**RSICC CODE PACKAGE PSR-379**

**1. NAME AND TITLE**

UTSG: Code System for Calculating the Nonlinear Transient Behavior of a Natural-Circulation U-Tube Steam Generator with Its Main Steam System.

**2. CONTRIBUTORS**

Gesellschaft fuer Reaktorsicherheit (GRS) mbH, Garching, West Germany through the NEA Data Bank, Issy-les-Moulineaux, France.

**3. CODING LANGUAGE AND COMPUTER**

IBM 3033; FORTRAN-IV (P00379I303300).

**4. NATURE OF PROBLEM SOLVED**

The code is based on a non-linear theoretical model describing the steady-state and transient behavior of a vertical natural-circulation U-tube steam generator together with its main steam system. The steam generator is considered to consist of a heat exchange section, a top plenum, a down-comer region and a main steam system (with a sequence of relief and/or safety valves, isolation, bypass, turbine-trip and turbine-control valves and a steam turbine). Possible perturbations from outside can be: inlet water temperature, inlet water mass flow and system pressure on the primary side, feedwater temperature, feed-water mass flow and outlet steam mass flow disturbed by actions of the different valves within the main steam system on the secondary side.

**5. METHOD OF SOLUTION**

The theoretical model consists of a set of analytical state and nonlinear ordinary differential equations of first order:

The steady-state values are determined by putting in the set of equations the time derivatives equal to zero and solving the resulting set of nonlinear algebraic equations by using the solution method of a linear algebraic equation system in a recursive way.

Eliminating the steady state parts from the differential equation system and regarding the variables as absolute difference values (with respect to its steady state), the calculation of the transient behavior of the system is performed in a normal-precision way, thus avoiding the more capacity and time-consuming double-precision procedure. The solution of the non-linear ordinary differential equation system of first order is obtained by applying the digital DIFSYS method, an explicit integration procedure based on a method established by Bulirsch and Stoer.

**6. RESTRICTIONS OR LIMITATIONS**

Maximum number of horizontal nodes of the heat exchange region to 7. Maximum number of radial layers of the heat exchanging tubes is less than or equal to 3. Maximum number of time points to be printed out is less than or equal to 501.

**7. TYPICAL RUNNING TIME**

The test case ran in 4.6 CPU seconds on an IBM 3033.

**8. COMPUTER HARDWARE REQUIREMENTS**

UTSG was tested at the NEA Data Bank on an IBM 3033. With minor source changes, it was executed on an IBM RS/6000 at RSICC.

**9. COMPUTER SOFTWARE REQUIREMENTS**

UTSG was developed under the IBM MVS operating system and requires a Fortran compiler. If no plots are needed (for which special plot routines are necessary), substitute a dummy routine for subroutine PLTFCT. The routine SCALST will then be surplus.

**10. REFERENCES**

**a:) Included in document:**

A. Hoeld, " UTSG - A Digital Code for Calculating the Nonlinear Transient Behaviour of a Natural-Circulation U-Tube Steam Generator with Its Main Steam System" Program Description, GRS-A-426 (March 1980).

** b:) background information:**

A. Hoeld, "A Theoretical Model for the Calculation of Large Transients in Nuclear Natural-Circulation U-Tube Steam Generators (Digital Code UTSG)," Nucl. Eng. and Design 47, 1-23 (1978).

**11. CONTENTS OF CODE PACKAGE**

Included are the referenced document in 10.a and a DS/HD diskette which includes the Fortran source and test case transmitted in both DOS and Unix compressed formats.

**12. DATE OF ABSTRACT**

April 2000.

** KEYWORDS:** HEAT TRANSFER; PWR