Introduction: My father lectured at ORNL a number of times on Plutonium handling. I wanted to call attention to this lecture, because of the contrast between my father's methodical, rational and scientific approach to plutonium and the the approach of Helen Caldicott. My father is every bit as aware of the dangers of plutonium as Caldicott, but unlike Caldicott he does not approach the subject through fear. Rather he sees the problem of handling plutonium as a challenge to be mastered. Thus his goal is to place a barrier between the plutonium and the human body. In the case of scientific research in the lab, this barrier is provided by a Glove Box . Careful attention is paid to contamination, and the removal of contaminated material. The object is never under any circumstances to allow plutonium to come into conduct with the human body. It should be noted that in a reactor there are far more barriers between human body and plutonium than in the lab. Three Mile Island showed the success of the American nuclear safety approach. Despite a partial core meltdown, there was no public exposure to plutonium.
Caldicott is unaware of the real problems of working with plutonium. It should be noted that my father always worked with bomb grade plutonium, or with plutonium surrogates. He never attempted to work with reactor grand plutonium in a lab for good reason. Reactor grade plutonium was far too radioactive to be a safe research material in a glove bok environment.
HIGH LEVEL PLUTONIUM HANDLING
(An Environmental Sciences Division Seminar Presented by C. J. Barton, February 27, 1974)
In my talk today, I shall discuss the hazards of plutonium handling, the philosophy of plutonium handling, glove boxes, and plutonium handling techniques. I shall not discuss the problem of handling mixtures of a -and y-active materials. I have drawn material for my talk from a number of sources, including my experience and visits to plutonium facilities at Los Alamos and Argonne National Laboratory, the ORNL Health Physics Manual, the book, "Glove Boxes and Shielded Cells," edited by G. N. Walton, and my chapter on Glove Boxes in Technique of Inorganic Chemistry, Volume III.
The toxicity of 239pu stems from the fact that it is an alpha emitter with a half-life of 24,400 years and that it is deposited predominately in the bones and liver. It is reported to be adsorbed from the gastro-intestinal tract only to the extent of about 0.003%, while from 1 to 10% of the inhaled dose may be adsorbed, depending mainly upon particle size and solubility. A small amount may be absorbed through the skin and through contaminated cuts and puncture wounds, but lung absorption is potentially the most important route of entry into the body.
Once in the body, plutonium is excreted extremely slowly. Table 1 shows maximum permissible body burdens (MPBB) which have been established for body burdens of various plutonium isotopes and maximum permissible concentrations (MPC) of these isotopes in air. 2
The maximum permissible body burden of 0.04 ~c of 239pu was established by comparison with 226Ra for which a considerable amount of clinical information exists. This is equivalent to approximately 0.6 ~g or 6 x 10-7 g of Pu. This amount is provided by a sphere of Pu02, 55 ~ in diameter. On the basis of animal experiments, it is estimated that introduction of 20 to 70 mg of 239pu into systemic circulation would result in a 50% chance of death within 30 days and that an individual surviving beyond the 30-day period would surely succumb eventually to chronic or delayed effects of such a dose. Smaller doses may have a long-delayed effect, such as bone cancer. Although there are some individuals who are known to have a body burden in excess of the presently accepted limit, no published case involving death or even serious body damage from exposure to plutonium has come to my attention. A 27-year follow-up study of 25 individuals exposed to Pu during the early Los Alamos operations was published in the November 1973 issue of Health Physics. Although some were estimated to have several times the MPBB, all were in good health and working, most as successful executives. It should be mentioned that one of the difficulties involved in working with plutonium and other a-emitters is the limited sensitivity of most continuous methods of monitoring for airborne a-material. This type of monitoring is usually achieved by pulling air through filter material at a measured rate for a period varying from 5 minutes to 8 hours or more, and then counting the a's on the paper, either with automatically activated counters or with manually operated counters. Some are equipped with alarms based on rate of increase in air count. It is necessary to live with a rather high background in this type of monitoring due to the radon and thoron content of air or to allow this activity to decay overnight, but some investigators have devised techniques for minimizing this interference. The allowable concentration of airborne plutonium is 9 d/m/M3 for an 8-hour working day. Other types of a-monitoring routinely carried out usually involve determining surface contamination of gloves, floors, or other surfaces either by wiping with a filter disc and counting the filter or by use of a portable alpha meter. Permissible surface levels are in the range 1 to 20 d/m/cm •2
The generally accepted philosophy of plutonium handling should be obvious from the discussion of plutonium toxicity. It is, briefly, to take every precaution possible to avoid any exposure of operating personnel to more than trace quantities of plutonium. The ORNL Health
Physics Manual, Section A-7, provides a guide to the type of laboratory required to handle various amounts of radioisotopes, as shown in Table II.
Included in the very high toxicity classification are all the plutonium isotopes, while tritium and l4c fall in the lowest toxicity class.
Glove boxes are required in Type A and B laboratories, while chemical hoods vented through absolute filters are adequate for Type C. Bench top operations are permitted in Type D laboratories. Modifying factors to be applied to the figures in Table II are shown in Table III.
Some qualities considered desirable for Pu glove boxes include tightness, fire resistance, convenience of operation and decontamination, high degree of visibility, provision for safe entry and for removal of contaminated materials, and moderate cost. Some of these requirements are, to some extent, mutually exclusive.
Laminated glass is the preferred window material because it is more resistant to heat than existing plastics, but it is far from being an ideal construction material. Plastic window materials are available which are more fire resistant than Lucite, but these are currently not approved for use at ORNL. The perfect window material has not yet been fabricated.
Stainless steel is one of the more common materials for construction because of its resistance to most corrosive atmospheres, ease of fabrication, ease of decontamination, and good structural properties. However, when hydrochloric acid must be used in the glove box, it is necessary to provide a protective coating on the stainless steel surfaces and some people at ORNL and elsewhere feel that in such cases one may as well use a less expensive construction material, such as mild steel. A glove box of the type used in a number of ORNL laboratories is shown in Figure 1. A type of box construction originally developed at Argonne is based on woven fiber glass impregnated with plastic. This type of construction is attractive for applications requiring resistance to corrosion by Hel, but it is not being used for high-level activity work at ORNL because of limited heat resistance.
One of the facts of life which must be kept in mind in planning glove box work is the average length of the human arm. For small installations this is usually accomplished by making the box small enough so that any part of the interior of the box can be reached from a single pair of gloves. In larger installations, several pairs of strategically located gloves may be required. "Free-standing" glove boxes which can be approached from all sides provide advantages for some types of operations.
Glove port covers, or interior closure plugs, make a definite improvement in the fire resistance of glove boxes when the gloves are not in use. Glove boxes for plutonium work are nearly always operated at a pressure of 0.5 to 1 in. below that of the laboratory, and the air entering the glove box and leaving it must be filtered by high-efficiency fire resistant filters.
I now turn to the subject of glove box assemblies. Although techniques are available for safely transferring plutonium and plutoniumcontaminated materials into and out of glove boxes, this is, in general, a time-consuming operation. Whenever operations must be performed with plutonium materials which cannot all be performed in one box, it is common practice to connect several glove boxes together through connecting chambers generally referred to as interlocks. The boxes on both sides of the interlock are provided with doors so that the glove boxes need not be opened to each other during transfers. Such assemblies vary from very simple ones containing two or more interconnected boxes to very elaborate installations. Two views of an assembly in Building 4501 that I helped to plan are shown in Figures 2 and 3.
It is necessary to assume that any equipment or material which has been exposed to a glove box atmosphere containing plutonium is contaminated. The most widely used method for removing such materials and plutonium samples from glove boxes is the plastic bag technique, using spec: ports similar to glove ports. After the contaminated material is transferred into a plastic bag of suitable size, the bag is twisted and taped for several inches. A cut is then made through the center of the taped section, and both ends of the cut are immediately covered with more tape Some installations prefer to effect the sealing by means of a portable heat sealing device as shown in Figure 4. A cut is made through the middle of a broad seam or three narrow seams "sewed" in the bag at the appropriate distance above the contaminated material. Either method gives good results when properly handled, but neither should be regarded as foolproof. Disposal of contaminated material is made in a controlled area.
My work in the Y-12 area involved heating mixtures containing PuF3 mixed with various other fluorides in a stainless steel glove box and determining the solubility of plutonium in these mixtures by a filtration method. Other research performed in this box included thermal analysis studies of PuF3 systems and examination of fused mixtures with a polarizing microscope to identify crystalline phases. Later studies in Building 4501 involved principally 231pa with 233pa as tracer.
The prevention of fires in plutonium facilities is regarded as the best method of avoiding release of material from this type of accident.
Consequently, the use of flammable materials, such as solvents, should be minimized or eliminated wherever possible in glove box work with plutonium. Good housekeeping is essential.
In conclusion, I want to leave with you the idea that plutonium in any amount should be treated with a great deal of respect, that solid plutonium-containing materials should be handled in such a manner that they are never exposed to the laboratory air, that high-level plutonium work should only be performed in well-planned and well-constructed facilities having adequate provisions for monitoring for escape of plutonium, and that eternal vigilance is the price of safety in plutonium work.
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