MODULE 4.0: NUCLEAR CRITICALITY SAFETY CONTROLS

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MODULE 4.0: NUCLEAR CRITICALITY SAFETY CONTROLSPurposeWelcome to Module 4.0 of the Nuclear Criticality Safety DirectedSelf-Study Course! This is the fourth of five modules in this directedself-study course. The purpose of this module is to assist you inidentifying nuclear criticality safety controls and limits, and inevaluating the appropriateness of selected applications in the workenvironment.This directed self-study module is designed to assist you inaccomplishing the learning objectives listed at the beginning of themodule. There are seven sections in this module plus supplementalreading material. The module has self-check questions and activitiesto help you assess your understanding of the concepts presented inthe module.Before You BeginIt is recommended that you have access to the following material:Trainee GuideComplete the following prerequisite:Module 3.0 Nuclear TheoryHow to Completethis Module1. Review the learning objectives.2. Read each section within the module in sequential order.3. Complete the self-check questions and activities within thismodule.4. Check off the tracking form as you complete the self-checkquestions and/or activities within the module.5. Contact your administrator as prompted for a progress reviewmeeting.6. Contact your administrator as prompted for any additionalmaterials and/or specific assignments.7. Complete all assignments related to this module.8. Ensure that you and your administrator have dated and initialedyour progress on the tracking form.9. Go to the next assigned module.

Module 4.0: Nuclear Criticality Safety ControlsTABLE OF CONTENTSLearning Objectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Nuclear Criticality Safety . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .How Is Nuclear Criticality Safety Achieved? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .How Is This Control Generally Exercised? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Nuclear Criticality Accident . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Self-Check Questions 4-1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Nuclear Criticality Safety Control Factors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Mass . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Control Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Workplace Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Volume . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Control Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Workplace Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Enrichment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Control Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Workplace Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Geometry or Shape . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Control Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Workplace Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Interaction and Separation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Control Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Workplace Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Moderation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Control Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Workplace Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Reflection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Control Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Workplace Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Concentration and Density . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Control Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Workplace Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Neutron Absorbers or Poisons . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Control Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Workplace Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Heterogeneity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Control Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Workplace Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Self-Check Questions 4-2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Types of Controls . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Administrative Controls . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Engineered Controls . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Definitions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .How NCS Control Factors Work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Activity 1 - Control by Practice/Equipment Design . . . . . . . . . . . . . . . . . . . . . . . . . . . .Self-Check Questions 4-3 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Hierarchy of Nuclear Criticality Safety Control Factors . . . . . . . . . . . . . . . . . . . . . . . .Primary Controls . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .USNRC Technical Training CenterNuclear Criticality 4-264-264-274-274-294-304-314-310905 (Rev 3)Directed Self-Study

Module 4.0: Nuclear Criticality Safety ControlsSecondary Controls . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Activity 2 - Hierarchy of NCS Control Factors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Self-Check Questions 4-4 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Double Contingency Principle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Key Elements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Definitions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Example . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Interpretation Issues . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Selection of Nuclear Criticality Control Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Evaluation Criteria for Compliance with Double Contingency . . . . . . . . . . . . . . . . . . .Facility Considerations for Control Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Example of an Application . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Activity 3 - Hierarchy of Control Parameters for Double Contingency Application . . . .Activity 4 - Loss of Nuclear Criticality Control at Fuel Fabrication Facility . . . . . . . . . .Activity 5 - Nuclear Criticality Safety Measurements Not Performed for EnrichedUranium Solution Storage Tanks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Activity 6 - Potential Loss of Nuclear Criticality Controls . . . . . . . . . . . . . . . . . . . . . . .Activity 7 - General Electric, Nuclear Fuel and Components Manufacturing (NFCM) .Activity 8 - Large Deposit of UO2 F2 at the K-25 Site . . . . . . . . . . . . . . . . . . . . . . . . . .Activity 9 - Thermosyphon Evaporator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Activity 10 - Arc Melt Furnace . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Self-Check Questions 4-5 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Conservatism . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .General Considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .What Are the Potential Pitfalls of Excessive Conservatism? . . . . . . . . . . . . . . . . . . . .Examples of Conservative Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Activity 11 - Conservatism in Nuclear Criticality Safety . . . . . . . . . . . . . . . . . . . . . . . .Self-Check Questions 4-6 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Nuclear Criticality Accident Alarms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Purpose . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Regulatory Guidance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Background . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Components . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Detection Criterion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Neutron vs Gamma Detectors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Criticality Accident Alarm System Logic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Failure Warning . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Testing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .How Does a Criticality Alarm System Work? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Activity 12 - Criticality Accident Alarms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Self-Check Questions 4-7 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Progress Review Meeting Form . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Module Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .USNRC Technical Training CenterNuclear Criticality 804-804-804-814-834-854-870905 (Rev 3)Directed Self-Study

Module 4.0: Nuclear Criticality Safety ControlsLIST OF TABLESTable 4-1.Table 4-2.Table 4-3.Table 4-4.Table 4-5.How Nuclear Criticality Safety Control Factors Work . . . . . . . . . . . . . . . . . . . .Factors in Seven Nuclear Criticality Accidents in U.S. Processing Plants . . . . .Example of the Application of the Double Contingency Principle . . . . . . . . . . .Examples of Conservative Applications in NCS Analyses . . . . . . . . . . . . . . . . .Comparison of Regulatory Guide 3.71 and ANSI/ANS 8.3 . . . . . . . . . . . . . . . .4-284-324-424-724-77LIST OF FIGURESFigure 4-1.Figure 4-2.Figure 4-3.Figure 4-4.Figure 4-5.Figure 4-6.Figure 4-7.Figure 4-8.Figure 4-9.Figure 4-10.Figure 4-11.Figure 4-12.Figure 4-13.Figure 4-14.Figure 4-15.Figure 4-16.Figure 4-17.Figure 4-18.Figure 4-19.Nuclear Criticality Safety Control Factors . . . . . . . . . . . . . . . . . . . . . . . . . . . .Mass . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Volume . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Enrichment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Geometry or Shape: Neutrons Leak Out and Do Not Cause Fissions . . . . .Geometry or Shape: Neutrons Causing Fissions . . . . . . . . . . . . . . . . . . . .Interaction and Separation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Moderation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Examples of Good Moderators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Reflection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Examples of Reflection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Concentration and Density . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Neutron Absorbers or Poisons . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Heterogeneity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Examples of Double Contingency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Fault Tree Example . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Solvent-Extraction Process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Nitrate Waste and Solvent-Extraction Process Waste Flow . . . . . . . . . . . . .Schematic of Deposit Location Showing Nondestructive Assay NeutronMeasurement Count Rates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Figure 4-20. Thermosyphon Evaporator (Process Engineer's Drawing) . . . . . . . . . . . . . .Figure 4-21. Arc Melt Furnace . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Figure 4-22. Power Trace for the French CRAC 13 . . . . . . . . . . . . . . . . . . . . . . . . . . . . .USNRC Technical Training CenterNuclear Criticality -174-194-214-384-414-504-514-564-644-674-780905 (Rev 3)Directed Self-Study

Module 4.0: Nuclear Criticality Safety ControlsLearning Objectives 4.14.1.1Upon completion of this module, you will be able to identifynuclear criticality safety controls and limits and recognizeexamples of workplace applications.Define nuclear criticality safety.4.1.2Describe nuclear criticality safety control factors.4.1.3Identify examples of control factors.4.1.4Identify administrative and engineered controls.4.1.5Identify the hierarchy of control factors.4.1.6Distinguish between primary and secondary controls.4.1.7Define the double-contingency principle.4.1.8Define the role of contingencies in establishing nuclearcriticality safety limits.4.1.9Identify conservatism as it relates to nuclear criticality safety.4.1.10 Distinguish between neutron and gamma criticality alarms.4.1.11 Given a scenario, determine compliance with NRC RegulatoryGuide 3.71 and ANSI/ANS 8.3.USNRC Technical Training CenterNuclear Criticality Safety4-10905 (Rev 3)Directed Self-Study

Module 4.0: Nuclear Criticality Safety ControlsLearning ObjectiveWhen you finish this section, you will be able to:4.1.1Define nuclear criticality safety.NUCLEARCRITICALITYSAFETYIntroductionNuclear criticality safety has been defined as the protection againstthe consequences of an inadvertent nuclear chain reaction, preferablyby the prevention of the reaction. Nuclear criticality safety foruranium facilities involves both preventive measures to maintainprocess conditions and mitigative measures to reduce the overall riskof nuclear criticality.How Is NuclearCriticality SafetyAchieved?Nuclear criticality safety is achieved by controlling one or more factorsof the system within subcritical limits.How Is This ControlGenerallyExercised?Control may be exercised:Administratively through procedures (e.g., by requiring that amass not exceed a posted limit)By physical restraints (e.g., by confining a solution to acylindrical vessel with diameter no greater than the subcriticallimit)Through the use of instrumentation (e.g., by keeping a fissileconcentration below a specific limit by devices that measureconcentration and prevent its buildup through reflux in achemical system)By chemical means (e.g., by prevention of conditions thatallow precipitation, thereby maintaining concentrationcharacteristic of an aqueous solution)By relying on the natural course or credible course of events(e.g., by relying on the nature of a process to keep the densityof uranium oxide less than a specified fraction of theoretical)Nuclear CriticalityA nuclear criticality accident is an unintentional, uncontrolled nuclearUSNRC Technical Training CenterNuclear Criticality Safety4-20905 (Rev 3)Directed Self-Study

Module 4.0: Nuclear Criticality Safety ControlsAccidentfission chain reaction. All operations with fissile materials should beperformed to prevent the establishment of nuclear chain reactionsand the sudden release of energy. When released, this energy wouldbe in the form of heat and ionizing radiation. The ionizing radiationmight be lethal to nearby personnel. Also, damage to equipmentcould possibly cause fission products generated by the incident toescape. Damaged equipment could result in the interruption ofoperation schedules and could lead to the release of sufficientmaterial to provide a contamination problem or an environmentalhazard.The achievement of nuclear criticality safety depends on controllingeither the mass of the fissile material or neutron behavior.USNRC Technical Training CenterNuclear Criticality Safety4-30905 (Rev 3)Directed Self-Study

Module 4.0: Nuclear Criticality Safety ControlsSelf-Check Questions 4-1Complete the questions. Answers are located in the answer key section of the Trainee Guide.1. What is nuclear criticality safety?2. Nuclear criticality safety control may be exercised by:3. Nuclear criticality safety is dependent on controlling either:You have completed this section.Please check off your progress on the tracking form.Go to the next section.USNRC Technical Training CenterNuclear Criticality Safety4-40905 (Rev 3)Directed Self-Study

Module 4.0: Nuclear Criticality Safety ControlsLearning ObjectivesWhen you finish this section, you will be able to:4.1.2Describe nuclear criticality safety control factors.4.1.3Identify examples of control factors.NUCLEARCRITICALITYSAFETY CONTROLFACTORSIn order to maintain a system in a subcritical state, factors that canaffect nuclear criticality must be controlled. These factors are: MassVolumeEnrichmentGeometry and ShapeInteraction and separationModerationReflectionConcentration and densityNeutron absorber or poisonsHeterogeneityThese factors are interdependent. See Figure 4-1. Changes made toone of the factors may affect the parameters on the other factors. Inthis module, each factor is presented separately and it is assumedthat everything else remains constant.Figure 4-1. Nuclear Criticality Safety Control FactorsGeometryor ShapeInteraction &SeparationMass & tion &DensityNeutronAbsorbers orPoisonsHeterogeneityThecombinedeffects of several factors determine whether an array of nuclear material canbecome critical. Changes made to one of the factors may affect the parameters onthe other factors.USNRC Technical Training CenterNuclear Criticality Safety4-50905 (Rev 3)Directed Self-Study

Module 4.0: Nuclear Criticality Safety ControlsMASSMass is the amount of fissionable material present. See Figure 4-2.The critical mass is the amount of a fissionable material that willsupport a fission chain reaction. The critical mass varies for differentfissionable elements and different fissionable isotopes. Mass controlis generally discussed with regard to solids. If the amount of materialis kept small enough, neutrons will escape and the nuclear criticalityis prevented.Control ExamplesControl examples may include:Established limits on the amount of fissionable materialallowedVolume favorable designsWorkplaceApplicationsWorkplace applications for mass may include:Container identificationPosted mass limitsHoles in waste basketsLimited-size containersFigure 4-2. MassUSNRC Technical Training CenterNuclear Criticality Safety4-60905 (Rev 3)Directed Self-Study

Module 4.0: Nuclear Criticality Safety ControlsVOLUMEVolume is generally used for control of solutions or powders. Limitsare placed on the capacity of containers available for use in an area.Smaller containers allow neutrons to escape more easily. Containersshould be geometrically favorable for specific fissionable material.See Figure 4-3.Control ExamplesControl examples may include:Fixed geometry by constructionEstablished limits on the amount of fissionable materialallowedGeometrically favorable designsWorkplaceApplicationsWorkplace applications for volume may include:Limited-volume process equipmentContainer identificationPosted volume limitsLimited-size containersHoles in waste basketsOverflow proceduresFigure 4-3. VolumeUSNRC Technical Training CenterNuclear Criticality Safety4-70905 (Rev 3)Directed Self-Study

Module 4.0: Nuclear Criticality Safety ControlsENRICHMENTEnrichment refers to the percentage of fissionable isotope. Enrichedmaterial has more than the normal or natural amount of thefissionable isotope. The higher the enrichment, the smaller the massneeded to sustain a chain reaction. When U-235 atoms becomemore plentiful in a given mass of uranium, the distance between themis smaller and they are more likely to be struck by a neutron andfission. When all other conditions are equal, the higher theenrichment, the smaller the mass required to cause a nuclearcriticality. See Figure 4-4.Control ExamplesControl examples may include:Equipment, operation, or storage conditions must be designedfor the highest enrichment that could possibly be usedORVery stringent separation of areas/facilities handling differentenrichmentsOption 1: Treat all materials as if maximum enrichment.Option 2: Use higher limits for lower-enriched materials.WorkplaceApplicationsWorkplace applications for enrichment may include:Color codingComputerized Special Nuclear Materials (SNM) inventoryContainer sizePhysical separation of areasUnique fittingsPhysically distinct containersMultiple samplingAccountability practicesUSNRC Technical Training CenterNuclear Criticality Safety4-80905 (Rev 3)Directed Self-Study

Module 4.0: Nuclear Criticality Safety ControlsFigure 4-4. EnrichmentUSNRC Technical Training CenterNuclear Criticality Safety4-90905 (Rev 3)Directed Self-Study

Module 4.0: Nuclear Criticality Safety ControlsGEOMETRY ORSHAPEGeometry refer to the size and shape of the material or container.For a given type of material, geometrically favorable designs are usedto achieve a large ratio of surface area to volume so the neutronshave a greater chance of escaping. See Figure 4-5.Figure 4-5. Geometry or Shape: Neutrons Leak Out and Do NotCause FissionsFor a given amount of material, the sphere is the most reactiveshape. The sphere has the smallest surface area for its volume;therefore, it is considered the least favorable shape. A neutrongenerated inside has a better chance of causing other fissions beforeit escapes into the environment. See Figure 4-6.Control ExamplesControl examples may include:Geometrically favorable dimensions that confine the materialsto subcritical limitsWorkplaceApplicationsWorkplace applications for geometry may include:Long, thin bottles; storage tanks; and extraction columnsPlanar arraySpecially designed equipment that maintains the desiredspacingSlab tanksUSNRC Technical Training CenterNuclear Criticality Safety4-100905 (Rev 3)Directed Self-Study

Module 4.0: Nuclear Criticality Safety ControlsSlab trays (for fuel rods)Figure 4-6. Geometry or Shape: Neutrons Causing FissionsNOTE:"Favorable geometry" means favorable for safety. It doesNOT mean favorable for sustaining a fission chain reaction.INTERACTION ANDSEPARATIONInteraction occurs when neutrons from one location can reach andenter material at another location. Individual units are kept separatedat distances determined by the Nuclear Criticality Safety (NCS) staff.When two or more subcritical units are brought closer to each other,they may become critical since each may gain extra neutrons. SeeFigure 4-7.Control ExamplesControl examples may include:Specially designed equipment that maintains the desiredspacingWorkplaceApplicationsWorkplace applications for interaction and separation may include:USNRC Technical Training CenterNuclear Criticality SafetyStorage racks that allow only specific numbers of containerswith predetermined spacingMechanisms that secure storage containers only atpredetermined and properly spaced locations4-110905 (Rev 3)Directed Self-Study

Module 4.0: Nuclear Criticality Safety ControlsStated separation criteriaBirdcagesMovement control procedures“Bumpers” to maintain spacing between carts and/or racksLimit to the number of cartsFigure 4-7. Interaction and SeparationMODERATIONModeration is the ability of a material to slow down a neutron. If aneutron hits a nucleus of equivalent mass, it can lose almost all of itsspeed. If it hits a heavier nucleus, it will not be slowed down asmuch.It is much harder for a fast-moving neutron to cause a fission. Theslowing down of neutrons increases the probability of causing afission. The fissile atom can "catch" a slow neutron more easily thana fast neutron. It's similar to a golfer trying to sink a putt. SeeUSNRC Technical Training CenterNuclear Criticality Safety4-120905 (Rev 3)Directed Self-Study

Module 4.0: Nuclear Criticality Safety ControlsFigure 4-8.Figure 4-8. ModerationItems containing hydrogen and/or carbon are good moderators.Hydrogen nuclei are similar in mass to neutrons. Carbon, while notas low in mass as hydrogen, is also a very effective moderatorbecause it absorbs neutrons poorly. See Figure 4-9.Figure 4-9. Examples of Good ModeratorsUSNRC Technical Training CenterNuclear Criticality Safety4-130905 (Rev 3)Directed Self-Study

Module 4.0: Nuclear Criticality Safety ControlsControl ExamplesControl examples may include:Limiting or excluding moderating materialsControlling concentrationWorkplaceApplicationsWorkplace applications for moderation may include:Water-tight containersSloped covers on storage racksAdequate drainageFire fighting equipment that does not use waterDoors shut upon initiation of sprinklersMoisture samplingGlove box logbooksPersonnel access controlStorage racks that allow only specific numbers of containerswith predetermined spacingControl oil for gear boxesREFLECTIONReflection refers to the bouncing back of neutrons into the fissionablematerial, providing subsequent chances to produce a fission. SeeFigure 4-10. Escaping neutrons continue moving away in a straightline unless they hit something in their path. Under these conditions,less fissionable material is needed to become critical.USNRC Technical Training CenterNuclear Criticality Safety4-140905 (Rev 3)Directed Self-Study

Module 4.0: Nuclear Criticality Safety ControlsFigure 4-10. ReflectionControl ExamplesControl examples may include:Maintaining spacing limits to control proximity of reflectors (forexample, floors, walls, ceilings) to fissionable materialsUsing physical barriersWorkplaceApplicationsWorkplace applications for reflection may include:Adequate drainageFire fighting equipment that does not use waterMechanisms that secure storage containers only atpredetermined and properly spaced locations with adequateseparation from walls, floors, and ceilingsPersonnel access controlsComposition controls (for example, use of poisoned reflectormaterials, such as cadmium sheets)Metal guardsFigure 4-11 shows examples of good reflectors.WaterOilPlastic 1 inch in thicknessUSNRC Technical Training CenterNuclear Criticality Safety4-150905 (Rev 3)Directed Self-Study

Module 4.0: Nuclear Criticality Safety ControlsGraphiteThe human bodyIn addition, all construction materials, such as

Nuclear Criticality Safety Directed Self-Study Learning Objective When you finish this section, you will be able to: 4.1.1 Define nuclear criticality safety. NUCLEAR CRITICALITY SAFETY Introduction Nuclear criticality safety has been defined as the protection against the conseque

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