type fuel element containing erbium-doped uranium-zirconium hydride (UZrHx -Er) fuel (see Chapter 2).

Neutronics Analysis

A number of key neutronics analyses were performed for a range of reactor core states, including the beginning-of-life, middle-of-life, and end-of-life states. These studies included analyses of:

•  Power distributions (for use in the thermal/hydraulic analyses), including (1) total fuel assembly power and core power distributions; and (2) axial and radial power distributions in the maximum power fuel assembly;

•  Shutdown margins as a function of fuel burnup; and

•  Key reactivity parameters, including (1) delayed neutron fraction2; (2) prompt neutron lifetime3; (3) control element worth4; and (4) prompt temperature coefficient.5

The neutronics of the reactor core were modeled using Los Alamos National Laboratory’s Monte Carlo n-Particle code, version 5 (MCNP5) with the core nuclear reaction database ENDF/B-VII maintained by the National Nuclear Data Center. In addition, Argonne National Laboratory’s REBUS codes for analysis of fuel cycles were used for the burnup analysis. Finally, some confirmatory analysis was performed using the HELIOS two-dimensional generalized-geometry lattice physics transport code.6

Several challenges were associated with performing these analyses at Wisconsin. First, sufficient information was not available on the operational history of the HEU core to be able to calculate fuel composition for use in benchmarking the model. As a substitute, analysts worked backwards to estimate the composition of the fuel using the current critical conditions for the core. This does not provide a benchmark but gives some confidence in the validity of the model. Second, large computing resources were required


2 Delayed fission neutrons are neutrons emitted spontaneously from decay of a fission product from a prior fission event, whereas prompt neutrons are neutrons emitted from the fission process directly. The delayed neutron fraction is the ratio of the mean number of delayed fission neutrons per fission to the mean total number of neutrons per fission (prompt plus delayed).

3 The prompt neutron lifetime is the average time between the emission of neutrons and either their absorption in or their escape from the system.

4 The control element worth is the negative reactivity change caused by inserting a control element into the reactor. UWNR has five separate control elements.

5 The prompt temperature coefficient is the change in reactivity per degree change in fuel temperature.

6 This confirmatory analysis was a two-dimensional deterministic analysis coupled with a one-dimensional diffusion approximation.

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