Evaluation of external radiation exposure of workers engaged in loading of radioactive material shipping cask between ship and truck by crane

Abstract

In the marine transportation of radioactive material shipping casks, a loading/unloading cargo work has been carried out using a large crane between a ship and a truck at the interface of land and sea. At that time there is a radiation exposure situation where a worker should approach a cask for handling, tie-down and dosimetric inspection. For a spent fuel cask whose surface dose rate is relatively high, it is necessary to evaluate a radiological risk for cargo workers as one of the planned exposure situations since there is no valid shielding between cask and worker for a long time. In order to instantly evaluate worker dose under any working environment, it is necessary to prepare a simple method that accurately calculate a dose rate map around a cask of interest. In this study, simple calculation methods developed for a spent fuel cask have been surveyed to determine the best fit parameters for a representative type-B wet cask of Japan NFT14P. Then, worker dose is practically evaluated by using the parameters under working environment of cask loading in Japan. There assumed to be three types of cargo workers; handler, inspector and spotter. It is found that the highest individual dose is evaluated to 1.70 μSv per cask loading for an inspector on a carrier of truck and the annual cumulative dose will be well below the annual public limit of 1mSv. The collective dose per loading is amounted to about 17 man-μSv in a total of 19 workers. The occurrence of latent effects can be negligible but the dose values are still higher compared to estimated exposures for escort and off-link population along a road route in U.S. land transportation of spent fuel.