Transactions of the Korean Nuclear Society Virtual Spring Meeting July 9-10, 2020 Fuel Performance Uncertainty to Rod Burst Power in LBLOCA Analysis Joosuk Lee and Young-Seok Bang Korea Institute of Nuclear Safety 62 Gwahak-ro, Yusong-gu, Daejeon, 305-338, Republic of Korea Tel: +82-42-868-0784, Fax: +82-42-868-0045 Email: jslee2@kins.re.kr 1. Introduction 2. Analysis Details 2.1 Burst power analysis condition Recently developed acceptance criteria of emergency APR1400 plant with 16x16 ZIRLO cladding fuel was core cooling system (ECCS) by Korea Institute of used for large-break LOCA safety analysis. Design Nuclear Safety (KINS) has three modeling parameters of fuel rod, operating conditions, and base requirements, and one of the requirements deals with irradiation power history were obtained from Ref. [4]. the consideration of fuel relocation and dispersal during Initial conditions of fuel rod before accident were loss-of-coolant accident (LOCA) [1]. And under certain calculated by FRAPCON-4.0 code [5], and transient conditions, zirconium alloy cladding of fuel rod can be fuel behaviors for a LOCA period were analyzed by the ruptured due to the excessive plastic deformation integrated code of FRAPTRAN-2.0P1 and MARS- during LOCA. And if sufficient amounts of fuel pellet KS1.4. Current available version of integrated code is were dispersed into the core, coolability can be V1129sig. It has additional models to predict the impaired. In this safety concern, KINS has been thermal behavior of fuel rod due to the formation of developing a methodology to predict fuel rod burst in a crud and oxide layer. And features for fuel uncertainty core-wide during LOCA, and to support the regulation analysis are implemented. of this issue [2]. In the methodology, fuel rod power For the LOCA analysis, reactor core in APR1400 before LOCA was used as a measure for the assessment was divided into a hot channel and an average channel, of rod burst. Also uncertainty parameters related to the and a hot rod was allocated in the hot channel. Hot performances of fuel and ECCS were identified. Fuel channel represents single hot assembly. In this study, behaviors by combining those parameters were assessed the same linear heat generation rate (LHGR) was using a statistical method. Through this process, limit imposed on both the hot rod and hot assembly. This curves of power to burst were derived, and fraction of means that each rod in the hot assembly has the same fuel rod burst in APR1400 during LOCA was evaluated LHGR. But during this process total reactor power was preliminarily. maintained by adjusting the power of average channel. But, authors’ previous work has some limitations. Top-skewed cosine shape power profile was used in the One of them is that the curves and sensitivity analysis analysis. results were produced with the FRAPTRAN standalone code with the fixed thermal-hydraulic boundary 2.2 Considered factors and assessment conditions for the selected hot assembly. As a result, For the cladding burst assessment, two different thermal-hydraulic conditions that do not reflect the cladding burst criteria are used. One is a well-known actual conditions were used, which may lead to less strain-based NUREG-0630 fast ramp criterion [7] and accurate predictions. Thereby, assessment of rod burst the other is a stress-based rupture criterion, which is power and sensitivity analysis by considering the actual modeled in FRAPTRAN. Two different cladding system thermal-hydraulic behaviors is strongly required. deformation models are also used. One is FRACAS-I Meanwhile, as a part of audit methodology model and the other is BALON2 model. Details of development program for the proposed ECCS rule these models are described in the ref. 6. Analyzed cases revision in Korea, KINS has been developing an with given condition are listed in Table 1. Burst curves integrated code between US Nuclear Regulatory were developed with fuel burnup from 0 to 70 Commission (NRC) fuel performance code, MWd/kgU. FRAPTRAN and system thermal-hydraulic code, MARS-KS [3]. Table 1. Analysis condition for burst power in LOCA. In this paper, best-estimate power to burst curve Case # Deformation LHGR Hot rod was estimated with the integrated code of FRAPTRAN Burst criteria model LHGR Hot assembly and MARS. And effects of fuel burst criteria and 1 NUREG-0630 BALON2 deformation model on the burst curve were also 1.0 2 FRAPTRAN assessed. Accordingly, impacts of fuel performance 3 FRACAS-I NUREG-0630 uncertainty and combined uncertainty to the burst Ref. BALON2 Fixed hot assembly LHGR (12.74kW/ft) power were re-evaluated.
Transactions of the Korean Nuclear Society Virtual Spring Meeting July 9-10, 2020 Fig. 1. Best-estimate required peak fuel power for rod burst as a function of fuel burnup with changing cladding burst criteria and deformation model The authors have identified uncertainty parameters, Burst criteria change from strain-based NUREG- such as related to the fuel rod manufacturing, to the 0630 to stress-based one in BALON2 model do not models of computer code and thermal-hydraulics [2]. give any meaningful differences on burst power. As can Among them, 10 and 26 parameters for manufacturing be seen in Fig. 1, the burst power derived from stress- and for models of fuel rod chosen in this study. They based criterion (case 2) is almost same as the strain- are listed in Table 1. Impacts of these parameters to the based ones (case 1). This is due to the characteristics of burst power were assessed at 0 and 60 MWd/kgU fuel ballooning and burst process of the BALON2 model. burnup. Root sum squared (RSS) method was used for Typically, when the BALON2 model was activated and the assessment of combined uncertainty. deformation proceeded, the required time to reach the cladding failure strain or failure stress is very short, 3. Results and Discussion such as less than about 1~2 s. Meanwhile, changing of cladding deformation model 3.1 Required fuel power for rod burst from BALON2 to FRACAS-I gives some differences. Fig. 1 shows a peak LHGR that is required to the rod As can be seen in Fig. 1, when the FRACAS-I model burst based on the best estimate values of the was activated with the strain failure criterion (case 3), parameters as a function of fuel burnup. Here, the ‘best the required power was about 0.4~1.0 kW/ft higher estimate’ means that the calculation was made without than the BALON2 model cases (case 1, 2). This implies any tolerance or bias, listed in Table 1. As BALON2 the selection of deformation model for burst prediction deformation model and NUREG-0630 strain failure is important in the integrated code. criterion were activated (case 1), the required burst power at 0 MWd/kgU was 12.2 kW/ft, and as burnup 3.2 Influencing factors to rod burst moved to 10 MWd/kgU, it was increased to 13.4 kw/ft. Table 2 shows the changes of required peak power for rod burst ( ∆ P _burst ). These changes are assessed However, the burnup moved further from 10 to 70 MWd/kgU, it was reduced slowly and continuously based on the case 1 condition, listed in Table 1. In until reaching to 11.5 kW/ft. Such a burst power general, manufacturing uncertainties revealed a small evolution behavior is generally similar with the effect to the burst power, such as less than 0.9 kW/ft. previous work [2], as shown in Fig. 1, with a little Cladding inner diameter has induced 0.9 kW/ft at zero deviation. At fresh fuel condition, the required power is burnup. similar between two cases, but burnup increased to 10 In model uncertainties, fuel thermal conductivity, MWd/kgU, the integrated code shows lower burst fission gas release (FGR), cladding yield stress showed power than the Ref. case. And above that burnup the a relatively strong influence. Fuel thermal conductivity difference is gradually reduced, and finally even higher has induced 0.4 and 2.1 kW/ft at 0 and 60 MWd/kgU. fuel burst power is attained above 50 MWd/kgU. These At fresh fuel, FGR has no influence, but as burnup are clearly caused by the difference of hot assembly moved to 60 MWd/kgU, its impact intensified such as LHGR, which in turns affects the thermal-hydraulic 1.5 kW/ft power change. Cladding yield stress showed conditions in the assembly. 0.9 and 1.5 kW/ft changes at 0 and 60 MWd/kgU,
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