Comparison of nodal diffusion theory methods for analysis of main steam line break accidents in PWR

The unified nodal method (UNM) neutronics code, SUNCARDS, is coupled with the thermal hydraulics code, MARS, into a thermal-hydraulic/reactor kinetics coupled code, MARS/SUNCARDS. The coupled MARS/SUNCARDS code is then applied for the analysis of OECD/NEA pressurized water reactor main steam line break benchmark (MSLB) phase II and III problems. The fact that the UNM can produce several different nodal method solutions to two-group neutron diffusion equations is utilized to examine the effects of nodal computational solution options on the consequences of the MSLB accidents. It is shown that the predictions made by various nodal methods are almost indistinguishable in that the differences in the computational accuracy of different nodal solution methods do not show up in the various benchmark solutions. It is concluded that thermal hydraulic feedback effects may be described by the finer nodalization that may be compatible with nodal neutronics calculations. (author)