0 Tilman Drath, Ingo D. Kleinhietpaß, Marco K. Koch Lehrstuhl für Energiesysteme und Energiewirtschaft (LEE) Ruhr-Universität Bochum (RUB) 11 th International QUENCH Workshop Forschungszentrum Karlsruhe, October 25-27, 2005 Analysis of the TMI-2 Accident Using ATHLET-CD

1 Outline TMI-2 Accident ATHLET-CD Short Description Modelling with ATHLET-CD Calculational Results Conclusions

2 TMI-2 Accident Three Mile Island Unit 2 (TMI-2): was planned by a number of organisations (i.e. Babcock & Wilcox Inc.), is banked on the named-giving island in the midst of the Susquehanna River near Harrisburg, USA, beside Unit 1 being still in commercial operation since September 1974, was completed as 2-Loop American PWR, and started operation in the end of 1978 with an electrical power of 0.9 GW el and a thermal power of 2.7 GW th. Google Earth

3 TMI-2 Accident The TMI-2 accident was initiated by problems on the secondary side of the plant. These problems finally led to: the loss of the main feed water pumps and the turbine, an insufficient low heat dissipation in the steam generators, as well as a rise of the primary pressure, and finally the reactor shutdown on the 28 th of March 1979 at 04:00:37 h local time.

4 TMI-2 Accident Sequence of Events and Primary Pressure History PORV Failure!

5 TMI-2 Accident Postulated Configurations of the TMI-2 Core Before Reflood (174 min) After Reflood and before Relocation (224 min) Endstate Configuration

6 ATHLET-CD Short Description ATHLET-CD:Analysis of THermal-hydraulics of LEaks and Transients – Core Degradation The code is being developed by the Gesellschaft für Anlagen und Reaktorsicherheit (GRS) mbH in Germany. It deals with the analysis of anticipated and abnormal plant transients, small and intermediate leaks, as well as large breaks in LWRs. The ATHLET code covers the spectrum of design basis and beyond design basis accidents (without core degradation) for PWRs and BWRs. The CD part represents an add-on to ATHLET with modules describing core melting, fission product release, as well as fission product and aerosol transport processes in the primary system. Modular structured Code ATHLET-CD

7 ATHLET-CD Short Description Gesellschaft für Anlagen und Reaktorsicherheit (GRS) mbH Modular Structure of ATHLET-CD

8 Modelling of the TMI-2 Accident ATHLET-CD Nodalisation of the Plant Loop A Loop B

9 Modelling of the TMI-2 Accident ATHLET-CD Nodalisation of the Core ROD1 (Inner Ring) ROD2 ROD3 ROD4 (Outer Ring) Core Reflector Bypass Bypass Cross Connections

10 Results of the TMI-2 Calculations Calculated and Measured Primary Pressure History B-Pump Transient High Pressure Injection End of run using MESOCO! Heat transfer to the coolant is over-predicted due to the under-estimation of core degradation because of still missing late phase models.

11 Results of the TMI-2 Calculations Calculated and Measured Leakage Mass (PORV) The mass flow rate is calculated with the critical discharge model and shows a qualitative good result of the accumulated leakage mass. Measuring inaccuracy during the trip due to the operation beyond design limits.

12 Results of the TMI-2 Calculations Calculated and Measured Pressuriser Water Level Calculated mixture and water level only differ, if the PORV (block valve) is opened, which means a good reproduction of the pressuriser behaviour. Measuring inaccuracy during the trip due to the operation beyond design limits.

13 Results of the TMI-2 Calculations Calculated Water-/Steam Distribution History

14 Results of the TMI-2 Calculations Calculated and Postulated Core Configuration before Reflood (174 min)

15 Results of the TMI-2 Calculations Calculated and Postulated Core Configuration after Reflood and before Relocation (224 min) Debris bed and molten pool behaviour are not calculable at present!

16 Results of the TMI-2 Calculations Calculated History of the Core Configuration

17 Results of the TMI-2 Calculations Calculated and Measured Hydrogen Production The H 2 generation is under-predicted during the reflood due to: 1.) fuel rod failure and material relocation into deeper core sections, 2.) temperatures being too low to contribute significantly to the oxidation rate in the lower core regions, 3.) no access of the coolant to the melt because of blockage formation.

18 Results of the TMI-2 Calculations Calculated Outer Oxide Layer Thickness (240 min) Fuel rod failure: ROD1 and ROD2! Permanent water level!

19 Results of the TMI-2 Calculations Calculated Fission Product and Structure Material Transport into the Primary System (240 min) Transport Rate [%] of Initial Inventory Group of Species ATHLET-CD 2.0A/1.1K Estimation of Akers et al. (Nuclear Technology, Vol. 87, pp ) I, Cs, Xe53 %50 % Te, Sb5 % 1.6 % (available for Sb only) Sr3 %3.5 % U, Ag-St0.4 %-

20 Conclusions The thermal-hydraulics are predicted quite well compared to the TMI-2 data, especially if the measuring inaccuracy during the trip due to the operation beyond design limits is taken into account. The core configurations are calculated in good agreement compared to the postulated status of the core at ascertained times of the sequence. The H 2 generation is under-predicted during the reflood because of fuel rod failure and material relocation into deeper core sections, where the temperatures are too low to contribute significantly to the oxidation rate. Additionally, no access of the coolant to the melt exists because of blockage formation. The calculated fission product and structure material release as well as their transport in the primary system represent plausible results. The calculation of the TMI-2 accident using ATHLET-CD in the current program version 2.0A/1.1K shows a good overall code performance. But the code has, of course, still a potential of improvements in modelling physical phenomena especially concerning the late phase of core degradation and the behaviour and coolability in the lower head.

21 Acknowledgement This work is sponsored by the German Federal Ministry of Economics and Labour (BMWA ). The authors thank for support and interpretational hints from Dr. Klaus Trambauer (GRS mbH). Special thanks to all colleagues from the FZK for hosting the QUENCH Workshop.

22 Tilman Drath, Ingo D. Kleinhietpaß, Marco K. Koch Lehrstuhl für Energiesysteme und Energiewirtschaft (LEE) Ruhr-Universität Bochum (RUB) 11 th International QUENCH Workshop Forschungszentrum Karlsruhe, October 25-27, 2005 Analysis of the TMI-2 Accident Using ATHLET-CD