NCSU Workshop to its Safety Considerations

Xe-100: Aspects of Design Important to its Safety Considerations

Eben MulderX-energy: SVP, CNO

NCSU Workshop

August 17-18, 2019

© 2019 X Energy, LLC, all rights reserved

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


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


• 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


• 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


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


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



- Leakage from HPB into building and structures- Activation of impurities

Instrumentation line failure,small & large pipe breaks, plate-out, liftoff

MELCORXSTERMFLOWNEX



- Transport throughout building to the environment

Plate-out, liftoff XSTERMMELCORSTAR-CCM+



- Atmospheric dispersion- Inhalation, Ingestion

Postulates XSTERMSTAR-CCM+ MACSS

US/DOE Codes X-energy in house code Commercial NQA-1 Code Legacy codesColor Legend:


Neutronics & TF Feedback Design



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


● 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


● 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


Set of Identified LBE’s


VSOP Core modelEquilibrium cycle

Model Design and Description


● 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


● 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


● 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


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)


VSOP Core modelDesign Considerations

Example: Coupled Neutronics / Thermofluidics


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


Xe-100 200 MWth Rx: In-Core Power Distribution


Xe-100 200 MWth Rx: Axial Flux Distribution


Xe-100 200 MWth Rx: Thermal Flux Distribution


Xe-100 200 MWth Rx: Radial Flux Distribution


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


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


Xe-100 200 MWth Rx: Proliferation Resistance


Xe-100 200 MWth Rx: RCSS Characteristics


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


Depressurized Loss of Forced Coolant

Time (hours)

Long transient accident analysis ≈ 120 hours


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


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


● 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


Documents

NCSU Workshop to its Safety Considerations