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Post Irradiation Examination of Thoria-Plutonia Mixed Oxide Fuel in Indian Hot Cells

J. L. Singh, Prerna Mishra, J. S. Dubey, H. N. Singh, V. D. Alur, P. K. Shah, V. P. Jathar, A.

Bhandekar, K. M. Pandit, R. S. Shriwastaw, P. M. Ouseph, Nitin Kumawat, P. M. Satheesh,

G. K. Mallik and Arun Kumar

Post Irradiation Examination Division

Nuclear Fuels Group

Bhabha Atomic Research Centre

Trombay, Mumbai-400085

Abstract Mixed oxide (MOX) fuel clusters containing ThO2+4%PuO2, and ThO2+6.75%PuO2 fuel pins were

irradiated in the pressurized water loop of the Indian research reactor CIRUS, to burn up in the range

of 20 GWd/T(HM). The ThO2+4%PuO2 fuel elements had free standing cladding made of Zircaloy-2

and the ThO2+6.75%PuO2 had collapsible Zircaloy-2 cladding. The fuel clusters had performed well

during irradiation with no apparent indications of failure. The techniques used for the post irradiation

examination (PIE) of these fuels in the hot cells included visual examination, fuel pin diameter

measurements, leak testing, gamma scanning, gamma spectrometry, ultrasonic testing, eddy current

testing, ceramography, metallography, beta gamma autoradiography and measurement of released

fission gases. Micro hardness measurement of cladding and evaluation of mechanical properties using

ring tension test were also carried out. This paper elaborates on the techniques and the results of the

PIE carried out on ThO2+4%PuO2 fuel.

1. Introduction

India has about four times more thorium resources than uranium. Utilization of thorium for large scale

energy production is a major goal in the three stage nuclear power programme[1]

. Hence, research and

development in fabrication, characterization and irradiation testing of thoria based fuels is necessary.

A six pin cluster, consisting of ThO2-4% PuO2 fuel pellets, had undergone irradiation testing in the

pressurized water loop (PWL) of CIRUS thermal reactor up to a burn-up of 18.5 GWd/t. Post

irradiation examination (PIE) of the fuel pins from the cluster was carried out at BARC hot cells

facility. Non-Destructive Examination of the fuel pins from the cluster has been carried out by visual

examination, fuel pin diameter measurements, leak testing, gamma scanning, gamma spectrometry,

ultrasonic testing, eddy current testing. Micro structural characterization on two fuel pins, TH-5 and

TH-2 from the cluster, has been done using optical microscopy, scanning electron microscopy, β-γ

autoradiography and α-autoradiography techniques. This paper presents the salient observations of the

examinations carried out on the irradiated fuel pins.

2. Fabrication details

The experimental six pin cluster (AC-6) consisting of ThO2-4% PuO2 fuel pellets, encapsulated in a

free standing Zircaloy-2 cladding, was fabricated at the Radio Metallurgy Division (RMD). The

cluster consisted of short-length fuel pins of about 0.5m length and the sketch of a typical

experimental MOX fuel pin is shown in Fig 1. Table 1 gives the details of the fabricated fuel pins.

3. Irradiation history

The AC-6 experimental cluster was irradiated in the PWL of CIRUS research reactor. The thermal

neutron flux in the loop was 5x1013

n/cm2/sec and the temperature and pressure of the coolant in the

loop was 240oC and 105Kg/cm

2 respectively. Peak linear heat rating and peak burn-up in the fuel pins

was 40 kW/m and 18.5 GWd/t respectively.

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Table 1 Details of the Thoria based MOX fuel pins

Cluster AC-6

Clad type Free standing

Number of pins 6 (5 ThO2-4% PuO2 + 1 Helium filled pin)

PuO2 enrichment 4%

Pellet diameter 12.22 ± 0.01 mm

Pellet length 12.0 ± 1.0 mm

Pellet density 92-94% TD

Stack length 435 mm

Cladding outer wall diameter 14.3 mm

Cladding wall thickness 0.8 mm

Cold plenum length 20 mm

4. Non-destructive testing

4.1. Visual examination

Visual examination was carried out on the individual pins using a wall mounted periscope. All the fuel

pins were found with deposit of loose white powder which may have come from the storage water

pool containing aluminum cladded research reactor fuel assemblies. The fuel pins were cleaned by

cotton damped with alcohol. No abnormality was visible on the surface.

4.2. Diameter measurement

A diameter measuring set up was made to be suitable to keep the fuel pin straight on the platform. The

diameter was read from the dial gauge display through the periscope. The diameter of the irradiated

fuel pins was found to be within the manufacturing tolerances.

4.3. Leak testing

Leak testing was carried out in the hot cell using liquid nitrogen and alcohol leak test. During the test

each pin was immersed in liquid nitrogen for 5-7 minutes. Subsequently the fuel pin was transferred to

get immersed in a tank filled with alcohol. There was no leak observed in the fuel pins.

4.4. Ultrasonic testing

Two 10 MHz line focused immersion probes were fitted at an angle in the probe carriage for detection

of axial and circumferential defects. Multi-channel ultrasonic flaw detector was used for slow helical

scan combining axial probe translation and rotation of fuel pin. Surface roughness signals were

predominant making interpretation difficult.

Fig. 1. A typical experimental MOX fuel pin

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4.5. Eddy current testing

Eddy current testing (ECT) was carried out on 4 fuel pins except pin TH-5, which could not pass

through the eddy current testing coil. Defect signals were obtained from the cladding of pin TH-2 as

shown in Figs 4 a and b. The fuel pin is pulled up slowly and signals are recorded by the computer.

4.6. Gamma scanning

Gamma spectroscopy and scanning using the HPGe detector and multi channel analyzer/scaler

(MCA/MCS) were carried out on these elements inside the hot cell. Co-60 and Cs-137 sources were

used for calibration. The spectra obtained from these elements revealed the presence of Cs-137, Cs-

134, Eu-154 and Tl-208. Fig. 5. Shows a typical gamma ray spectrum.

Fig. 5. Gamma ray spectrum of (Th-4%Pu)O2 fuel pin

Fig. 2. Loading of fuel pin to Ultrasonic scanner

using master slave manipulator

Fig. 3. Two channels of ultrasonic

flaw detector showing surface signals

Plenum spring

Defect signal

Fig. 4 b. ECT signal from defect location of TH-2

Length of the fuel

pin

Fig. 4 a. ECT setup inside the hot cell

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Multi-axial pellet cracks

Plenum spring

4.5. Neutron radiography

The fuel pin TH-4 suspected to be defective during eddy current testing was subjected to neutron

radiography in the test reactor Circus. The bottom and top end plugs were found intact. Multi-axial

cracks in the pellet were observed in the neutron radiograph shown in Fig. 6. The plenum spring was

also observed to be free from any distortion.

5. Destructive testing

5.1. Fission gas analysis

Fuel pins were punctured for the measurement of the amount of released fission gases. The volume of

the released fission gas and the void volume in the fuel pin were measured to arrive at the pressure of

the gas inside the fuel pin. A dual column gas chromatograph and a quadrupole mass spectrometer

were used to analyze the chemical composition and isotopic composition respectively of the collected

gases. Fission gas analysis was carried out on the fuel pins TH-2, TH-4 and TH-5. The fission gas

pressure in the pin TH-4 and TH-5 was 4.4 and 3 atmosphere respectively and the gases were He, Xe

and Kr. The fuel pin TH-2 did not show fission gas, indicating it to be a leaky pin which was also

found defective by ECT.

5.2. Metallographic examination

Metallographic examination of two fuel pins, TH-5 and TH-2 has been carried out to study the fuel

restructuring, cladding oxidation and hydriding. β-γ autoradiography of the metallographic samples

was carried out to study the distribution of the fission products (mainly Cs137

) across the cross section.

α-autoradiography was carried out to analyze the distribution of plutonium in the fuel[2]

. Radiographs

are shown in Fig. 7a, 7b and 7c.

The white spots are the region of low activity, probably PuO2 agglomerate created local high

temperature and cesium migrated to other location. This observation is still under investigation. Radial

Bottom plug

Insulation pellet

Fig. 7a. Photo macrograph

Fig. 6. The neutron radiograph of fuel pin TH-4 shows no abnormality after irradiation.

Fig. 7b. β-γ autoradiograph

Fig. 7c. α-autoradiograph

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cracks were observed in the fuel. No columnar grain formation or grain growth was observed in the

fuel. The β-γ autoradiographs revealed asymmetric distribution β-γ activity across the fuel cross

section. Higher activity was observed in the cracks in the fuel. The α-autoradiograph of the fuel

sections revealed a uniform Pu-activity along the cross-section. Uniform oxide layer was observed on

the outer surface of the cladding, whereas a discontinuous oxide layer was noticed on the inner

surface. Average oxide layer thickness on the outer and inner surface of the cladding was 1.3 µm and

0.9 µm respectively.

5.2.1. Clad metallography at defect location

Metallographic samples were taken from fuel pin, TH-2, in which a defect location was identified

during eddy current testing and no fission gas was obtained during fission gas measurement.

Examination of the cladding revealed a massive hydride blister. Fig. 8.a. shows the photo macrograph

of a section from the pin with a defect region in the cladding. View of the hydride blister at high

magnification is shown in Fig. 8.b.

5.2.2. Scanning electron microscopy (SEM)

The cellulose acetate replica prepared from the fractured surface of the fuel from pin TH-5 was

examined under SEM. The grain size was measured from the impression of the grains on the

replicating tape. Fig. 9a. reveals the grain morphology observed in the fuel. The average grain size

was found to be 14µm. A bimodal grain size distribution with grains up to 30µm in size and cluster of

fine grains of 2-3µm size were also observed among the larger grains as, shown in Fig. 9b.

Fig. 8a. Photo macrograph of the TH-2 fuel pin cross section. Fig. 8b. Massive hydride blister

in the clad and subsequent cracking of clad

(a) (b)

Fig. 9a. SEM photograph showing grain morphology and 9b. bimodal grain size distribution

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Examination of the fractured fuel surfaces revealed grains with as-fabricated pores at the grain edges

and corners. At a few locations, very fine fission gas bubbles were observed on grain faces and there

was evidence of interlinking of the fine bubbles as shown in Fig. 10a. A magnified image of the

interlinked features present on the grain face is shown in Fig. 10b.

6. Conclusions

1. Absence of visible grain growth or columnar grain formation was absent.

2. Radial cracks were present in the fuel pellet cross section.

3. Measured grain size and grain morphology was similar to the as-fabricated fuel with an

average grain size of 14 µm. A few grains up to 30 µm in size and at some locations were

observed. Clusters of fine grains of 2-3 µm were also observed. Bimodal grain size

distribution occurred in some regions of the fuel.

4. Reduced porosity was seen in the central portion of the fuel.

5. Submicron size fission gas bubbles on the fuel grain surfaces in the central region and inter

linkage was also observed.

REFERENCES

[1] JOURNAL OF NUCLEAR MATERIALS 383 (3008) 119-121, Utilization of Thorium in

Reactors.

[2] 11th INTERNATIONAL CONFERENCE ON CANDU FUEL, CANADA, OCTOBER 17-20,

Post-Irradiation Examination of Candu Fuel Bundles Fuelled with (Th, Pu)O2.

Fig. 10a. presence of fine bubbles on the grain faces and 10b. Magnified image shows interlinking of

fission gas bubbles in TH-5