Actinide decay contributes a maximum of approximately 20-30% of the total decay heat for a wide range of irradiation conditions up to cooling times of ~ 108 sec. (Fig. 6). The most significant contributions to actinide decay heat arise from 239U and 239Np for cooling times < 106 sec, while the a decay of 238Pu, 241Am, 242Cm and 244Cm become more important at much longer times (Fig. 7). Assuming a decay-heat contribution of 30% by 239U and 239Np gives rise to a maximum uncertainty in the total decay-heat predictions of 2% for uranium-based thermal reactors at cooling times up to 106 sec. At longer cooling times, the actinide contribution to decay heat is dominated by 242Cm, and the contribution to the overall uncertainty in decay heat ranges from 0.5% to 6% over the cooling time 108 to 3 x 109 sec. Overall, there would appear to be no serious problems associated with the actinides in decay-heat calculations.

Fig. 4. Uncertainties in total decay heat for 235U(T) without taking correlation effects into account

j —— TOTAL

——- DECAY CONSTANT

——- INDEPENDENT YIELD

……… CUMULATIVE YIELD

——- ENERGY

Uncertainties in total decay heat for 238Pu(F) without taking correlation effects into account

T able 5: Main parameters contributing to the uncertainties in total

decay heat for 238Pu(F)

Fig. 6. Percentage contribution of actinides to total decay heat from typical fuel of different reactor systems

Fig. 7. Percentage contribution from individual actinides to total decay energy release rates [5 MW/Te(U) end of life rating 30 GWd/Te(U)] for gas-cooled reactor fuel (AGR)