Criticality Safety Limits and Controls

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Criticality SafetyLimits and Controls

• Based on Notes from the
• Criticality Safety Short Course
• University of New Mexico
• Howard Dyer - ORNL

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Overview - Scope of Topic

• Difference between a limit and a control
• Identification of limits (parameters)
• Physical
• Nuclear
• Safety Margin
• Identfication of controls
• Administrative
• Engineered
• Implementation of limits and controls

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Limits and Controls

• A limit is (usually) the numerical value of a
  parameter that must be controlled within a
  defined allowable range
• Example limits with numerical values
• Max. 1000 g U
• Max. 1 liter container
• Min. 24 in. E-T-E separation
• H/U lt 0.088

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Limits and Controls

• Example limits without numerical values
• For solutions only
• Performed only on day shift
• Verify .... before starting
• Provide drain holes in plastic bags
• A control is the apparatus, instructions,
  process, actions, etc., by which the limit is
  maintained within its allowable range.

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Limits

• Subcritical limits vs. actual process limits
• Subcritical limits (from ANS Standards)
• Just subcritical
• No contingencies considered
• No margin of safety
• About 1 margin of subcriticality (beyond the
  uncertainties)
• Actual process limits
• Must account for contingencies
• Usually must define a safety margin in terms of
  margin of safety or margin of subcriticality
• Many parameters may need to be limited
• Usually permits an increase in the ANS Standards
  subcritical value because multiple parameters are
  being controlled

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Actual process limits

• What is available to be limited?
• What value should it be limited to?
• How to determine the value of the limit?
• What safety margin should be included?
• margin of safety
• margin of subcriticality
• How to proceed from identifying limits to
  identifying controls?

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Nine parameters for control
1. Mass 2. Density 3. Assay 4. Geometry -
Volume - Dimensions 5. Reflection 6.
Interaction 7. Moderation - Internal -
Interstitial 8. Composition 9. Neutron absorbers
amount of fissile material ( n
generation) neutron escape ( n
leakage) neutron absorption ( absorption)
physical characteristics
nuclear characteristics
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Parameters are not independent variables

• A change in MASS causes a change in VOLUME
• A change in DENSITY causes a change in
  COMPOSITION and GEOMETRY
• A change in INTERNAL MODERATION causes a change
  in COMPOSITION, DENSITY, and VOLUME
• A change in any parameter will probably cause
  changes in at least one other parameter
• A single parameter may impact more than one
  nuclear characteristic, e.g. INTERNAL MODERATION
  changes absorption generation

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Nuclear characteristics are not independent

• A change in the rate of neutron production will
  cause a change in the rate of neutron absorption
  and leakage
• A change in the rate of neutron leakage will
  cause changes in the rate of neutron production
  and absorption
• A change in the rate of neutron absorption will
  cause changes in the rate of neutron production
  and escape

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Calculating keff
The calculation of keff is determined from the
nuclear characteristics of the system, which are
influenced by the physical parameters
rate of n prod
keff
rate of n abs rate of n leakage
kinf

1 (M2)(Bg2)
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Crit. Safety Analysts Responsibility
The criticality safety analyst (you) must
understand the interdependency of changes in a
parameter upon the other parameters, and the
effect these changes will have upon the nuclear
characteristics of the system.
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Parameter studies

• Parametric study - how keff changes as a
  parameter changes
• Two aspects to be considered
• Sensitivity of keff to the change
• Selection of the value (may be impacted by the
  method of control)

keff
f( )
13

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Parameter studies...

• Key fix as many of the parameters as possible
  remember that other parameters may not be
  independent
• Example A 4-liter beaker is to be used to
  dissolve oxide in acid.
• What is/can be fixed (non-variable)?
• Assume 100 assay 3
• GEOMETRY (vol. and dims.) 4
• Assume full REFLECTION 5
• Single unit, no INTERACTION 6
• Assume no NEUTRON POISONS 9
• COMPOSITION (oxide and acid) 8

Relative proportions of oxide and acid will
vary will be considered in MODERATION or DENSITY
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Parameter studies...

• Example What is variable?
• MASS 1
• DENSITY 2
• INTERNAL MODERATION 7

For this example, DENSITY f(
MODERATION) MODERATION f( DENSITY)
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Example Full Beaker
Mass U varies VOLUME constant
x
keff
x
x
0
high
H/U
Density U
low
high
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Example Fixed Mass Beaker
Mass U constant VOLUME varies
10 kg
keff
5 kg
1 kg
0
high
H/U
high
Density U
low
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Safety Margin

• What safety margin (in terms of margin of safety
  or margin of subcriticality) should be included?
• Examples
• Reference to facility guides and handbooks
• Y-1272, GAT-225, etc. (with margin of safety
  included in data)
• Use of standard hand calculational techniques
  (solid angle, limiting surface density, etc.)
• Computer code calculations (keff, with parameters
  limited to satisfy some margin of subcriticality
  criteria)
• Normal conditions, keff 2s lt 0.90
• Accident conditions, keff 2s lt 0.95

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Safety Margin

• Examples, cont
• Comparison to critical or subcritical data
• ANS Standards, TID-7016, TID-7028, LA-10860-MS,
  ARH-600, etc.
• Safety margin?

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Selecting the parameter limit
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Controls

• Nuclear criticality safety is achieved by
  exercising control over
• the mass and distribution of the fissile
  material, and
• the mass and distribution of all other materials
  associated with the fissile material
• Parametric study (comparison to critical or
  subcritical data, reference to safety guides,
  keff calculations, etc.) identifies
• what to control (the parameters) and
• the limits (values) of the parameters, but
• not how to control them

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Types of controls

• Engineered controls
• Active - Elec./Mech./Hyd./Pneu. actuated hardware
  that senses a process variable and provides an
  automatic response.
• Passive - Constructed such that
  human/Elec./Mech./Hyd./Pneu. intervention is not
  needed to maintain subcriticality during
  off-normal conditions
• Administrative controls
• Any action(s) required which is dependent upon
  operator performance

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Controls

• Active Engineered Controls (sensor
  activated)Examples
• thermostat turns heater off
• liquid level sensor starts pump
• load cell closes supply line
• pressure switch starts pump
• computer controlled

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Controls

• Active Engineered ControlsConcerns
• the sensor (tolerance, drift, calibration)
• the actuator (motive force, time response)
• pre-operational verification
• importance to subcriticality (redundancy)
• detection of malfunction or failure (of the
  sensor and of the actuator)
• maintenance and configuration control

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Controls

• Passive Engineered Controls (fixed by
  design)Examples
• favorable (safe) geometry
• rigid/fixed spacing
• fixed poisons (Raschig-rings)
• equipment limitations
• natural forces gravity (vacuum breaks), physical
  chemistry

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Controls

• Passive Engineered ControlsConcerns
• initial design
• correct installation
• pre-operational verification
• continued effectiveness
• detection of change

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Controls

• Administrative Controls (operator
  actions)Examples
• mass limits
• spacing limits
• composition limits
• product piece count
• instructions, signs, training

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Controls

• Administrative ControlsConcerns
• requires operator thought and action each time
  control function is needed
• difficult to detect non-adherence
• difficult to declare unlikely (re Double
  Contingency)

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Controls

• Order of preference
• 1st - Passive Engineered Controls (minimum
  human intervention)
• 2nd - Active Engineered Controls (moderate
  human intervention)
• 3rd - Administrative Controls (total human
  intervention)

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Controls

• Controls must fit the need (for subcriticality)
• accuracy of controlling the parameter value
• sensitivity of keff to changes in the value
• ability to limit the parameter
• Controls must be functionally achievable
• mass scales, records and logs
• composition, H/U lab analyses
• spacing fixed racks, painted spots
• Temp/pressure T P indicators
• fixed poisons visual lab analysis
  concentration control

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Controls

• Controls must be identified (usually negotiated
  with operations personnel)
• in criticality evaluation
• in operating procedures
• in OSR/TSR/SAR
• Controls must be understood by operators

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Operating Limits and Controls (SUMMARY)

• Difference between a limit and a control
• Identification of limits
• Physical parameters
• Nuclear characteristics
• Safety Margin
• Margin of safety
• Margin of subcriticality
• Identification of controls
• Engineered
• Administrative
• Implementation of limits and controls