Methods of Criticality Control By Mohammed S Saba

Methods of Criticality Control By Mohammed S. Saba Department of Nuclear Safety, Physical Security and Safeguards Nigerian Nuclear Regulatory Authority, Abuja NNRA In-house Virtual Training on Criticality Safety Management

Objectives Explain the main factors that affect criticality Describe how these factors can be controlled Understand the implications of choice of criticality control State the hierarchy of criticality controls Briefly describe why fissile material assay is used for fissile material accountancy 2/1/2022 2

Factors affecting criticality 2/1/2022 3

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Factors affecting criticality…. . 1. Mass [/Volume] ØIf there is insufficient fissile mass, criticality will not be achievable ØNote that mass may refer to total mass of an element (i. e for U, the total mass of U-235 and U 238) 2. Enrichment [/Isotopic composition] Ø As U is enriched from natural to low (<20%), intermediate (LEU) or high levels (HEU) the fuel mass required for criticality is reduced Ø Increased conc. of particular isotopes 2/1/2022 5

Factors affecting criticality…. . 3. Concentration [/Density] Ø As fissile isotopes become more dispersed, the chances of them seeing neutrons released in the fission of another fissile isotope is reduced Ø There is more than enough fissile uranium in seawater that, if accumulated, construction of a critical assembly would be possible. However, its so widely dispersed, criticality does not result Ø This is different to enrichment 2/1/2022 6

Factors affecting criticality…. . 4. Geometry [/ Volume] Ø The smaller the surface area to volume ratio, the lower the probability of neutron leakage and thus the higher the keff Ø Spherical (or near-spherical) shapes are worst Ø A given mass of fissile material may achieve criticality if accumulated in a cylindrical drum but not if dispersed across the floor. 2/1/2022 7

Factors affecting criticality…. . 5. Absorption [ or Poisons] Ø Presence of absorbers will compete for neutrons with fuel Ø Certain types of absorber (neutron poisons) like boron are often mixed in solution with, or sometimes incorporated in the containment of, fissile materials Ø Control elements (i. e. rods) can also be used to maintain sub-criticality 2/1/2022 8

Factors affecting criticality…. . 6. Moderation ØThe presence of hydrogenous (or other low atomic number, i. e. Be, C) materials will slow down fast neutrons and thus increase the probability of fissile capture 2/1/2022 9

Factors affecting criticality…. . 7. Reflection Ø Moderating materials can also reflect escaping neutrons back into a system containing fissile materials 2/1/2022 10

Factors affecting criticality…. . 8. Interaction ØClosely related to absorption and scattering, competing nuclear reactions will also compete for neutrons with fuel ØSometimes they may also lead to the introduction of additional neutrons (i. e. muon interactions) ØAll interactions will be sensitive to temperature (density) changes through changes in mean free path and microscopic cross-section. 2/1/2022 11

Criticality Safety by Design • The possibility of a criticality excursion is best considered (but not always possible) during the design stage of a facility • Designing safety into equipment is more reliable than to base safety on process or procedures • Engineered controls can include safe geometry containers, small volume vessels, spaced arrays, etc. 2/1/2022 12

Double Contingency Factor Philosophy • Two or more engineered controls may be employed in a facility, ideally controlling independent routes to criticality • This independence is the basis of the double contingency philosophy • Operators should incorporate sufficient safety features so that two unlikely, independent, concurrent changes would have to occur …. before the system can become “Critical”. 2/1/2022 13

Passive or active controls Ø Whilst engineered controls may include the use of neutron absorbers, it is safer, if these are passive (i. e. boronated concrete) walls rather than active (i. e. a movable control rod) Ø Active controls require additional controls to ensure their continued effectiveness. 2/1/2022 14

Instrumented controls • To monitor the continued safe condition of each of • • the eight factors affecting criticality, pressure sensors, flow rate meters, and radiation detectors may be employed in nuclear facilities Through use of control logic, these sensors control elements (i. e. valves) within the plant These systems must have known reliability as well as independence, diversity, redundancy and separation 2/1/2022 15

Example of instrumented controls • In a nuclear facility with safe geometry tanks • • • containing at high concentration fissile solutions an earthquake might be a common cause failure This may lead to damage of the tanks causing the release of their contents to the floor The sprinkler system may be activated adding additional moderator to the floor, presenting a criticality hazard Seismometers detect event and close isolation valves and trip supplies that would cause the overfill 2/1/2022 16

Operational controls Controlled movements • Fissile materials are moved around such that a critical mass or distance is not exceeded Batch control • The concentration or mass per batch is limited • The number of batches may be limited Moderator exclusion • i. e. preventing the introduction of additional moderators 2/1/2022 17

Operational controls… Material accountancy • The amounts and location of fissile materials within a facility is known and logged at all times Process monitoring • Concentrations and masses, etc, are monitored during and/or after each batch is processed Written procedures • Personnel follow a pre-assessed safe procedure without deviation 2/1/2022 18

Hierarchy of criticality controls 1. 2. 3. 4. Passive engineered controls (lowest risk) Active engineered controls Instrumented controls Operational controls (highest risk) 2/1/2022 19

Independence of controls In-line with the double contingency factor, there needs to be more than one independent criticality control, i. e. • Safety geometry containers (passive engineered) • Control by neutron absorbers (active engineered) • Batch control (operational control) 2/1/2022 20

Sub-Critical Limits • Limiting value assigned to a controlled parameter that • • results in a sub-critical system under specified conditions Allows for uncertainties in calculations But not for contingencies (double batching etc) 2/1/2022 21

Operating Limits Operational limits may deviate from sub-critical limits due to the incorporation of additional safety factors Actual systems may allow limits to be increased 2/1/2022 22

Single and Multiple Parameter Limits Compliance with single parameter limits assures subcriticality for any operation However, limits are small for fuel cycle facilities Multiple parameter limits provide substantial relief but need additional operational controls, e. g. verification of concentration 2/1/2022 23

Assemblies Real operations, usually involve storage of multiple units at some stage For criticality control need to consider • Changes in the moderator, i. e. flooding, spraying, oil, snow, or the introduction of bubbles between rows • Changes in the coupling between units and reflectors such as introduction of additional units/ reflectors, improper stacking of units, or collapse of frameworks 2/1/2022 24

Choice and selection of criticality controls Handling of a single batch of fissile material with a mass below that specified for sub-criticality will provide the least criticality risk It is not normally economical to build a facility with this degree of restriction Another economic consideration is the use of engineered controls – whilst they provide the most robust means criticality control, they can also be over -engineered 2/1/2022 25

Choice and selection of criticality controls… Cost-effective solutions are acceptable as long as risks are demonstrated to be ALARP Complexity of systems or written procedures may lack user-friendliness or be difficult to understand Simplicity is normally the best option Like all engineering solutions – criticality control involves some degree of trade off 2/1/2022 26

Complexity Some plants may be so complex that the route to criticality may be difficult to foresee (e. g. UKAEA Windscale accident in B 203, 1970) What is considered safe (and approved) is, in fact, not safe! Defence in depth is important Shielding and alarms are essential back up. No lives lost at Windscale 1970. 2/1/2022 27

Conclusions • Criticality accident risks will not disappear as long as significant quantities of fissile material exists. • However, with appropriate support from management, diligence on the part of criticality staff and operators, and adherence to codified fundamental safety principles and guidance, accident likelihood can be maintained at the current low level or possibly reduced further. 2/1/2022 28

Conclusions (2) 2/1/2022 29

References 1. LANL. 2000. A Review of Criticality Accidents. LA-13638. US Do. E, Washington D. C. May 2000. 2. IAEA Criticality Safety in the Handling of Fissile Material. SSG-27. IAEA, Vienna. May 2014. 3. Bowen , D. G. , & Busch , R. D. (2006, November). Hand Calculation Methods for Criticality Safety – A Primer. 4. NTEC N 13 - Criticality Safety Management Lecture note by Dr Kirk Atkinson, 2016. 2/1/2022 30

THANK YOU 2/1/2022 31