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Vorträge und Posterpräsentationen (mit Tagungsband-Eintrag):

G. Sdouz, G. Sonneck:
"Accident Management Strategies for Accidents with Large Containment Leaks in VVER-1000 Reactors";
Vortrag: Proceedings of the 10th Internaitonal Topical Meeting on Nuclear Reactor Thermal Hydraulics (NURETH-10),, Seoul, Korea; 05.10.2003 - 09.10.2003; in: "Proceedings of the 10th Internaitonal Topical Meeting on Nuclear Reactor Thermal Hydraulics (NURETH-10),", American Nuclear Society, - (2003), ISBN: 89-88852-11-7; S. 287.



Kurzfassung:
The goal of this work is the investigation of the influence of different accident management strategies on the thermal-hydraulics in the containment during a severe accident with a large containment leak from the beginning of the accident. The increasing relevance of terrorism suggests a closer look at this kind of severe accidents.
Normally the course of severe accidents and their associated phenomena are investigated with the assumption of an intact containment from the beginning of the accident. This intact containment has the ability to retain a large part of the radioactive inventory. In these cases there is only a release via a very small leakage due to the untightness of the containment up to cavity bottom melt through.
In the "Deutsche Risikostudie Kernkraftwerke" different possibilities of leakages from the containment as part of the safety analysis of the containment are described The range of the leakages from the beginning of the accident is covered by three different representative leakage sizes. The maximum leakage of the containment has a diameter of 300 mm. In addition the source term behavior is greatly influenced by the thermalhydraulics in the containment which is dependent of the accident management strategy.
The focus in this study is on the "Station Blackout"-sequence (or TMLB` in the WASH-1400 nomenclature) in VVER-1000-type reactors under different accident management strategies. The results can be compared with the outcome of a previous study assuming no large containment leak from the beginning. The calculations were performed using the Source Term Code Package (STCP), hydrogen explosions are not considered.
In this study four scenarios have been investigated:
- The electricity cannot be restored up to the end of the accident, therefore no spray and LP ECC (Low Pressure Emergency Core Cooling) systems including the coolers are available during the accident.
- All spray and LP ECC systems including the coolers are available after 10 hours, i.e. after the core-concrete interaction starts.
- All spray and LP ECC systems, but no coolers, are available after 7 hours, i.e. just before the onset of the core-concrete interaction.
- One spray- and one LP ECC system, but no coolers, are available after 7 hours, i.e. just before the onset of the core-concrete interaction.
The results of the first two scenarios can be compared directly with results of accident sequences without a large containment leak from the beginning.
Up to the melt-through of the cavity bottom the thermal-hydraulic phenomena are almost identical to the case without the large containment leak. All of the phenomena occur slightly delayed due to the containment leak. In addition to the untightness of the containment a large containment leak allows the fission products to leave the containment from the beginning of the accident. There are three main releases of the fission products. The first part of the source term stems from the core melt (3,6 h), the second part originates from the pressure increase caused by the core slump (5,3 h) and the third part begins at the time of the start of the core-concrete interaction (9,3 h). Due to the large leak there is no fourth release of the volatile fission products at cavity bottom melt throuh as normal. From the comparison with the case without a large containment leak it can be shown that the intact containment retains the nuclides up to a factor of 6000.
In a previous study it was demonstrated that different accident management strategies (scenarios 2 to 4) reduce the source term considerably. This effect depends on the volatility of the fission products. The released mass of the high volatible nuclide groups as CI and CH can be reduced by a factor of two, for the low volatile groups the reduction can be as large as three orders of magnitude. However the large containment leak from the beginning diminishes this source term reducing effect. In this case accident management cannot reduce the releases of the volatile fission products. The reducing effect for the low volatile nuclides is diminished to a factor of 15.
The thermal-hydraulic phenomena which reduce the fission product releases applying different accident management strategies during a severe accident sequence with a large containment leak are discussed in this paper. As main result the influence of certain measures on the different nuclides in this specific accident type is assessed.

Schlagworte:
Source Term, Accident Management, VVER-1000 Reactor

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