Three Loading Patterns for ⁹⁹Tc Transmutation in PWR Burnable Poison Rods

Using the MCNP code, we explore three different ⁹⁹Tc loading patterns in pressurized water reactor (PWR) burnable poison rods (BPRs). We also calculate the effects on the PWR k_eff and boric acid concentration readjusted amount after loading ⁹⁹Tc transmutation material into PWR BPRs. Finally, we carry out the transmutation rate and depletion calculation.

After the ⁹⁹Tc transmutation material is mixed homogeneously with burnable poison (BP) in the BPRs, k_eff slightly increases. As the amount of ⁹⁹Tc coating on the BPRs increases, k_eff decreases gradually and slightly; this result is similar to the tendency of k_eff to decrease after applying a thin-layer coating of minor actinide in the BPRs. Our calculation results show that as the coating thickness of ⁹⁹Tc in the water gap of BPRs increases, k_eff decreases correspondingly. The more BPR water gaps are filled in with ⁹⁹Tc transmutation material, the sharper is the decrease of k_eff. If ⁹⁹Tc fills in the water gaps of 12 BPRs of each fuel assembly, the coating thickness is 0.02 cm, and the corresponding total ⁹⁹Tc coating amount is 206.76 kg. This is the annual ⁹⁹Tc yield of more than three PWRs.

Our calculations also indicate that the more ⁹⁹Tc is loaded into the PWR, the more boric acid concentration needs to be reduced in the coolant. For instance, if the ⁹⁹Tc coating thickness in the water gaps is 0.02 cm, when ⁹⁹Tc fills in 12 BPR water gaps of each fuel assembly, a boric acid concentration of 75 parts per million must be reduced from the PWR primary coolant to allow the PWR return to criticality. The transmutation rate and burnup calculation results indicate that ⁹⁹Tc mixed homogeneously with BP may be a satisfactory ⁹⁹Tc loading pattern in ⁹⁹Tc transmutation in PWRs.