HFIR dissolution nuclear criticality analysis information

Abstract

The LTB function covers an H/X range of 90 < H < 2,800. The function was found over its range: LTB (90) = 0.993 and LTB (2,800) = 1.001. This indicates that the lowest possible LTB for these plutonium solutions is found at the lowest H:X value and is 0.993, which is higher than the LTL for uranium solutions. Therefore, an LTL of 0.990 (implies bias = 0.01) for uranium solutions over the range of applicability provided in SRNS-IM-2009-00035 is used. 2.6.3 Minimum Subcritical Margin (MSM) The biased k eff does not incorporate a MSM. The MSM, by definition, must be large enough to protect against reasonable changes to the system being analyzed leading to an unsafe condition using the evaluated k safe. The MSM depends on several items and is used to ensure subcriticality for systems with calculated k eff less than k safe. SRNS-H8200-2014-00021 Attachment 1-HFIR Dissolution Nuclear Criticality Analysis Page 12 of 71 2.7 VALIDATION OF ORNL MODEL In order to validate that the SCALE model provided by ORNL is a reasonable and accurate model for criticality safety evaluations, SCALE was used to perform a Monte Carlo integration of the volumes of the many units in the model. Along with calculating the volume of each unit, SCALE also calculates the total mass of each unit and provides this data in a table in the output. SCALE also provides a "mixing table" in the output which gives the weight fraction of each nuclide for every material number. These two pieces of data (overall mass and nuclide-specific weight fractions) can be combined to determine how much mass is being modeled in the input. When this technique was performed on the ORNL model of HFIR, the SCALE output mass calculations agree to within 1 gram 235 U based on the Appendix A mass values for both the inner and the outer element. This indicates that the ORNL model is a reasonable and accurate model for criticality safety evaluations. 2.7.1 SCALE 6 vs. SCALE 5 The original input provided by ORNL was written for SCALE 6, and it included many isotopes that do not exist in the validated SCALE 5 cross section libraries. To run the input in SCALE 5, all the isotopes were removed (commented out) such that only H, O, Al, and the U isotopes remained. Three SCALE 6 cases were run, 1.) with all original isotopes, 2.) with only boron removed, 3.) with all isotopes removed except H, O, Al, and U isotopes. The lowest k eff is for the case with all the original isotopes. When boron is removed, k eff increases. When the remaining isotopes are removed, k eff increases further. This indicates that modeling the fuel as consisting of H, O, Al, and U overestimates the calculated k eff , and use of the modified SCALE 5 inputs is conservative. 3.0 NORMAL CONDITIONS 3.1 WITH FRESH ACID The mass of the inner element is 2,595.8 g 235 U. The mass of the outer element is 6,804.4 g 235 U. Also, there are 4,571.7 moles of Al in each HFIR core. A core consists of an inner element and an outer element. These values are given in the HFIR NCSE Scope of Work Memo, SRNS-H8100-2014-00004 and have be...