2.1. Characteristics of low density MOX pellet fabrication
In the ordinary MOX fuel fabrication process, feed PuO2 and UO2 powder are blended together with recycled MOX powder to meet fuel specification, pressed into the pellets and sintered at high temperature around 1700 degrees centigrade. Normally, the sintered density of this pellet reaches to around 95 % of theoretical density and this value is close to density specification of MOX pellet for other reactors. Because MONJU adopts low density MOX pellet to decrease smear density of fuel, it is necessary to decline the density of MOX pellet by approximately 10% relative lower than ordinary sintered density. The addition of pore
former is, therefore, necessary during powder treatment steps. This pore former is required to be decomposed and removed from the matrix of MOX pellet completely with other organic additives such as binder and lubricant during de-waxing step prior to sintering, which results in the formulation of closed pores within MOX pellet to meet the specification of density. The original process flow for low density MOX pellet fabrication in PFPF is shown in Figure I. In this flow, the paraffine type additive was assumed to used as pore former and mixed into blended MOX powder with lubricant after granulation.
Table I. Typical specifications for MONJU MOX pellet
Item Nominal Value of Specification
Equivalent Fissile Content (For initial load fuel)
0.153 (For inner core fuel) 0.224 (For outer core fuel) Uranium Enrichment 0.2 wt%
Impurities (typical) C: 300 ppm N: 200 ppm
Volatiles:180ও/gMOX (including moisture)
O/M Ratio 1.97
Pellet Density 85.0
Dimensions of Pellet Diameter: 5.40 mm
Height: 8 mm (referenced value)
PuO2or
MOX Powder UO2Po wder
Ball M illing
MOX Powder UO2Po wder
Ball M illing
FIG. 1. The original process flow for low density MOX pellet fabrication in PFPF.
2.2. Decay heat by plutonium and its influence on organic additives
In the PFPF, a size of production lot for MONJU fuel fabrication is about 36 kilograms of MOX with about 20 and 30 wt.% of plutonium contents. After operation of each process step, these 36 kilograms of MOX powder are divided into two portions and each of them is accommodated into the powder transfer container. At the design stage of PFPF, the power transfer container was also designed to accommodate about 18 kilograms of MOX powder (about 3.2 ~ 4.7 kgPu) and was made of aluminum taking decay heat by plutonium into account. The heat generation rate from plutonium accommodated in this container was about 10 watts/kg of plutonium as shown in Ref. [1]. This heat generation rate was not significant, however the thermal conductivity of MOX powder is quite low, so the actual temperature of MOX powder at the center part of container raised up to about 180 degrees centigrade, while the surface temperature of container didn’t raise significantly thanks to the radiation fins attached to the outer surface of container. An approximate expression for the thermal conductivity of MOX powder was calculated by using measured temperature of MOX powder in the container as shown in Figure 2. On the basis of this expression, the thermal conductively of MOX powder at 100 degrees centigrade is estimated at about 0.12 watts/(m͠) for 3 g/cm3of powder density. The temperature distribution of MOX powder within this container is calculated by using this thermal conductivity of MOX powder and measured temperatures at center and surface of container as shown in Figure 3. This temperature distribution of MOX powder within the container gave an affect on the organic additives such as Zinc-Stearates and pore former blended into the MOX powder, especially large affect to them in the portion close to the center of container. The temperature of MOX powder in this portion reached to about 180 degrees centigrade as shown in Figure 3, and this temperature was beyond the melting point of these organic additives as shown in Table II.
FIG. 2. Approximate expression of thermal conductivity of MOX powder.
6JGTO CNEQPFWEVKXKV[QH/ 1 :RQY FGTCVCDQWV͠
3RZGHU'HQVLW\'
7KHUPDO&RQGXFWLYLW\.
K (W/m͠)=0.05D(g/cc) - 0.04
Applicable Range of Plutonium Content : 20
㨪
40 wt%(g/cc) (W/m͠)
Fin
㨪 30͠ 30͠ 㨪 60͠ 60͠ 㨪 90͠ 90͠ 㨪120͠ 120͠ 㨪150͠ 150͠ 㨪180͠
Rang of Temperature 115 mmǾ
316 mmǾ
Material : Aluminum Thickness : 3mm
Maximum Capacity : 14Liter
Amount of charged MOX powder : 20kg
Plutonium Content : 22wt.%
Maximum Temperature : 180͠
FIG. 3. Configuration of powder transfer container and temperature distribution of MOX powder within container.
Table II. Melting point of organic additives
Organic Additives Melting Point (degrees centigrade) Zinc-Stearates (Binder, Lubricant) 140
Paraffin Type Pore Former 85 ~ 87
Because of this temperature distribution, a certain parts of the organic additives were destructed by decay heat. This destruction of organic additives created poor quality of closed pore and wide distribution of pellet densities, which resulted in lower yield than expected production level.
2.3. Improvements and stabilization for powder treatment process
Improvements and stabilization for powder process should be required for resolving the situations described in farmer sections. For this purpose, the investigations with small scale testing conducted focusing on the following points:
(1) For the alternative pore former; selection and utilization of a heat stable material being stable at least up to 200 degrees centigrade with optimized particle size;
(2) For the powder transfer container; introduction of new designed one that could decline the temperature of accommodating MOX powder under 80 degree centigrade;
(3) For the blending procedure of pore former with MOX powder; development of new procedure to improve the quality of closed pore distribution in the sintered pellet;
(4) For the sintering operation; adjustment on the selected pore former.