An improved method for hydrogen deflagration to detonation transition prediction under severe accidents in nuclear power plants
Introduction
The containment building in commercial Nuclear Power Plants (NPPs) that performs a confinement function is the last barrier of defense-in-depth strategy to prevent the release of radioactive substances. Its integrity combined with other containment system components, such as fuel cladding and reactor vessel under both normal operation and severe nuclear reactor accidents play a crucial role in controlling fission products [1], [2]. Studies of large-scale hydrogen detonations inside containment building have been a highlighted aspect of reactor safety design since it is the primary threat that could breach the structure. Resulting from the oxidation of Zircaloy fuel rod cladding by high-temperature steam in the case of loss-of-coolant accidents in light water reactors, the combustion or explosion of hydrogen and ensuing dramatic pressure spike may exceed design pressure of the containment building and give rise to confinement failure [2], [3], [4]. According to an analysis performed by Yanez, J., et al. (2015), the devastating explosion that occurred in Fukushima-Daiichi Unit 1 reactor on March 12th, 2011 involved amount of 130-kg hydrogen, as the product of various chemical reactions following the dearth of cooling water [5].
A number of studies have developed and assessed hydrogen risk management systems for nuclear power plants, such as hard venting containment system, passive autocatalytic recombiners (PARs) and igniters. These devices are specially designed for mitigating hydrogen accumulation in reactor containment before its concentration could reach a flammable level during Severe Accidents (SAs) [2]. Hydrogen passive recombination is a perspective technical solution and PARs have decades of commercial practices in the nuclear energy industry.
Inside a recombiner, plates glazed with special catalyst form flow channels on which hydrogen and oxygen molecules react and produce steam and heat [6], [7]. The presence of PARs changes mole fraction of oxygen in the containment air. The combustible gaseous mixture that primarily consists of hydrogen, steam, and air, in which the decrease of oxidant proportion sees a concomitant proportion increase of inert gas - nitrogen; that is to say, the hypothetical hydrogen burning or explosion would occur under inerting condition.
Computational codes that perform simulations and analyses of this combustible mixture often use simplified combustion limits as shown on the Shapiro diagram at 1 bar (Fig. 1) [8]. Each point on the diagram represents a mixture's composition, the flammability of which could be determined by position on the diagram. Because the combustion types are disparate in mechanisms, various computational codes are applied to different regions on the Shapiro diagram. Computational Fluid Dynamics tools such as ANSYS CFX, COM3D and REACFLOW are applicable to hydrogen deflagrations starting from moderate ignition [9]. Detonation simulations require specialized codes such as DET3D and DEST that were developed primarily for the applications in the nuclear energy industry [10]. The occurrence of deflagration to detonation transition can be determined by using certain criteria. There is an unstated assumption in the flammable limits that the ratio of oxygen and nitrogen in the dry air is unchanged. This assumption, however as discussed above, might not hold, while PARs are functioning during SAs in containment buildings as the ratio declines.
The presented study proposed a new method that improved the accuracy of nuclear safety analysis codes, since some cases in which the old method may yield erroneous and unreasonable results.
The HYDRAGON code, a three-dimensional computational fluid dynamics computer program, has been developed specially for hydrogen risk analysis and prediction in NPPs [11], [12], [13], [14]. The code was employed for evaluating the hydrogen combustion phenomena using both the old and the new method. In order to corroborate the viability of this new method, comparisons of results stimulated by the HYDRAGON code and experimental data covering a wide spectrum of experimental configurations were demonstrated.
Section snippets
Theory of hydrogen risk criteria
Hydrogen combustion types depend on a variety of parameters, such as temperature, pressure, turbulence, and composition of gaseous mixtures. The myriad combinations of these parameters engender a wide spectrum of combustion regimes ranging from slow flames to violent detonation [2], [15], [16]. Even begin with a weak ignition, combustion waves could accelerate and experience the transition to fast combustion or detonation.
For a flammable mixture of hydrogen, air, and steam starting spontaneous
Improved DDT prediction method and hydrogen mitigation
According to a report issued by OECD/NEA in 2002, as one of major hydrogen mitigation measures inside containment building of large pressurized water reactors (PWRs), the implementation of PARs has been extended particularly in western European countries [25]. For instance, all containments in French PWRs are equipped with PARs having hydrogen removal capacity of around 110 kg per hour at 4% hydrogen [20]. Despite the variety of PARs in size and shape, the working principle for PARs is almost
Validation of the new method
With the purpose of validating the proposed modification on original λ criterion correlation, the HYDRAGON code that has been developed specially for the containment's hydrogen analysis was employed. The HYDRAGON is a three-dimensional computational fluid dynamics code that has the ability to predict the flowing and distribution of compressible gases. It consists of several modules that are dedicated for nuclear power plant hydrogen risk analysis, including steam wall-condensation module,
Conclusions
The presented work proposed a new method for deflagration to detonation transitions’ prediction of combustible gaseous mixtures. The assumption made by the previous DDT criterion that air composition is immutable may be unwarranted under some circumstance in which oxygen is being consumed by the recombiners inside the containment during severe accidents of nuclear power plants. The new method has been developed to address solving such problems.
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The modification made to the old DDT criterion that
Acknowledgments
The authors of this paper appreciate the financial support from National Natural Science Foundation of China (series number 11705189) and National Key Laboratory of Science and Technology on Reactor System Design Technology, Chengdu, China.
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