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Known Issues with SCALE 6.2

Topic: Nuclear Science and Technology

Since SCALE 6.2 was released on April 28, 2016, end users and the SCALE development team have identified a few issues that impact the performance of the code package in some cases. Several of these issues, the most important ones, are addressed with the SCALE 6.2.1 Update, which is recommended for all SCALE 6.2 users. The issues not resolved by this update are planned to be addressed in a further patch to the current release or in the next release of SCALE. The identified issues are presented below, along with suggested user approaches to overcome existing limitations.

KENO V.a Requires Cuboidal Outermost Region to Enable the Use of Albedo Boundary Conditions

In all versions of SCALE, the Monte Carlo code KENO V.a only implements the use of non-vacuum albedo boundary conditions (e.g., mirror, periodic, white) when the outermost geometry region of the model is a cuboidal region. This limitation is noted in the user documentation in the section on Albedo data, where it is stated that “Albedo boundary conditions are applied only to the outermost region of a problem. In KENO V.a this geometry region must be a rectangular parallelepiped.”

It was recently discovered that—beginning with the release of SCALE 6.1 in 2011—KENO V.a will accept non-compliant input that specifies albedo boundary conditions for non-cuboidal outer shapes and will then attempt to complete the calculation. For example, a user can specify a cylinder as the outermost region and add a mirror boundary condition on the top or bottom to effectively double the volume of the system considered. A user could also add a mirror boundary condition to both the top and the bottom of the cylinder to simulate a bounding case of an infinite system. While these scenarios are accepted and perform as expected in KENO-VI, KENO V.a requires the addition of a cuboidal region (typically an empty void region) to enable the use of these albedo boundary conditions.

For calculations using KENO V.a in SCALE 6.1–6.2.2 with non-compliant input in which albedo boundary conditions are applied but without the required cuboidal outermost region, the calculation will proceed without warning, and an underestimation of k-eff often results. The magnitude of underestimation in k-eff can vary widely, depending on the system modeled and the desired boundary conditions, but it can exceed several percent in k-eff.

It is strongly recommended that users who rely on albedo boundary conditions in KENO V.a review their input models to ensure that the outermost region is a cube or cuboid, per the documentation requirement. Note that input models that were generated and applied with SCALE 6 and earlier versions that included the check for the cuboidal outer boundary will continue to produce the expected results with SCALE 6.1–6.2.2.

In testing the extent of this issue by placing mirror boundary conditions on non-cuboidal outer shapes, it was found that cylinders oriented along the x-, y-, or z-axis most often produce non-conservative results without warning. The calculation will terminate prior to completion for cases in which a sphere is the outermost shape. The calculation will terminate with an error message for cases in which a hemicylinder or hemisphere is the outermost shape. The calculation performs as expected for cases in which a cube or cuboid is the outermost shape.

This issue applies to all SCALE 6.1–6.2.2 sequences that implement KENO V.a, including CSAS5, TSUNAMI-3D-K5, T5-DEPL, and STARBUCS. No other SCALE sequences are impacted by this issue. The error condition for the attempted use of albedo boundary conditions on non-cuboidal outer shapes in KENO V.a will be restored in the pending release of SCALE 6.2.3, thus preventing users from inadvertently entering non-compliant input.

Incorrect power normalization for TRITON timetable SWAP cases

Corrected in SCALE 6.2.1

The TRITON t-depl sequence in SCALE 6.2 contains a new capability to “swap” materials in the timetable block. This capability is designed to allow users to swap in and out control rods and burnable absorber materials during depletion simulations. A bug was recently discovered in the “swap” implementation that causes an incorrect power normalization. Specifically, it was observed that power is incorrectly applied to materials that are swapped out, resulting in an incorrect power normalization for all materials. This bug may not be apparent to users, because the materials associated with the swap function are correctly placed in the geometry and the calculated k-eff behavior may look correct; however, the power assigned to those materials is incorrectly calculated. The magnitude of the impact increases with increasing fuel burnup. The error can be seen by inspecting the power summary table in the TRITON output file (search for “Material powers” or “Transport k”). In the “Total Power” column of this table, users will notice that materials that are swapped out at a certain time still have power applied to them. This issue is corrected in SCALE 6.2.1. SCALE 6.2 users are advised to refrain from using the SWAP function until receiving the SCALE 6.2.1 update.

Incorrect depletion analysis when using TRITON ASSIGN

Corrected in SCALE 6.2.1

An error that impacts depletion analyses that use the ASSIGN function was introduced into TRITON for SCALE 6.2. The error leads to incorrect calculation of mass and volume for some of the depleted materials, and consequently of the specific power for these materials. The error was introduced in SCALE 6.2 and corrected in SCALE 6.2.1. Users are advised to refrain from using the TRITON ASSIGN function until receiving the SCALE 6.2.1 update.

Multigroup temperature interpolation issues for threshold reactions

Corrected in SCALE 6.2.1

SCALE provides continuous energy cross section libraries at several different temperatures, the highest temperatures being between 1200K and 2400K. A new Doppler broadening method was implemented into CRAWDAD for generating temperature-dependent PW cross section libraries for CENTRM calculations. Internal studies show that the new method is superior for generating cross section libraries at temperatures between 1200K and 2400K compared to the previous interpolation method (based on linear interpolation as a function of square root of temperature). However, an issue was identified for the new method when generating a cross section for a threshold reaction such as Th-232 fission. In this scenario, the new method produces small negative cross sections at or near the energy threshold of the reaction (i.e. where the cross section approaches zero). CRAWDAD was updated to set the cross section to zero in this type of scenario. Calculations where negative cross sections are produced will not run to completion and other calculations are not affected. This issue is resolved in the SCALE 6.2.1 update.

Incorrect units printed in the TRITON system mass summary output table

Open Issue

The TRITON depletion sequences produce a system mass summary output table (see SCALE 6.2 documentation Section One column header of this table states “Fractional HM Mass (g)”. The values of this column correspond to fractional heavy metal (HM) mass, which is a unitless quantities. Users should be aware that the units are not grams as the column header suggest. Moreover, the system mass summary table provides units for the HM mass of each material as well as the units of the normalization factor required to normalize the mass to 1 metric tonne of HM in the system. For example, for the 2D depletion (t-depl) sequence, the units of the HM mass should be “g/cm” and the unit of the normalization should be “cm”. Whereas,for the 3D KENO sequences (t5-depl and t6-depl), the displayed units should be “g” and “no units”. An issue was identified in TRITON that the units displayed in the table are always “g/cm” and “cm” regardless of the geometry's dimension. Users should be aware that for 3D depletion calculations (and 1D slab or sphere depletion), the output file displays the wrong for the mass summary table. However, the data in the tables correspond to units that are based on the geometry’s dimension, as indicated in the example above for 2D and 3D depletion. This issue is NOT addressed in the SCALE 6.2.1 update.

Change in branch order in the TRITON output file

Designed Output Change

The TRITON branch calculations have been modified in SCALE 6.2 compared to previous releases. In previous SCALE versions, the nominal calculation (i.e. branch 0) was first performed, followed by the branch calculations (branch 1 through branch N). This order was also reflected in the generated “txtfile16” file, which archives few-group cross sections at each time step and branch step. In SCALE 6.2, TRITON performs the branch calculations before the nominal calculation. The change in the order that the nominal and branch calculations are performed was applied in SCALE 6.2 to better facilitate the new in-memory design of the TRITON depletion data structures. It is important to note that this change only impacts the calculation order, not the archive order in the txtfile16. Therefore, any post-processing utility designed for previous versions of TRITON remain valid. This marks a permanent change in the calculation order in TRITON and users should be aware that the output file and txtfile16 are ordered differently.

Resonance self-shielding for unreferenced compositions in TRITON calculations

Change in Default Behavior

SCALE has historically treated unreferenced compositions (i.e. compositions that are not explicitly identified in the CELLDATA block) as infinite homogeneous media for multigroup resonance self-shielding calculations. Unreferenced compositions undergo BONAMI and CENTRM self-shielding calculations based on the infinite homogeneous treatment (if parm=bonami, only BONAMI is called). To reduce run-time for depletion calculations, TRITON has historically performed only BONAMI infinite homogeneous media self-shielding calculations for the unreferenced compositions, irrespective of the specification of "parm". TRITON in SCALE 6.2 further reduces runtime by treating the compositions as infinitely dilute instead of as an infinite homogeneous media, meaning that resonance self-shielding calculations are not performed unless the composition is explicitly identified in the CELLDATA block. In most cases this update has a minor effect on results, as most compositions require self-shielding definitions. However, several compositions (e.g. structural materials) may be unreferenced, and the infinitely dilute treatment can lead to some noticeable effects. To test for consistency, infinite homogeneous media self-shielding can be explicitly added to the cell block for any unreferenced composition (e.g. "inf 10 end" to specify infinite homogeneous medium treatment for mixture 10 or "inf 10 20 30 end" to treat mixtures 10, 20, and 30 using only one line of input). Additionally, users can test for consistency by comparing eigenvalue differences between TRITON and CSAS calculations, as CSAS continues to provide the traditional approach of treating unreferenced compositions as infinite homogeneous media. This issue is NOT addressed in the SCALE 6.2.1 update.