Answer to Question #25 Submitted to "Ask the Experts"

Phosphogypsum (PG) has been used extensively in cement, wallboard, and other building materials in Europe, Japan, and Australia. In at least some cases, this has been because of the absence of low-cost natural gypsum and/or scarcity of long-term storage space. In the United States, use has been very limited in the past and there is not any current use. Largely because of the naturally occurring radioactivity associated with this material, the U.S. Environmental Protection Agency (EPA) rules prohibit any disposition other than indefinite storage except under a special permit.

Worldwide, there is a continuing interest in various uses of by-product PG as a resource material—for the phosphate industry this would be a solution to the requirement for continued storage of the accumulated by-product at phosphoric acid chemical plants; potential users are interested in locations where it is more readily available than mined natural gypsum. Prerequisites to this use include developing processes for removal of impurities that affect the strength and quality of the product, removal of the radioactivity contained in the PG (or at least determining that it does not present an unreasonable radiation hazard), and obtaining relief from regulatory prohibitions.  

²³⁸Uranium and its decay series are naturally associated with phosphate deposits of marine origin. In the production of phosphoric acid by the wet process (acidulation of phosphate rock with sulfuric acid), a calcium sulfate (phosphogypsum) by-product is produced and the ²²⁶Ra contained in the phosphate rock appears in the PG. The ²²⁶Ra content of the PG is related to the ²²⁶Ra content of the input phosphate rock, which varies with the place of origin. Concentrations from various sources around the world range from a few hundred Bq kg^-1 to 1,500 Bq kg^-1 (several pCi g^-1 to 40 pCi g^-1). In its 1993 report on the Sources and Effects of Ionizing Radiation, UNSCEAR (United Nations Scientific Committee on the Effects of Atomic Radiation) quoted a typical value of 900 Bq kg^-1 (24 pCi g^-1).

As a matter of information, concentrations in PG derived from the major sources of phosphate rock in the United States are in Central Florida, in the range of 740 to 1,300 Bq kg^-1 (20 to 35 pCi g^-1); North Florida, on the order of 480 to 550 Bq kg^-1 (13 to 15 pCi g^-1); and Idaho, 850 Bq (23 pCi g^-1). The radiological concern in use of PG in building materials would be from the resulting gamma radiation levels and from the concentrations of indoor airborne ²²²Rn (hereafter simply designated Rn) that might result as a consequence of the contained ²²⁶Ra.

Are the potential radiation and airborne radioactivity levels significant? This correspondent has reviewed literature up to 1993. There doesn't seem to be an abundance of published literature on radiation doses and airborne Rn concentrations in actual structures built with PG-containing building materials. In its 1993 report, UNSCEAR quoted dose estimates from a 1972 report of O'Riordan, et al., of the United Kingdom's NRPB (National Radiation Protection Board). These authors estimated that, for a residential building in which 4.2 tonnes of by-product gypsum would have replaced established building materials, the absorbed dose rate in air from the gamma radiation would be 0.07 µGy h^-1 (7 µrad h^-1) and the effective dose rate from inhalation of Rn progeny would be 0.6 mSv y^-1 (60 mrem y^-1).

For full-time occupancy, the gamma radiation field would result in an annual effective dose equivalent of about 0.5 mSv (50 mrem) and the PG-attributable gamma plus Rn progeny annual effective dose would be about 1.1 mSv (110 mrem). These doses can be compared to the recommendation that the average annual dose to a member of the general public from all human-caused sources should not exceed 1 mSv (100 mrem) above background and that the dose due to a single practice should not exceed some fraction (values in the range of 10 to 30 percent are used) of the 1 mSv (100 mrem).

This can also be addressed from another approach. A number of groups have proposed regulatory approaches in which derived limits for concentrations of radionuclides in building materials are specified on the basis of modeling from criteria for gamma dose, airborne Rn exposure, or gamma plus Rn progeny total effective dose. Published derived concentration limits include those that have been proposed by the NEA-OECD, by the NRPB of the United Kingdom, in the Netherlands, in Sweden, and in Germany (the Federal Republic of Germany in 1980).

The various concentration limits applicable to ²²⁶Ra range from 150 Bq kg^-1 (4 pCi g^-1) to 900 Bq kg^-1 (25 pCi g^-1). As was stated earlier, the ²²⁶Ra concentrations in PG from various sources around the world range from a few hundred to 1500 Bq kg^-1 (several to 40-45 pCi g^-1). Thus it can be seen that many PGs will have ²²⁶Ra concentrations that exceed the more restrictive of the proposed limits and some will exceed even the most liberal limits. Several statements have appeared concerning the radiological impact of the use of PG in building materials.

SENES Consultants of Richmond Hill, Ontario, Canada, in a 1987 report prepared for the Ontario Ministry of the Environment, observed average ²²⁶Ra concentrations in Canadian PG of 700 Bq kg^-1 (19 pCi g^-1) and stated . . . "Radioactivity in building materials has the potential to expose large numbers of people. This exposure rate could result in relatively large additional exposures compared to that due to the natural background of about 2 mSv y^-1. The foregoing indicates that the development of criteria to limit such possibilities should be encouraged." Several years later in its 1993 report, UNSCEAR considered the typical ²²⁶Ra concentration in phosphogypsum to be 900 Bq kg^-1 (24 pCi g^-1) and stated that "Significant radiation exposures can occur if such by-products are used in the building industry."

This is a topic that deserves further study. In its 1993 report, UNSCEAR qualified its assessment with the statement: "These estimates are highly uncertain and need to be confirmed by measurements in dwellings that have been constructed using known amounts of phosphogypsum." One researcher has stated that the use of PG is contingent upon standards for permissible levels of radiation and/or radioactivity for each application and that the development of standards would, in turn, stimulate research, including radium reduction studies. There are methods of producing PG with lower concentrations of ²²⁶Ra and the costs of doing this vs. the value of the product needs to be factored into the total by-product utilization equation.

The above discussion is based on my review of the literature up to 1993. It should be informative to also look at literature published since that time. The most productive effort would be to look at work originating in the European community and in Japan since it is in those parts of the world, rather than North America, that PG has been used most extensively for wallboard and the resource availability reasons are the more compelling.

Charles E. Roessler, CHP, PhD