Pellet-cladding Interaction Failure Research Articles

Atomistic modelling of iodine-oxygen interactions in strained sub-oxides of zirconium

In water reactors, iodine stress corrosion cracking is considered the cause of pellet-cladding interaction failures, but the mechanism and chemistry are debated and the protective effect of oxygen is not understood. Density functional theory calculations were used to investigate the interaction of iodine and oxygen with bulk and surface Zr under applied hydrostatic strain (−2% to +3%) to simulate crack tip conditions in Zr to ZrO2, using a variety of intermediate suboxides (Zr6O, Zr3O, Zr2O and ZrO). The formation energy of an iodine octahedral interstitial in Zr was found to decrease with increasing hydrostatic strain, whilst the energy of an iodine substitutional defect was found to be relatively insensitive to strain. As the oxygen content increased, the formation energy of an iodine interstitial increased from 1.03 eV to 8.61 eV supporting the idea that oxygen has a protective effect. At the same time, a +3% tensile hydrostatic strain caused the iodine interstitial formation energy to decrease more in structures with higher oxygen content: 4.56 eV decrease in ZrO compared to 1.47 eV decrease for pure Zr. Comparison of the substitutional and interstitial energies of iodine, to the adsorption energy of iodine, in the presence of oxygen, shows the substitutional energy of iodine onto a Zr site is more favourable for all strains and even interstitial iodine is favourable between strains of +1-5%. Although substitutional defects are preferred to octahedral interstitial defects, in the ordered suboxides, a 3% tensile strain significantly narrows the energy gap and higher strains could cause interstitial defects to form.

Optimal operation strategy of LWR based on the PCI online prognosis model

Abstract The pellet-cladding interaction (PCI) is one of the major issues in fuel rod during the power change operation in light water reactors (LWR). The frequent power transient will lead to the PCI failure, which results in the radioactive leakage. Unfortunately, the failure cannot be detected on time during operation. For solving this problem, the PCI results is calculated and predicted before the power transient, and the results is used for optimizing the operation strategy. As everyone knows, the calculation of the fuel is a cumbersome work, which is not fit for online evaluation. At first, we present a PCI online prognosis model in this paper by the method of radial basis function neural network (RBFNN). Based on this online model, an optimal operation strategy is present then. The PCI evaluation results is treated as the feedback of the strategy, which will go to the power control system to decide to carry out the power plan or to modify it. The presented operation strategy used the fast online prognosis model to predict the PCI status, which is used as a feedback signal to the power control system. The optimal operation strategy is tested by the experimental data from the references, and the results demonstrate its effectiveness.

Optimal operation strategy of LWR based on the PCI online prognosis model

Abstract The pellet-cladding interaction (PCI) is one of the major issues in fuel rod during the power change operation in light water reactors (LWR). The frequent power transient will lead to the PCI failure, which results in the radioactive leakage. Unfortunately, the failure cannot be detected on time during operation. For solving this problem, the PCI results is calculated and predicted before the power transient, and the results is used for optimizing the operation strategy. As everyone knows, the calculation of the fuel is a cumbersome work, which is not fit for online evaluation. At first, we present a PCI online prognosis model in this paper by the method of radial basis function neural network (RBFNN). Based on this online model, an optimal operation strategy is present then. The PCI evaluation results is treated as the feedback of the strategy, which will go to the power control system to decide to carry out the power plan or to modify it. The presented operation strategy used the fast online prognosis model to predict the PCI status, which is used as a feedback signal to the power control system. The optimal operation strategy is tested by the experimental data from the references, and the results demonstrate its effectiveness.

Effect of power history on pellet-cladding interaction

In this paper, the mechanical pellet-cladding interaction (PCI) is studied under various power histories. We find that the power history significantly affects possible PCI failure. PCI during the thermal transient period of power ramping occurs in such a way that crack-mouth opening increases regardless of when contact is initiated, before or during power ramping. The crack-mouth opening continues to increase further during the thermal steady state (during the power ramp). Conversely, PCI initiated either during the thermal steady-state power ramp or during the normal operation (with no power ramp) leads to decrease in crack-mouth opening. As a consequence, PCI initiated before power ramping or during the transient period of power ramping leads to high local tensile hoop stress. In contrast, PCI initiated during the thermal steady state of power ramping or during the normal operation without power ramping leads to local compressive hoop stress. In the former case, PCI failure near the crack tip might be anticipated (by either brittle failure during the transient power ramp or failure during the steady-state power ramp), whereas in the latter case, it is suppressed.

3D simulation of a power ramp including fuel thermochemistry and oxygen thermodiffusion

This paper presents 3D simulations of power ramps in pressurized water reactors with the fuel performance code ALCYONE, which is part of the computing environment PLEIADES. The code has been upgraded to couple the description of irradiated fuel thermochemistry already available with oxygen transport taking into account thermodiffusion. The impact of oxygen redistribution during a power transient on irradiated fuel thermochemistry in the fuel and on chemically reactive gas release from the fuel (I(g), I2(g), CsI(g), TeI2(g), Cs(g), Cs2(g)) is studied. The simulations show that oxygen redistribution, even if moderate in magnitude, leads to the reduction of metallic oxides (molybdenum dioxide, cesium molybdates, chromium oxide) at the fuel pellet center and consequently to the release of a much greater quantity of gaseous cesium. Pellet-Cladding Interaction failure propensity is shown to decrease in consequence of the thermodynamically favourable reaction of iodine with released cesium in the free volume of the rod.

Co-extrusion of Zircaloy-2 and Zr-Sn alloy for double clad tube manufacturing: Numerical simulation and experimental validation

Zircaloy-2 cladding in boiling water nuclear reactors is prone to brittle failure due to pellet-cladding interaction (PCI). The simultaneous effects of stress corrosion cracking and cladding stresses are the main failure mechanisms which limit the life of fuel clad tubes. Zircaloy-2 clad tube, with the inner surface lined with dilute Zr-0.3 wt %Sn layer is a potential and effective protection against PCI failure. Due to the difference in flow behavior of two dissimilar alloys (softer in the core and harder in the sleeve), the manufacturing of double-clad tubes requires proper selection of process parameters. In the present study, manufacturing process has been developed at actual scale to produce metallurgically bonded Zr-Sn lined Zr-2 double clad. Further, this paper systematically brings out the billet assembly methodology using copper jacketing and vacuum degassing. The constraints present in defect-free metallurgical bonding have been overcome by a series of industrial co-extrusion trials of Zr-Sn and Zr-2 composite billet assembly suggested by FEM-based simulation. The co-extrusion process parameters such as strain rate (ram speed), temperature and co-extrusion ratio were suitably optimized using FEM-based analysis of co-extrusion. The metallurgical bond integrity, microstructure evolution and liner thickness variation were characterized by FEG-SEM. The interface between Zr-2 and Zr-Sn was found to be continuous.

An Out-of-Pile Method to Investigate Iodine-induced SCC of Irradiated Cladding

Pellet Cladding Interaction (PCI) during power changes can be a limiting factor for nuclear reactors operation as it can cause fuel cladding failure by Iodine-induced Stress Corrosion Cracking (ISCC) [1]. In the as-fabricated condition, there is a gap between the fuel cladding and the pellets. During reactor operation when fast power transient changes occur, the fuel experiences power ramp and the pellets expand more than the cladding, such that the gap is reduced and eventually closed. When the fuel and the cladding are in direct contact, further fuel swelling and thermal expansion produces tensile stresses in the cladding. Moreover, pellet cracking occurs causing the stress to increase locally in the cladding at the location of the cracks. ISCC initiates at locations where both the stress and the iodine content locally reach critical levels. The cracking initiates at the inner surface of the cladding and propagates towards the outer surface [2, 3].ISCC can be investigated out of pile by means of an expanding mandrel test technique [4, 5]. In this design of the mandrel test equipment, the fuel pellet is simulated using a ceramic sheath with 4 longitudinal slits to facilitate the transport of iodine inside the cladding, which contains a Zr slug (mandrel) inserted in the center. The Zr slug is axially compressed and expands radially forcing the surrounding ceramic sheath to crack. The ceramic fragments can strain the cladding with further compression of the Zr slug.Different versions of the mandrel technique have previously been used to test unirradiated cladding. However, previous studies show that ISCC is sensitive to the condition of the cladding inner surface as well as the material strength, thus the irradiated cladding is expected to behave differently with respect to ISCC than unirradiated cladding.To evaluate the susceptibility to PCI failure of a given fuel rod, power ramp tests are performed in experimental reactors [2]. However, a good out of pile test to complement single ramp tests is crucial to improve the understanding of the mechanisms behind the PCI failures as well as the effect of different parameters.Within the SCIP program<sup>3<sup/>, in addition to ramp testing, a mandrel testing technique that enables out-of-pile testing of irradiated cladding has therefore been developed. The equipment is designed to investigate critical parameters that are known to affect PCI failures such as: irradiation, the strain rate, pellet performance (expansion and cracking), manufacturing (missing pellet surface) and cladding properties (texture, hardness, liner composition etc) [2].The tests are conducted under carefully controlled environmental conditions. Ar gas (99,99%) is passed through a temperature-controlled bath with iodine crystals (reagent grade 99,8%).The effect of the following parameters can be investigated:- Strain rate,- Iodine concentration,- Oxygen potential,- Temperature,- Different crack pattern of the pellet or missing chip,- Cladding properties.The mandrel equipment at Studsvik is specially designed and installed in a hot cell to enable testing on irradiated cladding. The mandrel test technique has been qualified within SCIP using unirradiated cladding. The effects of stress/strain rate, iodine concentration and oxygen potential, on the ISCC behavior were investigated by mandrel testing coldworked non-liner Zircaloy-2 cladding materials.After qualification of the test equipment, several tests have been performed on irradiated non-liner recrystallized Zircaloy-2 claddings. The tests were performed at a temperature of 320°C and iodine partial pressure in the range of 0 to 200 Pa. The tests were run by controlling the strain rate with an initial fast strain rate (21.6%/h) up to 0.6% strain from the initial diameter followed by a slow strain rate (0.36%/h) up to a strain of 2% and with a holding time of 20 hours or until failure. After the mandrel tests, the samples have been characterized by visual inspection, profilometry, LOM and SEM<sup>4<sup/>.The results of the irradiated recrystallized Zircaloy-2 claddings showed that in the presence of iodine partial pressures in the range of 60 to 200 Pa, ISCC occurred at 0.7-0.78% total (elastic + plastic) diametral strain. The low strain to failure as well as typical ISCC features like fluting, observed at the fractured surfaces, are consistent with previous hot cell examinations of PCI failures, and thus the new expanding mandrel technique seems promising.

A new phenomenological criterion for pellet–cladding interaction rupture

This study is concerned with structural integrity assessment of Pressure Water Reactor's (PWR) fuel rods under pellet–cladding interaction (PCI) loading condition. An important experimental and research cooperative program between EDF, AREVA-NP and the Atomic Energy Commission CEA is achieved in order to get a better understanding of the mechanisms possibly leading to PCI failure, as well as to qualify a PCI resistant rod design. The objectives of this work are: first, to improve the understanding of the pellet mechanical properties impact on cladding local loading with 3D simulations results, and second, to propose a new phenomenological rupture criterion for a better assessment of the failure risk. In this study fuel behaviour modelling under nominal and transient loading conditions is achieved with a multi-dimensional simulation tool called ALCYONE, included in the new fuel software PLEIADES currently co-developed by the CEA and EDF. Cladding loading due to mechanical interaction during power transient stage is first analysed through pellet–cladding interfacial stresses computed in the 3D simulation. Then, a 2D model is proposed in order to establish a correlation between interfacial loading and stress concentration in the cladding. In order to assess the failure risk under PCI a phenomenological criterion based on the membrane circumferential stress in the cladding and shear stresses at pellet–cladding interface is proposed. To compute the shear loading at pellet–cladding interface a new parameter (called W rθ ) is introduced. Based on 3D calculations of PCI, it is shown in this paper that pellet fracture properties can have a significant effect on PCI loading.

3D fuel cracking modelling in pellet cladding mechanical interaction

This study is concerned with the modelling of fuel behaviour and of pellet–cladding interaction (PCI). A new fuel software (PLEIADES) is currently co-developed by the Atomic Energy Commission (CEA) and Electricité de France (EDF). This software includes a multi-dimensional FE program (ALCYONE) devoted to Pressure Water Reactors (PWR) fuel rods. PCI studies are mainly undertaken with the 3D model of ALCYONE. The objectives of this work are twofold: first, to propose a constitutive model for the fuel pellet which accounts for the stress relaxation of the material resulting from cracking and creep, second, to estimate the impact of the pellet cracking on PCI. In this paper, a mathematical formulation which couples a viscoplastic law for creep with a multi-surface plastic softening law for cracking is detailed, leading a two inelastic strains model. Mesh dependency is overcome thanks to a material parameter related to the finite element size. The 3D calculations of PCI presented in this paper show that the considered modelling of fuel cracking is consistent with the experimental knowledge available on crack development under irradiation. A parametric study is then presented which leads to the conclusion that the tangential stresses at the pellet cladding interface and hence the risk of PCI failure are significantly reduced when the fuel tensile strength is divided by two.

Pellet-clad interaction (PCI) failures of zirconium alloy fuel cladding — A review

This review summarizes the history of the appearance and cure of pellet-cladding interaction (PCI) failures during the operation of Zircaloy clad UO 2 fuel in a number of reactors. The work carried out to permit unrestricted operation of reactors without causing PCI failures has led to the universal adoption of the CANLUB-graphite coated cladding in CANDU reactors, and to the wide adoption of Zr-liner cladding in BWRs. There has only been a low incidence of PCI failures in PWR cladding, and the problem has not loomed large enough to require the adoption of either of the above protective methods in these reactors, although experimental liner cladding has been tested. The extensive work on the mechanism of PCI failures (leading to the conclusion that an SCC process induced by fission product iodine is the most probable cause) is summarised.

An Evaluation of the Influence of Fuel Design Parameters and Burnup on Pellet/Cladding Interaction for Boiling Water Reactor Fuel Rod through In-Core Diameter Measurement

The influence of design parameters and burning on pellet/cladding interaction (PCI) of current boiling water reactor fuel rods was studied through in-core diameter measurement. Thinner cladding and a smaller diametral gap enhanced the PCI during startup. At constant power, fuel with SiO/sub 2/ added greatly reduced PCI due to relaxation. The fuel with a small grain size greatly reduced PCI due to densification. Preirradiation of rods up to 23 MWd/kgU caused a large PCI not only in a small gap but also in a large gap rod. Relaxation and permanent deformation was small. In the power increase experiment, one rod experienced PCI failure. The spurt times of coolant radioactivity coincided well with the sudden drop of cladding axial strain and marked crack opening at the rod surface. The estimated hoop stress predicted by FEMAXI-III was 350 MPa at the failure.

軽水炉燃料のペレット‐被覆相互作用 ＩＩ ＢＷＲ型燃料棒

The infuence of bum-up and fuel design parameter on the pellet-cladding interaction (PCI) of 8×8 BWR fuel was studied. Obtained results are as follows: (1)It is revealed that the PCI was enhanced with increase of burn-up and with decrease of diametral gap. (2)At burn-up of 15 MWd/kgU, a rod was subjccted power cycling to 826 times with range of 30-45kW/m with cycling period 20-25min. No failure occurred in the course of experiment. The PCI was elastic and caused repeatedly at every cycle. The magnitude of the PCI decreased with increasing cycles. It was revealed from PIE that there was little rod pressure difference between cycled and nontycled rod. The FP gas release rate between the rods was diffusion and dependent on square root of holding time at power. (3)The PCI failure was dependent on fuel burn-up. The failure threshold of this experiment was equal to or less than that of the rods ramped under HBWR condition. The rod failed by PCI showed little relaxation and accompanied significant reduction of axial elongation at the failure.

In-pile eddy current test on PWR fuel rod failed by pellet-cladding interaction.

It is believed that pellet-cladding in-teraction (PCI) of light water reactor fuel has a potential to cause failure. To understand the PCI induced fuel failure (PCI failure), it is first necessary to understand the phenomenon of microscopic crack initiation and propagation (CIP) which initiates from the internal surfaces of Zircaloy cladding.To date, however, there have been many technical difficulties with direct CIP measurement of an irradiated fuel rod. The Halden Project has recently developed a technique with eddy-current test (ECT) which allows direct measurement of the CIP phenomenon. The use of in-pile ECT technique itself is not unique and has been used widely(1)(9).However, the application of ECT to in-pile experiments to detect CIP phenomenon leading to PCI failure, is new. This paper describes the first results of ECT on in-pile experiments carried out at the Halden Boiling Water Reactor (HBWR).

Comparisons between Measured and FRAP-T4-Predicted Fuel Rod Failure Probability under Pellet-Cladding Interaction Conditions

The results of power ramp tests and associated cladding failure calculations performed as part of FRAP-T4 independent assessment studies have been analyzed. The 90 ramp tests considered are characterized in terms of relevant fuel design and operating parameters for Zircaloy-clad UO/sub 2/ fuel rods. The use of alternative gap closure and fuel deformation models strongly affects results of the current stress-dependent failure probability model. The influence of both mechanical and chemical failure mechanisms can be separated to some extent by interpreting the concurrent effects of both the ramp-induced stress levels and the relative amounts of retained fission gas available for promoting stress corrosion. A relatively simple modeling approach can meet the basic mechanistic requirements for calculating pellet-cladding interaction failure probability.

Observations on Damage Accumulation during Iodine-Induced Stress Corrosion Cracking of Zircaloy Cladding

A predictive model has recently been proposed for pellet-cladding interaction (PCI) failures of water reactor fuel rods. The model is based on the assumption that PCI failures are attributable to stress corrosion cracking (SCC) of the Zircaloy cladding induced by fission product iodine. Because the kinetics of iodine-induced SCC of irradiated Zircaloys are not well-documented at present, the PCI model is based largely on failure-time data from constant stress tube pressurization experiments.