Shield primary

In one electroless plating technique, which reportedly achieves attenuation levels of 70 dB in layers of 1-2 microns thick, the molded part is immersed in an aqueous solution of copper salt, reducing agent, and initiator to obtain copper deposition as the primary shielding layer. Nickel is then added to form an outer layer that provides corrosion resistance and improved impact strength. 

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. 

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). 

The total mass of the primary shield is approx. 90.5 mg. Of this, 9.2 mg is due to the conical part, whose four segments are approximately equal in weight. The triangular segments of the cylindrical section each have a mass of approx. 3.7 mg, while the square ones have a mass of approx. 16.6 mg. The maximum activation is 1.55x10 Bq/g. 

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 primary shield is demolished by remote operation from the enclosure. The planning is to install a platform below the primary shield in the reactor cavity on which parts of the primary shield can be deposited. The parts of the primary shield are then disassembled by means of a saw which can be carried by the crane it is applied to the component, braced, and thus allows horizontal and vertical cutting. 

As already mentioned and shown in Figure 9.1, the primary shielding mechanism is reflection for which shield should possess free charge carriers (electrons/holes) that can interact with incident EM field. Therefore, electrical properties of ICP-based blends and composites are extremely important and their detailed understanding is of paramount importance, for regulation of attenuation as well as shielding mechanism. 

Shielding requirements of the surrounding structures are described in Section 12.3. Reactor coolant system shielding permits limited personnel access to the containment building during power operation. The reactor vessel sits in a primary shield well. This and other shielding reduces the dose rate within the containment and outside the shield wall during full power operation to acceptable levels. 

As far as light water cooled reator systems are concerned the recommendations made in 2.4.1 (a), (b), (c), (d) and (e) apply in principle with respect to demolition and removal of the primary shield wall. In addition it is recommended that work be done to assess the feasibility of constructing the inner part of the primary shield wall to contain water tanks. 

The region of the primary shield wall adjacent to the active core. Ihe activated zone would extend approximately 1.0m to 1.5m radially into the wall from the reactor cavity face and similar dimensions above and below the active core region. See figures 48 to 52. 

A comparison of activity levels in the above itens has been compiled by Gregory fQj. year and 100 year decay times. Total activity levels and mean specific activity levels have been extracted and are shown in table 2. Inspection of these values shows that the primary shield wall has significantly lower values of both total and mean specific activity levels than either the RPV or reactor Internal structures. No other plant or structures become activated during normal operation of the plant. 

Difficulties occur in trying to design for ease of demolition and yet fulfil the required structural functions for normal operation and postulated fault conditions. Typically, in posttilated fault conditions the primary shield wall is subject to large reactions from the embedded anchorages for the reactor pressure vessel as a result of high transient pressures within the reactor cavity and inspection gallery. 

Alternatively, to reduce the amounts of activated materials, it may be possible to replace part of the inner portions of the existing shield wall structure with a series of "water tanks" containing borated water which can easily be drained. This scheme, shown in figures 53 and 54, could replace as much as the innermost 600 mm of concrete, without major detriment to the structural strength. Such a scheme would reduce the volume of activated material of the primary shield wall and may also reduce the level of activity of the remaining structures but would have no effect on the level of activity in the vessel support system. The material content of the tanks themselves and their resultant activity would need to be considered. 

Section showing possible location for water primary shield wall. tanks in PWR... 

Fig.53. SECTION SHOWING POSSIBLE LOCATION OF WATER TANKS IN PWR PRIMARY SHIELD WALL...

For post-accident iodine control and to minimize corrosion of the stainless steel in the containment, the pH of the water in the IRWST and thus of the recirculated containment spray solution, is maintained at a minimum of 7.0 as recommended in SRP Section 6.5.2 and DTP MTEB 6-1. Disodium phosphate stored in baskets in the IRWST holdup volume becomes immersed in water during a LOCA and the resulting solution overflows into the IRWST. The stainless steel baskets, which are attached to the primary shield wall of the holdup volume, have a solid top and bottom with mesh sides to permit submergence of the disodium phosphate. The baskets are below the IRWST spillway but at a sufficient elevation to prevent an inadvertent submergence during normal operation. Access is provided to the baskets for inspection and sampling. 

The primary shield supports the thermal shield, fuel tube assemblies, control rods and ball hoppers It also restrains leakage of the reactor gas atmosphere ... 

Total Weight (at upper primary shield floor) About 2m (at primary shield wall) About 13,000 ton... 

The primary shield consists of a large mass of concrete surrounding the reactor vessel (Section 12.3.2.1 of Reference 6.1). The secondary shield consists of the concrete compartment walls around the principal components of the reactor coolant system the reactor vessel, the steam generators, the pressuriser, the reactor coolant pumps and the associated piping that part of the chemical and volume control system within the containment is also located in a shielded compartment the regenerative heat exchanger, the letdown heat exchanger, the filters, the demineralisers and the letdown lines. 

The main source of radiation is the reactor core, which emits gamma rays and neutrons. These are attenuated by the reactor internal components and by the reactor vessel, but further external shielding, the primary shield, is still necessary to limit the neutron activation of components and stmctural materials. 

The primary shielding design needed for the radiation emmited from reactor core is reported in Section 12.2.1.2.2 of Reference 6.1. 

During reactor operation, the shield building protects personnel occupying adjacent plant structures and yard areas from radiation originating in the reactor vessel and primary loop components. Internal to the containment, the reactor vessel is shielded by the concrete primary shield and by the concrete secondary shield, which also surrounds the primary components. 

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