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; Haga, Kazuo
PNC TN9410 91-205, 55 Pages, 1991/05
A transportable reactor has been studied in one of activities of frontier research in PNC. Since the reactor is going to be used at an secluded place in the earth, on the surface of the moon or at the deep sea bottom, the operation of the reactor requires wireless communications. Based on the present status of technology, a preliminary study has been performed in this report on a wireless operation method of the reactors on the moon and at the deep sea bottom. A wireless operation system of the reactor on the moon is supposed to exist technically on the extension of a present space communication system and a difficult problem does not seem to remain at the development stage of the wireless operation system. Concerning the wireless operation system of the reactor at the deep sea bottom, a few problems remain to be solved in the field of acoustic communications in sea water. However they seem to be solved technically in future. It takes about three seconds for an electric wave to go and come back between the reactor on the moon and the earth. It takes also about four seconds for an acoustic wave to reach the reactor at the deep sea bottom from the sea surface. Therefore, urgent control of the reactor by wireless communications is impossible in both cases. The urgent control must be performed by the reactors themselves.
; Haga, Kazuo
PNC TN9410 91-185, 62 Pages, 1991/05
Conceptual design of a 200kWe power source for a 6.5 km class research submersible was performed by using a sodium and a lithium cooled high temperature reactors and a closed Brayton cycle system. The SPECTRA, which had been already designed as a portable reactor, was employed in the conceptual design. A pressure hull, made of high strength steel containing 10 % Ni, had the shape of two spheres connected horizontally, which had the same inner diameter of 3 m. Heat removal from the reactor system was by heat transfer to the sea water after thermal conduction through the pressure hull. The conceptual design work confirmed the technical availability of this system as the power source for the deep sea research submersible.
Nomura, Norio; Haga, Kazuo;
PNC TN9410 91-125, 75 Pages, 1991/04
Within a framework of transportable reactor study, a conceptual design of power source to lunar base or space station is being performed. In the lunar base power plant SPECTRA-L, dose equivalent at the area around the reactor can be reduced till the natural back ground level by the shield of regolish. But, it is not easy to reduce the dose equivalent to an acceptable level the limit of dose equivalent from the reactor. Because shields abobe the reactor are only thin regolish whose thickness are 2m, and plant equipments such as boiler, condenser, alternator, and turbine. In case of SPECTRA for the space station, shield must be furnished from the beginning. In this report, shielding structure of SPECTRA is analyzed seeking for an optimum one. The analytical condition are as follows. (1)Thickness of shield is less than 1 m. (2)Weight is as light as possible. The finally obtained shielding structure is a layer by Beryllium, tungsten, lithium-hydrogen, and SUS304. These total thickness is 90cm. Using this shield, the dose equivalent rate at 25m apart point from the reactor, where life module will be located, is estimated to be 14.4
Sv/h. This value is more than a proposed limit of dose equivalent from the reactor (40mSv/y : 4.6 Sv/h). But it is expected that the actual dose equivalent is less than the limit value owing to the plant equipment in the direction and the life module shield of cosmic ray.
Nomura, Norio; Haga, Kazuo;
PNC TN9410 91-107, 40 Pages, 1991/03
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.
Nomura, Norio; Haga, Kazuo;
PNC TN9410 91-100, 73 Pages, 1991/03
Liquid Metal cooled Fast Reactor is a good candidate of a large-scale energy supply system to a manned lunar base because of the compact structure and being free of refueling. A 300 kWe transportable reactor SPECTRA-L is being studied as the power source on the moon. Because the reactor system is launched by a launch vehicle, safety evaluation is necessary to the launch failure accident. We examined (i) the possibility of recriticality, and (ii) the influence of nuclear fuel leakage to the environment in the case of reactor damage. The followings are the main findings of this preliminary study. (1)Under-criticality is maintained even the reactor falls into water or crashes against the earth. (2)The external exposure dose by a radiation cloud of released fuel is less than the natural radiation. (3)The internal exposure dose by inhaling the cloud is less than 1 mSv/year which is a reco㎜ended dose limit to the public. (4)The surface radioactive density increased of land by the accident is be less than 0.4 Bq/square centimeter, which is a limit for things contaminated by alpha radiation to be transported from a controlled area, regardless the whether. This estimation is based on a leakage of five percent fuel, but the exposure dose would be far less than the estimated from the following reasons. (1)Nuclear fuel is in a ceramic form called pellets, and they are inserted in fuel cladding, and contained in the coolant material (metal) and the reactor vessel. (2)The nuclear fuel does not break into fragment as aerosol by the accident.
