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"Nuclear Safety Since Chernobyl", Special Report of Nucleonics Week, April 18-25, 1996
E. O. Adamov, Y. M. Cherkashov, L. N. Podlazov, Y. M. Nikitin, I. A. Stenbok, O. Y. Novoselsky, A. I. Ionov, N. N. Ponomarev-Sepnoy, E. V. Bourlakov, S. D. Malkin, A. V. Krayushkin, M. N. Babaitsev, K. P. Checherov, A. A. Abagyan, V. N. Vasekin, and I. M. Kisil', "Chernobyl Accident Causes: Overview of Studies over the Decade," IAEA International Conference, One Decade After Chernobyl: Nuclear Safety Aspects, Vienna, Austria, pp. 46-83, April 1-3, 1996.
E. O. Adamov, Y. M. Cherkashov, N. N. Ponomarev-Sepnoy, and E. V. Bourlakov, "Upgrading of RBMK Reactors: Safety Enhancement Measures, " IAEA International Conference, One Decade After Chernobyl: Nuclear Safety Aspects, Vienna, Austria, pp. 84-102, April 1-3, 1996.
P. Loizzo, A. Galati, F. Norelli, D. Lavrencic, L. N. Podlazov, B. I. Raskatov, D. F. Khokhlova, and V. E. Trekhov, "Italian-Russian Cooperation to Improve RBMK Safety, Nuclear Energy in Central Europe: Present and Perspectives, Portoroz, Italy, 13-16 June 1993.
Abstract - On September 1980 ENEA and RDIPE started a common research program for studying the Chernobyl accident. The two groups of specialists agreed about the methods of calculation, the evaluation of the results and a common organization to implement a new three-dimensional code. The first phase of the accident started with the insertion of the safety rods which can produce, with the configuration of April 1986, the insertion of a positive reactivity. The analysis of this phase led to the evaluation of the total static reactivity in the real configuration, and to an accurate comparison between calculated and experimental local neutron fluxes. The cooperation, still continuing on reactor modelling is now being extended to the fuel design and safety analysis, to the study of boiling instabilities within the framework of the International Boiling Instability Program, and to other aspects of RBMK reactor (high-power channel-cooled graphite moderated) safety improvements.
P. A. Landeryro adn A. Buccafurni, "Time-Independent Neutronic Analysis of the Chernobyl Accident," Nuclear Science and Engineering, Vol. 108, pp. 126-149, 1991.
Abstract - Estimates are made of the positive reactivity introduced through the growth of the coolant void fraction in the Chernobyl reactor at both the average burnup value given by the Soviets and the maximum value. Using Monte Carlo models, various possible axial burnup distributions, displacer models, conditions in the control channels, and control rod positions are considered in calculating the insertion of positive reactivity by the manual and emergency control rods, that is, the "positive scram." Two possible scenarios are examined for a second reactivity peak: (a) creation of a mixture of fuel, water, and cladding in a number of central fuel channels, resulting in the explosion of these channels, and (b) uniform vaporization throughout the entire reactor, resulting in reactor depressurization. From the data presented in this study, it can be concluded that vaporization of the cooling water in the fuel channel gave the highest reactivity contribution to the Chernobyl accident. The positive reactivity due to insertion of the manual and emergency control rods played only a minor role in the reactivity balance of the accident.
J. M. Martinez-Val, J. M.Aragones, E. Minguez, J. M. Perlado, and G. Velarde, "An Analysis of the Physical Causes of the Chernobyl Accident," Nuclear Technology, Vol. 90, pp. 371-388, June 1990.
Abstract - The initiating events and propagating mechanisms of the Chernobyl accident are the subject of this analysis. The neutronics and thermohydraulics of RBMK reactors under different regimes are studied. It is found that the reactor response to a loss of pumping power was a reactivity trip that could not be fully overcome by the Doppler effect because of the neutronic importance of hydrogen captures under the conditions before the accident. This very high importance was induced by an incorrect hydraulic regime being established before the accident in order to conduct an electromechanical experiment. This experiment was responsible for the loss of pumping power that triggered the accident.
P. S. W. Chan and A. R. Dastur, "The Physical Basis for the Void Reactivity Effect and its Dependence on Absorber Rod Configuration in the RBMK-1000," Nuclear Science and Engineering, Vol. 103, pp. 283-288, 1989.
Abstract - The components of the void reactivity effect in the RBMK lattice are obtained using multigroup multidimensional lattice codes. The relative magnitudes of the reactivity change due to changes, on voiding, in neutron absorption and in neutron moderation are compared for several lattice configurations that include either absorber rods or followers of various materials. This has led to the identificaiton of the mechanism that makes the void coefficient dependent on the number of absorber rods in the core. In line with these results, it is shown that replacing the graphite in the follower with nonmoderating materials reduces the void coefficient dependence on the number of absorber rods and is an economic method that may have potential in the void coefficient reduction program.
P. S. W. Chan and A. R. Dastur, "The Sensitivity of Positive Scram Reactivity to Neutronic Decoupling in RBMK-1000," Nuclear Science and Engineering, Vol. 103, pp. 289-293, 1989.
Abstract - The sensitivity to the axial neutron flux distribution of the positive reactivity that may have been introduced on initiation of scram in Chernobyl-4 has been evaluated. It is found that the scram reactivity is positive and its size is remarkably insensitive to a wide range of axial flux distortions provided the flux shape is concave, which is characteristic of neutronic decoupling of the core. In contrast, the scram reactivity is negative when flux shapes are convex, i.e., those that are a characteristic of strong neutronic coupling. This indicates that unless there were a significant number of control absorbers present in the core just before the accident to provide a convex flux shape, the chances that some positive scram reactivity was inserted to initiate the power pulse are high.
M. Rajamaki and F. Wasastjerna, "On the Reactivity Effects of Nuclear Fuel Fragmentation with Reference to the Chernobyl Accident," Nuclear Science and Engineering, Vol. 101, pp. 41-47, 1989.
Abstract - The reactivity effects caused by fragmentation of nuclear fuel and by simultaneous cooling of the fragments are described. A series of light water reactor (LWR) cases and three speculative scenarios for the Chernobyl accident are considered. Calculations were carried out with the LWR cell burnup code CASMO-HEX. Fragmentation is described by increasing the number of fuel pieces while decreasing their diameter. Cooling is considered to occur as quasi-stationary. Relative movement of the fragments and the coolant is taken into account by varying the water/fuel ratio. Under certain circumstances, substantial reactivity increases are found to occur in both reactor types. These may have contributed signifcantly to the severity of the Chernobyl accident.
M. Sobajima and T. Fujishiro, "Examination of the Destructive Forces in the Chernobyl Accidnet Based on NSRR Experiments," Nuclear Engineering and Design, Vol. 106, pp. 179-190, 1988.
Abstact - The possible causes of the destruction of the Chernobyl reactor core were examined by making use of the Nuclear Safety Research Reactor (NSRR) experimental results concerning the destructive forces generated by a fuel failure. A complementary experiment with Chernobyl reactor conditions was performed in order to observe the fuel failure behavior and the resultant vessel pressure rise, etc. Also, generation of hydrogen from the fuel rod cladding and the consequent system pressure rise were estimated base on the experiments. These examinations let to the conclusion that the most probable cause of the core pressure tube rupture in the accident was a static pressure rise due to rapid energy release from fragmented fuel. Other phenomena such as the hydrogen generation and molten fuel contact to the tube wall might have contributed to the tube rupture. The water hammer force is also estimated to have been large enough to break tubes even using conservative assumptions.
P. S. W. Chan, A. R. Dastur, S. D. Grant, J. M. Hopwood, and B. Chexal, "The Chernobyl Accident: Multidimensional Simulations to Identify the Role of Design and Operational Features of the RBMK-1000," International Topical Conference on Probabilistic Safety Assessment and Risk Management, Zurich, Switzerland, August 30 to September 4, 1987.
Abstract - A multidimensional analysis of the Chernobyl accident was carried out to identify the role of the design and operating features of the RBMK-1000 and thereby identify implications on other reactor concepts. The results show that assumptions regarding the preaccidnet fuel burnup and flux distributions are major derterminants of the size and shape of the power pulse, especially due to their influence on effective system void reactivity and on the amount, if any, of positive scram reactivity.
F. Reisch, "Techincal Note: How Chernobyl Happened: A Second Opinion," Nuclear Safety, Vol. 28, No. 1, pp. 43-45, January-March 1987.
B. Sorensen, "Chernobyl Accident: Assessing the Data," Nuclear Safety, Vol. 28, No. 4, pp. 443-447, October-December 1987.
Abstract - Data presented in the official U.S.S.R. report to the International Atomic Energy Agency on the Chernobyl Atomic Energy Station accidnet are critically assessed. Special attention is given to the derivation of release fractions from fallout measurements, a procedure demonstrated to involve large elements of uncertainty. Further comments relate to estimates of plume rise and deposition velocity. A comparison is made with the predictions of previously published theoretical reactor safety studies.
M. Ishikawa, S. Shiozawa, T. Wakabayashi, N. Ohnishi, and H. Mochizuki, "An Examination of the Accident Scenario in the Chernobyl Nuclear Power Station," Nuclear Safety, Vol. 28, No. 4, pp. 448-454, October-December 1987.
Abstract - The accident scenario in the Chernobyl Nuclear Power Station was examined by using EUREKA-2 analyses and the experimental results from the Nuclear Safety Research Reactor. The accidnet scenario is characterized by two power excursions, pressure tubes bursting from rapid steam generation resulting from molten fuel dispersion into the coolant, heatup caused by zirconium-water or zirconium-air reaction and subsequent burning of graphite rings, and a graphite moderator fire. These events can be explained quantitatively without conflict with the report submitted by the U.S.S.R. State Committee to the International Atomic Energy Agency Experts' Meeting in August 1986.
T. Wakabayashi, H. Mochizuki, H. Midorikawa, Y. Hayamizu, and T. Kitahara, "Analysis of the Chernobyl Reactor Accident(I) Nuclear and Thermal Hydaulic Characteristics and Follow- Up Calculation of the Accident," Nuclear Engineering and Design, Vol. 103, pp. 151-164, 1987.
Abstract - A follow-up calculation was made on the accident of Reactor No. 4 of the Chernobyl Nuclear Power Plant based on the literature and accident report published by the USSR. The analysis code system used had models perticuliar to a pressure tube type reactor, of which the accuracy had been verified by the experimental facilities at the O-arai Engineering Center and the test made at the "Fugen" Nuclear Power Plant. The anaysis data were prepared based on plant specifications and its operation history obtained from those published literature and accident reports. The analysis was composed of (1) a calculation of the nuclear and thermal-hydraulic characteristics, and the graphite heating and temperature distributions which were the basis data for the follow- up calculation of the accident, (2) an analysis of the plant behavior before the test started, using these basic characteristics, and (3) a follow-up calculation of the power increase which occurred after the test started. The analytical results were found to agree well withthe data published by the USSR. It was confirmed from these analyses that the main factors causing the accident were the increased enthalpy at the core entrance caused by the test made at low power level and the increased void fraction due to reduced coolant flow rate, in addition to the nuclear characteristics and performance of the control system peculiar to the Chernobyl Nuclear Power Plant.