Evaluation of Measured LWR Spent Fuel Composition Data for Use in Code Validation End-User Manual

Burnup credit (BUC) is a concept applied in the criticality safety analysis of spent nuclear fuel
in which credit or partial credit is taken for the reduced reactivity worth of the fuel due to both fissile
depletion and the buildup of actinides and fission products that act as net neutron absorbers.
Typically, a two-step process is applied in BUC analysis: first, depletion calculations are performed
to estimate the isotopic content of spent fuel based on its burnup history; second, three-dimensional
(3-D) criticality calculations are performed based on specific spent fuel packaging configurations.
In seeking licensing approval of any BUC approach (e.g., disposal, transportation, or storage) both
of these two computational procedures must be validated. This report was prepared in support of
the validation process for depletion methods applied in the analysis of spent fuel from commercial
light-water-reactor (LWR) designs. Such validation requires the comparison of computed isotopic
compositions with those measured via radiochemical assay to assess the ability of a computer code
to predict the contents of spent fuel samples.
In the performance of a thorough validation effort for commercial spent fuel depletion, it is
necessary to assemble a database of appropriate measurement data for use in code comparison.
It is recommended that the following issues be considered with respect to such a database:
1. The database should be of a sufficient size to obtain a good statistical sampling of trends
between measured and calculated results.
2. The database should include a wide but applicable range in key depletion factors, including
burnup, initial U enrichment, moderator density, temperatures, soluble 235 boron concentration
(where appropriate), and core and assembly design.
3. Each set of data in the database should have a compilation of design and operating data as
complete as possible, with minimal but clearly identified assumptions.
4. The database should not overemphasize data taken from unusual or non-mainstream reactor
designs even when such data are readily available and in a usable form. This strategy will
avoid any biasing toward off-normal designs.
5. It is important to recognize that few (if any) measurements that are currently available were
performed with BUC validation as a consideration. Hence, the type of fuel samples analyzed
or the quality of the actual measurements may not be consistent with validation analysis
requirements. Such experiments should be avoided when possible, or only appropriate
subsets of data should be included. In addition, outliers found after an analysis is completed
may be excluded from statistical trending and determination of computational biases, but only
if the reason for outlying behavior can be identified, and would not be expected under normal
operating conditions.
The purpose of this report is to address the availability and appropriateness of measured data
for use in the validation of isotopic depletion methods. Although validation efforts to date at ORNL
have been based on calculations using the SAS2H depletion sequence of the SCALE code system,1
this report has been prepared as an overview of potential sources of validation data independent of
the code system used. However, data that are identified as $in use# in this report refer to earlier
validation work performed using SAS2H in support of BUC. This report 2,3 is the result of a study of
available assay data, using the experience gained in spent fuel isotopic validation to date and with a
consideration of the validation issues described earlier. This report provides recommendations for
the suitability of each set of data for validation work similar in scope to the earlier work.
Recommendations in this report are based solely on a broad ad hoc survey of available
literature and the expertise of the authors. In cases where a measurement is judged unsuitable
because of the lack of detailed information, it may be possible to obtain the missing data only through
a reactor facility, utility company, or government agency. No effort was made to acquire such data
in the performance of this literature survey, nor to assess the ease or difficulty of acquisition of such
data. Data sets were classified as very complete, fairly complete, or incomplete. "Very complete"
indicates minimal assumptions are necessary in model development. "Fairly complete" means that some assumptions are necessary to create a model, but that such assumptions are reasonable.
"Incomplete" data is missing essential parameters, and model development is not possible.
For completeness, this report also includes a description and ranking for each of the sets of data used
in the analyses reported in refs. 2 and 3, as well as data used in a follow-up report being prepared at
Oak Ridge National Laboratory (ORNL).
This report provides recommendations for further validation efforts for 19 reactors:
10 pressurized-water reactors (PWRs), 7 boiling-water reactors (BWRs), and 2 test reactors. The
recommendation for use for each set of chemical assay data is graded as In Use (used in refs. 2 and
3 or in ongoing analyses), Recommended, Not Recommended, and Insufficient Data. Explanations
are provided when data are categorized as Not Recommended. Experiments classified as Insufficient
Data appear to be consistent with the needs of BUC, but available documentation is inadequate, and
further operational and/or design data would be necessary to make a full recommendation.