NEA-1098 THARC-S.

1. NAME OR DESIGNATION OF PROGRAM:  THARC-S.

3. DESCRIPTION OF PROGRAM OR FUNCTION

THARC is a multi-subchannel program to predict the multidimensional transient thermo-hydraulic behaviour of Liquid Metal Fast Breeder Reactor (LMFBR) subassemblies during postulated flow and power transients. THARC-S provides the time history of subchannel coolant temperatures, fuel rod temperatures, channel wall temperatures, heat fluxes and upstream boiling boundaries. Only single coolant phase is taken into account  by this version of the program.

4. METHOD OF SOLUTION

- Application of the subchannel concept with (flow-dependent) inter-   subchannel mixing.
- Application of the "hybrid" method for transient conduction in   fuel rods and channel (wrapper) wall segments.
- Lagrangian treatment for solving subchannel coolent energy   equations.
- Combined analytical time-dependent temperature solutions for sold    and coolant regions.
- Computation of upstream void propagation for imposed inlet flow   decay neglecting liqhid-coolant flow restribution effects.

5. RESTRICTIONS ON THE COMPLEXITY OF THE PROBLEM

Storate requirements and computation time represent limitations to bundle size (number of subchannels, axial extent). Usually, characteristic regions of the subassembly (bundle) are selected for analysis, e.g. one-twelfth of  a full-size subassembly or one or two rows of subchannels across the subassembly in case of lateral power gradient. The maximum number of subchannels used so far is 60. For radial heat conduction calculations within fuel rods the latter may be discretized into 100 elements. Experience has shown that in general 20 is enough (15 in fuel, 5 in cladding). Present restrictions regarding fuel rod heat transfer (which however can be removed) are: (a) temperature independent physical properties; (b) no axial variation of materials.

6. TYPICAL RUNNING TIME

A typical IBM 370/165 run time for the LOF analysis of one-twelfth of a 271-rod subassembly (with 13 axial segments, fuel rod and channel wall discretization in 20 elements) from nominal conditions until boiling inception is about 10 minutes.

7. UNUSUAL FEATURES OF THE PROGRAM

The analytical time-dependent solution method allows the use of large computation time steps for transient conduction calculations. In the Lagrangian coolant heat transfer approach microtime steps are determined by the residence time of coolant slabs in an axial segment. The option NSTEEP provides the possibility to "trace" coolant slabs in their axial motion through the bundle at "macro" time intervals. This feature allows to compute LOF transients from nominal conditions until boiling inception in times which are independent of the steepness of the transient.

8. RELATED AND AUXILIARY PROGRAMS

The treatment of inter-subchannel mixing in THARC-S is similar to that in the steady-state heat transfer program HERA. The latter may also be used to provide the radial subchannel mass flow distribution as input for THARC-S.

10. REFERENCES:

- R. Nijsing, G. De Viries and W. Eifler:
THARC-S: A Computer Program for Transient Thermohydraulic Analysis of Liquid
Metal Cooled Rod Bundles with Emphasis on Single-Phase Conditions, JRC Ispra
(Draft)

11. MACHINE REQUIREMENTS

Main storage requirements depend on problem size. For the problem described in item 10 with double-precision calculations, 700K bytes are required on an IBM 370/165 computer. Experience shows that the progrm may also be run with single precision.

13. OPERATING SYSTEM UNDER WHICH PROGRAM IS EXECUTED:

14. OTHER PROGRAMMING OR OPERATING INFORMATION OR RESTRICTIONS:
No plotting facility is provided with THARC-S.

15. NAME AND ESTABLISHMENT OF AUTHORS

   R. Nijsing, G. De Vries, W. Eifler
   Commission of the European Communities
   Joint Research Centre
   ISPRA Establishment, Italy

Electronic documentation