Criticality Safety Limits and Controls

1. Criticality SafetyLimits and Controls Based on Notes from the Criticality Safety Short Course University of New Mexico Howard Dyer - ORNL

2. 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

3. 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 < 0.088

4. 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.

5. 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

6. 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?

7. Nine parameters for control

8. 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

9. 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

10. Calculating keff

11. Crit. Safety Analyst�s Responsibility

12. 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)

13.

14. 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 dim�s.) #4 Assume full REFLECTION #5 Single unit, no INTERACTION #6 Assume no NEUTRON POISONS #9 COMPOSITION* (oxide and acid) #8

15. Parameter studies... Example: What is variable? MASS #1 DENSITY** #2 INTERNAL MODERATION** #7

16. Example: �Full� Beaker

17. Example: �Fixed Mass� Beaker

18. 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 < 0.90 Accident conditions, keff + 2s < 0.95

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

20. Selecting the parameter limit

21. 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

22. 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

23. 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

24. 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

25. 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

26. Controls Passive Engineered ControlsConcerns: initial design correct installation pre-operational verification continued effectiveness detection of change

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

28. 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)

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

30. 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

31. 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

32. 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