Archived Web Page: Draft Regulatory Document RD-364Joint Canada - United States Guide for Approval of Type B(U) and Fissile Material Transportation Packages

6.0 CRITICALITY EVALUATION

This section of the application should identify, describe, discuss, and analyze the principal criticality safety design of the package, components, and systems important to safety and describe how the package complies with the requirements of 10 CFR 71.15, Exemption from Classification as Fissile Material, 10 CFR 71.55, and 10 CFR 71.59 and Paragraph 528 of TS-R-1 as referenced in Subsection 16(4) of the PTNS Regulations, Paragraph 671 of TS-R-1 which is incorporated in Paragraph 7(1)(a) of the PTNS Regulations by reference to Paragraph 813 of TS-R-1, Paragraphs 672 of TS-R-1 as referenced in Subsection 1(1) of the PTNS Regulations and Paragraphs 673-682 of TS-R-1 which are incorporated in Subsection 1(1) of the PTNS Regulations by reference to Paragraph 672 of TS-R-1.

This section should address the structural and thermal effects on the packaging and its content under normal and hypothetical accident conditions in terms of material and geometry changes and subsequent effect on the criticality safety design. Any operational, fabrication, and maintenance requirements with respect to criticality safety importance for the package should be included in the application in Section 7, Package Operations, and Section 8, Acceptance Tests and Maintenance Program.

6.1 Description of Criticality Design

This section should provide a description of the criticality design which would include provisions in 10 CFR 71.31, Contents of Application, and 10 CFR 71.33 and Paragraphs 807 and 813 of TS-R-1 as referenced in Paragraph 7(1)(a) of the PTNS Regulations.

6.1.1 Design Features

This section should describe the design features of the package that are important for criticality control. Packaging design features important for criticality include the following:

1. Dimensions and tolerances of the containment system for fissile material;
2. Structural components that maintain the fissile material and neutron poisons in a fixed position within the package and in a fixed position relative to each other;
3. Location, dimensions, and concentration of neutron absorbing materials and moderating materials, including neutron poisons and shielding material;
4. Dimensions and tolerances of floodable voids and flux traps within the package;
5. Dimensions and tolerances of the overall package that affect the physical separation of the fissile material contents in package arrays; and
6. Information on control rod assemblies, shrouds, or other fuel assembly components included with fresh fuel or SNF as applicable to the criticality evaluation.

All information presented in the text, drawings, figures, and tables should be consistent with each other and with that used in the criticality evaluation. The drawings are the authoritative source of dimensions, tolerances, and material composition of components important to criticality safety.

6.1.2 Summary Table of Criticality Evaluation

This section should provide a summary table of criticality analysis results for the package for the following cases, as described in Sections 6.4 through 6.6:

1. A single package under the conditions of 10 CFR 71.55(b), (d), and (e) or Paragraphs 677, 678, and 679 of TS-R-1 which are incorporated in Subsection 1(1) of the PTNS Regulations by reference to Paragraph 672 of TS-R-1;
2. An array of undamaged packages under the conditions of 10 CFR 71.59(a)(1) or Paragraph 681 of TS-R-1 which is incorporated in Subsection 1(1) of the PTNS Regulations by reference to Paragraph 672 of TS-R-1; and
3. An array of damaged packages under the conditions of 10 CFR 71.59(a)(2) or Paragraph 682 of TS-R-1 which is incorporated in Subsection 1(1) of the PTNS Regulations by reference to Paragraph 672 of TS-R-1.

The maximum value of the effective neutron multiplication factor (k_eff), any stochastic uncertainty, the biases and associated uncertainties, and the number of packages evaluated in the arrays should be specified in the table. The table should also show that the sum of k_eff, two standard deviations, and the biases with their associated uncertainties adjustment does not exceed 0.95 for each case. Therefore, a package is considered subcritical, under the regulatory conditions, if it can satisfy the following relationship:

k_eff + 2σ + Δk_u ≤ 1 - Δk_m Eq. 6-1

Where

keff	=	the calculated value obtained for the package or array of the packages
σ	=	is the standard deviation of the keff value obtained with Monte Carlo analysis (the value for this parameter is set to zero if a deterministic method is used)
Δku	=	an allowance for the calculation bias and uncertainty as discussed in Section 6.8
Δkm	=	a required margin of subcriticality (minimum of 0.05 depending on the sensitivity of keff to the system parameter uncertainties)

Therefore, Equation 6-1 can be rewritten as follows:

k_eff + 2σ + Δk_u ≤ 0.95 Eq. 6-2

6.1.3 Criticality Safety Index

This section should provide the Criticality Safety Index (CSI) in accordance with 10 CFR 71.59(b) or Paragraph 528 of TS-R-1 as referenced in Subsection 16(4) of the PTNS Regulations, based on the number of packages evaluated in the arrays and show how it was calculated.

The CSI should be consistent with that reported in Section 1, General Information, of the application. The value of N, which is the number of packages allowed for transport, should be specified.

6.2 Fissile Material Contents

This section should describe in detail the fissile material allowed in the package in accordance with10 CFR 71.33 or Paragraphs 807 and 813 of TS-R-1 as referenced in Paragraph 7(1)(a) of the PTNS Regulations. This section should describe in detail the fissile material in the package.

Specifications for the contents used in the criticality evaluation (e.g., shielding, thermal, containment) should be consistent with those in Section 1, General Information, and throughout the application. Specifications relevant to the criticality evaluation should include fissile material mass, dimensions, enrichment, physical and chemical composition, density, moisture, and other characteristics dependent on the specific contents. Any differences from the specifications in Section 1 or other sections should be clearly identified and justified. Because a partially filled container may allow more room for moderators (e.g., water), the most reactive case may be for a mass of fissile material that is less than the maximum allowable contents. Therefore, a minimum mass may have to be specified.

If the package is designed for multiple types of contents, the application may include a separate criticality evaluation and propose different criticality controls for each content type. Any assumptions that certain contents need not be evaluated because they are less reactive than evaluated contents should also be properly justified.

For SNF, specifications relevant to criticality evaluation should include the following:

1. Types of fuel assemblies, plates, or rods (e.g., BWR/PWR) and vendor/model as appropriate;
2. Dimensions of fuel (including any annular pellets), cladding, fuel-cladding gap, pitch, and rod length;
3. Number of rods or plates per assembly and locations of guide tubes and burnable poisons;
4. Materials and densities;
5. Active fuel length;
6. Enrichment (variation by rod if applicable) before irradiation;
7. Chemical and physical form;
8. Mass of initial heavy metal per assembly or rod;
9. Number of fuel assemblies or individual rods per package; and
10. Other components when included in the criticality analysis or which have non-negligible effect on k_eff.

The conditions of the SNF assemblies, including missing or replaced fuel rods, should be described. In general, the description of the contents should be sufficient to permit a detailed criticality evaluation of each type or to support a conclusion that certain types are bounded by the evaluations performed. If the contents include damaged fuel, the maximum extent of damage should be specified. Any cans or canisters used as part of the content for the packaging should be described.

6.3 General Considerations

This section should address general considerations used to evaluate the criticality of the package. These may apply to the criticality evaluations of a single package and arrays of packages under both normal conditions of transport and hypothetical accident conditions.

6.3.1 Model Configuration

This section should describe and provide sketches of the calculation model used in the calculations. The sketches should identify the materials used in all regions of the model. Any differences between the model and the actual package configuration should be identified and justification given that the model is conservative. In addition, the differences between the models for normal and accident conditions of transport should be clearly identified and justified.

Within the specified tolerance range, dimensions should be selected to result in the highest reactivity. For example, cavity sizes and poison thickness should be considered in the manner that maximizes reactivity.

Deviations from nominal design configuration should be considered. For example, the contents of a powder packaging can be positioned at varying locations and densities, the fuel assemblies might not always be centered in each basket compartment, and the basket might not be exactly centered in a spent fuel package. The relative location and physical properties of the contents within the packaging should be justified as those resulting in the maximum multiplication factor.

For fuel assembly packages, the fully flooded scenario should address preferential flooding and include flooding of the fuel-cladding gap. In addition, variable water density should be considered for possible system reactivity peaks.

Due to the capabilities of modern computer codes, homogeneous modeling should not be used. If homogenization is used in the model, this section should demonstrate that it is applied correctly and/or conservatively.

6.3.2 Material Properties

This section should provide the appropriate mass densities and atomic number densities for materials used in the models of the packaging and contents. Material properties should be consistent with the condition of the package under the tests specified in 10 CFR 71.71 and 10 CFR 71.73 or Paragraphs 719-724 and 726-729 of TS-R-1 which are incorporated in Subsection 1(4) of the PTNS Regulations by reference to Paragraph 716 of TS-R-1. Materials relied on for criticality control must remain in place and be effective.

No more than 75% of the specified minimum neutron poison concentration should generally be considered in the criticality evaluation unless a higher percentage can be justified.

The differences in material condition between normal conditions of transport and hypothetical accident conditions should be clearly identified. Materials relevant to the criticality design, such as poisons, foams, plastics, and other hydrocarbons, should specifically be addressed.

Neutron absorbers and moderators (e.g., poisons and neutron shielding) should be properly controlled during fabrication to meet their specified properties. Such information should be discussed in more detail in Section 8 Acceptance Tests and Maintenance Program, of the application.

Materials should not degrade during the service life of the packaging to the point affecting adversely the package performance. Specific controls should be in place to ensure effectiveness of the packaging during its service life. Such information should also be discussed in more detail in Section 8 of the application.

6.3.3 Computer Codes and Cross-Section Libraries

This section should describe the basic methods used to calculate the effective neutron multiplication constant of the package to demonstrate compliance with the fissile material package standards. This should address the following:

1. A description of the computer program and neutron cross-sections used;
2. The basis for selecting the specific program and cross-sections; and
3. Key input data for the criticality calculations, such as neutrons per generation, number of generations, convergence criteria, and mesh selection.

At least one representative input file and one output file (or key sections of these files) for a single package, undamaged array, and damaged array, should be included in the application. The calculation should properly converge and the calculated multiplication factors from the output files should agree with those reported in the evaluation.

6.3.4 Demonstration of Maximum Reactivity

This section should include a demonstration that the most reactive configuration of each case listed in Sections 6.4 through 6.6 (single package, arrays of undamaged packages, and arrays of damaged packages) has been evaluated. All assumptions and approximations should be clearly identified and justified.

For packages with multiple SNF assembly types for the content, all assembly types should be analyzed or the bounding fuel assembly type should be justified and analyzed.

This section should identify the optimum combination of internal moderation (within the package) and interspersed moderation (between packages) as applicable. The following should be considered:

1. Moderation by water and any hydrogen-containing packaging materials, such as polyethylene;
2. Preferential flooding of different regions within the package; and
3. Partial loadings (i.e., fissile masses less than the maximum allowable mass).

6.3.5 Burnup Credit for Irradiated Fuel Packages

In designing the criticality control system for irradiated fuel packages, if the applicant is relying on the reduced reactivity of fuel assemblies due to the depletion of fissile material and production of neutron absorbing isotopes, biases and uncertainties in predicting isotopic inventory and reactivity of irradiated fuel assemblies in packages should be established. This is further explained in Section 6.8 of this document. The amount of irradiation necessary with respect to reactivity to load the fuels into packages should be presented as a function of initial enrichment and any restrictions on the conditions during irradiation. In addition, a final independent burnup verification measurement should be performed prior to loading irradiated fuel assemblies into packages for shipment.

Both 10 CFR 71.83, Assumptions as to Unknown Properties, and Paragraph 673 of TS-R-1 which is incorporated in Subsection 1(1) of the PTNS Regulations by reference to Paragraph 672 of TS-R-1 require that when the properties of the fissile material are not known, those properties that give the maximum neutron multiplication will be assumed.

In addition, Paragraph 674 of TS-R-1 which is incorporated in Subsection 1(1) of the PTNS Regulations by reference to Paragraph 672 of TS-R-1 requires that:

1. The irradiation history that results in maximum neutron multiplication shall be assumed; and
2. Prior to shipment, a measurement for confirming the conservatism of the isotopic composition shall be made.

The applicant should include descriptions of benchmarking experiments used to establish biases and uncertainties associated with depletion and criticality models used for the irradiated fuel in the packaging, the bounding irradiation history parameter values, and the method for burnup verification measurement.

6.4 Single Package Evaluation

6.4.1 Configuration

This section should demonstrate that, as a design basis in accordance with 10 CFR 71.55(b) or Paragraph 677 of TS-R-1 which is incorporated in Subsection 1(1) of the PTNS Regulations by reference to Paragraph 672 of TS-R-1, a single package is subcritical under normal or accident conditions, whichever is more reactive, with the following assumptions:

1. Fissile material in its most reactive credible configuration consistent with the condition of the package and the chemical and physical form of the contents;
2. Water moderation to the most reactive credible extent, including water in-leakage to the containment system as specified in 10 CFR 71.55(b) or Paragraph 677 of TS-R-1; and
3. Full water reflection on all sides of the containment system as specified in 10 CFR 71.55(b)(3) or Paragraph 678 of TS-R-1 which is incorporated in Subsection 1(1) of the PTNS Regulations by reference to Paragraph 672 of TS-R-1 or reflection by the package materials, whichever results in the maximum reactivity.

Paragraph 678 of TS-R-1 requires that the “confinement system” shall be closely reflected by at least 20 cm of water or such greater reflection as may additionally be provided by the surrounding material of the packaging. The “confinement system” consists of those components of the packaging that maintain the geometric configuration of the fissile material inside the packaging with respect to criticality safety. In addition, 20 cm of water is believed to be equivalent to a “full reflection” as inferred from 10 CFR 71.55(b)(3). However, if a layer of water thicker than 20 cm makes the system more reactive, the larger water thickness should be used as part of the design basis.

In addition, this section should also demonstrate, in accordance with 10 CFR 71.55(d) or Paragraph 679(b) of TS-R-1 which is incorporated in Subsection 1(1) of the PTNS Regulations by reference to Paragraph 672 of TS-R-1, that the package content would be subcritical when the package is subjected to the normal conditions of transport.

Furthermore, this section should also demonstrate, in accordance with 10 CFR 71.55(e) or Paragraph 679(c) of TS-R-1 which is incorporated in Subsection 1(1) of the PTNS Regulations by reference to Paragraph 672 of TS-R-1, that the package content would be subcritical when the package is subjected to the accident conditions of transport.

6.4.2 Results

This section should present the results of the single package evaluation and should also address the additional specifications of 10 CFR 71.55(d)(2)-(d)(4) or Paragraph 679 of TS-R-1 which is incorporated in Subsection 1(1) of the PTNS Regulations by reference to Paragraph 672 of TS-R-1 under normal conditions of transport.

The results of the most reactive case for the single package analysis should be consistent with the information presented in the summary table discussed in Section 6.1.2. If the package can be shown to be subcritical by reference to a standard, the standard should be applicable to the package conditions.

6.5 Evaluation of Package Arrays under Normal Conditions of Transport

6.5.1 Configuration

This section should evaluate, in accordance with 10 CFR 71.59(a)(1) or Paragraph 681 of TS-R-1 which is incorporated in Subsection 1(1) of the PTNS Regulations by reference to Paragraph 672 of TS-R-1, an array of 5N packages under normal conditions of transport. The evaluation should consider the following factors:

1. The most reactive configuration of the array (e.g., pitch and package orientation) with nothing between the packages;
2. The most reactive credible configuration of the packaging and its contents under normal conditions of transport (noting that if the water spray test has demonstrated that water would not leak into the package, water in-leakage need not be assumed for this case); and
3. Full water reflection on all sides of a finite array.

6.5.2 Results

This section should present the results of the analyses for arrays and identify the most reactive array conditions. The results of the analysis should be consistent with the information presented in the summary table discussed in Section 6.1.2.

The appropriate N value should be used in determination of the CSI. The appropriate N should be the smaller value that ensures subcriticality for 5N packages under normal conditions of transport or 2N packages under hypothetical accident conditions, as discussed in the next section.

6.6 Package Arrays under Hypothetical Accident Conditions

6.6.1 Configuration

This section should evaluate, in accordance with 10 CFR 71.59(a)(2) or Paragraph 682 of TS-R-1 which is incorporated in Subsection 1(1) of the PTNS Regulations by reference to Paragraph 672 of TS-R-1, an array of 2N packages under hypothetical accident conditions. The evaluation should consider the following factors:

1. The most reactive configuration of the array (e.g., pitch, package orientation, and internal moderation);
2. Optimum interspersed hydrogenous moderation;
3. The most reactive credible configuration of the packaging and its contents under hypothetical accident conditions, including in-leakage of water; and
4. Full water reflection on all sides of a finite array.

6.6.2 Results

This section should present the results of the analyses for arrays and identify the most reactive array conditions. The results of the analysis should be consistent with the information presented in the summary table discussed in Section 6.1.2.

The appropriate N value should be used in determination of the CSI. The appropriate N should be the smaller value that ensures subcriticality for 5N packages under normal conditions of transport or 2N packages under hypothetical accident conditions.

6.7 Fissile Material Packages for Air Transport

6.7.1 Configuration

This section should evaluate a single package under the expanded accident conditions specified in 10 CFR 71.55(f) or Paragraph 680 of TS-R-1 which is incorporated in Subsection 1(1) of the PTNS Regulations by reference to Paragraph 672 of TS-R-1. The evaluation should consider the following factors:

1. The most reactive configuration of the contents and packaging under the expanded accident conditions;
2. Full water reflection; and
3. No water in-leakage.

6.7.2 Results

This section should present the results of the analyses for the single package and identify the most reactive contents and packaging conditions. The results of the analysis should be consistent with the information presented in the summary table discussed in Section 6.1.2.

6.8 Benchmark Evaluations

This section should include a description of the methods used to benchmark the criticality calculations. The computer codes for criticality calculations should be benchmarked against critical experiments. The same computer code, hardware, modeling methodology, and cross-section library used to calculate the effective multiplication factor values for the package should be used in the benchmark experiments. This section should present the results of calculations for selected critical benchmark experiments to justify the validity of the calculation method and neutron cross-section values used in the analysis.

The International Handbook of Evaluated Criticality Safety Benchmark Experiments, issued September 2005 [13], provides a source for selecting applicable critical experiments in benchmarking the computer codes and cross-sections used for designing the packages.

6.8.1 Applicability of Benchmark Experiments

This section should describe selected critical benchmark experiments that were analyzed using the method and cross-sections given in Section 6.3. This section should show the applicability of the benchmarks in relation to the package and its contents, noting all similarities and resolving all differences. The benchmark experiments should have, to the maximum extent possible, the same material, neutron spectrum, and configuration as the package evaluations. Key package parameters that should be compared with those of the benchmark experiments include type of fissile material, enrichment, H/X atomic ratio (dependent largely on rod pitch and diameter for fuel assemblies), poisoning, reflector material, and configuration. References that give full documentation on these experiments should be provided. Computer codes such as Tools for Sensitivity and UNcertainty Methodology Implementation (TSUNAMI), developed by Oak Ridge National Laboratory as part of the SCALE 5.1 package [12], may be used to assess similarities of packages with critical systems for benchmarking purposes.

The overall quality of the benchmark experiments and any uncertainties in experimental data should be addressed. The uncertainties should be treated in a conservative manner. Results of the benchmark calculations as well as the actual nuclear and geometric input parameters used for those calculations should be provided.

6.8.2 Bias Determination

This section should present the results of the benchmark calculations and the method used to account for biases and uncertainties in calculations (i.e., Δk_u in Eq. 6-2) and the contribution from uncertainties in the experimental data. This section should show a sufficient number of appropriate benchmark experiments and that the results of the benchmark calculations were appropriate to determine the bias for the package calculations. In search of biases, parameters such as pitch-to-rod diameter, assembly separation, and neutron absorber material should be considered. Statistical and convergence uncertainties should be addressed. Only negative biases (results that under predict k_eff) should be considered, with positive bias results treated as zero bias.

In quantifying Δk_u for computer codes and cross-sections used in designing burnup-credit spent fuel packages, biases and uncertainties from both depletion and criticality computer codes should be included. In addition, biases due to axial and horizontal variation of the burnup within a spent fuel assembly should be considered. Furthermore, the effects of reactor operating history on the reactivity of discharged spent fuel assemblies should be addressed.

6.9 Appendix

The appendix should include a list of references, applicable pages from referenced documents, justification of assumption or analytical procedures, test results, photographs, computer code descriptions, input and output files, and other supplemental information. Input files for representative or “most limiting” cases for a single package and arrays of damaged and undamaged packages should specifically be included.