Fuel – Prismatic Design

Japan Atomic Energy Agency

Training Course on

High Temperature Gas-cooled Reactor Technology

October 19-23, Serpong, Indonesia

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Prismatic Type Fuel (HTTR)

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Coating Layers of the Coated Fuel Particle

• The 1st Buffer layer provides free volume for stable

fission gases and CO gas generated by fission. The

damage to the IPyC layer by the fuel kernel swelling

and by recoil of fission fragments is also protected.

• The 2nd Inner PyC layer, the second layer, prevents

corrosion of the UO2 kernel in the SiC layer coating.

The PyC layer almost completely retains short-lived

fission gases.

• The 3rd SiC layer is the strongest layer and is the

pressure vessel against internal pressure during

irradiation. This layer also acts as the primary barrier

to the release of metallic fission products.

• The 4th Outer PyC layer, the final layer, protects the

inner SiC layer from mechanical failure during

handling of coated fuel particles. The PyC layers

undergo irradiation-induced shrinkage as a result of

fast neutron exposure and the OPyC layer places a

compressive load on the SiC layer.

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Fuel Fabrication Process

Optimize fluidization & Continuous coating Optimize warm pressing

Uranyl nitrate solution

UO2 particle

Buffer coating (C2H2+Ar) IPyC coating (C3H6+Ar) SiC coating (CH3SiCl3+H2:MTS) OPyC coating (C3H6+Ar)

TRISO coated particle

Graphite powder Binder

Overcoat particle

Overcoat particle

Warm pressing Preheating Heating

Fuel compact

Graphite sleeve

Fuel rod Graphite block Burnable poison (B4C)

Fuel assembly

HTTR fuel is fabricated in commercial-scale fuel fabrication plant by Nuclear Fuel Industries, Ltd.

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Fuel Fabrication Process (Kernel)

Outer gelation method U3O8 Powder

Uranyl nitrate (UO2(NO3)2)

Broth solution

Vibrational dropping into NH4OH

ADU ((NH4)2U2O7) particles

Calcination

UO3 particles

Reducing & sintering

UO2 kernels

Nitric acid

Additives

Q =(pD3/6) f Q : flow rate of metal solution D : diameter of droplet f : frequency of vibrating nozzle

HTR Fuel Pamphlet, NFI, Ltd. Tokai Works

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Fuel Fabrication Process (Coated Fuel Particle)

UO2 kernels

1st layer (Low-density pyrolitic carbon)

• C2H2 (w/ Ar) 2C + H2

2nd layer (High-density pyrolitic carbon)

• C3H6 (w/ Ar) 3C + 3H2

3rd layer (Silicon Carbide)

• CH3SiCl3 (w/ H2) SiC + 3HCl

4th layer (High-density pyrolitic carbon)

• C3H6 (w/ Ar) 3C + 3H2

TRISO (TRIstructural ISOtropic) - coated fuel particle

HTR Fuel Pamphlet, NFI, Ltd. Tokai Works

Chemical vapor deposition (CVD) on the surface of the particle by the coater with fluidizing bed

Reaction for 1st, 2nd & 4th layer : Pyrolysis of hydrocarbon.

Reaction for 3rd layer : Pyrolysis of Methyltrichlorosilane. H2 as the carrier gas is important to prevent the generation of SiC+C.

Coating temperature : Approx. in range of 1,350 - 1,650 °C (Different from each process)

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Fuel Fabrication Process (Coated Fuel Particle)

Gas

Source Gas Inlet

Commercial scale (3 kg-U) coater • Optimizing the particle

fluidization mode.

Once-through coating process • Coating from 1st to 4th layers

continuously in order to avoid failures by unloading and loading processes.

C2H2 + Ar : 1st C3H6 + Ar : 2nd, 4th CH3SiCl3 + H2 : 3rd

Furnace with fluidizing bed

Appearance of particles spouting

(in the dummy bed)

HTR Fuel Pamphlet, NFI, Ltd. Tokai Works

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Fuel Fabrication Process (Coated Fuel Particle)

HTR Fuel Pamphlet, NFI, Ltd. Tokai Works

• Overcoating

Coating TRISO particles with graphite matrix by rotating vessel to prevent particle failures due to direct contacting each other.

Weight fraction before carbonization

Natural graphite : 64 wt.%

Petroleum coke : 16 wt.%

Phenol resin binder (m.p. 143°C) : 20 wt.%

• Warm pressing with metal dies

CFP packing fraction : 30 vol.%

• Heat treatment

Pre-heating at 800 °C in N2 for carbonization of binder

Heating at 1,800 °C in vacuum for degassing

TRISO-coated fuel particle

Overcoating

Warm pressing

Preliminary heat treatment

Heat treatment

Fuel compact

Graphite powder

Binder

Graphite matrix

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Fuel Failure Mechanism

Plant states Causes of Fuel Failure Failure Mechanism

Normal operation

Abnormal condition

Kernel Migration

Pd-SiC

Internal pressure

Abnormal temp.

RIA

SiC thermal degradation

Internal pressure

SiC oxidation

Temp. gradient

Nuclear fission

Oxidation

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Kernel Migration (Amoeba Effect)

Diffusion in solid phase

C + (1/2)O2 CO

C + O CO

Hot side

Cool side

Thermal diffusion

through UO2

Vapor-phase diffusion through 1st (buffer) layer

Deposit

K. Hayashi, JAERI-M89-162(1989)

An example of “amoeba effect” of coated fuel particle with UO2 kernel

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Pd-SiC Interaction

The Pd-SiC interaction in the HTTR design reference fuel was investigated intensively in PIEs.

Minato, et al., J. Nucl. Mater. (1990)

1470oC, 3.84%FIMA 1260oC, 4.4%FIMA

• Irradiated SiC-TRISO coated low-enriched (4-20wt% of 235U) UO2 particles has been observed by an optical microscope and electron probe microanalyzers.

• Following reaction would have occurred at the corroded area.

2Pd + SiC Pd2Si + C

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Pressure Vessel Failure

• The stresses act on the coating layers are assumed to be caused

by pressure of fission gases and CO gas by fast neutron-induced shrinkage of the

PyC layers. • Through-coatings failed particle and SiC-failed

particle are categorized.

Through-coatings failed particle

Intact particle SiC-failed particle

Sawa, Tobita, Nucl. Tech. (2003)

SEM image of the failed particle (91F-1A irradiated up to 90GWd/t)

300mm

30mm

Outer PyC surface

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Fuel Failure Mechanism

Plant states Causes of Fuel Failure Failure Mechanism

Normal operation

Abnormal condition

Kernel Migration

Pd-SiC

Internal pressure

Abnormal temp.

RIA

SiC thermal degradation

Internal pressure

SiC oxidation

Temp. gradient

Nuclear fission

Oxidation

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SiC Thermal Degradation

2320oC for 25min

2200oC for 25min

2320oC for 64min

The transition temperatures of -SiC (as-deposited) to -SiC vary at the temperature up to 2200 ˚C.

The smooth surface is composed of thin-plate grains of 6H polytype -SiC.

Tabular grains composed of thin plates appear in the -SiC matrix with small pores.

Composed of only -SiC

Kurata, et al., J.Nucl.Mater.(1980)

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SiC Thermal Degradation

The behavior of the coated fuel particles up to 2400 oC was investigated with a furnace installed in a hot cell. It was found that the coating layers of the HTTR coated fuel particles would maintain their intactness below 1600 oC. In general, design target for the maximum fuel temperature is set as 1600 oC to maintain the integrity of coating layer.

1.0

0.8

0.6

0.4

0.2

0

Temperature [oC] F

ailu

re f

raction [

-]

1600 1800 2000 2200 2400 2600 2800

* INL, INL/EXT-10-18610 (2010).

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SiC Thermal Degradation

W. Shenk, et al. J. Nucl. Mater. 171, 19 (1990).

Figs. Fractional releases of 85Kr (left) and 137Cs (right) from irradiated HTGR fuel as a function of heating time at constant temperature.

Fractional releases of Kr and Cs nuclides from the irradiated HTGR fuel element are significantly low at 1600 °C for 500 hours.

The HTGR fuel keeps its integrity under the accident condition.

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Internal Pressure - RIA

According to the RIA demonstration test by NSRR, it was revealed that fuel failure by reactivity insertion occurs at UO2 melting point due to pressure increase because of the immediate thermal expansion of fuel kernel.

The parametric simulations for RIA accident showed that temperature difference is observed between fuel kernel and fuel compact. The fuel kernel temperature differs depending on the gap size and contact surface area between kernel and coating layers.

K Sawa, et al., JAERI-M 92-175 (1992)

Tem

per

atu

re (

oC

)

2000

1000

0 0 10 20

Tem

per

atu

re (

oC

)

3000

2000

1000

0 0 10 20

Elapsed time (s) Elapsed time (s)

Gap Gap

Fuel compact

Fuel kernel

Fuel compact

Fuel kernel

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SiC Oxidation

K. Minato and K. Fukuda, IAEA-TECDOC784(1993)

In case of the SiC-C-O2-He system, two modes of the oxidation behaviors of SiC have been characterized in this study;

The active oxidation SiC(s) + O2(g) SiO(g) + CO(g)

The passive oxidation SiC(s) + 3/2O2(g) SiO2(s) + CO(g)

Acitive oxidation causes decrease in the thickness of the SiC coating layer and to lead to the coating failure or loss of coatings together with extensive fission product release The active-to-passive transition of oxidation of SiC+C occurred at lower temperatures than that of SiC at a given initial O2 pressure. Under the passive oxidation conditions, protective layer of solid SiO2 would be formed on the SiC coating layer below the melting point (1723oC) of SiO2.

Fuel Design Approach in HTTR

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The initial failure fraction in the coating layers of the coated fuel particles shall be less than 0.2% in terms of the sum of exposed uranium and SiC defects.

The coated fuel particles shall not fail systematically under normal operating condition that tis,

a. The penetration depth of Pd-SiC interaction which is the corrosion depth of the SiC layer by a fission product of Pd shall not exceed the thickness of the SiC layer of 25μm, because the fully-penetrating Pd-SiC interaction is thought to lead to a loss of the fission product retention function of the SiC coating layer.

b. The kernel migration shall not exceed the thickness of 90 μm, which is the sum of the first and second layers.

The coated fuel particles shall be designed so as to avoid, in principle, failure considering irradiation-induced damage and chemical attack throughout the full service period, that is, the additional failure fraction in the coating layers of the coated fuel particles shall be less than 0.2% during the full service period.

The maximum fuel temperature shall not exceed 1600oC at any AOOs to avoid fuel failure, thus in other words from fuel side, the coating layers of the fuel particles shall maintain intact at any anticipated transient below 1600oC.

K Sawa, et al., J. Nucl. Sci. Technol., 36(8) 683-690 (1999)

Fuel Failure Detection in HTTR

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• In the HTTR, the continuous and reliable measurement of the coolant activity is carried out to evaluate the fuel performance and the fission gases release behaviors during the operation.

• The fission gases are released from the through-coatings failed particle and from the uranium contamination in the fuel compact matrix.

• The evaluation of fuel and fission gas behavior during the operation is carried out by the primary coolant radioactivity measurement and analytical modeling of fission gas release.

Ionization chamber of PCR

instrumentation

Manual sampling test of the

primary coolant

Precipitators

FFD system

Fuel Performance in HTTR

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Fra

ctional re

lease

of

fissio

n g

as (

88K

r)

Operation date in 2010

10-9

10-8

10-7

10-6

10-5

10-4

(U.S.A.) Fort St. Vrain

(Germany) AVR

Continuous high temperature operation

Operational limit of the HTTR : 1×10-4