Answer to Question #10396 Submitted to "Ask the Experts"

We are preparing a "hot room" to work with ³²P and ¹⁴C in my laboratory (Universidade Federal do Rio de Janeiro). We are installing a stainless-steel sink and we would like to know if it is appropriate for that kind of radioactivity as some people say that stainless steel reflects strong beta radiation.

The choice of sink materials may depend, at least in part, on the radiological characteristics of radionuclides being handled, but you must also consider the effects that certain chemicals will have on the sink material. Among the common materials used for laboratory sinks, stainless steel may be a reasonable choice if you are not using chemicals that are likely to present corrosion problems, such as halogen acids (e.g., HCl and HBr) and sulfuric acid. In such instances other choices such as polyolefin or epoxy might be preferred.

Regarding its applicability for disposal of ³²P wastes, it is true that stainless steel will produce somewhat more beta particle backscatter than nonmetallic sinks. This is because the higher atomic number steel is more efficient at backscattering beta radiation than are lower atomic number materials. Stainless steel might be expected to produce backscatter that is two to three times greater than that from synthetic materials such as polypropylene. The end result of this is that if ³²P is on the stainless-steel sink surface, as opposed to being on a lower atomic number synthetic-sink surface, the total dose rate at a fixed point above the surface might be increased by about 25 percent. Also, there will be a small increase in bremsstrahlung radiation from beta particle interactions in the stainless steel compared to other typical synthetic materials, because the bremsstrahlung radiation yield is proportional to the atomic number of the medium.

In my opinion, the relatively small potential increase in dose associated with the use of the stainless-steel sink does not, in itself, mitigate against its use, especially if you implement reasonable housekeeping rules. If you will be pouring significant amounts of waste ³²P solutions into the sink, it is generally desirable to attempt to pour the waste directly into the drain line in the sink so as to avoid spreading much contamination over the sink surface and increasing exposure to people near the sink. Sink surfaces should be monitored and cleaned on a regular basis. The stainless steel has the advantage that, if chemical corrosion is not a concern, the sink surface may be easier to clean, especially as the surface ages compared to some synthetic surfaces. Many of the latter tend to be softer than stainless steel and more subject to deep scratches and surface damage that can trap radioactivity. Stone or ceramic sinks have good abrasion resistance but they are more likely to get damaged if objects are dropped in them.

In the end, you will have to make the judgment as to which sink best fits your needs and budget. Assuming that reasonable housekeeping is implemented, I would not reject the stainless-steel sink based on radiological considerations.