Improved methods of determining temperature transients in reactor systems are desired because of recent interest in Pressurized Thermal Shock (PTS) issues. The research presented herein was performed in support of the Nuclear Regulatory Commission's effort to re-evaluate its existing PTS rules. These rules are particularly important to the re-licensing of aging nuclear power plants. The much advanced computational power available to industry may offer a tool that allows the accurate calculation of temperatures inside the reactor vessel while not being inaccessibly expensive. It is proposed that an off-the-shelf Computational Fluid Dynamic (CFD) code, STAR-CD, can be a competitive tool in solving the thermal hydraulic domain of a reactor system. A comparison of the methodology and accuracy of the code types that have been previously used in PTS and one that has not been used extensively, CFD, is provided. A review of the literature shows that computer codes have been validated for solving PTS scenarios. The highly specialized program, REMIX, has been utilized extensively from 1986 to 1991 to interpret accident scenarios in reactor systems. Other programs are also available that can calculate downcomer temperatures including system and CFD type codes. Three codes representing the three different types of programs available are described in detail in the literature review section. Data appropriate for assessing a program's ability to calculate the response of a system to a PTS scenario is available from the current matrix of PTS tests being completed at the APEX-CE facility of the Oregon State University Nuclear Engineering department. The facility is a reduced scale integral test facility originally built for modeling the then-proposed AP-600 plant designed by Westinghouse. For the current test series, the facility was modified to model the Palisades nuclear power plant, a Combustion Engineering Pressurized Water Reactor (PWR). Two of the tests were chosen for their PTS typical conditions to compare with calculations of STAR-CD, REMIX, and RELAP. The computer models in each of the programs were either created, modified from a previous version, or the calculations for the comparisons were contributed. The downcomer temperatures at several locations and cold leg temperature gradients, where available, were extracted from the data and calculations and compared. Comparisons are presented in chapter 5 with graphs, along with some interpretation of the comparisons. It was found that STAR-CD agreed best with the data set in the downcomer and is the only program that calculated the temperature gradient in the cold legs. The agreement of STAR-CD with the cold leg data is also very good. REMIX and RELAP calculations agreement with data for downcomer temperatures are found to be good for all comparisons made, qualitatively more than quantitatively when contrasted with the STAR-CD calculations. / Graduation date: 2002