applied M5 corrosion model was the JRC's by [PERSON] and [PERSON] [15] (ICORRO = 39);  
The applied creep model was the JRC's by [PERSON] and [PERSON] [14], modification 02, date 22 May 2020 (ModClad (7) = 21);
* the cladding failure criterion was based on both cladding instability and overstress criteria (ICLAFIL = 3);
* the assumed initial enrichment of the fuel was 9% and its assumed porosity 5.0%; Initial fuel gap is 225 \(\upmu\)m.  
The simulated hoop stress in the cladding over time was designed to replicate the experimental values according to Ref. [8] and is shown in Figure 6.  
The TRANSURANUS simulation was performed after changing the 'thinning increment' from 2 to 0.1 \(\upmu\)m in the routine 'WeakeningOfClad' as the original version provided some non-physical jumps in the diameter displacement. The simulation results, compared with the experimental data from the HWR-658 test and the described model, are shown in Figure 7.  
The comparison in Figure 7 highlights notable differences between the experimental data, the developed creep model, and the predictions from TRANSURANUS. While the experimental results from the HWR-658 and HWR-755 tests [8, 9] serve as important benchmarks, their reliability is compromised by uncertainties related to instrumentation issues and potential misrepresentation of oxide growth and crud deposition effects. Such limitations, as detailed by [PERSON] [8] and [PERSON] [9], necessitate cautious interpretation of the experimental trends.  
The developed model, which incorporates mechanisms such as secondary creep and the influence of surface oxidation, provides a more comprehensive framework for predicting cladding behavior. Its alignment with steady-state creep trends supports its validity under typical PWR operating conditions. Nonetheless, the slight overestimation of creep deformation in the early stages suggests that refinements may be needed for more accurate modeling of primary creep effects.  
In contrast, the TRANSURANUS predictions underpredict creep deformation, particularly under conditions of elevated temperatures and high neutron flux. While the code offers conservative estimates that are suitable for bounding analyses, it falls short in capturing the nuanced interplay of the factors observed in M5 cladding behavior. Enhancing its creep modeling framework would improve its applicability for precise evaluations of advanced materials like M5.  
Overall, the findings underscore the value of the developed model as a reliable tool for understanding M5 cladding creep behavior under in-pile conditions. They also point to the need for refined experimental methodologies and further development of simulation codes to address gaps in predictive capability.