Multiphase Flow Phenomena in SGTR:

1
Multiphase Flow Phenomena in SGTR Importance
Ranking and Scaling Nam Dinh Division of
Nuclear Power Safety Royal Institute of
Technology (KTH) Stockholm, Sweden
2
Outline

• Multiphase Flow Phenomena in SGTR Context -
  Revisited

• Pressure Shock Wave

• Sloshing

• Steam Explosion

• Transportability of Steam Bubbles to the
  Reactor Core

• Concluding Remarks

3
15..25 MPa, 350..500 oC
SGTR
0.3 MPa, 500..600 oC
LFR 2000 MWth
EFIT
4
SGTR Safety Risk-Oriented Approach
Risk of SGTR R P(o)C(?)
Eliminate the intermediate HLM loop
Risk
Economy
Measures to reduce P of SGTR (materials, quality,
operation, maitenance)
Measures to reduce C of SGTR (design, control
systems, EOP)
What are Consequences? Systematic Approach?
5
SGTR-Induced Threats

• Dynamic Loadings and Impact on Reactor Equipment
  ? Causing Secondary Failures
• Transport of Steam to the Core and Core Voiding
  ? Reactivity Insertion with Potential
  for Power Excursion

• Rupture-induced pressure shock wave

• Steam Generation-Induced Sloshing

• Steam Explosion

• Steam Transport to the Reactor Core

6
System Behavior Primary Side
The first stage is related to the rupture moment,
and associated with dynamic interactions between
the discharged jet flow and molten lead. The
threat posed by this stage is the formation and
propagation a pressure wave. The second stage
is related to the formation and expansion of the
mixing zone that leads to lead displacement and
pool sloshing, with potential for mechanical
damages. The third stage is initiated by a
trigger that causes the pre-mixture to enter a
CCI regime and lead to an energetic steam
explosion. The fourth stage is transport of the
multiphase mixture toward the reactor core,
causing core voiding with potential reactivity
consequences.
Receiving Side
7
Today Messages

• The mechanical effect of dynamic and energetic
  threats are expected to be insignificant
• Careful treatment of the driving side
  (secondary loop)

• Prediction of core voiding is subject to
  multiphase flow patterns dynamics governed by
  bubble length scale (steam dispersal
  coalescence)
• Initial-phase data exist but more are needed
• New experiments in relevant flow regimes.
  Scaling.

• Safety-by-design Limiting design/operation
  conditions need to be established
• High-fidelity 3D CFD simulation of (lead, water,
  vapor) system
• Analytical experiments for constitutive
  relations
• Integral experiments for validation

8
Steam Generator Tube Rupture
Gas Space 0.1MPa,
Void fraction 10 --85
Water 14 MPa 335 oC
Liquid Lead (Pb)
?14 mm
Normal Operation
9
Accident Initiation Tube Rupture
Rupture site ? 1050 mm
Water 14 MPa 335 oC
10
Accident Situation Water-Lead Interactions
Depressurization Waves
High Pressure Discharge of Water/Steam into Lead
(HLM)
Water 14 MPa 335 oC
Accurate Simulation of the Secondary-Side
Dynamics is Important
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SGTR Multiphase Flow Phenomenology
Dynamic and Energetic Interactions
(Steam Explosion)

• Multi-fluid Mixing
• (Lead, Water, Steam)

Forces that Facilitate the Mixtures Transport
Formation of a Bubbly Mixture
Fine Bubbly Mixture
Transport of Voided Coolant to the Reactor Core
Again, Bubble and Droplet Sizes (Length Scales)
are Key
12
Secondary Side is the Driving Force
1
The SGTR interactions are limited by the
dynamics of the secondary (supply) side.
2
Failure location probability? System approach
? self-limiting threat!
3
EFIT AnsaldoNucleare

13
Primary Side Pressure Wave
Two-phase flashing and expansion similar to
Boiling Liquid Expanding Vapor Explosion (BLEVE)
due to a vessel burst. Characteristic length
and time scales are L (M RaTa/Pa)1/3, t
L/U, where the velocity scale is defined as
U 2E/M1/2 and the energy that drives the
expansion is determined as E M ?h0a M (h0
ha) with h0 and ha being the initial
(pre-BLEVE) mixture (liquid) enthalpy and mixture
enthalpy after flash evaporation (at ambient
condition), respectively.
14
Primary Side Pressure Wave
M -- the mass of instantaneous exposure can be
estimated from the volume formed by the breach
area (A) and pipe diameter (D), thus fairly small
volume (10-510-6 m3).
The ambient mixture enthalpy is ha related to the
saturation enthalpies of liquid and vapor as ha
xv hv,a (1- xv)hl,a , where xv is the mass
fraction of vapor after flash evaporation of a
superheated liquid. xv can be determined from the
isentropic expansion as xv (sl,0
sl,a)/(sv,0 sl,a),
15
Primary Side Pressure Wave
The pressure wave magnitude can be predicted and
shown to be negligible (say 0.1Pa) for structures
in a distance equal to a so-called energy-based
radius r determined as r (E/Pa)1/3. The
value ?h0a in a SGTR event can be found in a
typical range up to few (two-three) hundreds
kJ/kg. Consequently, r is predicted to be in a
fairly narrow range of 5-10 cm. Even with a
mass of order of liter (10-3 m3) suddenly exposed
to low pressure expansion, we would have r 0.5
m, and the same conclusion about negligible
loading on structures applies. Thus, the first
stage poses no significant threat to structures.
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Key Data

• Beznosov et al (2005)

liquid water
Water injection (at 30 MPa, 335 oC) into lead
at 0.8 MPa
a steamwater mixture, and 100350C, 125 MPa
steam were bubbled through 0.62 mm in diameter
openings (tube 14x2 mm), under a layer of lead
ranging in thickness from 100 to 3000 mm, at
temperatures 350600C
Limited expansion.
No explosion reported.

• Large fraction of liquid water upon discharge
  means limited (immediate) expansion, followed by
  gradual evaporation in film boiling mode

17
Expanding Bubble
As a reference case, we can assume that no mixing
occurs, so the two-phase mixture ejected from the
secondary circuit forms a steam cavity (large
bubble). We write mass balance for the steam
bubble (of characteristic radius R) as
fast
slow
where the first term in RHS is the steam supply
rate from isentropic expansion, and the second
term represents evaporation (by film-boiling heat
flux q) of water droplets of the same diameter
dp.
Compensating factors
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Steam Bubble Size Distribution
Water 22-24 MPa, 150-250 oC
Beznosov et al, 2005
14x2 mm tube 10 mm discharge 2000 mm depth
52 mm
Short wavelength due to high-pressure discharge.
19
Size distributions of water drops
Beznosov et al, 2005
92 does not boil
x7 ? final bubble radius
20
Primary Side Coolant-Coolant Interactions CCI
Can Explosion Occur? - Is pre-mixture
triggerable and detonable? If yes, - What
are ranges of pressure impulse? - What is
post-explosion mixture?
21
Multiphase Thermal Detonation
NON-PARTICPATING COOLANT
COOLANT
VAPOR
FUEL
(melt)
m-FLUID
PREMIXTURE
vO, PO
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Anatomy of Explosion
0.2 ms interval
KTH MISTEE synchronized video and Xray images.
23
Micro-Interactions Dynamics in FCI
KTH MISTEE Xray images
24
Analogy and Difference between FCI and CCI
FCI
For a postulated FCI with 1000 kg of oxidic
corium in the pre-mixture, the total energy
potential is 1.5GJ. Given triggerability and
detonation, a typically small fraction ? of this
energy (10 and less), or 150 MJ mechanical
energy.
CCI
For a postulated CCI with 10 kg of liquid water
in the pre-mixture (self-limiting liquid
inventory), the total energy potential is 20 MJ.
Given triggerability and detonation, a
typically small fraction ? of this energy (0.1-1
and less), or 20200 kJ mechanical energy.
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CCI Limiting Mechanisms Macro-Level
Short-lived premixture short time window for
steam explosion. The characteristic time period
?tEVA during which a water droplet (1 mm) is 60
s.
26
CCI Limiting Mechanisms Micro-Level

• High contact (interface) temperature, forming
  stable vapor film

• Stable bubble-wall surface due to high density
  of HLM

• No phase-change occurs at bubble wall

CCI
FCI
T gtgt
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Primary Side Core Voiding
Transportability of Steam Bubbles to the Reactor
Core and Reactivity Insertion depend on

• Smaller bubbles are more easily trapped in HLM
  flow

• Steam dispersal during water discharge

• Bubble distribution and coalesence during
  transport

• Convection (velocity) UC,DOWN ?
  UB,TER.

• Flow path geometry

• Forces (depth of mixture)

Bubble Size (Length Scale) is Key
28
Today Messages

• The mechanical effect of dynamic and energetic
  threats are expected to be insignificant
• Careful treatment of the driving side
  (secondary loop)

• Prediction of core voiding is subject to
  multiphase flow patterns dynamics governed by
  bubble length scale (steam dispersal
  coalescence)
• Initial-phase data exist but more are needed
• New experiments in relevant flow regimes.
  Scaling.

• Safety-by-design Limiting design/operation
  conditions need to be established
• High-fidelity 3D CFD simulation of (lead, water,
  vapor) system
• Analytical experiments for constitutive
  relations
• Integral experiments for validation

Next Step Scaling Support for SGTR Experiments