accident management strategies for mark i and mark iii … of presentation • background •...
Post on 14-May-2018
214 Views
Preview:
TRANSCRIPT
Accident Management Strategies for
Mark I and Mark III BWRs
E. L. Fuller
Office of Nuclear Regulatory Research
United States Nuclear Regulatory Commission
IAEA Workshop
Vienna, Austria
July 17-21, 2017
1
Outline of Presentation
• Background
• Assumptions for the analyses
• In-vessel recovery by injecting water into RPV
– MAAP 5 models for corium evolution and quenching
– Vessel penetration failure modeling
• Results and insights from in-vessel retention studies
– MAAP 5.03 and MAAP 5.04 analyses
• Ex-vessel mitigation
• Conclusions and insights
2
Background
• CPRR Rulemaking technical basis report discusses venting strategies
and water addition to mitigate the effects of core debris exiting the
vessel.
– Water addition and water management, along with venting through the wetwell,
ensure BWR Mark I and Mark II containment building integrity and minimization of
radionuclide releases to the environment
– Analyses suggest that sufficient time may be available for adding water to the
vessel to prevent vessel failure
• BWR SAMGs evaluate whether vessel has failed to determine which
operator actions to use
• In-vessel recovery analyses should consider water level in lower
plenum, corium constituents, corium-water and corium-structure
interactions, and vessel failure modes
• Ex-vessel corium cooling should consider corium-concrete
interactions and cavity/pedestal designs
• SAMGs for Mark III plants need to prevent major hydrogen burns
3
• All transients start with an ELAP and last 72 hr
• Industry (BWROG) EPG/SAG Rev. 3 is in place
• FLEX is in place both pre- and post-core damage
– 500 gpm injection into RPV or Drywell from external source at vessel
breach
– Provision for both Severe Accident Water Addition (SAWA) or Severe
Accident Water Management (SAWM)
• Control flow rate to prevent submerging the wetwell vent
• Recirculation pump leakage of 18 gpm per pump starts at the
time of the initiating event
• Initial buildup of water in the drywell from nominal leakage
• RCIC operation
– Suction from SP (option for suction from CST/SP)
– Flow rate nominally 600 gpm
– RPV level control via throttling of RCIC
Assumptions for the Analysis
4
Assumptions (continued)
• RPV pressure control
– Initial pressure control in 800 – 1000 psig band after 10 min
– Controlled depressurization after one hour
– Subsequent pressure control in 200 – 400 psig band for
continued RCIC operation
– Further depressurization after RCIC failure
• Containment venting (Mark I)
– Early venting from wetwell air space prior to core damage at
15 psig
• Not performed if RCIC already failed
– Close vent upon entry into SAG; reopen at PCPL (60 psig)
– Vent sizing consistent with industry assumptions
5
Relevant Models in MAAP 5.04 and
Results When No Water is Added
6
• Improved treatment of corium behavior in lower
plenum
• Addition of an important vessel penetration
model that results in reducing the time to vessel
failure
Principal Features
of Lower Plenum Modeling
• Corium relocation to water in LP
• Particle bed generation from corium melt jet breakup
• Corium structure – formation of oxide pool, crusts,
and metal layer
• Heat transfer within corium pool and between corium
and its surroundings
– Quenching debris bed
– In-vessel gap formation and cooling from gap
• Reactor vessel failure mechanisms
7
MAAP 5.04 Paints a Different Picture
Of Corium Behavior in LP and Failure
at Penetrations than Previous Codes
8
• Instrument tubes fill with molten core debris
in core region.
• Molten debris re-freezes in instrument tubes
and plugs form, particularly at the
penetration locations.
• After relocation into the LP, instant
stratification into a particle bed over metal
and oxidic layers no longer assumed, but is
calculated. The effect is to diminish the size
and role of the metal layer.
• Earlier vessel failure predicted due to failure
of closure welds from transfer of heat in the
plugs.
• Consequently, in-vessel recovery may be
less likely if water is added.
Failure mechanisms evaluated for
initial failure of RPV lower head
9
• CRLH - damage fraction due to creep of the lower
head wall
• CRCRD - damage fraction due to creep of ex-vessel
CRD tube
• TRPTN - debris plug melt-through of instrument
tubes at vessel weld: heat is transferred from plug
to weld.
• EJPT - ejection of instrument tubes from weld failure
• EJCRD - of CRD tubes from weld failure
• EJDR - of drain line from weld failure
• ABLH - jet ablation of the lower head
• OVLH - overlying metal layer attacking the vessel wall
10
Progression of RCS
Failure Models Added
in MAAP 4 through
MAAP 5.04. (Note: while
the illustration is for
BWRs, the models also
apply to PWRs)
Note that the other
penetration failure
models are also
included in MAAP 5.03
Results from Simulating
Mark I No Water Addition Cases
• Industry FLEX and (BWROG) EPG/SAG Rev. 3+ are assumed to be followed.
• Depressurizing RPV after RCIC failure makes in-vessel injection of fire water possible.
• Time is available after RCIC failure for injection into RPV to possibly prevent vessel failure.
• MELCOR 2.1 vessel failure due to lower head creep rupture.
• MAAP 5.03 vessel failure due to CRD tube ejection.
• MAAP 5.04 predicts earlier vessel failure (at 17.8 hr), at lower head penetrations due to weld
failure. If penetration failure models bypassed, vessel fails by creep rupture at 19.2 hr.
• MELCOR 2.1 and MAAP 5.03 predict venting before vessel failure and consequent early
iodine and cesium releases to the suppression pool and the environment.
• MAAP 5.04 releases to environment don’t start until venting begins.
11
Event Timing (hr) MELCOR 2.1 MAAP 5.03
MAAP 5.04
RCIC fails 9.6 9.6 9.6
Core uncovers 12.0 11.3 11.1
Core damage begins 13.7 11.7 11.4
Lower head dries out 18.1 19.9 16.2
Containment vented at 75 psia 14.9 22.8 18.9
Vessel fails 23.0 25.0 17.8
I release fraction to env. at 72 hr 2.3 E-1 7.8 E-3 1.1 E-2
Cs release fraction to env. at 72 hr 1.9 E-2 2.4 E-3 7.9 E-3
Hydrogen produced in vessel, kg 1195 790 700
12
MAAP 5.03 and 5.04 Pictures of Melt
Progression and Debris Evolution
(no water addition)
MAAP 5.04
• Smaller molten pool formed in core
region than for MAAP 5.03.
• Particulate debris, caused by jet
breakup of molten pour, aggregates in
lower plenum.
• Particulate debris melts to form an
oxidic pool, much crust, and a very
small metallic layer.
• Vessel failure at 17.8 hr from
instrument tube weld failure at the
bottom of the lower head.
MAAP 5.03
• Significant molten pool formed in core region,
with surrounding crusts.
• Axial flow through molten pool blocked by
crusts.
• Particulate debris, caused by jet breakup of
molten pour, in lower plenum predominates at
first, with rapid formation of molten metal and
oxidic layers below the particle bed. Oxidic
crust separates the molten layers.
• Vessel failure at 25 hr from ejection of CRD
tubes penetrating part-way up the lower head.
13
PB No Injection,
Vessel Damage Fractions (MAAP 5.04 Case)
• Welds at instrument tube penetrations fail due to heat transfer from internal corium plugs
where the tubes penetrate the vessel.
• Damage fractions increase rapidly at first; then increase gradually.
• CRD and drain line ejection penetration fractions are somewhat lower by vessel failure.
• RPV lower head creep damage fraction is still quite low at vessel failure.
IN-VESSEL RECOVERY
14
Benefits of Preventing Vessel
Failure by Water Addition to RPV
• Averts drywell liner melt-through and other containment failure
modes.
– No fission products or hydrogen in the reactor building
• If water is added soon enough, relocation of the corium into the
lower plenum may not occur
• Nearly all volatile fission products are deposited either in the
suppression pool or the RPV.
– Less than 1% are vented to the environment, and only after the vent is opened
– The sooner water is added the lower the releases
– The best outcome is to be able to add water before venting
• All lower-volatility fission products remain in the RPV
• Post-accident cleanup and maintenance of a safe stable state is
much easier
– Long-term cooling by using the RHR system
– No debris in the pedestal region or drywell
15
CsI Distribution at 72 hr (MAAP 5.04)
• Releases to environment don’t start until venting begins.
• Water addition suppresses revaporization and releases from RPV.
16
Time RPV
injection
begins, hr
Vessel
Failure
Fraction of CsI Inventory at 72 hr
RPV DW SP Env.
13 No 0.42 1.5 E-3 0.58 1.5 E-5
14 No 0.38 6.2 E-4 0.62 1.1 E-4
15 Yes (late) 0.17 6.5 E-4 0.82 3.2 E-3
24 Yes 0.11 1.3 E-3 0.89 5.0 E-3
none Yes 0.07 2.0 E-3 0.92 7.9 E-3
No vessel failure following
injection into RPV at 14.5 hr (MAAP 5.04)
17
Vessel Fails Following
Injection into RPV at 15 hr (MAAP 5.04)
18
• Injection occurred while water
was still in LP.
• Vessel failure delayed until
29.3 hr
• Weld failure from heat transfer
from plugged tube at bottom
of vessel
Ex-Vessel Mitigation
and Achievement of a
Safe Stable State
19
Debris Cooling in Mark I Pedestal Region following
Injection into RPV just after Vessel Failure (MAAP 5.04)
20
21
Delayed Injection is Better Than
No Injection at All (MAAP 5.03)
22
BWR Mark III Containment
and Shield Building
• Perry and River Bend have steel
containments inside annular
shield buildings.
• Grand Gulf and Clinton have
reinforced concrete containments
inside annular shield buildings.
• All four containments have
hydrogen igniters throughout.
• The design pressure is 15 psig for
each plant.
• Grand Gulf has a vent that can be
used to prolong RCIC until it fails,
and then again after core damage.
The others don’t.
• DW and WW communicate only if
the pressure difference between
them is large enough to uncover at
least one horizontal vent.
• If the SP water level gets high
enough it can flow over the weir
wall into the DW (and vice-versa).
23
• Pressurization after vessel failure is dominated by non-condensable gas generation
from core debris/concrete interactions.
• Containment failure is assumed at 64.3 psig (the median composite failure pressure in
the Perry IPE), at the equipment hatch level above the suppression pool (Comp. A).
• Containment survives more than a day.
• Small hydrogen burns early in scenarios.
Results for Perry Cases
with No Recovery
Results and Insights for Grand Gulf
No Recovery Cases
24
• The GGNS FLEX Integrated Plan states that igniters can be re-powered using a portable
hydrogen igniter generator.
• There is no burning in the 12 hr battery cases.
• The no dc power and 12 hour battery cases, respectively, have practically identical
pressure behavior.
Grand Gulf 12 hr Battery Cases:
Oxygen Concentration in Upper Dome
25
• The main purpose of early venting is to prolong RCIC operation.
• In addition, early venting significantly lowers oxygen
concentration, thus eliminating the possibility of Hydrogen and
Carbon Monoxide burning later.
• Is not so effective when the batteries fail early.
PNPP RECOVERY STUDY
• Recovering diesel/ac power allows a number of
actions to be taken
– Turning on RHR to cool suppression pool
– Using HPCS to add water to vessel and cool debris
either in-vessel or in the Pedestal region
– Activate the Suppression Pool Make-Up system to
initiate upper pool dump
– Vent the containment to lower pressure and remove
hydrogen from the containment atmosphere
• Global burns are an issue to be dealt with after
initiating recovery actions
• Recovery makes it possible to achieve a safe
stable state
26
27
• Recovery of RHR cools down the suppression pool and the containment, and condenses steam.
• HPCS is somewhat effective in limiting CCI and consequent Hydrogen and CO production.
• Large global hydrogen and CO burns are predicted in the dome after recovery.
PNPP Recovery Study: Batteries Lost at 6 hr;
Vary Time When RHR and 500 gpm HPCS are
turned on after diesel generator is available
Attributes of a Safe Stable State
• Low pressure in containment
• Water in suppression pool is cool and plentiful
• Sufficient water in Pedestal and Drywell to cool
debris and terminate core debris/concrete
interactions
• No flammable mixture of hydrogen and oxygen
• Core melt progression has stopped
• Radioactive releases have stopped
28
A Perry Safe Stable State Case: Containment Pressure
29
• Operator actions and events assumed are: diesel power and venting at 18
hr, upper pool dump at 19 hr, RHR, 500 gpm HPCS, igniters on at 20 hr, and
100 gpm SP make-up at 24 hr.
• Drywell pressure rises quickly following TIP tube failure at 10 hr and vessel
failure at 17.1 hr.
• Containment pressure decreases to ambient after venting.
A Perry Safe Stable State Case: Containment Temperatures
30
• Operator actions and events assumed are: diesel power and venting at 18 hr,
upper pool dump at 19 hr, RHR, 500 gpm HPCS, and igniters on at 20 hr, and
100 gpm SP make-up at 24 hr.
• Drywell temperatures are high because of persistent decay heat in core
debris. Spikes are due to TIP tube failure at 10 hr and vessel failure at 17.1 hr.
• SP water and containment dome temperatures are low by 48 hr.
Perry Safe Stable State Determination: Hydrogen Production
31
• Recovery provides sufficient water onto the core debris to terminate
hydrogen production and CCI.
• Earlier recovery significantly reduces hydrogen production from CCI.
• Venting as soon as possible after recovery further reduces hydrogen
production from CCI.
Conclusions and Insights
• Early controlled venting enables RCIC to run longer, and
delay core damage.
• Early venting in Mark III containments can diminish oxygen
concentrations in containment , lowering the likelihood of
hydrogen combustion later.
• Depressurizing RPV after RCIC failure makes in-vessel
injection of fire water possible.
• Time is available after RCIC failure for injection into RPV to
possibly prevent vessel failure.
• MAAP 5.04 predicts earlier vessel failure than MELCOR 2.1
and MAAP 5.03.
• Most likely vessel failure mode is instrument tube ejection
due to closure weld failure weld failure.
32
Conclusions and Insights (cont.)
• Fission product releases to environment don’t start
until venting begins.
• Water addition suppresses volatile fission product
revaporization and releases from RPV.
• Water addition into the pedestal or lower drywell, from either
the failed vessel or directly onto the corium, is very effective
in limiting concrete ablation and combustible gas production.
• Venting is an important mitigation action, even for Mark III
containments.
• For Mark III’s it is important to vent after ac power is restored,
and to wait awhile to turn igniters back on.
33
Backup Slides
34
Debris Bed Arrangement in Lower Plenum.
35
Quenching by Water Ingression to Debris Bed.
36
37
38
39
Molten Pool Mass in Core Region
for Various Injection Times (MAAP 5.03)
Recovery from Injection into RPV at
20 hr: Corium and Water in LP (MAAP 5.03)
40
• MAAP 5.03 predicts that recovery is still possible even if most of the corium is in the
LP and the LP has dried out.
• RCIC is assumed to fail at 9.6 hr, so more than 10 hr is available to begin injection to
prevent vessel failure.
Debris Cooling in Mark I Pedestal Region following
Injection into RPV just after Vessel Failure (MAAP 5.03)
41
42
• Flow rates for Wsat are 1000 – 2000 gpm. Significantly more energy is removed than what is
produced by decay heat. However, steam condensation causes the containment to be de-
inerted and a global burn results, failing the containment.
• A flow rate of 500 gpm easily removes the decay heat. However, a global burn results.
• Flow rates for Wvap are 100 – 200 gpm. Energy from the decay heat is removed, and no
global burns occur. Pressure remains elevated, however.
• 50 gpm is insufficient to remove all the decay heat, The containment fails from overpressure.
PNPP Recovery Study: Batteries Lost at 6 hr,
RHR and HPCS on at 24 hr: HPCS flow rate variations
43
• Even with no igniters, global burns calculated to occur when containment is de-inerted.
and ignition criteria are met, even if igniters are not energized.
• Suppression pool temperatures reduced to about 110 ºF from RHR cooling.
• Sufficient water added to Pedestal and Drywell to cool core debris.
PNPP Recovery Study: Batteries Lost at 6 hr,
RHR and 500 gpm HPCS, No Igniters
44
• Global Hydrogen (and Carbon Monoxide) burns are predicted when Oxygen concentration
in the dome increases while steam condenses. Once de-inerting is achieved, igniters
provide the sparks.
• CO and Nitrogen are not shown. CO behaves like Hydrogen, while Nitrogen is an inertant
like steam.
PNPP Recovery Study: RHR and 500 gpm HPCS
at 24 hr – Global Burn in Upper Dome at 65.7 hr
Upper Dome and Drywell Pressures
at Time of Global Burn
45
• Pressure rise in Drywell lags that in the containment dome (where the burn
originates).
• This leads to an implosive pressure load on the drywell
• Suppression pool bypass is unlikely because the pressure differential between
the Drywell and Wetwell is less than 30 psid and the likelihood of a stuck-open
vacuum breaker is very low.
Perry Safe Stable State Determination: Hydrogen Production
46
• Recovery provides sufficient water onto the core debris to terminate
hydrogen production and CCI.
• Earlier recovery significantly reduces hydrogen production from CCI.
• Venting as soon as possible after recovery further reduces hydrogen
production from CCI.
47
Perry Safe Stable State Determination: Cesium Release Fraction
Fission product releases reduced from no-recovery case results.
top related