Dryout Heat Flux for Core Debris Bed, (III)

Mixture of cylindrical steel pellets and Al₂O₃ balls, which simulated the intact fuel pellets and fragmented claddings, respectively, was inductively heated in a 50 mm I.D. pyrex glass cylinder filled with water, to investigate the coolability of TMI-2 type degraded core debris bed. The size of steel pellets was 11 mm dia. × 11 mm for BWR, 8 mm dia. × 12 mm for PWR and 5.5 mm dia. × 9 mm for FBR and Al₂O₃ balls were about 2 mm in diameter. The height of the debris bed was 25 cm or lower.

The dryout heat flux does not level off up to a bed height of 25 cm or over for the TMI-2 type bed while 8 cm or so in the bed of only steel balls. The dependence of dryout heat flux on the system pressure agrees with the Lipinski's 0-D model by adopting a proper equivalent diameter. When a simple number-weighted average is used as the equivalent diameter, the prediction gives a fairly good agreement with the experiment for FBR type bed but underestimations for the PWR and BWR type beds. It should be noted, that the small balls of less fraction, not the large pellets, substantially govern the dryout. When the coolant flow is allowed from the bottom, however, the dryout heat flux is enhanced up to the level for the complete vaporization of coolant, and small amount of mass flux or circulation head can greatly improve the coolability.     

Experimental Study on Coolability of Particulate Core-metal Debris Bed with Oxidization, (II) Fragmentation and Enhanced Heat Transfer in Zircaloy Debris Bed

The oxidization and coolability characteristics of the particulate Zircaloy debris bed, which is deposited under the hard debris and through which first vapor penetrates and then water penetrates, are studied in the present paper. In the vapor penetration experiments, it is found that Zircaloy debris particles are effectively broken into small pieces after making thick oxidized layer with deep clacks by rapid oxidization under the condition that vapor with 20 cm/s penetrates for 30 to 70 min at an initial debris bed temperature of 1,030°C. It is also confirmed in the water penetration experiments that the oxidized particle debris bed has potentiality of high coolability when water penetrates through the fully oxidized particle bed because of a high capillary force originating from those particles with deep cracks on their surfaces.

Based on the present study, a new scenario for the appearance and disappearance of the hot spot in the TMI-2 accident is posssible. The particulate core-metal debris bed is first heated up by rapid oxidization with heat generation when vapor can penetrate through the debris bed with porosities. This corresponds to the appearance of the hot spot. The resultant oxidized particulate debris bed causes a high coolability due to its high capillary force when the water can touch the debris bed at wet condition. This corresponds to the disappearance of the hot spot.     

Critical Heat Flux for Rod Bundle under High-Pressure,Low-Flow and Mixed Inlet Conditions

The authors have developed a new on-line corrosion product monitor (OCPM) to automatically measure the concentrations of corrosion products in PWR secondary water. The corrosion products of interest for this application are mainly iron and copper. The OCPM offers a number of outstanding features: (1) it measures trace metals on-line; (2) it allows simultaneous quantitative multi-element analysis; (3) its operation is fully automatic with an excellent lower detection limit; and (4) it is transportable and easy to operate. During its development stage, the OCPM was applied at an actual PWR plant and was able to successfully automatically analyze the concentrations of iron and other trace metals contained in the PWR secondary water, while maintaining operational stability without operator intervention for long periods of time. It was also shown that the use of this new system would greatly reduce the workload of plant water chemistry personnel, providing a better alternative to conventional chemical analysis methods. The OCPM is now being introduced into several PWR plants in Japan, with the objectives of conducting detailed studies of the corrosion product behavior in the plant systems and reducing the workload of plant personnel.     

Application of Zeolites to Remove Iodine from Dissolver Off-Gas, (I)

The oxidation behavior of Zr(Fe,Cr)₂ precipitates being present at the surface of Zircaloy-4 was examined with microprobe Auger electron analysis, focussing attention on the oxidation behavior of chromium and iron in the early stage of oxidation where the oxide film was coherent. Chromium formed a thin oxide layer at the top surface of the zirconium oxide film of precipitate and remained in a metallic state inside the oxide film. Iron was oxidized via dissolution in the matrix zirconium oxide near the top surface and remained in a metallic state inside the oxide film. Such variety of chemical state of chromium and iron with depth in the oxide film was attributed to the existence of oxygen potential gradient in the oxide film.     

Development of Reflux Vapor Trap for Sodium Cooled Fast Breeder Reactor, (I)

With the view of obtaining basic data required for designing durable reflux vapor traps of high performance for use in sodium-cooled FBR's, experiments were conducted to (a) select a suitable packing, and (b) to examine the effect brought on trapping performance by changing the gas flow rate, packing material, packing density and trap outlet temperature. The results indicated that:

(1) As trap packing, plane weave stainless steel mesh proved to ensure lower pressure drop through trap.

(2) Using the plane weave mesh packing, and with the trap outlet temperature kept at 130°C, the reflux vapor trap efficiency was found to exceed 99.6% in the range of vapor trap gas velocity below 1.3 m/s, and packing density below 0.07 g/cc. The efficiency decreased at outlet temperatures above 130°C.