Commercial Waste and Spent Fuel Packaging Program. Thermal simulation test of spent fuel in drywell storage

In the drywell storage concept, spent fuel assemblies are packaged in steel canisters and placed in a surface storage facility consisting of an array of steel-lined drywells. The spacing of the drywells in the array depends on the capability of the soil to dissipate heat and on the allowable maximum spent fuel temperature. The drywell demonstration activity at Hanford consisted of the operation of a full-scale mockup of a drywell storage unit, using an electrical heater to simulate the thermal power of a spent fuel package. This demonstration unit was operated for a 1-year period with a constant heat output of 1 kW while the drywell and surrounding soil temperatures were monitored by an array of thermocouples. The 1 kW thermal power roughly corresponds to the power output of a 5-year-old PWR spent fuel assembly. The temperature distributions were calculated using the heat conduction code HEATING5. The input thermal parameters were estimated from the measured data and recent Hanford soil studies. An estimate of the effective soil conductivity was obtained by a comparison of the measured temperature data with the calculated soil temperature distributions using a range of conductivities. Results indicated that evaporation of the soil moisture initially present around the drywell, and subsequent water vapor migration upward, are significant heat transfer mechanisms and tend to reduce the drywell temperatures below what would be expected in totally dry soil. The demonstration showed that the drywell storage concept is feasible from the heat transfer standpoint. 20 figures