Demolition of Building 12, an old plutonium filter facility

This report discusses the decommissioning and disposal of a plutoniumcontaminated air filter facility that provided ventilation for the main plutonium processing plant at Los Alamos from 1945 until 1973. The health physics. waste management. and environmental aspects of the demolition are also discussed. // / I. mSTORY A plutonium processing facility was built in Los Alamos in 1944, on what is now known as DP Mesa. The urgency at that time dictated that the facility be built as rapidly as possible, incorporating all the best construction ideas but using only those materials that were readily available. The process buildings were constructed with sheet metal ou 1. 22-m-high concrete wainscoting. Plaster, ou metal laths over metal studs, was used to give a smooth interior surface. The buildings were ventilated with a 60 OOO-m3/mincapacity central air exhaust b)'stem. This system handled air from rooms and fu'rne hoods, sparging of dissolvers, and venting of solution tanks. At that time exhausting air from the glove boxes was not believed necessary. Several years later the decision was made to vent these work enClosures. The air was exhausted, without being filtered, through the room air exhaust system. Particulates were removed from the exhaust ai~ by electrostatic precipitron units backed up by a single bank of American Air Filter Company Type PL-24 filters. This system was conSidered the best available for air clean-up at that time. 303 The filter building,designated Building 12, was completed and put into service in.May 1945. It continued in service for room a."ld process air until July I, 1959. That year another system was installed for the process air, and afterward only room air was handled in Building 12. Building 12 continued in service until February 1973, when new room air filtration systems were completed, one for each of the process buildings. n. DESCRIPTION OF FACUlTY The site plan (Fig. 1) shows the relationship of the process buildings to the filter building. The finished site is shown in Fig. 2. The filter building is on the left. Air from the rooms was exhausted from floor level, up vertical ducts through the roof, then to ducts mounted parallel to the roof, to the collector duct on the peak of the roof. All the ductwork was galvanized steel. In those ducts that handled chemical fumes, corrOSion began immediately, and small holes formed within a few years. Corrosion products and dirt drawn through the holes in the ductwork were deposited in the plenum of the filter building. The floor plan of Building 12 is shown in Fig. 3. The floor area for that portion housing the filters and precipItron units was 30.8 m by 19.5 m. The intake plenum was a trapezoidal area 23.5 m wide at its longest base, 7.6 m wide where the air entered the building, and 18. 9 m from that point to the rectangular portion of the building. The preclpltron units and filter banks were built in five sections. Each section had two large doors that could be lowered to isolate the area while filters were being changed or while work was being done on the precipltron unit. Access to the isolated section was by ladder from the second story of the building. The second story housed the doors when they were in the raised position. Figure 4 shows a side view of the building; and Fig. 5, a side view of the filter and blower area, shows the positions of the electrostatic prec1pltron units, the PL-24 filter bank, the common blower plenum, the exhaust blowers, and the doors used to isolate the sections. A front view of the building is shown in Fig. 6. The construction of Building 12 was constrained by the materials available at that time. The concrete foundation was made deeper and thicker because reinforcing steel was in short supply. The wall studs and floor and roof beams were wood. They were covered with two layers of gypsum board to give a smooth interior surface. This construction actually helped prevent the spread of contamination during demolition. Construction details will be discussed in Sec. IV. m. DECONI'AMINATION OF FACIUTY In 1960 the interior of the plenum and the largest portion of the air ducts were cleaned. About 3000 kg of dirt were removed from the building during this first cleaning operation, including several lrundred pounds of sand that had been used in sandblasting plutOnium parts. Samples of dirt removed were analyzed and showed a plutonium content ranging from 0.001 to 0.05 wt%. The data indicated that this dirt, which was packed in two O. 3-mm-thick plastic bags and placed in steel drums 239 for burial, conta!ned about 600 g of plutonium (93.5% Pu, 240 241 6% Pu, and 0.5% Pu). The precipltron units were 304 disassembled, removed fl'om the building, wrapped in several layers of plastic, and packed in plywood crates for burial. Over the next few years the building was cleaned several times. Each time the final operation was to wipe down the entire floor with wet rags. Immediately after this cleaning, the floor would have a swipe count of only a few hundred diSintegrations per minute, but the direct 2 count was still> 1 diS/min per 60 Cm. All the cracks, such as expanSion jOints, had a swipe count of > 100 000 dis/min. IV. DEMOI1TION OF BUILDING 12 A proposed procedure for the demolition of the bUilding was prepared by a member of the Engineering group and a member of the PlutoniUm Processing group. Their report was submitted for approval to the Demolition Committee, which was Composed of representatives from the Los Alamos Scientific Laboratory (LASL) and from the contractor that would do the demolition. The names of the groups represented are shown in Table I. Demolition work was started using this approved procedure; but as work progressed, conditions 'YElre sometimes encountered that necessitated a change in procedure.. Therefore, the Committee met every week to hear progress reports on the demolition and to review proposals for any change in the procedure.