4/2003 rev 2 i.4.9f – slide 1 of 50 session i.4.9f part i review of fundamentals module 4sources...

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