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Historical Overview: Computational Support for the UMLRR
Prof. John R. White

Early HEU to LEU Conversion Efforts
When I first arrived at the University in 1982 there were no computer models of the 1 MW UMass-Lowell research reactor (UMLRR). I began teaching some of the Reactor Physics courses in the Nuclear Engineering Program in 1985 and, shortly thereafter, my students and I started to develop some simple models of the existing HEU core configuration. At that time we used the LEOPARD code for cross section generation and the 2DB diffusion theory code for core physics studies.

In the late 1980s, we got involved in a program to convert the reactor from HEU fuel to a high-density uranium-silicide LEU fuel, with less than 20 w/o enriched U235. The early focus in this work was on benchmarking the models and methods using the startup data from the original HEU core. At this point we started using several modules from the AMPX system to generate cross sections for use with the VENTURE code for 2-D modeling of the reactor (both XY and YZ models were developed). Once the models and methods had been validated for the HEU core, our efforts then focused on determining the best assembly design and the most appropriate core configuration for the UMLRR using the new uranium silicide fuel. This work culminated in 1993 with the submission, and subsequent approval by the NRC, of a new LEU core design for the UMLRR. During this period, two students in particular, John Stoddard, Jr. and Robert Freeman, played pivotal roles in our HEU to LEU conversion work, and their hard work and creativity are still greatly appreciated!

Support for HEU Core Operations
After 1993 nothing happened for several years relative to the HEU to LEU conversion -- due primarily to a lack of funding to support fabrication of the new LEU fuel elements. In the interim, our efforts focused on the characterization of the existing HEU core and its experimental facilities. In particular, projects funded by North Atlantic Energy Corporation and Aspen Systems allowed us to continue upgrading our modeling and analysis capability for the UMLRR. During this time, we converted to using the SCALE system and the VITAMIN-B6 library for cross section generation and more detailed 1-D models in XSDRN-S and 2-D models in VENTURE and DORT were generated. A series of 47 neutron group and 20 gamma group 2-D DORT models, in particular, gave us our first look at the neutron and gamma space- and energy-dependent radiation fields throughout the reactor.

HEU to LEU Conversion Revisited
In 1999 the DOE authorized the fabrication of the uranium silicide LEU fuel elements for the UMLRR. We immediately embarked on a major effort to upgrade our LEU models from the early 1990s. The goals here were to provide a wide range of computational analyses to support the actual conversion and to fully characterize the experimental facilities for the new LEU core configuration. As part of this effort, all our original 2-D LEU core models were refined based on experience gained over the previous several years and a new detailed 3-D VENTURE model was generated. Some new visualization tools using Matlab were also developed. With the help of two students, Areeya Jirapongmed and Justin Byard, a complete pre-analysis was performed and, finally during the summer of 2000, the actual conversion took place. Everything went pretty much as planned and the LEU-fueled UMLRR has been in routine operation since August 2000. An overview of the actual startup evaluations and a comparison to our computer predictions was presented recently at the RERTR 2002 conference in San Carlos de Bariloche, Argentina -- thus, bringing closure to our HEU to LEU conversion project.

Fast Neutron Irradiator
In the summer of 2000 we also initiated a major effort to design, build, and install a large-volume experimental facility which would serve as a dedicated fast neutron irradiator (FNI) -- with a relatively high fast neutron fluence rate and relatively low gamma ray and thermal neutron components. This effort involved removing three beam ports from one side of the reactor and installing a completely new support grid and associated FNI components. A significant number of design calculations and the associated analyses were required to optimize the design of the new FNI facility. This project was recently completed in summer 2002, and the new FNI is now fully operational and ready for routine use for a variety of applications.

The Future
Who knows what the future will bring? For example, one of my students, Anthony Stevens, just completed an interesting study that addressed the safety implications of upgrading the UMLRR from a power level of 1 MW to 2 MW. In addition, the reactor control room has recently undergone a major renovation with a new digital control and data acquisition system. With just these two examples, it is clear that there are some real opportunities for additional research here!!! We shall see what develops…

Last updated by Prof. John R. White (January 2003)

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