Elsevier

Annals of Nuclear Energy

Volume 74, December 2014, Pages 12-23
Annals of Nuclear Energy

SARNET2 benchmark on air ingress experiments QUENCH-10, -16

https://doi.org/10.1016/j.anucene.2014.05.013Get rights and content

Highlights

  • Two similar QUENCH air ingress experiments were analysed with eight different codes.

  • Eight institutions have participated in the study.

  • Differences in the code were mostly small to moderate during the pre-oxidation.

  • Differences in the code were larger during the air phase.

  • Study has proven that there are physical processes that should be further studied.

Abstract

The QUENCH-10 (Q-10) and QUENCH-16 (Q-16) experiments were chosen as a SARNET2 code benchmark (SARNET2-COOL-D5.4) exercise to assess the status of modelling air ingress sequences and to compare the capabilities of the various codes used for accident analyses, specifically ATHLET-CD (GRS and RUB), ICARE-CATHARE (IRSN), MAAP (EDF), MELCOR (INRNE and PSI), SOCRAT (IBRAE), and RELAP/SCDAPSim (PSI).

Both experiments addressed air ingress into an overheated core following earlier partial oxidation in steam. Q-10 was performed with extensive preoxidation, moderate/high air flow rate and high temperatures at onset of reflood (max Tpct = 2200 K), while Q-16 was performed with limited preoxidation, low air flow rate and relative low temperatures at reflood initiation (max Tpct = 1870 K). Variables relating to the major signatures (thermal response, hydrogen generation, oxide layer development, oxygen and nitrogen consumption and reflood behaviour) were compared globally and/or at selected locations. In each simulation, the same input models and assumptions are used for both experiments, differing only in respect of the boundary conditions. However, some slight idealisations were made to the assumed boundary conditions in order to avoid ambiguities in the code-to-code comparisons; in this way, it was possible to focus more easily on the key phenomena and hence make the results of the exercise more transparent. Remarks are made concerning the capability of physical modelling within the codes, description of the experiment facility and test conduct as specified in the code input, and code limitations that might warrant additional research to support model improvements, especially the modelling of nitride formation and melt oxidation.

Introduction

Air ingress issues, first raised by Powers et al. (1994), have received considerable attention in recent years in view of the likely acceleration in the cladding oxidation, fuel rod degradation, and the release of some fission products, most notably ruthenium. This last issue is being addressed within the SARNET2 Source Term Work Package. In addition, the Paks NPP cleaning tank incident and the accident at Fukushima Daiichi drew attention to the possibility of overheated fuel assemblies becoming exposed to air outside of the reactor. As a consequence, air ingress is the subject of recent and continuing multinational efforts, notably within the European Commission (EC) 5th and 6th Framework Programme projects 5th Framework Programme,1998-2002, 6th Framework Programme, 2002-2006, the MOZART experiments (Clement, 2002) within the ongoing International Source Term Programme (ISTP) Clement and Zeyen, 2005, numerous investigations at KIT (Steinbrück, 2007, Steinbrueck, 2009) AEKI (Matus et al., 2008). Elsewhere, ANL performed a large programme of experiments (Natesan and Soppet, 2004) and the OECD Sandia Fuel Project (OECD/NEA Sandia Fuel Project (SFP), 2009) has recently been completed. A number of integral air ingress experiments have been performed under a range of configurations and oxidising conditions, namely AIT-1, AIT-2 (Hózer et al., 2003), QUENCH-10 (Q-10) Stuckert et al., 2004, Steinbrueck et al., 2006, QUENCH-16 (Q-16) Stuckert and Steinbrueck, 2014, Stuckert et al., 2013 and PARAMETER SF4 (International Science and Technology Centre, 2009).

The accumulated data have demonstrated that air oxidation is a remarkably complicated phenomenon governed by numerous processes whose role can depend critically on the oxidising conditions, the past oxidation history and the cladding material (e.g. alloying elements, geometry, manufacturing process). The knowledge and models for air oxidation do not yet cover the whole range of representative conditions. The post-test analyses of integral experiments and the safety analyses of severe accidents with air ingress scenarios showed that fast oxidation and temperature excursion can be initiated by relatively small effects in high temperature air. From both scientific and accident management points of view, it should be identified under which conditions a temperature excursion could be prevented and the damage of bundle could be limited in air ingress scenarios. The main aims of Q-10 and -16 were to investigate areas where data were comparatively sparse.

The Q-10 and -16 tests were proposed in the frame of the EC-sponsored LACOMERA (Miassoedov et al., 2006) and LACOMECO programmes (Miassoedov, 2009) as part of the collective investigation of air ingress into overheated nuclear fuel assemblies. Both experiments were proposed and supported by AEKI and received additional planning support by participating institutes within the European Framework Programmes.

The experiments focussed specifically on following phenomena:

  • Air oxidation after pre-oxidation in steam.

  • Air oxidation at increasing temperatures and possible transition from slow to rapid oxidation and temperature excursion.

  • Role of nitrogen under conditions of oxygen availability and starvation.

  • Formation of oxide and nitride layers on the surface of Zr.

  • Reflooding of oxidised and nitrided bundle by water, release of nitrogen.

  • Release of hydrogen during reflood.

  • Cooling achieved during reflood.

The above scientific objectives were defined in the proposed scenarios and agreed by the LACOMERA and LACOMECO experiment selection panels. The proposals included target scenarios characterised by the desired conditions during air ingress.

Q-10: the response to air ingress following significant preoxidation (up to 50% of cladding thickness) in steam with progressive consumption of oxygen at high air flow rate leading to possible oxygen starvation.

Q-16: the response to air ingress following limited preoxidation (<20% of cladding thickness) in steam with progressive consumption of oxygen at low air flow rate with a long period of oxygen starvation to promote the occurrence of possible zirconium-nitrogen reaction.

Q-10 was performed by KIT on 21 July 2004 and Q-16 on 27 July 2011, according to agreed test protocols following discussions among the participants. The results provided data on air ingress scenarios under contrasting conditions and hence offer an excellent opportunity to assess the modelling capabilities of reactor accident analysis codes under challenging conditions. The present benchmark exercise (SARNET2-COOL-D5.4) follows through on this opportunity. Table 1 summarises the main boundary conditions for Q-10 and Q-16 for each test phase as well as the main times and temperatures.

The aim of the benchmark was to assess the status of modelling air ingress sequences and to compare the capabilities of the various codes used for accident analyses to represent these sequences. The experimental boundary conditions were slightly idealised in the interests of clarity of code to code comparison and to focus on modelling of the most important aspects of the phenomenology. This idealisation could certainly be expected to have modified the results. However, separate simulations show that while the magnitudes of quantities were incrementally altered, the phenomenology was unaltered. In QUENCH-16 therefore, the minor fluctuations in boundary conditions, the shroud failure and the presence of steam during the air phase were not included. For the Q-10, the experimental values were used for the steam phase, but for the air phase constant power was used. The reason was to avoid the complication of a power transient superimposed on the thermal and oxidation transient and hence facilitate interpretation of the phenomenology. This will cause a slightly longer transient. Therefore, the reflood was set to start when a predefined maximum temperature was reached. The departure from the actual conditions meant that the code models could not be completely judged on the basis of quantitative comparison with the experimental results. This is judged to have been a necessary compromise to avoid the practical difficulties, both in performing the simulations with the exact conditions and also comparing the results. Finally, it was proposed to start with the Q-16 calculation and after completion to perform the Q-10 by using the same physical models and just changing the boundary conditions. In this way, it would be possible to assess how the modelling extends to different conditions.

Section snippets

General features of the participant codes

A general description of the codes that participated in the benchmark exercise is given below.

Q-16. results

The results are summarised in Table 3. The analysis of the benchmark results with the different codes/versions shows a very wide variety of the pre-oxidation results. EDF, GRS, INRNE and PSI calculated a closer agreement with the measured hydrogen production during this phase. IBRAE, IRSN and RUB calculated lower pre-oxidation. The axial temperatures follow a similar trend. The majority of the codes overpredicted the oxygen starvation period, they calculated fully oxygen consumption for about 30

Conclusions

The analysis of the two similar quench air ingress experiments with eight different codes and the collaboration of eight institutes have improved the understanding of cladding oxidation by air and hydrogen generation during the reflood. The quench air ingress experiments confirmed the different behaviour of oxidation in steam and air environments. All the calculations successfully predicted the entire sequence from initial heat-up to final quench of both experiments, using input models for the

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