Patent Number: 050193287
Section: summary

FIELD OF THE INVENTION AND RELATED ART STATEMENT This invention relates to a natural circulation type boiling light-water reactor. In a typical nuclear power plant employing a boiling light-water reactor, main steam generated in the boiling light-water reactor is supplied to a turbine system including turbines so as to convert thermal energy of the main steam into electric energy. Since the main steam generated contains radioactive substance, a reactor system, a turbine system, and a piping system for interconnecting therebetween are shielded by radiation shield structures, respectively. This results in the prevention of leakage of the radiation from the radioactive substance to the outside. The main radioactive substance contained in the main steam is nitrogen isotope .sup.16 N. .sup.16 N is produced by the following reaction of neutron with oxygen isotope .sup.16 O contained in light water in a reactor core or coolant (that is, reaction of neutrons with nuclei, i.e., charged particle production reaction or (n, p) reaction): EQU .sup.16 O+n.fwdarw..sup.16 N+.sup.1 H In ordinary coolant, .sup.16 N produced soon becomes anions of strong nonvolatility such as, NO.sub.2.sup.- or NO.sub.3.sup.-. Therefore, the amount of .sup.16 N contained in the main steam is small. In recent years, it has been practiced to pour deoxidizer into the coolant for the purpose of improving the quality of coolant. Pouring the deoxidizer into the coolant results in the reduction in oxidation potential of the coolant. Due to such reduction, cations such as NH.sub.4.sup.+, which are readily converted into volatile isotope .sup.16 N are increased. Further, there are known H.sub.2, NH.sub.3, N.sub.2 H.sub.4 and the like as substance to be poured into the coolant besides the deoxidizer. These substances are also important factors for the increase of .sup.16 N, the same as the deoxidizer is. To cope with this increase of .sup.16 N, not only a reactor pressure vessel but also the turbine and the piping systems are shielded by the radiation shield structures made of concrete or iron, with the result that the leakage of the radiation from .sup.16 N to the outside is suppressed satisfactorily. However, with the increase of .sup.16 N in the main steam, the shield structures have become heavy and thick and enlarged, thus bringing about an increase of the plant construction cost. OBJECT AND SUMMARY OF THE INVENTION It is therefore an object of the present invention to provide a natural circulation type boiling light-water reactor which makes a contribution to the reduction in weight and dimensions of shielding structures of turbine and piping systems. To this end, there is provided according to the present invention a natural circulation type boiling light-water reactor comprising: a pressure vessel divided into a steam/water chamber and a steam chamber; a reactor core disposed within the steam/water chamber to generate main steam which contains radioactive isotope .sup.16 N, the reactor core including a plurality of fuel elements; a shroud disposed within said steam/water chamber encircling the reactor core; a steam dryer assembly through which the main steam generated flows from said shroud into the steam chamber to reduce a wetness fraction of the main steam; a chimney connected at one end thereof to the shroud and extending within the steam/water chamber toward the steam chamber, through which the main steam flows together with said radioactive isotope .sup.16 N, the chimney being filled with light water as coolant and having the other end thereof opened toward the steam dryer assembly; a steam outlet through which the main steam generated is drawn out of the pressure vessel, the steam outlet being provided in a wall of the pressure vessel; and steam passage means through which the main steam generated flows from said shroud to said steam outlet via said steam dryer assembly. The inventory of the radioactive substance .sup.16 N in the pressure vessel depends upon the time period while .sup.16 N flows within the pressure vessel. According to the present invention, the radioactive substance .sup.16 N contained in the main steam generated by the reactor core is allowed to flow within the pressure vessel of the reactor taking a time period longer than a half-life of .sup.16 N (about seven seconds). Namely, according to the present invention, the time period while the main steam flows within the pressure vessel is prolonged by controlling the velocity of the main steam and/or increasing the length of the path for the main steam. Accordingly, the inventory of the radioactive substance .sup.16 N is reduced in the reactor pressure vessel so that the amount of .sup.16 N contained in the main steam directed from the reactor pressure vessel towards the turbine system becomes smaller. In consequence, it is possible to reduce the weight and dimensions of the shielding structures for the piping and the turbine systems. In addition, it is possible to further improve the shielding effect in case of the use of conventional shield structures. Effects and functions of the present invention will become more clear from the description of a preferred embodiment to be described in the following with reference to the accompanying drawings.