Experimental investigation on the causes for pellet fragmentation under LOCA conditions

Abstract

The study of the Loss Of Coolant Accident (LOCA) in a nuclear reactor has been under investigation since the beginning of the nuclear civil technology. The latest research revealed several new phenomena associated with fuel rod behaviour and the work presented in this dissertation is an experimental research focused on high burnup fuel fragmentation during a LOCA. The research primary objectives are: 1) the design and execution of a LOCA single effects tests to be performed in hot cell environment; 2) the study of fuel fragmentation and fragment size distribution under selected boundary conditions; 3) the study of the fission gas release and 4) the development of a semi-empirical correlation to obtain a first order description of the resulting fragments size distribution. The experimental test was designed based on a comprehensive review of past research LOCA tests and observations on fuel fragmentation. A particular attention was dedicated to the study of the IFA-650 LOCA test series performed at the OECD Halden Reactor Project. The experiment employed the use of Light Water Reactor (LWR) irradiated fuel with a burnup of 52 MWd/kgU and the heat-up was simulated with heat provided by an electric oven. The selected boundary conditions were: Rod segment 1: removal of the constraint provided by the cladding (rod ballooning without burst); Rod segment 2: removal of the constraint provided by the cladding and by the rod internal pressure (rod ballooning with burst); Rod segment 3: retain the constraint provided by the cladding (no rod ballooning). The parameters of the rod segments required to obtain the above mentioned boundary conditions were calculated with the fuel performance code TRANSURANUS. The experiment was successfully performed with the three selected boundary conditions achieved. The post experiment investigations included non-destructive investigation such as neutron radiography, visual inspection and axial diameter measurements by profilometry and destructive investigations such as ceramography of an archive pellet, puncturing, fragments sifting and analysis of the fission gas release. The neutron radiography showed a fragmentation of a limited number of pellets in correspondence to the burst region of rod segment 2 while the rod segment 1 that only ballooned presented cracks along the entire length. The rod segment 3 was shown to be mostly intact. The fission gas release during the transient was higher for the rod segment 2 (7.3%) followed by the rod segment 1 (6%) and rod segment 3 (3.5%). An effort was made to find a law to reproduce the fragment size distribution measured by sifting following the LOCA test. A fractal law correlation was derived starting from the study on the compressing mechanism behavior of concrete. The fractal coefficients were determined as a function of the fuel burnup, and of the individual contribution of the effects studied in this work: the cladding strain and the burst pressure. The experiment's results and the first modeling approach provide new insights and a be...

Modelling of fine fragmentation and fission gas release of UO2 fuel in accident conditions

“…This conclusion has been confirmed by out-of-reactor heating tests, in which pulverization and TFGR were measured for UO 2 fuel rodlets that did or did not experience cladding ballooning and burst during the test [30,31]. The results indicate that pulverization and fission gas release is enhanced in rods that experience cladding ballooning, and further enhanced in rods that also experience cladding burst with a sudden drop in internal gas pressure [30,31]. It is believed that the sudden loss of fuel mechanical constraint that the cladding ballooning and burst brings about helps trigger the fine fragmentation.…”

“…A pressure of about 50 MPa was reported to suppress pulverization, which means that mechanical constraint from pellet-cladding interaction may limit pulverization and transient fission gas release in high burnup LWR fuel rods [28,29]. This conclusion has been confirmed by out-of-reactor heating tests, in which pulverization and TFGR were measured for UO 2 fuel rodlets that did or did not experience cladding ballooning and burst during the test [30,31]. The results indicate that pulverization and fission gas release is enhanced in rods that experience cladding ballooning, and further enhanced in rods that also experience cladding burst with a sudden drop in internal gas pressure [30,31].…”

Summary of BISON Development and Validation Activities - NEAMS FY16 Report

“…• MTA-EK separate effects tests PUZRY [8] • EON segment 2 [9] • Halden integral fuel rod test IFA-650.2 [10] • Halden integral fuel rod test IFA-650.10 [11] In addition, the REBEKA separate effects tests [12] of cladding ballooning and burst under LOCA conditions were analyzed with BISON for a more extensive code assessment. Some of the main BISON results for the simulation of the FUMAC priority cases are presented hereinafter.…”

The role of nuclear thermal-hydraulics in the licensing of Atucha-II: The LBLOCA

“…After reflood no further mechanisms for fuel damage are expected: therefore nuclear fuel performance during LBLOCA is depicted hereafter by considering a list of physical processes causing degradation or rupture of fuel pins, namely occurring at high temperature and low coolant pressure:  H2 production following the Zircaloy-water chemical reaction discussed in section 3.3. Ballooning of a few or of several rods,Ammirabile & Walker, 2014: this causes obstruction to the coolant channels and interacts with QF progression. Burst (following ballooning) and fission gas release,Pontillon et al, 2001: the burst, following fuel fragmentation and relocation (see below) may cause release of long lived solid fission products into the coolant in addition to non-condensable gases. Fuel relocation,Kim et al, 2017, into the ballooned region which causes increase in local decay power production. This is preceded, depending upon burn-up, by fuel fragmentation (or even pulverization), Brankov, 2017, see alsoBianco et al, 2015. During the last two decades experimental research brought to better understanding of nuclear fuel weaknesses following in-core operation: high burnup is mainly concerned although"…fragmentation appears to almost always occur, regardless of burnup …" (reference is made here to the current US licensing limit of 62 MWd/Ton U), Raynaud, 2012. The weaknesses can be classified into three broad categories, synthesized by D'Auria et al, 2019: (a) Pellet Clad Mechanical and Chemical Interaction (PCMI and PCCI), noticeably involving fuel swelling and cracking associated with core power ramps and reactivity excursions, e.g.…”