Radiation area shielding

Pipes carrying radioactive materials are routed through controlled access areas properly zoned for the level of activity. Where it is necessary that radioactive piping be routed through corridors or other low radiation areas, shielded pipe ducts, or distance separation are provided. Where practicable, radioactive and non-radioactive piping are separated. 

To minimise personnel exposure from valve operations, motor operated, air operated or other remotely actuated valves are used where justified by activity levels and frequency of use. Where manual valves are used, either valve extenders or shielding is provided such that personnel need not enter a radiation area for valve operation. 

I Drums used to store radioiodine wastes must be kept in well-ventilated areas, away from occupied areas. Shielding may be necessary to prevent exposure if radiation levels are high. The radiation level at one meter per curie for I is 0.07 R/hr and for I is 0.22 R/hr. A lead sheet of 1.5 mm will reduce the radiation intensity from I by a factor of approximately 1,000 and 1 cm of lead will... 

A number of sensitivity studies were performed to determine the effect of parameter variations on the plant s overall heat balance and other parameters of interest. Of particular interest was the effect of parameter variation on required reactor thermal output, required radiator area, and reactor inlet temperature. The overall mass of the SNPP is minimized when reactor thermal output and required radiator area are near minimum values as the mass of the reactor, the reactor radiation shield, and the heat rejection segment dominate the overall SNPP mass. Minimizing the required thermal output of the reactor has many benefits as the mass of the fuel load, reactor structure, and reactor radiation... 

Power required to maintain pre-start-up temperatures above 200 K without solar illumination is estimated to be approximately 1500 Watts (see scoping calculations for plant support, shield, and reactor), which is below the power required for start-up. This assumes emissivity of 0.95 for the primary support radiator region ( 4 - 7 m as described previously), the shield, and the reactor envelope, and effective emissivity of 0.2 for the non-radiator regions of the primary support. If the radiator areas can be oriented toward the sun, the power to maintain temperature can be reduced significantly with proper selection of surface emissivities, and pre-start-up temperatures closer to room temperature. 

The physical properties of the Selenium also offer big advantages with respect to radiation shielding and beam collimation. Within the comparison of radiation isodose areas the required area-radius for a survey of 40pSv/h result in a shut off area that is for Selenium only half the size as for iridium. Sources of similar activity and collimators of same absorbtion value (95%) have been used to obtain values as mentioned in Table 3 below. 

Lead bricks are generahy used as temporary shields for radiation sources at nuclear power stations, research institutes, hospitals, and fuel reprocessing plants. Plat, rectangular bricks requite a double layer with staggered seams whereas the interlocking bricks requite only one course. Lead shot can be poured into inaccessible areas like a Hquid. 

Exposure is minimized by ehoiee of souree, by duration of exposure, by distanee from souree (at 1 m die radiation level is redueed almost 10-fold), and by shielding. The greater die mass per unit area of shield material die greater die shielding effieieney. Whereas a- and (3-partieles pose few problems (die former ean be absorbed by, e.g., paper and die latter by 1 em Perspex) y- and X-rays are not eompletely absorbed by shield material but attenuated exponentially sueh dial radiation emerging from die shield is given by ... 

Ground radiation is from deposited radioactive particles. The deposition rate from a radioactive cloud without rain (dry deposition) is so low that the ground radiation dose is about the same as the inhalation dose. A heavy rain, however, may wash out enough particles from the plume to make ground radiation the dominant contributor to the total dose in a limited area. Rain will also attenuate radiation by leaching the radioactivity to be shielded by the soil and by moving it to streams for further removal. 

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. 

The lower tubes in the shield bank in the convection section will receive heat by radiation from the radiant section. This can be allowed for by including the area of the lower row of tubes with the tubes in the radiant section. 

Non-airborne radiation can be countered by shielding the affected area with lead blankets or sheeting material. 

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