Design approach for the optimization of linear heat rating of MOX fuel pin

In FBRs, higher linear heat rating of fuel is desirable for economic operation without exceeding the safety limits prescribed for the fuel pin. Hence, for safe and reliable operation of the reactor, the fuel pin integrity is to be ensured throughout its residence time in the reactor core. The temperature of the clad, which forms the primary boundary, is limited from structural strength considerations. The fuel operating temperature should have sufficient safety margin with respect to the melting point. While the temperatures of the fuel and clad are not measured, only the coolant temperature is measurable through which the operating temperatures of clad and fuel are derived. Hence, design safety limits concerning the fuel, clad and coolant sodium, which are defined in terms of temperatures, have to be respected in the design of the fuel pin under all design basis events. Therefore, the operating linear heat rating of the fuel pin should be well below the incipient melting heat rating level. Determination of the safe operating and allowable linear heat rating levels should be robust and it should account for all the possible uncertainties that could get into the estimation of temperature of fuel, clad and coolant temperatures. Further, it should account for the possible instrumentation error in the neutron flux measurement and the time delay in effecting the safety action within which the power could reach higher levels. The power level at which the fuel pin can operate depends on the condition of the fuel whether it is fresh or conditioned. The uncertainties in the estimates are taken into account through hot spot factors and 15% over power margin is imposed in the design of the PFBR fuel pin. The paper brings out the details of the possible uncertainties and the methodology of their treatment in the design adopted for PFBR MOX fuel pin design. It is observed that the fresh PFBR MOX fuel can be operated at a peak linear power of 400 W/cm and could be enhanced to the rated power of 450 W/cm after few days of operation. (author)