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- 저자 : Freixa, Jordi (검색결과 3 건)
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Sharabi, Medhat,Freixa, Jordi Korean Nuclear Society 2012 Nuclear Engineering and Technology Vol.44 No.7
The pressurized water reactor APR1400 adopts DVI (Direct Vessel Injection) for the emergency cooling water in the upper downcomer annulus. The International Standard Problem number 50 (ISP-50) was launched with the aim to investigate thermal hydraulic phenomena during a 50% DVI line break scenario with best estimate codes making use of the experimental data available from the ATLAS facility located at KAERI. The present work describes the calculation results obtained for the ISP-50 using the RELAP5/MOD3.3 system code. The work aims at validation and assessment of the code to reproduce the observed phenomena and investigate about its limitations to predict complicated mixing phenomena between the subcooled emergency cooling water and the two-phase flow in the downcomer. The obtained results show that the overall trends of the main test variables are well reproduced by the calculations. In particular, the pressure in the primary system show excellent agreement with the experiment. The loop seal clearance phenomenon was observed in the calculation and it was found to have an important influence on the transient progression. Moreover, the collapsed water levels in the core are accurately reproduced in the simulations. However, the drop in the downcomer level before the activation of the DVI from safety injection tanks was underestimated due to multi-dimensional phenomena in the downcomer that are not properly captured by one-dimensional simulations.
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( Patrick Bolton ),( Xavier Freixas ),( Joel Shapiro ) 한국금융연구원 2012 금융리포트 Vol.2012 No.2
The collapse of so many AAA-rated structured finance products in 2007-2008 has brought renewed attention to the causes of ratings failures and the conflicts of interest in the Credit Ratings Industry. We provide a model of competition among Credit Ratings Agencies (CRAs) in which there are three possible sources of conflicts: 1) the CRA conflict of interest of understating credit risk to attract more business; 2) the ability of issuers to purchase only the most favorable ratings; and 3) the trusting nature of some investor clienteles who may take ratings at face value. We show that when combined, these give rise to two fundamental equilibrium distortions. First, competition among CRAs can reduce market efficiency, as competition facilitates ratings shopping by issuers. Second, CRAs are more prone to inflate ratings in boom times, when there are more trusting investors, and when the risks of failure which could damage CRA reputation are lower. We argue that regulatory intervention requiring: i) upfront payments for rating services (before CRAs propose a rating to the issuer), ii) mandatory disclosure of any rating produced by CRAs, and iii) oversight of ratings methodology can substantially mitigate ratings inflation and promote efficiency.
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