Heat Transfer From Electrically Heated Rods During a Simulated Loss-of- Coolant Accident.

Abstract

The loss of primary coolant from a pressurized water reactor as a result of an instantaneous rupture of a main circulating line' is called a LOCA and is perhaps the most serious accident in terms of radioactivity release from the fuel to the outer containment system that can be realistically postulated. Emergency Core Cooling Systems (ECCS) are provided to maintain continuity of energy removal from the reactor core following such an event. Paramount to the evaluation of the ECCS is knowledge of the fuel and fuel-clad temperatures at the end of the coolant loss period of 8 to l4 seconds. Heat-transfer data have not been available for the conditions of heater power (heat flux) and of transient coolant pressure, velocity, and temperature comparable to the environment around a fuel-rod bundle during a loss-of-coolant transient. Appropriate transient heat-transfer data have now been obtained with single Zircaloy clad heaters (0.5-in.-CD x 2-ft-long) and a seven-rod cluster in a closed cycle, circulating, 1500 psi pressurized, water loop at power levels of 17.5 kw/ft (^50,000 Btu/hr»ffc s) with cooling water near 500°P inlet and 1500 psi during a 12-second depressurization and coolant loss. Flow regimes associated with bulk-nucleate boiling and with thin film surface evaporation are clearly discernible where the temperature difference and heat flux data are considered. The high heat-transfer coefficients were observed for times .. greater than the heat-removal time constants of the heater rods and suggest that assumptions made in current calculations are conservative.

Occurrence of critical heat flux during blowdown with flow reversal. [LWR]

“…turbulence, 7 ** and stability of the liquid film65 on the heater wall during the initial phase of blowdown have all been proposed as plausible explanation for the time delay to CHF. But for time well into blowdown, annular flow with liquid film on the wall was suggested as the major mode of efficient heat transfer observed.75 In general, CHF during blowdown occurred at a very late time, corresponding to near or complete losr of the coolant mass in the test section 67,74,76. At sucn a point where high quality flow was experienced, 'dryout' is probably the cause of the temperature excursion.>r j hot leg break, CHF was usually reported to occur first at the exit and then propagate upstream,53 > 57 > 61 » 71 ,82 In the case, of nonuniform heat flux, bidirectional propagation of the CHF front which occurred originally at the maximum heat flux location was observed.^ Choice of Working FluidIn the study of steady-state CHF, success in scaling down of the water system has been achieved by a liquid having a lower latent heat of vaporization and operating at pressure having the same liquid/ vapor density ratio as the water system.…”