ncsu workshop to its safety considerations
TRANSCRIPT
Xe-100: Aspects of Design Important to its Safety Considerations
Eben MulderX-energy: SVP, CNO
NCSU Workshop
August 17-18, 2019
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Background● The Xe-100 is a 200MWt pebble bed High
Temperature Gas Cooled Reactor (HTGR)● The X-energy fuel design is based on proven TRISO
based UCO fuel developed and tested in the US● Reference design spec is similar to AGR 5/6 fuel
with 15.5% enrichment
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Design Safety Basis – Supported by Analysis Tools
Control Criticality Control Heat Removal Contain Fission Products
• Low power density• Low excess reactivity• Strong negative
temperature coefficient• Fixed phase moderator
(graphite) and heat transport fluid (helium)
• Online refueling• Large thermal inertia
• Low power density• Strong negative
temperature coefficient• Fixed phase heat
transport fluid• Large thermal inertia• Matched pressure vessel
surface area for passive heat removal
• High retention capability of radionuclides in coated particles (99.99%)
• High temperature tolerance during loss of forced heat removal
• Multiple independent physical barriers
Neutronics analysisThermal and flow analysis
Neutronics analysisThermal and flow analysis
• Coupled Neutronics and Thermofluidics analysis
• Fuel Performance, dust and radionuclide transport analysis
Safety Function
Design Selection /
Feature
Analysis Tools
Focus of this Presentation
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• Strong coupling between neutronics and thermal analysis needed (feedback)
Fuel Temperature
• Fuel depletion calculations are needed to predict burnup and fluence of a non-static core
Fuel Burnup
• Source term analysis takes fuel quality into account as well as manufacturing defects
Fuel Quality
Key Fuel Performance Factors
TRISO Coated Particle
UCO kernelPorous Carbon
Silicon Carbide Pyrolytic Carbon
Pyrolytic Carbon
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• Power profile obtained from strongly coupled neutronics and thermal flow calculations
Power
• Calculated using CFD with validated porous media approach
Heat Transfer
• Use extensive material property data as a function of temperature and fluence
Material Properties
Key Factors Directly Impacting Fuel Temperature
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Source Term Calculation Path Using XSTERM
Max dose at site boundary
Releases from building
Releases from Pressure boundary
Releases from fuel elements
(pebbles)
Releases from TRISO fuel particles
Source Term Path
IodineSilver
CesiumDust
Element / Isotope Form / State Mechanism Physical Phenomena Methods / Software Codes
Iodine, SilverStrontium Cesium, EuropiumXenon, Krypton
Gaseous FPsMetallic FPs
- Release from intact and failed TRISO particles into matrix graphite- Activation of impurities
Power, temperature, irradiation time, fast fluence, burnup, particle defects, contamination
VSOP-A, VSOP-99, MGT, SCALE, ORIGEN, PARCS,FLOWNEX, STAR-CCM+,XSTERM, GETTER, PARFUME
Iodine, SilverStrontium Cesium, EuropiumXenon, Krypton
Gaseous FPsMetallic FPsDust Particles
- Diffusion from pebble into the helium stream- Activation of impurities
Power, temperature, irradiation time, fast fluence, burnup, contamination
VSOP-A, VSOP-99, MGTSCALE, ORIGEN, PARCS,FLOWNEX, STAR-CCM+XSTERM, MELCOR
Iodine, SilverStrontium Cesium, EuropiumXenon, Krypton
Gaseous FPsMetallic FPsDust Particles
- Leakage from HPB into building and structures- Activation of impurities
Instrumentation line failure,small & large pipe breaks, plate-out, liftoff
MELCORXSTERMFLOWNEX
Iodine, SilverStrontium Cesium, EuropiumXenon, Krypton
Gaseous FPsMetallic FPsDust Particles
- Transport throughout building to the environment
Plate-out, liftoff XSTERMMELCORSTAR-CCM+
Iodine, SilverStrontium Cesium, EuropiumXenon, Krypton
Gaseous FPsMetallic FPsDust Particles
- Atmospheric dispersion- Inhalation, Ingestion
Postulates XSTERMSTAR-CCM+ MACSS
US/DOE Codes X-energy in house code Commercial NQA-1 Code Legacy codesColor Legend:
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Neutronics & TF Feedback Design
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© 2019 X Energy, LLC, all rights reserved
Xe-100 Reactor
Control rods
Pressure vessel
Core barrel
Graphite bottomreflector
Graphite topreflector
Graphite sidereflector
Circulators
Steam collection manifold
Helical coil tubes (not shown)
Steam Generator
Reactor
Pebble bed Key Technical Specifications:• 200MWt / 75MWe• Rankine Cycle• Helical steam generator at
565oC / 16.5MPa• Multi pass fuel cycle
Feed water inlet
● AOOs, include planned and anticipated events. AOOs doses must meet normal operation public dose requirements.
● DBEs are unplanned off-normal events not expected in the plant’s lifetime, but which might occur in the lifetimes of a fleet of plants. DBE doses must meet accident public dose requirements. DBEs are the basis for the design, construction, and operation of the structures, systems, and components (SSCs) during accidents.
● BDBEs, which are rare off-normal events of lower frequency than DBEs. BDBEs are evaluated to ensure that they do not pose an unacceptable risk to the public.
LBE Categories of Events
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● AOOs – event sequences with mean frequencies > 10-2 per plant-year
● DBEs – event sequences with mean frequencies < 10-2 per plant-year and > 10-4 per plant-year
● BDBEs – event sequences with mean frequencies < 10-4 per plant-year and > 5 × 10-7 per plant-year.
LBE Frequencies
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● AOOs – 10 CFR Part 20: 100 mrem total effective dose equivalent (TEDE) mechanistically modeled and realistically calculated at the exclusion area boundary (EAB). For the Xe-100, the EAB is expected to be the same area as the controlled area boundary.
● DBEs – 10 CFR §50.34: 25 rem TEDE mechanistically modeled and conservatively calculated at the EAB.
● BDBEs – NRC Safety Goal quantitative health objectives (QHOs) mechanistically and realistically calculated at 1 mile (1.6 km) and 10 miles (16 km) from the plant.
Acceptable Limits
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Set of Identified LBE’s
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VSOP Core modelEquilibrium cycle
Model Design and Description
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● The following important material properties are to be considered:– neutron cross sections;–material densities;– thermal conductivities as a f(Temp, Dose);– heat capacities as a f(Temp);– Emissivity (pebbles, reflectors, core barrel, RPV, etc.)
Material Properties
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● Max fuel temperature (normal operating conditions);● Design dependent power load follow capability, eg. 100% - 40% - 100%;● Excess reactivity defined by load following to override the equilibrium
xenon build-up;● Assumed max power production per pebble = 4.5 kW
(higher values achievable, TBD);● At any given time in the operating life of the plant:– reactor shutdown using the RCS, followed by the RSS,– reactor cold shutdown condition (100 °C) using the RSS only– Tmax (max fuel temperature) to remain below the experimentally proven
limit. Total additional radioactivity release from the core to the environment remaining below allowed public dose limits – (MGT / XSTERM analysis – Not the focus of this presentation)
Design Criteria
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● X-energy had developed a strong analysis capability in the following areas:– Reactor neutronics (VSOP-A, VSOP-99, MGT)– Reactor gas flow and thermal dynamics (STAR-CCM+ : NQA-1 compliant)– Graphite lifetime probability of failure prediction tools (inhouse) – Integrated system performance analysis (FLOWNEX Nuclear : NQA-1 compliant)– Plant simulation and Human Factors Engineering (SIMUPACT : NQA-1 compliant)– Source term analysis (XSTERM inhouse code suite for fuel performance and radio
nuclide release calculations)– Probabilistic Risk Assessment (PRA)
● X-energy will be using all of these tools to support a Risk Informed Performance Based (RIPB) licensing approach.
● We are also engaging with all of the US DOE Labs; each specialist are to help us develop V&V code development roadmaps, review analysis work and in some cases even perform high fidelity analysis using the DOE High Performance Computing (HPC) capabilities.
Analysis Tools
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Reactor Integrated Design Process
Initial Assumptions
& Inputs
Step 1: Initial Core Design
Step 2: Reactor geometry design (core structure
layout)
Complies with
require-ments
Yes
No
Step 3: Reactor Thermal & gas
dynamic design optimization
Update Initial assumptions
Complies with
require-ments
Yes
NoUpdate Initial assumptions
Step 4: Primary & secondary loop
Analysis
Complies with
require-ments
No
Yes
Next Iteration
Step 5: Source term analysis
Neutronic data
Initial assumptions & requirements- Power level- Fuel cycle- Power density- Enrichment- HM loading
Step 6: DEM Pebble dynamics
Step 8: Seismic Analysis
Step 7: Structural Design (FEA)
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VSOP Core modelDesign Considerations
Example: Coupled Neutronics / Thermofluidics
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RU Model Design and Description
• Main model assumptions
• Boundary conditions
• Geometry description
• Fuel management: 6-pass –pebble flow paths and speeds–statistical fuel representation (mixing of fuel re-introduced into the core)
• Spectrum zones
• Other model input data
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Xe-100 200 MWth Rx: In-Core Power Distribution
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Xe-100 200 MWth Rx: Axial Flux Distribution
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Xe-100 200 MWth Rx: Thermal Flux Distribution
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Xe-100 200 MWth Rx: Radial Flux Distribution
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Xe-100 200 MWth Rx: Temperature Coefficient of Reactivity
-8.00E-05
-6.00E-05
-4.00E-05
-2.00E-05
0.00E+00
2.00E-05
4.00E-05
6.00E-05
0 200 400 600 800 1000
T-C
oef
f [D
k-ef
f/ °
C]
Temp [°C]
Doppler Coeff
Moderator Coeff
Reflector Coeff
T-Coeff
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0.00E+00
5.00E+00
1.00E+01
1.50E+01
2.00E+01
2.50E+01
3.00E+01
3.50E+01
4.00E+01
4.50E+01
5.00E+01
0 200 400 600 800 1000
Gra
ph
ite
Dam
age
[dp
a]
Temp [°C]
Xe-100 200 MWth Reactor: Reflector Fast Fluence (E > 0.1
MeV) Loads
30 Years
40 Years
50 Years
60 Years
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Xe-100 200 MWth Rx: Proliferation Resistance
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Xe-100 200 MWth Rx: RCSS Characteristics
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V&V Exercise: CR-5 Control Rod Model on the ASTRA Cold-Critical Facility
Differential Reactivety of the CR5 Control Rod
-3
-2.5
-2
-1.5
-1
-0.5
0
0.5
0 50 100 150 200 250 300 350 400 450
Rod Insertion (cm)
Wo
rth
($
) Experimental
MCNP
VSOP
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Depressurized Loss of Forced Coolant
Time (hours)
Long transient accident analysis ≈ 120 hours
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Xe-100 200 MWth Rx: Temperatures vs Core Volume in a DLOFC
0
10
20
30
40
50
60
70
80
90
100
0 20 40 60 80 100 120
Co
re V
ol [
%]
Time [h]
1400 - 1500°C 1500 - 1600°C 1600 - 1700°C
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Use of High Fidelity Analysis
SG Bundle
CirculatorDetailed Pebble bed
DEM Pebble flow
RPV & Core Barrel
SG Structures
SG Flow
Spent Fuel
Core Barrel
Core Structures
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● X-energy has a strong, experienced analysis team to support the Xe-100 design and licensing process.
● Roadmaps (plans) have been developed to enhance our understanding of the steps needed to deliver high quality analyses, that will be compliant with NRC requirements, to support our design and licensing.
● We have fully engaged with the US DOE Labs and Universities to be involved in our analysis process in almost every facet (development, V&V, review and analysis) of the design.
● A key part of the analysis roadmaps that now requires execution is the V&V exercise upon which we have embarked.
Summary
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