Coolant void reactivity estimation of ATR by using gadolinia fuel rod

Fukumura, Nobuo*; not registered; not registered

It is a difficult problem to shift coolant void reactivity of ATR to more negative. However it has been clarified by critical experiments with using DCA that use of the fuel pins containing gadolinia is very effective for decreasing coolant void reactivity. Then the analytical study has been done in order to find the solution which has the negative coolant void reactivity of ATR. The lattice is composed of the 36-rod fuel cluster and the lattice pitch is 24.2 cm square. The average enrichment through the MOX fuel cluster is 2.53 or 2.88 w/o Pufis.. This lattice is almost the same one as the demonstration power plant of ATR which is now under designing. The calculation code which is used is WIMS ATR - CITATION code system. The calculational input parameter is as follows; (1)Position of gadolinia poisoned fuel pin. (2)Number of gadolinia poisoned fuel pin. (3)Concentration of gadolinia contained in the fuel pin. (4)Concentration of B in the DO moderator. (5)0, 20, 40, 60, 80 and 100% coolant void. The burnup calculation is up to 20 GWD/T. This is the core average value. The output of calculation is as follows: (1)Coolant void reactivity. (2)Reactivity coefficient of coolant void. (3)Local power peaking. (4)Decay curve of gadolinia concentration through burn up. According to the present study, the following understanding is clarified: (1)The best solution which has the negative coolant void reactivity value through burn up is the use of the 36-rod MOX fuel cluster which has the gadolinia poisoned fuel pin both in the inner and in the middle array of the cluster. (2)The number of the gadolinia poisoned fuel pin is 3 both in the inner and in the middle array. (3)Concentration of the gadolinia is 5 w/o. It is explained that the above effect is due to decreasigg the thermal neutron shielding of HO coolant by the strong neutron absorber gadolinia.