ML20155C156
| ML20155C156 | |
| Person / Time | |
|---|---|
| Site: | 07003089 |
| Issue date: | 10/22/1998 |
| From: | Woolley R UNITED STATES ENRICHMENT CORP. (USEC) |
| To: | Pierson R NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM), NRC OFFICE OF NUCLEAR MATERIAL SAFETY & SAFEGUARDS (NMSS) |
| References | |
| L-98-011, L-98-11, NUDOCS 9811020095 | |
| Download: ML20155C156 (3) | |
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USEC A Global Energy Company Docket No. 070-03089 October 22,1998 L-98-011 Mr. Robert C. Pierson, Chief Special Projects Branch Division of Fuel Cycle Safety and Safeguards Office of Nuclear Materials Safety and Safeguards U. S. Nuclear Regulatory Commission ATFN: Document Control Desk Washington, D.C. 20555
Subject:
AVLIS Criticality Code Validation Report
Dear Mr. Pierson:
We appreciate NRC's willingness to review the AVLIS criticality code validation report submitted on April 22, 1998. Since we have not yet provided the Nuclear Criticality Safety Chapter of the license application (Chapter 5), we understand that some additional information defining 'our approach to criticality safety may be required for you to understand the scope of application for the validated methods.
The nuclear criticality safety (NCS) analyses of AVLIS Enrichment Plant systems are currently being performed according to general rules for AVLIS criticality safety analyses to be included in the Nuclear Criticality Safety Chapter of the AVLIS license application. To support your review of the validation report, these are included in the next two paragraphs.
AVLIS requires that process designs adhere to the double contingency principle. The NCS analyses identify and determine the NCS limits. When determining limits from calculations, the credible worst case (or most reactive) combination of special nuclear material density, H/X ratio, solutions concentration, reflection, interaction, interspersed moderation, and measurement uncertainty is considered before nuclear criticality safety limits are established.
These conservative bounding assumptions are made at each step in the analysis process.
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cumulative effect is to provide a large margin of safety.
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In the NCS analysis, a determination is made of the relationship of k,y,
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Mr. Robert C. Pierson October 22,1998 L-98-011 Page 2 l
controlled parameter.
This relationship, along with an assessment of uncertainty for the l
controlled parameter, and the ability to detect and control process variations that affect the l
controlled parameter, is used to establish adequate NCS limits. This gives the analyst an understanding of the sensitivity of k,m., to changes in the controlled parameters. For each l
controlled parameter, the values of the parameter that correspond to the Failure and Safety Limit are determined. The Failure Limit is defined as the lowest point at which the system may be i
. critical. The calculation of the Failure Limit includes the uncertainties in the benchmark experiments, the cross section sets, and the computer code system. The Safety Limit is set below the Failure Limit as an added margin of safety (Failure Limit - 0.02). Both the Safety Limit and
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Failure Limit include appropriate allowances for any bias and uncertainty in data and calculation methods used as demonstrated in the validation, and are based on conservative modeling assumptions, as mentioned earlier. The analyst is responsible for ensuring that the SCALE 4.3 code is validated for the forms, moderation, materials, enrichment, and energy spectra in the bounding safety limit calculations.
The NCS analyses use the bias and margin of suberiticality specified in the validation report.
l The NCS analyses identify and explicitly determine the adequacy of the designs, controls, limits, and bounding assumptions necessary to satisfy the double contingency principle. For AVLIS, calculations may be performed to support uranium production at enrichments up to 10 wt.% "U.
2 The calculated k,m., values must' include appropriate allowance for any bias and bias L
uncertainty in data and calculation methods used as demonstrated in the validation report.
The codes and cross sections that make up the SCALE 4.3 system are based on physics principles and sixty years of nuclear physics research into cross sections and reaction mechanisms. This is not an empirical activity but one with a solid theoretical and experimental physics basis. The benchmarks provide checks on the physics included in the codes and cross sections for the materials and configurations ofinterest, and define the overall uncertainty in the process. It is not necessary or even possible that the benchmark experiment set cover every parameter over the full range of every other parameter in the AVLIS area of applicability.
~ We have selected benchmarks that adequately cover the AVLIS range of applicability, including the intermediate fission energy range. We acknowledge that the number of experiments in the j
intermediate energy range is limited. However, we believe (as demonstrated in our presentation to the NRC on July 16, 1998) that the experiments currently included in the validation report adequately cover AVLIS criticality safety analyses in the intermediate energy spectrum.
l When considering separator operations with temperatures high enough to vaporize uranium, the i
credible worst case scenario occurs only when water is added to the system. Without water, the
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Mr. Rober. C. Pierson t
October 22,1998 L-98-011 Page 3 system is substantially suberitical. Further, we know that high temperature leads to Doppler broadening of resonance cross sections, with a corresponding decrease in self-shielding, resulting in an. increase in resonance absorption.
This decreases the reactivity of the system for enrichments below 20 wt.%. When water and water vapor are present, the temperature will be near the boiling point. Therefore, the SCALE 4.3 code system will not be used to calculate very high temperature uranium systems, and will be used for elevated temperatures close to the boiling point of water, where cross sections are well known and benchmarks exist.
AVLIS materials, designs, and operating conditions as currently designed are covered by the validation report. This, plus the general rules for AVLIS criticality safety form the safety basis for criticality analysis of AVLIS systems. Thus we are requesting endorsement of the SCALE 4.3 codes and cross sections applied to AVLIS materials and systems, within the safety basis.
This correspondence contains no new commitments. Please contact me (301-564-3413) if you have any questions concerning this matter.
Sincerely, f
Y Robert Woolley Manager, AVLIS Nuclear Regulatory Policy and Licensing i
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