Partner und Internationale Organisationen
(Englisch)
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VTT (FIN), FORTUM (FIN), Nuclear Research Institute Rez plc (CZ), NRG, an ECN KEMA Company (NL), Centro de Investigaciones Energeticas Medioambientales y Tecnologicas (CIEMAT) (E)
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Abstract
(Englisch)
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The objective of this work was to generate a database and to develop and verify models to support accident management interventions with an attempt to mitigate the environmental activity release in steam generator tube rupture sequences either leading to or induced by severe accidents. Experimental investigations in four different facilities to study fission product retention in the secondary side of a steam generator defining the level of activity release to the environment were conducted. The current accident management actions foresee flooding of the secondary side through the emergency feed water system in an attempt to arrest the activity. Effectiveness of the planned accident management actions should determine the reduction in the source term in these accident types. There is currently no appropriate database and associated model estimating the source term from these types of accidents.
The first task (WP1) of the project included definition of the most important steam generator tube rupture accident sequences. This was done by using the existing PSA studies for the PWR and VVER?440 and by performing additional integral code calculations. The range of boundary conditions for the experimental programmes was determined based on expected conditions in the reference PWR and VVER plants.
In WP2 experiments were conducted in a scaled?down versions of steam generators representing western PWRs and VVERs. An improved mechanistic understanding of the local deposition in a PWR steam generator was greatly achieved with an experimental programme carried out in the ARTIST facility of PSI, especially for the cases where the bundle is flooded. The separate?effect tests were conducted at CIMAT (E) in support of the integral tests. This data was necessary for the model development and verification. Effect of different secondary side flooding procedures (timing, flooding rate, etc.) was also investigated in the ARTIST facility.
In WP3 the experimental results was applied to real steam generators by utilising system codes, which was equipped with the new developed models. Models for individual phenomena were based on separate effect tests, which were then implemented in calculations of the integral experiments.
In WP4 the database and the analytical model were used to assess the effectiveness of different accident management procedures. Important accident scenarios for the reference PWR and VVER?440 plants were analysed.
Summary and Conclusions of the PSI part of the SGTR project (WP.2.1) per 31.12.2002
The following conclusions can be drawn from the investigations of integral effects in a vertical SG bundle:
· When the bundle is dry, and the full break flow directed into the bundle, the DF is typically small, i.e. between 2.5 and 3. There is strong evidence that the aerosols (at least the type used, TiO2 used with 0.020 mm AMMD primary particle) disintegrate into smaller particles because of the sonic conditions at the break. This obviously promotes particle escape from the secondary and lowers the overall DF. Further investigation needs to be performed to determine the influence of the type of aerosol used.
· With the dry bundle, and a small flow reproducing the far-field velocities, the DF is of the order of 5, implying better decontamination than with the full flow. This can be explained by the somewhat lower particle disintegration than witnessed with the larger flow. The far-field retention implies a DF of the order of 1.9 DF per stage, which, for SG with 9 or more stages, can translate in overall DF's of several hundreds when the break is located near the tube sheet region.
· With a bundle flooded just above the break and a steam/noncondensable mixture, the DF is between 45 and 112 for the full flow and 482 for the small flow (typical of far-field). This implies again that the far field stages are more efficient at trapping aerosols than the break stage. For plant applications and AM, this suggests that one should attempt to flood the bundle such that several far-field stages are covered with water.
· For the far-field conditions, under a flooded bundle and in presence of steam, the DF is roughly of the same order regardless of the water height, i.e. in the range from 482 to 1081. A large fraction of the aerosols is scrubbed at the break level because of strong diffusiophoresis and impaction of the incoming jet on the water interface. The additional water head beyond the break stage has only a secondary influence on the magnitude of decontamination.
· For the far-field conditions, under a flooded bundle and in absence of steam, the DF increases exponentially from 124 to 5739 when the water height in the bundle increases from 1.30 m to 3.6 m. The aerosol removal rate is roughly constant with height, and hence the DF is solely a function of residence time in the pool (pool height).
· When steam is present in the carrier gas under flooded secondary, condensation inside the tube causes aerosol deposition and produces blockage near the break, with a subsequent primary pressure rise. This has implications for real plant conditions, as aerosol deposits inside the broken tube will cause more flow to be diverted to the intact tubes, with a corresponding reduction in the source term to the secondary.
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