Assessment of post-LOCA long term cooling considering the in-vessel downstream effect

- Overall method was discussed to calculate the long-term cooling phase following a LOCA.
- Pressure drop calculated by in-vessel downstream effect test was simulated.
- Chemical deposition layer was modeled as 4th layer on the fuel heat structure.
- Additional calculations using a transient fuel performance code FRAPTRAN were conducted.

A method is discussed to calculate the long-term cooling phase following a Loss-Of-Coolant-Accident (LOCA) considering the In-vessel Downstream Effect (IDE) using a thermal-hydraulic code, MARS-KS1.3. There are two major concerns in considering the in-vessel downstream effect; the first is a simulation of blockage effect of debris at the core inlet and the second is an incorporation of the effect of chemical deposition at the fuel surface. Regarding the debris blockage effect, the present method takes into account the results of the tests which were conducted using a prototypical fuel assembly to determine the maximum pressure drop which corresponds to the maximum potential debris accumulation. And the result of the calculation by LOCA Deposition Model (LOCADM), the thickness and thermal conductivity of the debris layer, was used. Also, additional calculations using a transient fuel specific performance code, FRAPTRAN is conducted to confirm the validity of the present method. Using the present method the peak cladding temperature during post-LOCA long-term cooling phase considering in-vessel downstream effect can be predicted for evaluating the compliance to the acceptance criteria.