Biological shield

CP-1 was assembled in an approximately spherical shape with the purest graphite in the center. About 6 tons of luanium metal fuel was used, in addition to approximately 40.5 tons of uranium oxide fuel. The lowest point of the reactor rested on the floor and the periphery was supported on a wooden structure. The whole pile was surrounded by a tent of mbberized balloon fabric so that neutron absorbing air could be evacuated. About 75 layers of 10.48-cm (4.125-in.) graphite bricks would have been required to complete the 790-cm diameter sphere. However, criticality was achieved at layer 56 without the need to evacuate the air, and assembly was discontinued at layer 57. The core then had an ellipsoidal cross section, with a polar radius of 209 cm and an equatorial radius of309 cm [20]. CP-1 was operated at low power (0.5 W) for several days. Fortuitously, it was found that the nuclear chain reaction could be controlled with cadmium strips which were inserted into the reactor to absorb neutrons and hence reduce the value of k to considerably less than 1. The pile was then disassembled and rebuilt at what is now the site of Argonne National Laboratory, U.S.A, with a concrete biological shield. Designated CP-2, the pile eventually reached a power level of 100 kW [22]. 

The nuclear reactor also must be shielded against the emission of radioactive material to the external environment. Suitable radiation controls include both thermal and biological shielding systems. Radiation from alpha particles (a rays) and beta particles ((3 rays) has little penetrating power, but gamma rays have deep penetration properties. Neutron radiation is, however, the primary area of risk. Typically, extremely thick concrete walls are used as a neutron absorber, but lead-lined concrete and special concretes are also used. 

Previous experience in the production of plutonium 238 revealed the need for the double alpha containment of the cells where the alpha-emitter isotopes with high specific activity are handled. Thus all the hot cells are equipped with a double ventilation system which provides ventilation of alpha-cells and ventilation between alpha-cells and biological shields. Alpha detectors continuously monitor the exhaust circuits. 

To ensure safe shipment and observe the requirements of both the Russian Register and the International Supervisory Bodies to MVs, the new vessel should be equipped with double bottom, double sides, necessary number of cofferdams, biological shielding and physical protection devices, radiation control systems and equipment to ensure the above-water floodability. 

Naoumov V.A., Rubin I.E., Dneprovskaya N.M. et al. (1996) Description of neutron attenuation in the biological shield using the nethod of probabilities of penetrations Preprint IPE-17, Institute of problems of energetics of AS of Beloruss, Minsk (in Russian). 

The storage facility represents a tunnel supported by lightweight reinforced-concrete constructions made on basis of BelgTASM concrete mixes providing for reliable biological shielding. 

M. Itoh, K. Watanabe, M. Hatakeyama, M. Tachibana (2002) Determination of 41Ca in biological-shield concrete by low-energy X-ray spectrometry. Anal Bioanal Chem., 372 532-536... 

Research reactors are widely used for scientific investigations and various applications. Neutrons produced by research reactors provide a powerful tool for studying matter on nuclear, atomic, and molecular levels. Neutrons, often are used as probes by nuclear and solid state physicists, chemists, and biologists. Neutron experiments can also be performed outside the biological shield by means of installed beam tubes. Additionally, specimens can be positioned in or near research reactor cores for neutron irradiation, e.g. to produce radioactive isotopes for medical or research use. 

It is essential that people and the surrounding countryside be adequately shielded from possible exposure to radioactive nuclides. The entire reactor is enclosed in a steel containment vessel. This is housed in a thick-walled concrete building. The operating personnel are further protected by a so-called biological shield, a thick layer of organic material made of compressed wood fibers. This absorbs the neutrons and beta and gamma rays that would otherwise be absorbed in the human body. 

As with reactor sources targets are strongly contained and heavily shielded. Their biological shielding is similar to, but thicker than, that found on reactors ( 3.1.1.1). The extra shielding is required since most neutrons from spallation targets remain unmoderated and very penetrating. 

The reactor vessel is surrounded by several physical barriers. These barriers protect the surroundings from a core accident and protect the core from damage caused by external effects. In a PWR these barriers are the reactor tank (about 15—25 cm steel), the biological shield (1.5—3 m of concrete), and the outer containment. The whole primary circuit, including pressurizer dome, steam generator, and connecting pipes, is surrounded by concrete. 

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