Loss Of Coolant Accident Scenario Research Articles

Fretting Wear Behavior of APMT Steel at 350°C for Reactor Fuel Cladding Application

ABSTRACTIn support of a recent surge in research to develop an accident tolerant reactor, accident tolerant fuels and cladding candidates are being investigated. Relative motion between the fuel rods and fuel assembly spacer grids can lead to excessive fuel rod wear and, in some cases, to fuel rod failure. Based on industry data, grid-to-rod-fretting (GTRF) has been the number one cause of fuel failures within the U.S. pressurized water reactor (PWR) fleet, accounting for more than 70% of all PWR leaking fuel assemblies. APMT, an Fe-Cr-Al steel alloy, is being examined for the I2S-LWR project as a possible alternative to conventional fuel cladding in a nuclear reactor due to its favorable performance under LOCA conditions. Tests were performed to examine the reliability of the cladding candidate under simulated fretting conditions of a pressurized water reactor (PWR). The contact is simulated with a rectangular and a cylindrical specimen over a line contact area. A combination of SEM analysis and wear & work rate calculations are performed on the samples to determine their performance and wear under fretting. While APMT can perform favorably in loss of coolant accident scenarios, it also needs to perform well when compared to Zircaloy-4 with respect to fretting wear.

Impact of Pressure Relief Holes on Core Coolability for a PWR during a Large-Break Loss-of-Coolant Accident with Core Blockage Using RELAP5-3D

During a loss-of-coolant accident (LOCA), fibrous debris and other particles generated by the jet impingement may be transported to the sump, accumulate, or even penetrate through the strainers, reaching the reactor core. Pressure relief holes and other plant-specific features may provide alternative paths to the coolant under debris-generated core blockage scenarios and can play a major role in core coolability. A typical four-loop pressurized water reactor was modeled using RELAP5-3D to simulate the reactor system response during large-break LOCA scenarios under hypothetical full core blockage conditions. Pressure relief holes were included in the input model to study the effects of these alternative flow paths on the core coolability. The comparison of the simulation results obtained with two different models (with and without pressure relief holes) proved the effectiveness of these alternative flow paths in providing sufficient flow to the core to remove the decay heat during the long-term cooling phase, maintaining the cladding temperature sufficiently below the safety limits at any time after the core blockage occurred. The results presented in this paper not only confirmed the importance of including specific geometric features of the reactor system (generally neglected) when simulating core blockage scenarios but also provided evidence that even under certain extreme core blockage conditions, core coolability may still be guaranteed.

Experimental study of pressure drops through LOCA-generated debris deposited on a fuel assembly

Under post loss-of-coolant accident (LOCA) conditions, it is postulated that debris can be generated and transported to the containment sump strainer. Some of the debris may pass through the strainer and could challenge the long-term core cooling capability of the plant. To address this safety issue, in-vessel downstream effect tests for the advanced power reactor (APR) 1400 were performed. Fibrous debris is the most crucial material in terms of causing pressure drops, and was prepared in this study to satisfy the fiber length distribution obtained through a strainer bypass test. Sensitivity studies on pressure drops through LOCA-generated debris deposited on a fuel assembly were performed to evaluate the effects of water chemistry and fiber length distribution. The pressure drops with debris laden pure water were substantially less than those with debris laden ordinary tap water. The experiment with fiber length distribution suggested by WCAP-16793 showed lower pressure drops than those with the APR1400 specific fiber length distribution. All the experimental results showed that the pressure drops in the mock-up fuel assembly were less than the available driving head at each LOCA scenario.

A dynamic event tree informed approach to probabilistic accident sequence modeling: Dynamics and variabilities in medium LOCA

In Probability Safety Assessments, accident scenario dynamics are addressed in the accident sequence analysis task. In an analyst-driven, iterative process, assumptions are made about equipment responses and operator actions and simulations of the scenario evolution are performed. To calculate how scenario dynamics and stochastic variabilities may affect the results of this process in terms of estimated risk, this work applies Dynamic Event Trees (DETs) to more comprehensively examine the accident scenario space. Alternative event tree models are developed and the core damage frequency is quantified to reveal the effects of different delineations of the sequences and of the bounding assumptions underlying success criteria. The results from a case study on Medium-break Loss of Coolant Accident scenarios in a Pressurized Water Reactor are presented, considering the break size, available injection trains, and the timing of rapid cooldown and the switchover to recirculation. The results show not only that estimated risk can be very sensitive to the numerous assumptions made in current accident sequence analysis but also that bounding assumptions do not always result in conservative risk estimates, thereby confirming the benefits that DETs provide in terms of characterizing scenario dynamics.

Development of Virtual Sensor Networks to Support Accident Monitoring Systems


 
 
 In a nuclear power plant (NPP), most of the systems are linked due to processes of fluid flow, heat transfer etc., and their natural tendency to respond to changes during accident conditions. These relationships can be utilized to develop smart applications for plant accident monitoring and management. In this research, the statistical relationships among the process parameters have been analyzed. It has been embarked that the characteristics of a safety system during a particular interval can be estimated by utilizing the other affected parameters, employing statistical correlation and regression models developed from the simulation data offline, when evaluated for the same set of conditions on accident sequence and safety systems. The proposed methodology has been demonstrated for a specific loss of coolant accident scenario using correlation coefficient and neural networks, for the time interval when containment spray system was initiated at the particular stage of accident progression and remained operational for some designed time. Virtual sensor networks were constructed for the estimation of reactor vessel level during that time period, which demonstrates the realization of methodology. The estimations from such virtual sensor networks are expected to improve by utilizing the importance measures and concepts to generalize the neural networks. Also, correlation voting index (CVI) provides a capability to select a set of related outputs, which would be used as a yardstick for comparing results in case, missing or uncertain inputs are present.
 
 

Correlation analysis between team communication characteristics and frequency of inappropriate communications

The characteristics of team communications are important since large process systems such as nuclear power plants, airline, and railways are operated by operating teams. In such situation, inappropriate communications can cause a lack of situational information and lead to serious consequences for the systems. As a result, the communication characteristics of operating teams should be understood in order to extract meaningful insights to address the nature of inappropriate communications. The purpose of this study was to develop a method to evaluate the characteristics of team communications based on social network analysis and compare them with the frequency of inappropriate communications. In order to perform the analysis, verbal protocol data, which were audio-visual recorded under training sessions by operating teams, were used and interfacing system loss of coolant accident scenarios were selected. As a result of the study, it was found that the frequency of inappropriate communications decreased when more operators perform the same types of role as other operators, since they can easily and effectively back up each other. Also, the frequency of inappropriate communication is decreased for teams which provide a relatively large communication content that acknowledge or confirm another communication content.

The Integral Test Facility Karlstein

The Integral Test Facility Karlstein (INKA) test facility was designed and erected to test the performance of the passive safety systems of KERENA, the new AREVA Boiling Water Reactor design. The experimental program included single component/system tests of the Emergency Condenser, the Containment Cooling Condenser and the Passive Core Flooding System. Integral system tests, including also the Passive Pressure Pulse Transmitter, will be performed to simulate transients and Loss of Coolant Accident scenarios at the test facility. The INKA test facility represents the KERENA Containment with a volume scaling of 1 : 24. Component heights and levels are in full scale. The reactor pressure vessel is simulated by the accumulator vessel of the large valve test facility of Karlstein—a vessel with a design pressure of 11 MPa and a storage capacity of 125 m3. The vessel is fed by a benson boiler with a maximum power supply of 22 MW. The INKA multi compartment pressure suppression Containment meets the requirements of modern and existing BWR designs. As a result of the large power supply at the facility, INKA is capable of simulating various accident scenarios, including a full train of passive systems, starting with the initiating event—for example pipe rupture.

Quantification and local distribution of hydrogen within Zircaloy-4 PWR nuclear fuel cladding tubes at the nuclear microprobe of the Pierre Süe Laboratory from μ-ERDA

Hydrogen content and its distribution in in-core materials of nuclear plants are known to have a strong influence on their behaviour, especially on their mechanical properties but also on their corrosion resistance. This point has to be largely investigated in the case of the nuclear fuel cladding (Zr based alloys) of pressurized water reactors (PWR).Two situations have been considered here, with regards to the hydrogen content and its spatial distribution within the thickness of the tubes:(1)Irradiated fuel cladding tubes after a nominal period under working conditions in a PWR core.(2)Non-irradiated fuel cladding previously exposed to conditions representative of an hypothetical “loss of coolant accident” scenario (LOCA).As far as micrometric distributions of H were required, μ-ERDA has been performed at the nuclear microprobe of the Pierre Süe Laboratory. This facility is fitted with two beam lines. In the first one, used for non-active sample analysis, the μ-ERDA configuration has been improved to reduce the limits of detection and the reliability of the results. The second one offers the unique feature of being dedicated to radioactive samples. We will present the nuclear microprobe and emphasize on the μ-ERDA configuration of the two beam lines. We will illustrate the performance of the setup by describing the results obtained for Zircaloy-4 cladding both on non-irradiated and irradiated samples.

Simulation and Analysis of Experimental Blowdown and Small-Break Loss-of-Coolant Scenarios with RETRAN-3D

RETRAN-3D, a system analysis code currently employed by the nuclear industry in studies covering a wide variety of operational and accident scenarios, has not been extensively validated for application to loss-of-coolant accident (LOCA) scenarios.The results of the in-depth analysis of two experimental loss-of-coolant transients, namely, Test No. 9 in the French OMEGA facility, and the International Standard Problem 26 (ISP-26) in the Japanese ROSA-IV Facility are discussed. The OMEGA test simulated the blowdown phase of a double-ended cold-leg break, whereas the ISP-26 test simulated a small break (5%) in a full height, volume (1/48), and power (~1/342) scaled facility representing a typical two (or four)-loop pressurized water reactor (PWR) system.The RETRAN-3D results for the OMEGA test show good estimates of the important system parameters, with the best agreement corresponding to the use of the dynamic-slip flow model. A sensitivity analysis on the break flow showed that the Henry/Fauske-Isoenthalpic Expansion critical flow model yields the best results, which are significantly improved with a refined nodalization upstream of the break.The ISP-26 was also simulated using the dynamic-slip flow model. The results indicate that the code is able to calculate a small-break LOCA with a model including the main PWR system components and to reproduce the principal physical processes in a reasonable manner.In summary, this assessment shows the ability of RETRAN-3D to model LOCA scenarios in a reasonable way and also points to areas where further model improvement could result in more accurate simulations.

Short-Term Hydrogen Production Issues for Iter

The possibility of a beryllium-steam reaction during severe accidents in the International Thermonuclear Experimental Reactor (ITER) is a safety concern because the hydrogen produced from this reaction could pose a flammability or detonation hazard. The physical mechanisms governing the production of hydrogen are examined, and the sequence of events during a postulated ex-vessel loss-of-coolant accident (LOCA) are presented. A MELCOR simulation of an ex-vessel LOCA with simultaneous failure of the plasma shutdown system indicates that an in-vessel breach of the coolant system occurs because of first-wall melt-through. For the ITER interim first-wall/shield-blanket (FW/SB) design, this accident results in ∼67 kg of hydrogen being produced. A similar simulation for the divertor predicts only 0.3 kg of hydrogen because of additional cooling experienced by the divertor during the blowdown of coolant into the vacuum vessel. There is evidence to indicate that beryllium evaporation from the first wall at a surface temperature of 1100°C is enough to cause plasma termination through beryllium evaporation. This plasma termination occurs prior to first-wall melt-through and could minimize or eliminate significant hydrogen production. Sensitivity studies were performed by varying the first-wall temperature at which plasma termination and in-vessel breach occurs for an ex-vessel LOCA scenario. This study shows that if the plasma is terminated before 150 s (i.e., a maximum first-wall temperature of 777°C) after the ex-vessel LOCA, the amount of hydrogen generated is ∼1 kg, which is well below the flammability limit of 10 kg and gives a reasonable margin for model uncertainty. Other sensitivity studies using the FW/SB model indicated a relatively weak dependence of the hydrogen produced on in-vessel and ex-vessel breach size. In addition, a 60% reduction in coolant inventory resulted in only a one-third decrease in hydrogen production from the base case. Preliminary calculations for an in-vessel LOCA indicate that 100 kg of 50-µm dust in the vacuum vessel could generate 2 kg of hydrogen.