Clad-to-coolant heat transfer during a Reactivity Initiated Accident (RIA) is of major importance because the clad temperature strongly influences its mechanical behaviour. The French ‘Institut de Radiprotection et de Sûreté Nucléaire’ (IRSN) and ‘Electricité de France’ (EDF) set up the PATRICIA experimental program that was performed in the PATRICIA loop of CEA in order to investigate the clad-to-coolant heat transfer under fast transients.
In this program, a stainless-steel tube centered in an annular channel was heated up by direct Joule effect. Both PWR conditions (15 MPa, 280°C, 4m/s) and NSRR conditions (0.1 MPa, room temperature, stagnant water) have been simulated. The heating rates were representative of the clad heating induced by the fuel during a RIA transient (PWR: 30 ms half-width power pulse, clad heating rate: ~ 3 000°C/s, NSRR: 5 ms half-width power pulse, clad heating rate: ~ 10 000°C/s).
An increase of the Critical Heat Flux (CHF) and a shift of the critical temperature have been observed: transient CHF around 6 MW/m² reached at Tsat + 55°C in PWR conditions at 3 000°C/s (instead of ~ 3 MW/m² at Tsat + 20°C in steady-state) and around 20 MW/m² reached at Tsat + 240°C in NSRR conditions at 10 000°C/s (instead of ~ 1 MW/m² at Tsat + 20°C in steady-state). The Burn Out (BO) seems to occur before the nucleate boiling regime is fully established and does certainly not correspond to steady-state hydrodynamic instabilities in columns of vapor.
At 10 000°C/s in NSRR conditions, transient vaporization of a liquid layer at the wall surface is likely even in the tests that did not reach the Burn Out. This progressive vaporization can induce transient heat fluxes as high as 20 MW/m². Provided the energy stored in the cladding is sufficient to counterbalance such high heat fluxes, the clad outer temperature can reach a level at which the vapor film becomes stable. In that case, the vapor film thermally insulates the wall from the liquid and the transient Burn Out occurs with a strong temperature increase concomitant to a fall of the heat transfer coefficient.
In PWR conditions at 3 000°C/s, the thermal boundary layer at the Burn Out is less developed than in steady-state conditions and the corresponding enthalpy gradient induces higher heat fluxes. The value of the local quality, which is a parameter that integrates most of the phenomenology of BO in convective flows, is close to the inlet quality (~-0.3) while slightly positive values are expected in steady-state conditions.
Boiling curves based on the PATRICIA experimental results have been implemented in the SCANAIR code. The validation has been successfully performed in PWR conditions on the PATRICIA-PWR experiments and in NSRR conditions on the PATRICIA-NSRR experiments and on the in pile NSRR tests.
A sensitivity study on a 4-cycle UO2 rod submitted to a 30 ms power pulse in PWR conditions showed that the occurrence and timing of the Burn Out are governed by the thickness of the zirconia layer and by the value of the Critical Heat Flux. A strong cliff edge effect has been evidenced: once the Burn Out is reached, clad temperature can reach 1000°C and the dry out phase lasts at least 2 seconds.