CANDU Fuel Design Lec Flashcards by Missy Balachorek

A nuclear reactor is a device that ____
and ____ a sustained ____ ____ _____

initiates
controls
nuclear chain reaction

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Nuclear reactors are used for ____
generation and in ___ of ships

electricity
propulsion

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Heat from ____ is passed to a
______(___or ___), which runs through _____

nuclear fission
working fluid (water or gas)
steam turbines

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A nuclear reactor needs a few basic functional requirements:
list 6

-fissionable material (fuel (U-235))
-moderator to thermalize (i.e., slow down) fast neutrons
-coolant to remove heat
-control system to control the number of neutrons
-shielding system to protect equipment and people from radiation
-system that combines all of the above to form a workable device

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label candu reactor
slide 6

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The purpose of a nuclear fuel is to ______, which can be used to do ____that can be converted to ____

generate heat
WORK
power

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The materials performance requirements
for fuels are:

The density of fissionable nuclei must be
sufficiently high to sustain a controlled chain reaction

High thermal conductivity is necessary to
transfer the heat from the fuel to the coolant

Radioactive materials inherent to the fuel and generated during fission must remain safely sequestered within the fuel

The cost of the complete fuel life cycle, from
fabrication to disposal or reuse, must be
competitive with alternative technologies

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Which is preferable U02, U308, or metal fuel?

. However, in terms of the possibility of a gap developing between the fuel and the carbon matrix, UO2 is certainly preferable because its oxidation is accomp

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High thermal conductivity is necessary to
transfer the heat from the fuel to the coolant
What does this mean for energy efficiency?

Overall, high thermal conductivity in nuclear reactor materials leads to more efficient energy conversion, better fuel utilization, and improved safety, all of which contribute to the overall energy efficiency of the nuclear power plant.

You said:

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Radioactive materials inherent to the fuel and
generated during fission must remain safely
sequestered within the fuel
What are the radiation safety implications?

ensuring controlled reactor operations, protecting workers and the public, and preventing environmental contamination. Fuel integrity is one of the most fundamental aspects of nuclear power plant safety.

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The unique characteristics of CANDU fuel:

Collapsible Zircaloy-4 sheathing to ensure
good fuel-to-sheath heat transfer and neutron economy

High density Natural UO2 pellets with good
dimensional stability

Short simple bundle configuration

No plenums (spring loaded chamber)

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No plenums (spring loaded chamber)
Why no plenums? Explain

Continuous Online Refueling Eliminates the Need for Fuel Longevity Considerations

Shorter Fuel Bundles Reduce Gas Build-up

Zircaloy Sheathing and Fuel Bundle Structure Handle Gas Accumulation

CANDU Uses Natural Uranium with Lower Burn-up

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Nuclear fuels are subjected to ____
levels of radiation damage:
mainly due to the ____of ___ fragments

Both___ and ____ defects are
created

Presence of ____ defects alter the
material and its properties

very high

slowing down
fission

point and extended

crystalline

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CANDU reactors use_____for fuel (____% U-235 and ___% U-238)

natural uranium
0.711
99.284

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U-235 has a___ fission cross
section (f), while U-238 has a
f of ____barns for
thermal neutrons

high
essentially zero

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Fast reactors use___ neutrons where the fission cross section, (f), is ____
for U238 than for U235.

fast
higher

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density (mass number A) of fissionable nuclei must be ____to sustain a controlled chain reaction

sufficiently high

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Fuel Element Design- Requirements

Provide good ___economy

Capable of operating at_____
Capable of withstanding _____ and ____during fuel management schedules

Should withstand ____peaks between ____

Withstand power output changes from _____to burned-up fuel as U-235 is depleted and the buildup of____,____, and fission products occurs

neutron
continuous high power
power cycling
power changes
adjacent bundles
highly reactive fuel
Pu-239, Pu-241

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Fuel Element Design- Temperature
What is heat transfer?

movement of heat due to a
temperature difference between a system and its
surroundings

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What is heat transfer?
The energy transfer always occurs from a ___ temperature to a ____temperature
The units of heat transfer are the ____. The unit
for the heat transfer rate is the ____

higher
lower

joule (J)
watt (W)

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There are three types of
heat transfer, depending on
the medium that transmits
heat:

Conduction requires
molecular contact
Convection requires the
movement of fluid molecules
(flow)
Radiation does not require
any medium
(electromagnetic)

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How is heat transfer calculated?
see slide 24

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Convective heat transfer results from _____

fluid moving across
a surface that carries heat away

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convective heat transfer eq
slide 27

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Dittus-Boelter Equation slide 28

Nu is the _

Nusselt number, representing the convective to conductive heat transfer ratio across the boundary layer.

➢ Re is the ___

Reynolds number, representing the ratio of inertial to viscous forces in the flow.

➢ Pr is the ___

Prandtl number, representing the momentum and thermal diffusivity ratio.

➢ n is an ____ ➢ n=0.4 for ___ ➢ n=0.3 for ___

exponent dependent on the heating or cooling condition: heating (Twall>Tfluid). cooling (Twall

slide 29 and 30 and 31 and 32

At a steady state, the heat produced within the fuel is transferred to the ___along the length "l ” and around the circumference of the element.

coolant

example slide 33/34/35

The advantage of using linear heat generation (q’), rather than the volumetric heat generation, is ___

that the same ∆T is obtained for a given q' regardless of the fuel radius.

The sheath outside surface is in contact with __

the coolant delivering the heat flux

The fuel-to-sheath heat transfer coefficient is dependent on ___, ___, and ____

conduction through the gas layer between the two surfaces heat transfer through point of solid-tosolid contact radiation between the two surfaces.

slide 41

This temperature difference __when solid-solid heat transfer ___ as pellet-sheath interface pressure (coolant pressure minus the gas pressure) ___. This plays an important role during accidents

decreases increases increases

slide 43

Fuel Element Design - Bundle Power The maximum linear element power of a 600 MW CANDU 6 reactor is ___ kW/m

50.9

The fuel element length and diameter are __ m and ___ mm, respectively (Note: CANDU 6 uses __- element bundles)

0.4953 13.08 37

The maximum surface heat flux is ___ kW/m2

1237

The bundle power under nominal design conditions is __ kW

800

What is an over-power incident?

An over-power incident occurs when nuclear fission deposits excessively high power in the fuel. Regional overpower protection (ROP) systems in the reactor monitor and arrest such incidents; however, they are analyzed to ensure safety margins.

The over-power could lead to __. To guarantee optimum cooling of the fuel elements, the sheaths must always be covered with a __. If they are not completely covered it is called __.

dryout. film of water “dryout”

What happens when reactor experiences an overpower incident?

Dryout Sheath temperature rise Irradiation damage and hydride embrittlement on Zircaloy anneals out Oxidation embrittlement of sheath Braze melting and attack on Zircaloy Zirconium-water reaction (exothermic) Bundle collapse Sheath melting Fuel melting (extremely unlikely) Pressure tube balloon or burst Heat transfer to moderator

As power increases, the coolant flow transitions from ____,____,_____,____and ____with a sudden increase to fuel sheath temperature

single phase liquid, nucleate boiling, critical heat flux, dryout and film boiling

slide 51 ans 52

CHF is defined as a phenomenon describing the _____, which abruptly decreases the heat transfer, causing localised overheating of the surface If the surface does not rewet quickly, it may lead to____

thermal limit of a heated surface, reached due to phase change of liquid coolant, such as bubble formation, bubble coalescence, and vapour film formation rapid oxidation, possibly melting, and burnout of that surface

An objective of the CANDU reactor fuel design is the ____

avoidance of fuel melting during normal operation and design-basis accidents

For all CANDU accident analysis cases at least ~__% margin to onset of fuel melting is available

The parametric limit for fuel melting is set at ___°C An interim limit of __°C, recommended by the regulator, is being used by the industry

2,822 2600

There is __% enthalpy margin available for fuel melting in CANDU reactors during large break loss-of-coolant accidents

Fuel burnup is ___

the amount of energy that is obtained per unit mass of uranium in the fuel

The average discharge bundle burn-up is ___MWh/kg(U) A bundle with average power of __ kW remains in the reactor for __ hours before being discharged

180 800 4340

A higher burnup ___uranium utilization but the risk of fuel failures ___ ❖ The fuel melting point is reduced by __°C for every ___ MWh/kg(U) of fuel burnup

increases increase 32 240

Five main processes are responsible for the release of fission gases from the UO2 pellet:

Atoms of fission gases migrate to the surface of the pellet, Grain boundaries move and sweep up fission gases, Bubbles move up the temperature gradient and may tend to pull grain boundaries along with them and sweeping up fission gases, Fission-gas bubbles located on grain boundaries grow and eventually interlink, and Cracking of UO2 at a power change releasing gas trapped in tunnels to the voidage in the fuel-to sheath gap

The objective of pellet design is to ___

minimize sheath stresses and strains

CANDU fuel design meets strict __ economy needed for natural uranium reactors Fuel pellet densities have to be as ___ as possible and the amount of parasitic material as __ as possible

neutron high low

Pellets have a ___ dish in one or both end faces to accommodate the __ of the hot centre core of the pellet stack to ___ pellet axial and diametral expansions

spherical volumetric expansion minimizE

The dish can also provide a -____

significant void volume for relieving fuel element internal gas pressure

SLIDE 63

exercises slide 64/65

Stress-corrosion cracking (SCC) led sheath failures occur from the combined action of

stress concentrations from pellet-clad-interaction (PCl) and corrosive fission products

SSC occurs because:

irradiation generates corrosive fission products, high ramped power and fuel temperature release more fission products into the gaps between the UO2 and the sheathing, and power ramping causes PCI and local tensile stress above some critical value for SCC

to combat fuel failure problem, by reducing stress concentrations in the sheath, a ____

graphite lubricant CANLUB was used between the sheath/UO2 interface

Stresses and strains develop on the -__due to ____

fuel sheath fuel and fuel sheath expansion and contraction

primary causes of sheath stress, strain and fatigue

Power ramps, coolant pressure fluctuations, irradiation hardening and fission product generation are some of the

The pellet-sheath interaction during irradiation can lead to___

defects in both the sheath and the pellet under certain conditions

The pellets are designed to minimize _____ while ensuring ____

sheath stresses and strains low fabrication cost and low variability of product for quality control

The fuel sheath may fail due to:

Overstrain (5% average sheath strain to the onset of ballooning – limit of uniform strain), Low Ductility (due to work hardening – athermal glide), Beryllium-Assisted Crack Penetration, Oxygen Embrittlement, Overstrain under Oxide Cracks Oxide strengthening models and 2% sheath strain at 1000°C, High Strain Rate (ballooning failure).

slide 69

The ___ is the driving force for sheath strain in accidents

fission gas pressure

Affects sheath liftoff and therefore the

Fuel-to-sheath heat transfer (decreases) Fuel temperature (increases)

slide 73 hooks law

exeercises slide 74 and 75

slide 76 plastic regime, power law relates stress/strain/strain rate

sldie 77

Fuel sheathing depending on wall thickness will creep down under the effect of _____ unless supported by the ____ In thin wall elements, primary collapse or wrinkling of the sheath is prevented by___, and by ensuring that the __

coolant pressure and irradiation the UO2 pellets controlling the diametral gap between pellet and sheath to small values specified wall thickness and mechanical properties are maintained

___ on the pellet is desirable to close the ____with finite amount of plastic strain as long as it does not ___ These ridges are characterized by __, and appear rapidly once the clad became unstable and the tip of the ridge has the potential to become ___

close the radial gap between pellet and clad become unstable to form permanent longitudinal ridges very large plastic strain a site for crack initiation under subsequent cyclic thermal/pressure loading

Non-uniform temperature in the coolant surrounding an element due to __

imperfect mixing

Non-uniform heat transfer between the sheath and coolant due to __

variations in sub-channel geometry and local flow conditions

Asymmetric heat generation in the pellets due to __

neutron flux gradients across an element

The thermal cycles on a fuel element as it heats and cools induces ___

high contact pressure between the pellet and the sheath

The sheath region at the pellet-to-pellet interface develops __ in this region the pellet rotates around their upper (contacted) corners to develop ___

pronounced necking of the sheath sheath sag

The wall thickness reduces in the necked regions with a larger reduction amount on the ____ then __ This can be interpreted as an effect of ___

bottom than the top of the sheath localization of the sheath longitudinal tensile strain in these regions

Sheath Corrosion & Hydriding ❖ The kinetics of cladding oxidation in air environment is important for ___ ❖ The knowledge of air oxidation kinetics of cladding at relatively ___ temperatures is essential in __

fueling machine accidents, structural failure of dry storage and transportation cask low assessing and determining the margin for clad integrity

Flow Impedance & Out-Reactor Hydraulic Tests ❖ In the absence of an axial gap both the ___ and __ together as a single structure and the resultant axial stiffness is___ ❖ However, the pellet is not always in __with the sheath because ____,____and____ ❖ This in turn ___ the net flexural rigidity of the fuel element and hence ___ its vibration frequency ❖ Lower power also reduces the ___

UO2 and the sheath act high compared to that of the sheath alone tight contact continued irradiation, the interfacial pressure between the pellet and the sheath relaxes due to creep lowers decreases interfacial pressure

An inherent safety feature of CANDUs is that the reactivity mechanisms do not __

penetrate the pressure boundary, the fuel channels

Other inherent safety features include ___as a back-up heat sink and____ providing protection against severe accident progression

moderator moderator and shield tank

R&D includes _

high-temperature materials behaviour, chemistry, and heat transfer in specialized laboratories for high-temperature experiments

slide 91

slide 92

A device that initiates and controls a sustained nuclear chain reaction is a___

nuclear reactor

The mass number of fissionable nuclei must be ____

sufficiently high to sustain a controlled chain reaction

Nuclear fuel-generated heat is converted to___ and to do so, its design must avoid ___and __

power melting during normal operation design-basis accidents

The fuel sheath must provide ___

good neutron economy, operate at high power, withstand power cycling and prevent fission products from leaking

The fuel channels in CANDU reactors are inherently safe because __

the reactivity mechanisms do not penetrate the pressure boundary