Simulation of Radiation Dose Rate Distributions around Casks in Ships and Vehicles by Using QBF Code for PC-9801 Computer

It has been believed for a long time that quite a lot of calculation time is unavoidable for the calculation of spatial distribution of radiation dose rate around casks even with high speed computers. Consequently, no such trials have ever been reported to simulate the detailed behavior of radiation dose rate distributions in a wide region of cask shipping vessels. On the basis of the background mentioned above, a computer code 'QBF' is programmed to simulate spatial distribution of radiation dose rates in a wide region of a cask system composed of steel walls and beams, and radiation shieldings as well as casks. For a typical example of such system, one can enumerate a vessel for shipping casks. In the simulation, the following effects and influences should be taken into account:- (1) Radiation shielding effect by structural materials and shielding materials around the casks. (2) Shadow effect among casks which resembles the partial masking of the sun by the moon. (3) Influence from the difference of the radiation dose rate on each cask surface as well as that from the difference of cask geometries. Consideration of all these effects and influences is necessary to understand the complicated behavior of radiation dose rate distributions. The QBF code is programmed on the basis of a new calculation principle. Program steps of this code are so short that one can apply this code to the very popular personal computer In Japan: PC-9801, to calculate spatial distribution of radiation dose rates. Required calculation time with this computer is short enough to apply this code to various situations of shipping casks. Validity of calculation method adopted in this code has been confirmed and reported already (H. Yamakoshi (1990)). As a partner of the QBF code in the present code system, a code 'GRAPH' is programmed and plays an important role. The code GRAPH visualizes the calculated dose rate distributions into a colored contour map, which covers a range of 12 decades of dose rate values with 12 colors. Each decade Is covered by 5 lines of the same color, where the 5 lines correspond to the factor of 2, 4, 6, 8 and 10, respectively. Demonstration of simulating dose rate distributions will be performed for neutrons and gamma rays around a vehicle and a cask shipping vessel. The main aim of the present report is to show that the behavior of the spatial distribution of the radiation dose rate in a wide region can be easily visualized in various aspects of cask shipment even by personal computers which are very popular today, and that the QBF code system is a useful tool to perform this simulation. A great merit of using this code system is that one can easily analyze, with high reliability, roles of various important factors such as shielding structures and cask positions in the behavior of dose rate distributions.