4. METHOD OF SOLUTION
An axial single channel model with equally spaced mesh intervals is used to represent the neutronics of the re- actor core. Two-group cross section data are normally supplied by means of file handling procedures that are automatically linked to lattice codes. The initial steady state calculation may be for a subcritical core with a fixed source or to determine critical poison concentration or moderator height. The neutronics solution has a number of unusual features to provide accuracy and rapid conver- gence.
A radial finite difference model is used for heat conduction through the fuel pin, gas gap and can. Appropriate convective, boiling or post-dryout heat transfer correlations are used at the can-coolant interface.
The hydraulics model includes the important features of the SGHWR primary loop including 'slave' channels in parallel with the 'mean' channel. Standard mass, energy and momentum equations are solved explicitly with backward values in time or space being used as appropriate. Circuit features modelled include pumps, spray cool- ing and the SGHWR steam drum. Steady state convergence is based on a search for the channel inlet coolant velocity that gives a circuit pressure balance; this is always very rapid and reliable.
Perturbations to almost any feature of the circuit model may be specified by the user although blowdown calculations resulting in critical or reversed flows are not permitted. Automatic reactor trips may be defined and the ensuing actions of moderator dumping and rod firing can be specified.
Two basic steady state calculations are possible. For the study of startup transients a fixed source distribution may be input, or a photoneutron source distribution calculated and used to determine the neutron flux in an initially subcritical core. The user speci- fies a required value of K-effective (less than unity) and the code simultaneously determines an appropriate multiplier on the fission yield cross sections to meet this requirement. The other type of steady state calculation is for a reactor at a specified power level (where fixed sources are normally ignored) and a critical search is made for the boron concentration or moderator height that gives K-effective equal to unity.