A dissertation submitted to the Department of Physics, University of Surrey, in partial fulfilment of the degree of Master of Science in Radiation and Environmental Protection
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- Appendix 4 HEPA Filters Figure 17
- Appendix 5 Radionuclide Stack Height Data
- Figure 18 Figure 19 Figure 20
- Figure 24 Figure 25 Figure 26
[29] Table 9 63Ni ~5% of ingested 63Ni is absorbed into the bloodstream via the intestines. ~20-35% of inhaled 63Ni is absorbed through the lungs. Of the 63Ni that reaches the blood, 68% is excreted in the urine and 2% remains within the kidneys, with a t1/2 of 5 hours. The remaining 30% is evenly distributed to all other body tissues. 63Ni is only a problem if it enters the body. External gamma exposure is not a problem as 63Ni does not emit significant gamma radiation. The beta particle emitted is more of a concern, which is of low energy but internally can have detrimental effects. Nickel also exhibits chemical toxicity. Acute toxicity following ingestion of high concentrations includes gastrointestinal, blood and kidney effects. Nickel can also cause skin allergies and lung problems. (The respiratory system is the primary target following acute inhalation of high concentrations of nickel). Nickel subsulphide, an insoluble form, has been classified as a carcinogen. The cancer mortality risk coefficients for ingestion are somewhat higher than for inhalation. Using the inhalation unit risk, the Environmental Protection Agency estimates a person would have a one-in-a-million chance of developing cancer if exposed daily over a lifetime to air containing 0.002µg/m3 nickel subsulphide. [26] 60Co Gastrointestinal absorption from food /water is the principle source of internally deposited Co in the general population. ~5-30% GI absorption, which depends on the chemical form and the amount ingested. 30% is a typical child value Co is an essential element found in most body tissues, with the highest concentration in the liver. Vitamin B12 is cobalt- containing, essential for red blood cell formation. Intestinal absorption of Co in this vitamin is high. 50% of Co that reaches the bloodstream is immediately excreted, mainly via urine. 5% of Co deposits in the liver and 45% deposits evenly in other bodily tissues. Of the Co that deposits in the liver and other tissues, 60% leaves the body with a biological half-life of 6 days, with 20% clearing with a biological half-life of 60 days. 20% remains in the body for much longer, and has a biological half-life of 800 days. External exposure is a concern because of the strong gamma radiation. Internally, 60Co presents a hazard from both beta and gamma radiation. Using the external gamma risk coefficient to estimate lifetime cancer mortality risk, if it is assumed that 100 000 people were continuously exposed to a thick layer of soil with an initial average concentration of 1pCi/g 60Co, then 6 of the 100 000 people would be predicted to incur a fatal cancer. (0.006%) [27]
Like normal hydrogen, 3H can bond with O2 to form H2O, forming tritium oxide/tritiated water, and is radioactive. Because tritium oxide is identical to normal water, it cannot be filtered out of the water. 3H radiation is unable to penetrate the skin, so it only poses a problem if it becomes inhaled or ingested – or indeed through a wound. 3H gas can obviously be inhaled, but only about 0.04% is retained more than a minute or so and is therefore an insignificant exposure hazard. 3H oxide immediately mixes with the body fluids and is eliminated like normal water. Generally, half the amount of H-3 is eliminated within 10 days and can be sped up by increasing fluid intake. [27] 55Fe 55Fe emits low energy x-rays and electrons which are absorbed in the dermis. It has a 80% uptake by the liver and a 1.3% uptake by the spleen. The remainder is assumed to be uniformly distributed to all other organs and tissues of the body. Iron is retained in organs and tissues with a biological half-life of 2000 days.1or 2% of an uptake of 55Fe is eliminated in urine during the first 24 hours, the rest is eliminated in faeces. [28] 14C 14C is present in the body at an amount of 0.1µCi in adults and behaves in the same manner as other carbon isotopes. Most of it is completely absorbed upon ingestion, moving rapidly from the gastrointestinal tract to the bloodstream. The fractional uptake of 14C by inhalation is strongly dependent on its chemical form. For CO2 gas and organic compounds, essentially all inhaled 14C is absorbed into the bloodstream, whereas CO gas has an absorption fraction of 40%. The absorption fraction for 14C on inorganic particulate aerosols is significantly lower. The 14C which enters the bloodstream after either inhalation or digestion is quickly distributed to all organs of the body, as for other isotopes of carbon. 14C is eliminated from the body with a biological half-life of 40 days. 14C is not an external hazard as it decays by emitting a weak beta particle, with no gamma radiation. It is of weak energy and unable to penetrate the skin. [24] Appendix 4 HEPA Filters 
Figure 17 These filters have ~99.995% efficiency at removing airborne particles 0.3µm in diameter. They are composed of a web of randomly arranged fibres, composed of fibreglass and ranging in diameter between 0.5&2.0µm. The key components affecting function are the fibre diameter, filter thickness, and face velocity. The air space between HEPA filter fibres is 0.3µm. Particles become entrapped by the fibres through a combination of 3 mechanisms:
Particles follow a line of flow in the air stream and enter an area within one radius of a fibre and stick to it.
Larger particles are unable to use the air flow around the fibres to their advantage and avoid entrapment and the particle embeds itself straight into the fibre. This effect increases with smaller fibre separation diameters and higher air flow velocity.
The smallest particles, especially those <1µm in diameter collide with gas molecules and are therefore slowed in their path through the filter. Diffusion becomes dominant at lower air flow velocity and predominates below the 0.1µm diameter particle sizes. Impaction and Interception predominate above 0.4µm where Diffusion and Interception dominate the region in between the two. In the Nuclear Industry, HEPA filters are often used in conjunction with activated carbon filters. Carbon Filters contain a form of carbon which has been processed to make it extremely porous and therefore has a large surface area available for adsorption. 1g of activated carbon has a surface area of ~500m2. Activated carbon is usually derived from charcoal. These filters allow for the removal of radioactive gases and particles before exhausted air is released into the atmosphere. HEPA filters need to be correctly installed into a filter house or frame in order to achieve correct results. In the Nuclear Industry, the housings are sometimes called trains. These filter houses are usually arranged in an array with 24 inch by 24 inch by 11.5 inch deep filters, having a nominal capacity of 1500cfm (0.7m3/s) each. [30] Appendix 5 Radionuclide Stack Height Data The following graphs show the doses of radionuclides that are emitted from stacks of two heights, one of 1metre, the other of 10 metres. With direct relevance to appendix 3, it can be seen from these graphs that each radionuclide not only has its own individual way in which it affects the body, but also affects humans at different lifestages with different emphasis. Children have higher metabolic rates than adults and the uptake pathways can be quicker and accumulation of the radionuclide more severe.
Table 10   
122.7cm 
 
 
100cm    
97.2cm The above diagram gives the dimensions of the RSV. The dose rates were calculated with the Operator at a distance of 1m. The Microshield computer program inputs the dimensions and re-creates the shape and then calculates the dose rates. The program was set to calculate the doses based on a build-up factor within the source. The density of the resin was calculated and a value of 1.15 used. The maximum dose rate must not exceed 2mSv/hr. This is used whenever a barrier must be set up in the event of contamination. From appendix 6 it can be seen that the contact dose rates are well below this limit. 
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Figure 20
Figure 21
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Figure 26
