The Biological Shield

The shield is penetrated by about one hundred holes of various sizes Embedded.in the concrete and welded to the steel plates which act as inner and outer form plates for the concrete are all the permanent liners for the experimental holes. One of the major problems in the construction of the reactor will be the alignment of these liners. As indicated in Section 2.1.1, the top of tank section E is the only reference point available, and accuracy is essential if the HB and DB inner liners are to fit into their respective locations in thetanksectionDwall. 

The MTR shield is being built of barytes concrete in which the gravel part of the mix is approximately 93% BaSO, It was at first thought that only certain critical sections of the shield would be made of. barytes, but, owing to difficulty of mixed pours, it was decided that a more satisfactory shield would be obtained if barytes was used throughout.. In order to simplify the cons.truction procedure and obtain a uniformly dense concrete, it is now planned to use the Prepack method for setting all the biological shield. 

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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). 

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. 

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. 

The reactor is controlled by the same rotary drums as in the first version. The total thickness of the biological shield is less and equals 1.75m. It consist of water layer in the pool (1.25m), a steel layer (0.4m) and an external layer of boron polyethylene (0.1m). Collimators are similar those of the first version. The heat, generated by the reactor is absorbed by the whole weight of the pool water, thanks to its high thermal capacity. During continuous 8-hour reactor operation the temperature effects no more then 8 C. All reactor effects, connected with change in its condition (temperature, void) have negative values. 

A gap of 30 mm is provided between the two concrete to take care of differential thermal expansion. The biological shield concrete is cooled by circulating water through 180 coils embedded in concrete. The biological shield cooling (BSC) system has two distribution headers, each have 100% capacity. Each header has six sub-headers with individual isolation valves. 

CONCEPT FOR DISMANTLING THE REACTOR VESSEL AND THE BIOLOGICAL SHIELD OF THE COMPACT SODIUM COOLED NUCLEAR REACTOR FACILITY (KNK)... 

Under the 9th Decommissioning Permit, the reactor vessel with its internals, the primary shield, and the biological shield are to be dismantled. A Europeanwide limited tendering procedure was first run for these activities, and at last the contract was made with Westinghouse Reaktor Germany. 

Another difficulty is caused by the depth of activation by fast neutrons, as a result of which not only the reactor vessel proper, but also the entire primary shield (60 cm of grey cast iron) and large parts of the biological shield must be disassembled and disposed of under remote control. 

After disassembly of the metal components, the thermal insulation made of fireclay around the reactor vessel must be removed This can be achieved either by cutting, as mentioned above, or by chipping, as in the later demolition of the biological shield. 

At the level of the reactor core, the primary shield made of cast iron with lamellar graphite, GG-20, is situated in a niche of the biological shield outside the thermal insulation (see Fig. 3). 

Perhaps it will be necessary, prior to demolition of the primary shield, to remove parts of the biological shield above the primary shield as far as the outside diameter of the primary shield. This makes the primary shield freely accessible from the top and from the inside. 

The reactor core is surrounded by a block of concrete of very high density (density 4.14 g/cm ), the biological shield, which was also activated by the neutron radiation emanating from the reactor core (see Fig. 3). The specific Co-60 activity of the concrete achieves a maximum of 8x 10 Bq/g, which means that most of the disassembly work must be carried out remotely. 

The depth of demolition of the biological shield is determined by the depth of activation of the concrete. According to the new German Radiation Protection Ordinance, a clearance level for Co-60 of 0.09 Bq/g must be observed for the unrestricted clearance of building rubbish. Probably, a total of 330 mg of very-high-density concrete must be disposed of as radioactive waste. 

FIG. 4. Demolition of the biological shield. Table 2 provides the data about the balance of the radioactive waste. TABLE 2. BALANCE OF THE RADIOACTIVE WASTE... 

In this chapter of the handbook an attempt will be made to describe the structure and function of all the parts of the reactor enclosed within and including the biological-shield. Since this- is not-a cons true ti.on manualt-details not required to explain the functions of any part will be omitted. Such details can be obtained from the reference. drawings and reports listed at the end of this chapteri >, , ... 

A 2-in. by 7-ft vertical slot in the biological shield from the bottom of the thermal. shield to the sub-pile room ceiling will house the curtain when it is not in use. To shield against radiation streaming, the lower portion of this slot- will be filled with 2-ft-deep removable barytes concrete blocks supported from the sub-pile room ceiling by 6-in.-square steel bars which serve as additional shielding. 

In addition to the above experimental holes, twelve 2-in.-I.D. holes are provided in the concrete. These holes, designated VC-1 through 9 and VC-11 through 13, penetrate the south wall of the biological shield to elevation 92 ft. Their location is indicated in Fig. 3.E around the south instrument cubicle (to the right of GM-1) and between the cubicle and the outer edge of the reactor top. It is expected that these holes will.be useful for thermocouples or for shielding measurements. 

The first approach to the design of the biological shield of the Materials Testing Reactor was the comparison of the neutron and y-ray fluxes incident on... 

SvMflementnry Shielding.Problems.. . In addition to the most important problem of determining the adequacy of the concrete portion of the biological shield, there are a number of other shielding problems relative to the design of the MTR and associated facilities. - These are listed as follows ... 

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