In the safety assessment of the underground siting of a nuclear power plant, it is necessary to know the transport behavior of gaseous fission products in rock. A new in-situ experimental method has been developed in order to obtain the parameters governing the air movement in rock. The experimental results obtained with this method have made it possible to explain the transport behavior. The preliminary analyses has shown that the gaseous fission products, when leaked underground, would be kept and contained there to a fairly hi~h degree.
Japan has an area of 377 thousand square kilometers covered by 70 percent of mountainous area and a population of 112 million. The flat parts are therefore densely populated and intensively utilized for various purposes. Under these circumstances, underground siting of nuclear power plant (NPP) is thought to be attractive from the view point of effective land use. Recently, a committee established by the Ministry of International Trade and Industry (MITI) pointed out that it is essential to comprehend the behavior of gaseous fission products in rock and seismic characteristics of underground structures in order to realize underground NPP construction. As for behavior of gaseous fission products in rock, however, there are few field data to be applicable for analyses. This paper describes a new method to determine the parameters governing the air movement in rock, the result of the in-situ measurements which, taken under the auspices of MITI, are the basis of the parameters, and the analytical result under the condition of a hypothetical accident of underground NPP.
BEHAVIOR MODEL OF FISSION PRODUCTS IN ROCK.
At a hypothetical accident such as a loss-of-coolant in an underground NPP, atmospheric pressure and temperature within the reactor cavern will increase and a part of fission products such as noble gas and iodine will be released from the reactor core. Therefore the gaseous fission products will leak into the surrounding rock from the reactor cavern. In this case it may be thought that these fission products will be transported to ground surface dominantly through gaseous phase rather than liquid phase. The behavior of gaseous fission products in rock can be generally expressed by Eqs. (1) and (2) (the Japan Society of Civil Engineers, 1974).
There are several reports concerning the flow of compressed air through rock (Binggeli, Verstraete and Sutter, 1964; Bernell and Lirdbof, 1965; DiBiageo and Myrvoll, 1972). However there still remain many problems to be solved regarding rock properties such as air permeability coefficient, effective porosity of rock, effective diffusion coefficient of fission products in rock, absorption and adsorption coefficients of rock for fission products, etc. In December 1978, an in-situ experiment was performed at Numappara Pumped-Storage Hydro Electric Power Station to examine those rock properties. Fig. 1 shows the experiment site and the geological profiles.
Testing holes. The experiment site is located about 100 m inward from the adit entrance and 50 m beneath the ground surface.