Initialising ...
Initialising ...
Initialising ...
Initialising ...
Initialising ...
Initialising ...
Initialising ...
*; Ohshima, Hiroyuki
PNC TN9410 88-006, 71 Pages, 1988/01
Reactivity Feedback Modeling in Super System Code (SSC) has been improved to analyze the whole plant thermal hydraulic response to an anticipated transient without scram (ATWS) in a liquid metal fast breeder reactor (LMFBR). First of all, two-dimensional (2D) fluid flow and heat transfer modeling of reactor upper plenum (UP) has been modified. The heat transfer between the coolant and the control rod driveline (CRD) can be evaluated based on the 2D, not one-point, temperature distribution calculated by the UP model. The CRD is included as a part of in-plenum structure, and the thermal expansion of it is evaluated assuming the elongation is proportional to the temperature rise of the CRD. The reactivity feedback effect is evaluated using the elongation and the control rod worth. SSC-L is now capable of treating the following reactivity feedback effects caused by; (1)fuel doppler, (2)sodium density, (3)fuel axial expansion, (4)thermal expansion of the in-core structure, (5)thermal expansion of the core support structure, and (6)thermal expansion of the CRD. Whole plant thermal hydraulics during the ATWS accident can be analyzed taking the reactivity feedback effect into consideration more realistically than ever. An ATWS accident, i.e. unprotected loss-of-heat-sink (ULOHS), has been analyzed using SSC-L as an example. It is impossible to mitigate the ATWS consequence without core damage if no design change is made. However, it is found that more than 7 minutes of grace time is available for the remedial action at least if the above mentioned reactivity model is used. The accident progression is not so rapid in general in the ULOHS accident. The relatively slow response implies reactor shutdown can be achieved by a manual scram or a shutdown system actuated at slower speed can be utilized for mitigating the ATWS consequence.In the present analysis of CRD thermal expansion, it is assumed that the deformation of the CRD is one-dimensional, linear and elastic. It ...
Ohshima, Hiroyuki; *; Ninokata, Hisashi
PNC TN9410 87-122, 62 Pages, 1987/08
In the safety analysis of ATWS (Anticipated Transient Without Scram) sequences, emphasis is placed on the thermohydraulics in reactor core and interactions between core and heat transport system (HTS) are not considered. However, if progress of the sequence is not so fast, thermohydraulics in reactor core and HTS should be calculated at the same time. A whole plant system code, such as SSC-L, is available for this purpose. Since SSC-L has not been applied to the ATWS analysis so far, reactivity feedback model in this code has something to be improved. If uncertainty in parameters used in the analysis is large, conservative assumptions are employed. Therefore, the results of simulations are conservative and have large uncertainty in general. In order to evaluate ATWS sequences, it is desirable to take reactor core and HTS interaction into consideration and to improve accuracy of reactivity feedback model in SSC-L as well as decreasing uncertainty of the input data. Grace period available for mitigating the ATWS by the operator recovery action can be also evaluated from the whole plant thermohydraulics. Therefore, reactivity calculation module in SSC-L has been modified and improved in this study. Thus the reactivity feedback effects calculated in SSC-L are as follows: (1)Fuel doppler, (2)Sodium density and void (3)Fuel axial expansion, (4)Thermal expansion of the core internal structure, and (5)Thermal expansion of the core support structure. For the purpose of checking performance of the new model, SSC-L has been applied to the simulations of ULOHS (Unprotected Loss of Heat Sink) accidents and the results are consistent in our perception. This model should be validated by experiments and SSC-L is to be extensively applied to the safety analysis of LMFBR plants in future.
