Severe Nuclear Power Plant Research Articles

Radiolytical oxidation of gaseous iodine by beta radiation

Abstract Iodine is one of the most radiotoxic fission product released from fuel during a severe nuclear power plant accident. Within the containment building, iodine compounds can react e.g. on the painted surfaces and form gaseous organic iodides. In this study, it was found out that gaseous methyl iodide (CH3I) is oxidised when exposed to beta radiation in an oxygen containing atmosphere. As a result, nucleation of aerosol particles takes place and the formation of iodine oxide particles is suggested. These particles are highly hygroscopic. They take up water from the air humidity and iodine oxides dissolve within the droplets. In order to mitigate the possible source term, it is of interest to understand the effect of beta radiation on the speciation of iodine.

Experimental investigation on single-phase pressure losses in nuclear debris beds: Identification of flow regimes and effective diameter

During a severe nuclear power plant accident, the degradation of the reactor core can lead to the formation of debris beds. The main accident management procedure consists in injecting water inside the reactor vessel. Nevertheless, large uncertainties remain regarding the coolability of such debris beds. Motivated by the reduction of these uncertainties, experiments have been conducted on the CALIDE facility in order to investigate single-phase pressure losses in representative debris beds. In this paper, these results are presented and analyzed in order to identify a simple single-phase flow pressure loss correlation for debris-bed-like particle beds in reflooding conditions, which cover Darcean to Weakly Turbulent flow regimes.The first part of this work is dedicated to study macro-scale pressure losses generated by debris-bed-like particle beds, i.e., high sphericity (>80%) particle beds with relatively small size dispersion (from 1mm to 10mm). A Darcy–Forchheimer law, involving the sum of a linear term and a quadratic deviation, with respect to filtration velocity, has been found to be relevant to describe this behavior in Darcy, Strong Inertial and Weak Turbulent regimes. It has also been observed that, in a restricted domain (Re=15 to Re=30) between Darcy and Weak Inertial regimes, deviation is better described by a cubic term, which corresponds to the so-called Weak Inertial regime.The second part of this work aims at identifying expressions for coefficients of linear and quadratic terms in Darcy–Forchheimer law, in order to obtain a predictive correlation. In the case of monodisperse beds, and according to the Ergun equation, they depend on the porosity of the medium, empirical constants and the diameter of the particles. Applicability of the Ergun equation for debris-bed-like particle beds has been investigated by assessing the possibility to evaluate equivalent diameters, i.e., characteristic length allowing correct predictions of linear and quadratic terms by the Ergun equation. It has been observed that the Sauter diameter of particles allows a very precise prediction of the linear term, by less than 10% in most cases, while the quadratic term can be predicted using the product of the Sauter diameter and a sphericity coefficient as an equivalent diameter, by about 15%.

Tools for placing the radiological health hazard in perspective following a severe emergency at a light water reactor (LWR) or its spent fuel pool.

Experience from past nuclear and radiological emergencies shows that placing the radiological health hazard in perspective and having a definition of "safe" are required in order to prevent members of the public, those responsible for protecting the public (i.e., decision makers), and others from taking inappropriate and damaging actions that are not justified based on the radiological health hazard. The principle concerns of the public during a severe nuclear power plant or spent fuel pool emergency are "Am I safe?" and "What should I do to be safe?" However, these questions have not been answered to the satisfaction of the public, despite various protective actions being implemented to ensure their safety. Instead, calculated doses or various measured quantities (e.g., ambient dose rate or radionuclide concentrations) are used to describe the situation to the public without placing them into perspective in terms of the possible radiological health hazard, or if they have, it has been done incorrectly. This has contributed to members of the public taking actions that do more harm than good in the belief that they are protecting themselves. Based on established international guidance, this paper provides a definition of "safe" for the radiological health hazard for use in nuclear or radiological emergencies and a system for putting the radiological health hazard in perspective for quantities most commonly measured after a release resulting from a severe emergency at a light water reactor or its spent fuel pool.

Atmospheric Dispersion of Radioactivity from Nuclear Power Plant Accidents: Global Assessment and Case Study for the Eastern Mediterranean and Middle East

We estimate the contamination risks from the atmospheric dispersion of radionuclides released by severe nuclear power plant accidents using the ECHAM/Modular Earth Submodel System (MESSy) atmospheric chemistry (EMAC) atmospheric chemistry-general circulation model at high resolution (50 km). We present an overview of global risks and also a case study of nuclear power plants that are currently under construction, planned and proposed in the Eastern Mediterranean and Middle East, a region prone to earthquakes. We implemented continuous emissions from each location, making the simplifying assumption that all potential accidents release the same amount of radioactivity. We simulated atmospheric transport and decay, focusing on 137Cs and 131I as proxies for particulate and gaseous radionuclides, respectively. We present risk maps for potential surface layer concentrations, deposition and doses to humans from the inhalation exposure of 131I. The estimated risks exhibit seasonal variability, with the highest surface level concentrations of gaseous radionuclides in the Northern Hemisphere during winter.

Evaluation of an effective diameter to study quenching and dry-out of complex debris bed

Many of the current research works performed in the SARNET-2 WP5 deal with the study of the coolability of debris beds in case of severe nuclear power plant accidents. One of the difficulties for modeling and transposition of experimental results to the real scale and geometry of a debris bed in a reactor is the difficulty to perform experiments with debris beds that are representative for reactor situations. Therefore, many experimental programs have been performed using beds made of multi-diameter spheres or non-spherical particles to study the physical phenomena involved in debris bed coolability and to evaluate an effective diameter. This paper first establishes the ranges of porosity and particle size distribution that might be expected for in-core debris beds and ex-vessel debris beds. Then, the results of pressure drop and dry-out heat flux (DHF) measurements obtained in various experimental setups, POMECO, DEBRIS, COOLOCE/STYX and CALIDE/PRELUDE, are presented. The issues of particle size distribution and non-sphericity are also investigated. It is shown that the experimental data obtained in “simple” debris beds are relevant to describe the behavior of more complex beds. Indeed, for several configurations, it is possible to define an “effective” diameter suitable for evaluating (with the porosity) some model parameters as well as correlations for the pressure drop across the bed, the steam flow rate during quenching and the DHF.

Development of radiation risk assessment simulator using system dynamics methodology

The potential magnitudes of radionuclide releases under severe accident loadings and offsite consequences as well as the overall risk (the product of accident frequencies and consequences) are analyzed and evaluated quantitatively in this study. The system dynamics methodology has been applied to predict the time-dependent behaviors such as feedback and dependency as well as to model uncertain behavior of complex physical system. It is used to construct the transfer mechanisms of time dependent radioactivity concentration and to evaluate them. Dynamic variations of radio activities are simulated by considering several effects such as deposition, weathering, washout, re-suspension, root uptake, translocation, leaching, senescence, intake, and excretion of soil. The time-dependent radio-ecological model applicable to Korean specific environment has been developed in order to assess the radiological consequences following the short-term deposition of radio-nuclides during severe accidents nuclear power plant. An ingestion food chain model can estimate time dependent radioactivity concentrations in foodstuffs. And it is also shown that the system dynamics approach is useful for analyzing the phenomenon of the complex system as well as the behavior of structure values with respect to time. The output of this model (Bq ingested per Bq m−2 deposited) may be multiplied by the deposition and a dose conversion factor (Gy Bq−1) to yield organ-specific doses. The model may be run deterministically to yield a single estimate or stochastic distributions by “Monte-Carlo” calculation that reflects uncertainty of parameter and model uncertainties. The results of this study may contribute to identifying the relative importance of various parameters occurred in consequence analysis, as well as to assessing risk reduction effects in accident management.

Level 2 analysis to estimate LERF risk from a thermally induced rupture of a steam generator tube defect

This paper outlines the Level 2 portion of a methodology for determining the incremental induced steam generator tube rupture large early release fraction caused by an actual through-wall defect. This defect was responsible for the minor steam generator tube leak that occurred in September 2002 at the Comanche Peak Steam Electric Station Unit 1. In order to quantify the performance of the defect over the operating cycle, a range of defect lengths were input to the PROBFAIL computer code [Kenton, M., 2001. PROBFAIL: A Computer Code for Evaluating the Likelihood of Steam Generator Tube Rupture in Severe Nuclear Power Plant Accidents, CREARE TM-2138], using appropriate boundary conditions derived from MAAP4 [Henry, R., et al., May 1994. MAAP4—Modular Accident Analysis Program for LWR Power Plants, Computer Code Manual, EPRI Research Project 3131-02] runs. From the analysis of the calculated times of burst for each assumed defect length, the minimum through-wall defect length necessary for tube burst to occur prior to hot leg or surge line creep rupture was calculated. The probability that the defect would actually have this length was then estimated by determining the fraction of the cycle for which the defect would be at least that long. The methodology development and implementation relied on MAAP4 runs, which are discussed extensively in connection with their role in: (1) guiding the construction of the accident progression event tree, (2) generating relevant information for probability assignments in the various underlying fault trees and (3) obtaining boundary conditions of pressure and temperature for use in PROBFAIL. The overall increment in LERF due to the existence of the defect was calculated to be approximately 4E−08.

Experimental results of integral effects tests with 1/10th scale zion subcompartment structures in the Surtsey test facility

Three integral effects tests (IET-1, IET-3, and IET-6) were conducted to investigate the effects of high-pressure melt ejection on direct containment heating. A 1:10 linear scale model of the Zion reactor pressure vessel (RPV), cavity, instrument tunnel, and subcompartment structures were constructed in the Surtsey test facility at Sandia National Laboratories. The RPV was modeled with a melt generator that consisted of a steel pressure barrier, a cast MgO crucible, and a thin steel inner liner. The melt generator/crucible had a hemispherical bottom head containing a graphite limitor plate with a 4 cm exit hole to simulate the ablated hole in the RPV bottom head that would be formed by tube ejection in a severe nuclear power plant accident. The reactor cavity model contained 3.48 kg water with a depth of 0.9 cm that corresponded to condensate levels in the Zion plant. 43 kg iron oxide/aluminum/ chromium thermite was used to simulate molten core debris. The molten thermite in the three tests was driven into the scaled reactor cavity by slightly superheated steam at 7.1, 6.1, and 6.3 MPa for IET-1, IET-3, and IET-6 respectively. The IET-1 atmosphere was pre-inerted with nitrogen, while the IET-3 atmosphere was nitrogen with approximately 9.0 mol% O2. The IET-6 atmosphere was nitrogen with 9.79 mol% O2 and 2.59 mol% pre-existing hydrogen. In IET-1, approximately 233 g mol hydrogen were produced but almost none burned because oxygen was not available. In IET-3, approximately 227 g mol hydrogen were produced and 190 g mol burned. In IET-6, approximately 319 g mol hydrogen were produced and 345 g mol burned. The peak pressure increases in the IET-1, IET-3 and IET-6 experiments were 0.098, 0.246, and 0.279 MPa respectively. In IET-3 and IET-6 hydrogen burned as it was pushed out of the subcompartments into the upper region of the Surtsey vessel. In IET-6, although a substantial amount of pre-existing hydrogen burned, it apparently did not burn on a time scale that made a significant contribution to the peak pressure increase in the vessel.

Determination of the stability and fragmentation length of a melt jet in water

The fragmentation of a high-temperature melt jet in water is one of the principal mechanisms underlying the formation of a coarsely dispersed water-steam-melt mixture in the onset and development of a hypothetical severe nuclear power plant accident with core meltdown. Under certain conditions the melt can mix explosively with the water in such a mixture with potentially detrimental results for the reactor housing. The rate of fragmentation of a melt jet largely governs the characteristics of the resulting mixture and its capacity to detonate.

The Growth of Hygroscopic Particles During Severe Core Melt Accidents

Fission products and other compounds released during severe nuclear power plant accidents will form aerosol particles, which include water-soluble compounds such as cesium hydroxide (CsOH), cesium carbonate, and cesium iodide. These hygroscopic particles will grow in a humid environment, and thus their settling rate is increased significantly at high relative humidities. This paper evaluates the hygroscopicity of CsOH and other water-soluble compounds released under severe accident conditions. The effect was incorporated into the kinetic particle growth model based on coupled mass and heat transport to evaluate the growth rates of single particles at different atmospheric conditions. Finally, the kinetic growth model for hydroscopic particles was included in the NAUA aerosol code to predict the general behavior of aerosols released into the containment atmosphere. A sensitivity analysis of this model was carried out to guide further work on important parameters and to decrease computing time. It is concluded the hygroscopic properties of radioactive cesium can, in favorable conditions, suppress the release of radioactive materials (source term) by orders of magnitude.