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Nuclear Fuels : Property Evaluation

3.1 HIGH PLUTONIA MIXED OXIDE FUEL

As a part of the high plutonia (44% PuO2) MOX fuel development

for FBTR, new thermo-physical property data which is not

available in literature has been generated and highlighted in the

following paragraphs.

The coefficient of linear thermal expansion data of MOX fuel

was generated as a function of temperature (ambient to 1873K)

using a dilatometer under flowing Argon gas. The average thermal

expansion was found to be 12.52 x 10-6/K (ambient to 1273 K).

The same for MOX fuel containing 30% PuO2 is 10.65 x 10-6/K.

Thermal conductivity of the fuel was determined from the

experimentally measured thermal diffusivity by laser flash method

between 873 � 1773K. At 1273K thermal conductivity of MOX

(44% PuO2) was found to be 1.803 W/m.K which is lower than

that of MOX containing 30% PuO2 (2.3265 W/m.K).

The purity of sodium in terms of �O� content varied from highly

pure, pure and impure sodium. Test capsules, after isothermal

annealing at 973-1073K for times varying from 30 hrs. to 500

hrs. were evaluated by visual examination, X-ray radiography,

X-ray diffractometry, weight change and metallography. All these

examinations indicated that there was no sign of incompatibility

between the fuel and sodium and the pellets retained the

integrity. The results of these studies have made a very positive

impact on the possibility of utilizing this fuel for FBTR.

After approval of the safety committee MOX fuel is expected to

be used in the hybrid core of FBTR which will consist of both

mixed oxide ( 44% PuO2) and mixed carbide fuel (MK I) containing

70% PuC.

Thermal conductivity of MOX (44% PuO2) fuel

Fuel coolant chemical compatibility is one of the most important

aspects for the safe operation of the fuel inside the reactor. A

number of compatibility test capsules containing fuel pellets and

sodium were made to simulate different reactor conditions.

Different types of test carried out were:

..... MOX fuel pellets immersed directly intoMOX fuel pellets immersed directly intoMOX fuel pellets immersed directly intoMOX fuel pellets immersed directly intoMOX fuel pellets immersed directly into

liquid sodium;liquid sodium;liquid sodium;liquid sodium;liquid sodium;

..... Precracked fuel pin containing fuel pellets inPrecracked fuel pin containing fuel pellets inPrecracked fuel pin containing fuel pellets inPrecracked fuel pin containing fuel pellets inPrecracked fuel pin containing fuel pellets in

liquid sodium.liquid sodium.liquid sodium.liquid sodium.liquid sodium.

Schematic Diagram of MOX / Sodiumcompatibility capsule

Microstructure of as-received MOX(44% PuO2 pellet)

Nuclear Fuel Cycle BARC HIGHLIGHTS 17

Nuclear Fuels : Property Evaluation

Microstructures of MOX (44% PuO2) at different locations after compatibility testat 973K for 150 hrs. showing no reaction product at the pellet/sodium interface.

The grey areas within the pellet are cracks filled with araldite

BARC HIGHLIGHTS Nuclear Fuel Cycle18

Nuclear Fuels : Property Evaluation

A fuel pin undergoes thermal cycling during power ramp or shut

down/restart of a reactor. Any second phase precipitation may

lead to dimensional changes which is undesirable.

The dimensional stability of the fuel was tested by thermal cycling

(ambient to 1473 K) in a push rod type dilatometer under inert

cover gas and monitoring the change in length. No abrupt

change in the dimension of the pellet could be observed after a

number of cycling.

Dimensional Stability of MOX (44% PuO2)after thermal cycling