Conceptual design study of transportable reactor SPECTRA-L for lunar base (II); Study on the dose-bquivalent limit in space

Nomura, Norio; Haga, Kazuo; not registered

Conceptual design of the SPECTRA-L power plant for lunar base is being performed as one of the transportable reactor study. It is necessary to establish an acceptable limit of dose equivalent in space as well as that on the earth so as to design the shield to the reactor. Because the natural radiation dose in space is higher than that on the earth, terrestrial dose equivalent limit can not be directly applied. In this report, we examined the dose equivalent limit on the moon based on the terrestrial one, then analyzed the shielding effect of lunar soil (regolish) which is expected as shielding material in this plant. The study led us to propose some limits on the acceptable radiation in the human activity in space, that is 2.0 Sv in life and 0.4 Sv/year, consequently 0.04 Sv/month. The yearly limit is less than 0.5 Sv which is admitted in the ICRP Publication 60 to emergency situation. A limit of 40 mSv/year is suggested to the radiation from nuclear reactor at the place where stuffs of lunar base is easily accessible. The suggested value is 10% of the proposed yearly limit to the natural radiation in space. A one-dimensional analysis on the shielding effect of the regolish covering SPECTRA-L showed that the neutron dose equivalent rate of the reactor is attenuated to the background level of space at a point about 10 m apart from the reactor center and to that on the earth at about 12 m. Lastly, it is emphasized that the exposure to radiation should be reduced by the following methods. (1)Automatic operating system of the plant. (2)Development of materials for easy-working protection clothes and radiation shield using magnetism ln the living area. (3)Minimum exposure to radiation, especially in the living area, following the spirit of ALARA. (4)Underground base and underground transport routes to other facilities.