Self-Limiting Power Excursion Tests of a Water-Moderated Low-Enrichment UO₂ Core

Knowledge of the Doppler coefficient associated with the nonuniform temperature distribution conditions obtained in a reactor runaway is of importance to reactor safety considerations of low-enrichment oxide cores. Measurement of this dynamic coefficient has been made at Spert in an investigation of the kinetic behavior of a water-moderated, UO₂-fuel-rod reactor, by means of self-limiting power excursion tests covering a range of initial asymptotic reactor periods from 31 sec to 3.2 msec. In the long-period region, reactor shutdown was provided both by various heat-transfer-dependent mechanisms effecting loss of moderator from the core and by Doppler broadening. In the short-period region below 7 msec, where the reactor period is small compared to the thermal time constant of the UO₂ fuel rods and reactor shutdown is provided essentially by the Doppler reactivity alone, the data indicate R_c(t_m) = −6.2 , where R_c(t_m) and E_m are, respectively, the compensated reactivity and energy (Mw-sec) at the time of peak power. An additional reactivity effect, positive in sign and resulting from systematic bowing of the fuel rods during the transient power burst, yielded a significant change in burst shape behavior. The fuel rod bowing effect was separated from other feedback effects by performing two series of tests, with and without mechanical constraints on the fuel rods. In the shortest period test, the maximum power was about 7500 Mw, the total energy released in the burst was about 110 Mw-sec, and the maximum pressure measured was less than 8 psi. No damage occurred as a consequence of this or any other test.