4/2003 rev 2 i.4.9f – slide 1 of 50 session i.4.9f part i review of fundamentals module 4sources...
TRANSCRIPT
4/2003 Rev 2 I.4.9f – slide 1 of 50
Session I.4.9f
Part I Review of Fundamentals
Module 4 Sources of Radiation
Session 9f Fuel Cycle – Fuel Fabrication
IAEA Post Graduate Educational CourseRadiation Protection and Safety of Radiation Sources
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Object is to convert enriched UF6 into UO2 fuel pellets, suitable for use as fuel in a reactor
Fuel Fabrication
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Fuel Fabrication Overview
Large, industrial-type facilities
Generally good construction
Confinement not containment of Special Nuclear
Material (SNM)
No shielded areas
Generally operators/people involved/intertwined with
the process
Low radiation and airborne hazards
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Basic Chemical Approaches
“Wet” process chemistry
hydrolyze UF6 in solution precipitate with ammonia compounds calcine/reduce to UO2
ADU = ammonium diuranate
“Dry” process chemistry
hydrolyze UF6 with steam convert to UO2 with steam/H2
IDR = Integrated Dry Route
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Importance of Fuel
First two layers of confinement: Fuel form itself (Metal) cladding
Must be high quality - “Perfect” Leakers often require
reactor shutdown Special handling/canning of
leaking spent nuclear fuel (SNF)
Money, radiation dose and waste if wrong
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Importance of Fuel
Fuel around for “decades”
about 1 year after fabrication usually 3 cycles (about 5 years) in reactor minimum of 5 years in wet SNF storage minimum of 20 years in dry SNF storage some power reactor fuel 35+ years old Repository - 100+ years
Fuel is the “tail that wags the dog”
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Fuel Considerations
Enriched UF6 not suitable for fuel
Requires chemical conversion to more stable and robust form
Requires mechanical activities, cladding, and assembly
Fuel requires high density to achieve adequate nucleonics and properties
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Chemical Forms of Uranium Fuel
UO2 (a compromise) is used in most power reactors (LWRs, PHWRs, AGRs, RMBKs) as cylindrical pellets
Pebble bed would use coated UO2 and would probably be a UO2/UC mix
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Nuclear Fuel Enrichment
Enrichment Levels
PWR: 2.5-4.5% BWR: 3-5% CANDU/PHWR: 0.71% Naval/Research: up to 100% Gas/graphite: 0.71-20% FBR/LMFBR/IFR - 0.2 (blanket) to 30%
(driver); 15-25% fissile (Pu) typical
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Nuclear Fuel CoreTime and Quantities
Core irradiation time, years CANDU/PHWR: < 1 PWR/BWR: 4-5 Naval/research: 1 - 20+ Gas/graphite: 0.5-3 typical, some > 5 FBR/LMFBR/IFR: 3-5 (driver)
Physical quantities small about 10,000 MTHM/yr world about 2,000 MTHM/yr US U.S. SNF about 50,000 tonnes All U.S. SNF would fit on a football field 7.6 m
deep, subcritical
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Typical PWR Fuel Load
1,000 MWe nominal 193 assemblies 51,000 fuel rods 18,000,000 fuel pellets
Typical reject/rework rates
1-3% on pellets 0.1-0.3% on rods very low for assemblies
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UF6 received from enrichment facility in cylinders
Cylinders removed from package, weighed, and transferred to UF6 storage pad
UF6 CylindersArriving at Facility
Fuel Fabrication
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Greatest EnvironmentalHazards in Fuel Fabrication
Whether wet or dry …
chemical conversion of UF6 into UO2
chemical operations in scrap/recovery
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Ceramic Process andFinal Fuel Fabrication
Ceramic Process
Pretreat Pelletize (green) Sinter Grind Wash/dry Inspect
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Sample Sintered Pellets
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What are Burnable Poisons?
Materials in fuel that limit reactivity for part of the reactor operating cycle (absorb some neutrons)
“Poison Rod” like a weak control rod no fuel, just the neutron poison
“Poisoned Rod” contains fuel and poison poison in fuel pellets or as separate pellets in rod
Gadolinia and erbia typical poisons due to large neutron cross-sections
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Mechanical Process Steps
Mechanical Process
Prepare rods Load pellets Seal rods Make assemblies/Inspect Store, prior to transportation
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Why zirconium?
Capable of withstanding high T, P and radiation for years
Structural strength (for tubing)
Corrosion resistance in most coolant environments
Low thermal neutron absorbance Zr 0.185 b (1 barn = 1E-24 cm2) Hf 10.2 b (common impurity)
Reactor grade Zr requires < 100 ppm Hf
Alloys (mainly Zr, some Sn - 1%) Zircaloy-2 (BWR typical) Zircaloy-4 (PWR typical) Others - “Zirlo”
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Fuel Pellet “Stacks”
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Fuel Rods
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Spacer GridsSkeleton Assemblies
BWR Grid
PWR
BWR
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The completed fuel assembly is washed and inspected
Fuel Assemblyin Fixture
Fuel AssemblyClean Check
Assemblies
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Visual Inspection
PWRAssembly
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Storage
Assemblies stored in racks to
preclude water accumulation
maintain minimal separation/ distances
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Fuel Assemblies
1,000 MWe Reactor - about 100 MTHM in core
30-34 MTHM in refueling, every 18 months
60-70 assemblies per refueling (PWR)
PWR and BWR assemblies different BWR smaller size, weight, but about same height BWR more void space and channels PWR assembly about 0.5 MTHM BWR assembly about 0.2 MTHM
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PWR/BWR Assemblies
PWR17 x 17
BWR9 x 9
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Typical Scrap Materials
Off-specification pellets
Solids, residues, cleanout from processes (ADU, UOx)
Filter materials, blowback
Machined scrap - from grinding etc.
Dust from the ceramic process hammer mills, attritors granulating/slugging
anything containing uraniumeven incinerator ash
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Upon final acceptance of the fuel assembly, units are packed in shipping containers for transfer to utility power reactor site
Fuel Assembly Packing Shipping Container Loading
Fuel Fabrication
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AssembledFuel Bundle
At the Nuclear Power Plant, new fuel assemblies are inspected and loaded into the reactor corewhere the 235U in the fuelpellets fissions producingheat for electric powergeneration
Fuel Fabrication
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What is MOX?
MOX contains plutonium Mixed uranium-plutonium OXide fuel
Can be reactor or weapons grade Pu
A one-third core approach “essentially” same as LEUO2
Matrix is sintered DUO2 pellets
5-8% Pu in pellets
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Experience with MOX
European experience positive: over 20 reactors licensed for MOX (one-third) over 15 reactors using MOX MOX burnup license limit: 42,000 MWD/MTHM several fuel fabrication facilities Melox/France is the largest - dry powder
processing (about 200 MTHM/yr capacity; licensed at 105)
several minor incidents but no accidents
U.S. experience limited test assemblies, FBR fuel wet processing, generally OK some contamination concerns
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MOX Trends?
French, Swiss - continuing Germany - “some MOX activities” Britain - “waiting” Japan - “planning” Russia/FSU - valuable resource
Environmental Safety and Health impact: low, no discernable trend fuel fabrication doses, impact comparable to U facilities
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Use of Weapons Pu
Short irradiation and low burnup
Uses Pu from dismantled weapons
Typically 90%+ fissile Pu
Requires purification from Ga, Am-241 in-growth
Weapons Pu starts as metal, not as the oxide
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ES&H Concerns
Pu and MOX powder more radiotoxic than UO2 fuel powder
Room release example: 1 mg in nominal room, 1 minute exposure, nitrate = 0.35 Sv inhalation dose
Ground release example: at 100 meters, 0.32 g, 1 hour exposure = 1Sv (from Pu-239)
Uranium quantities would have to be 100 times larger to give the same doses
More radioactive/gamma, particularly for reactor Pu Criticality Once pelletized, sintered, in rods
essentially no impact
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UF6 release
Criticality
Chemicals used in process
Fuel Fabrication Hazards