Mitigating Damage Flashcards
What is Core Damage?
Core Damage is
- PCS activity greater than Tech Spec
- Fuel Geometry altered
- Core can not complete cycle
What are the the Probabilistic Safety Assessment “Dominant scenarios that lead to core damage?”
PSA Dominate Scenarios
82% involve SBLOCA or LOOP
–35% are result of SBLOCA with failure to maintain PCS inventory
–40% due to LOOP with failure of one or both D/Gs AND random secondary heat removal failure
What are the Probabilistic Safety Assessment “Key Operator Actions?”
PSA Operator Actions
- Initiate Aux Spray during a SGTR (EOP-5.0)
- CDF x 13.15
- Make-up to the CST-2 (EOP-7.0)
- CDF x 4.123
- Close Demin M/U P-936 C/B after Load Shed (EOP Suppl 2/ONP-2.1)
- CDF x 2.792
- Manually isolate a failed open ASDV (EOP-6.0)
- CDF x 1.935
- Initiate once-through-cooling (EOP-7.0/9.0 - HR-3)
- CDF x 1.868
- Initiate Emergency Boration (EOP-9.0 - RC-2)
- CDF x 1.609
- Raise AFW flow to unaffected S/G (EOP-5.0)
- CDF x 1.401
- Align SDC at CST-2 depletion (EOP-5.0/9.0)
- CDF x 1.229
How do you determine if subcooling margin is sufficient to ensure adequate core cooling?
Subcooling
SMM
CET/Pressures
HL vs CL Temps if PCP running
What is Cladding oxidation ?
Cladding oxidation
- It is a ziconium water reaction.
- Zirconium oxides and a thin layer of Zirc Oxide forms.
- This prevents further oxidation.
What is partial flow blockage?
Partial flow blockage
Blockage of coolant access to the fuel.
Indicated by specific incore thermocouples ‘heating up’, while other areas of the core ‘unaffected’. Superheated conditions indicated
What is fragmentation?
Fragmentation
The thermal shocking of zirconium fuel cladding by the re-introduction of cooling flow to overheated and now heavily oxidized fuel.
If 25% of the clad is ZrO2, the clad will shatter
What is core slump ?
Core slump
It is the processes of candling, decay heat and zirconium-water heat production causing core temperatures to rise to point of collapse
How does cladding failure form a Eutectic Mixture?
Cladding Failure/Eutectic Mixture
Cladding will melt at 3375oF. Liquefaction and dissolution of UO2 into molten clad can occur above this point. Resultant molten material is called “eutectic mixture
- Alloy which is composed of the fuel, zirconium or anything else (in area) combining
- Alloy that is easily fused
- Molten eutectic material will seep from the cracks of the cladding, “candling”
- Eutectic mixture solidifies/melts at about 2450oF (i.e., by definition of “eutectic”: at lower temp than each of the pure composite components)
At what temperature does Fuel Melt occur and how does new fuel compare to old fuel?
Fuel Melt
Uranium oxide (fuel) melting point is approximately 4900 to 5100oF (older fuel melts at a slightly lower temperature than new fuel)
What parameters indicate a change in core geometry and the extent of core damage?
Damage Indicators
- Nuclear instrumentation (power anomalies: reactor power indications will read higher [slow trend] due more neutron leakage in areas of core that are uncovered; or lower [rapid trend] due to refilling of down comer area)
- High core & coolant temperatures (superheat)
- High (or unexplainably low) PCS pressure (readouts)
- Super-heated primary conditions (loss of subcooling)
- Inadequate coolant inventory (Pzr level, RVLMS indication)
- Inadequate cooling flow (fluctuating PCP amps if operating)
- Radioactive anomaly (containment area and gamma radiation monitor alarms/indicators)
- Containment environment abnormal (H2, Temp, Press Level, Humidity)
What methods/equipment are available to aid the operator in determining the extent of core damage?
Damage Assessment
- Chemistry Sample for Isotopes Present
- EI procedures and methods which can be used to determine extent of fuel damage
- EI-11 “Determination of Extent of Core Damage”
–Requires LOCA in progress (i.e., release to Containment)
–Uses RIA-2321 & 2322 (Containment High Range Monitors)
–Can use RIA-2315 as last resort (Personnel Air Lock Monitor)
- EI-7 “Emergency Post Accident Sampling And Determination of Fuel Failure Using Dose Rates”
–Replaces PASM Panel method (PASM removed)
–Uses Dose Rates at NSSS Sample Panel Piping (while sampling)
- Critical Functions Matrix (pg 100 PPC)
Does the presence of I-131 in a PCS sample indicate fuel failure?
I-131 Presence
No, occurs via Halogen spiking/tramp uranium
What isotopes in a PCS sample indicate cladding failure?
Cladding Failure
Xe-131 & 133
I-131, 133, & 135
Released via Clad burst & gap diffusion
What isotopes in a PCS sample indicate pellet overheating
Fuel Pellet Overheating
Cs-134; Te-132
via Grain boundary diffusion
What isotopes present in a PCS sample indicate pellet melt?
Fuel Pellet Melting
Ba-140; La-140
via Escape from molten fuel
What are the potential problems of cooling a degraded core and what policies and procedures are used to diagnose and mitigate the situation?
Degraded Core Cooling
- Potential problem of cooling a degraded core is largely the blocking of any coolant access to the fuel
- Subsequent damage can occur when coolant is reestablished due to brittle failure shattering of hot fuel resulting in further reconfiguration and flow blockages
SAMG’s used to mitigate.
What are the conditions for and the effects of molten fuel penetration of the reactor vessel?
Vessel Failure
Vessel thermal attack occurs when core drops into lower region of vessel.
Once the core drops, localized vessel failure occurs in a few minutes (assuming no additional cooling water is encountered).
What is Creep Failure?
Creep Failure
- It can occur in the PCS due to extremely high temperature superheated steam and gases melting the steel (surge line, hot legs, etc.).
- These failures can occur before or without high pressure being pre
What are the heat removal mechanisms for a SBLOCA?
Heat Removal with SBLOCA
100% power and a SBLOCA occurs High PCS pressure will restrict SIS cooling flow (not available for feed and bleed, etc.).
Until PCS pressure is low enough to permit SI flow, steam generators MUST be used for core heat removal (natural circulation will probably be required). Another option is to make the leak size larger = once-through cooling. Further, using PZR spray to lower PCS pressure to allow SI flow is another option.
What is and when do we use Once Through Cooling?
Once Through
Used during a SB LOCA where system pressure is greater than the discharge head of HPSI pumps and no main or aux spray is available (last resort of EOP‑9.0)
Loss of steam generators above shutdown cooling operation zone. (EOP-7.0 & 9.0 only) SI through core and out PORV
What is Hot Leg Injection, its advantages and disadvantages?
Hot Leg Injection
Avoids boron precipitation problems which can begin to occur 29 hours after a cold leg break
During hot leg injection one-half (½) of the HPSI flow enters the hot leg and flows downward through the core
Advantage of hot leg injection
- Ensures core cooling or coverage by injecting core cooling water via hot and cold legs
- Dilutes excess boric acid concentrations
Disadvantage
If hot leg injection is administered too early (e.g., 2 hours) with a hot leg LOCA, required core cooling may be lost through break; the injection water may all be entrained by the steam exiting the core and can be carried out the break. Results in less than analyzed SI flow reaching the core. This can cause fuel damage
What is Reflux Cooling?
Reflux Cooling
Also referred to as reflux boiling. Occurs in the tubes via hot legs.
What can happen by operating PCPs with voiding?
PCP voiding
If voiding exists during primary coolant pump operation destruction of the pump may follow (loose parts hazard in reactor which could fail more fuel)
Pump voiding may be verified by vibrations and erratic and lower amperage indications
Why do incores read high during core uncovery?
Self-powered neutron detectors at high temperatures initially give off large negative currents (thermionic emission) until such time as they fail, around 2400F
How could excores could indicate voids?
If voiding exists, neutrons will travel further, due to lack of moderator. (Reduced moderating cross sectional area.)
Slugs of water in the reactor vessel annulus will cause the erratic power indication.
Control Room start-up neutron indication will be erratic for fluctuating voids.
Source Range Excores at TMI indicated up to 10,000 cps (and stable) when core was uncovered and vessel level was stable. When downcomer was refilled, the counts rapidly went down to about 400 cps. Normal post shutdown indication should be around 100 cps.
When are PPC P/T curves not to be used?
PPC P/T curves
PPC P/T curves are not to be “trusted”/used per yellow flashing alarm at the bottom of the PPC screen if 175oF or 3# exceeded (degraded containment)
What are the 10 CFR50.46 Emergency Core Cooling Acceptance Criteria
10 CFR50.46 Emergency Core Cooling Acceptance Criteria
- Peak clad temp <2200oF
- Cladding oxidation <17%
- Hydrogen generation <1% of hypothetical
- Coolable geometry maintained
- Long-term cooling available
