2965 2966 2967 2968 2969 2970 2971 2972 2973 2974 2975 (203) DRAFT REPORT FOR CONSULTATION Figure 15 137 Cs Wound, Weak Category; predicted values (Bq per Bq intake) following acute intake. (204) In comparison, if the 137 Cs in the contaminated wound site is assumed to be present in particles <strong>of</strong> irradiated power reactor fuel, then it can be given parameter values <strong>of</strong> the <strong>Part</strong>icle Category. In this case, dissolution and absorption to blood are much slower than for the Weak Category, and the urine and faecal excretion patterns exhibit a pseudo-equilibrium pattern after about 10 days, lasting for several years (Figure 16). (205) 86
2976 2977 2978 2979 2980 2981 2982 2983 2984 2985 2986 2987 2988 2989 2990 2991 2992 2993 2994 2995 2996 2997 2998 2999 3000 3001 3002 3003 3004 3005 3006 3007 3008 3009 3010 3011 3012 3013 3014 3015 3016 3017 3018 DRAFT REPORT FOR CONSULTATION Figure 16 137 Cs Wound, <strong>Part</strong>icle Category; predicted values (Bq per Bq intake) following acute intake. (206) The presence <strong>of</strong> wounds, abrasions, burns or other pathological damage to the skin may greatly increase the ability <strong>of</strong> radioactive materials to reach subcutaneous tissues and thence the blood and systemic circulation. Although much <strong>of</strong> the material deposited at a wound site may be retained at the site, and can be surgically excised, soluble (transportable) material can be transferred to the blood and hence to other parts <strong>of</strong> the body. (207) As noted in Section 3.1, the assessment <strong>of</strong> internal contamination resulting from wounds is in practice treated on a case-by-case basis using expert judgement. In many cases, the amount <strong>of</strong> a radionuclide transferred from a wound site to blood may be assessed directly from urine bioassay data. No dosimetric models are recommended by <strong>ICRP</strong> for calculating doses from radionuclides transferred from wound sites to blood and to other organs and tissues, and no dose coefficients are given. 3.5 Biokinetic Models for Systemic <strong>Radionuclides</strong> 3.5.1 General patterns <strong>of</strong> behaviour <strong>of</strong> systemic radionuclides (208) <strong>Radionuclides</strong> entering blood may distribute nearly uniformly throughout the body (e.g., 3 H as tritiated water), they may selectively deposit in a particular organ (e.g. 131 I in the thyroid), or they may show elevated uptake in a few different organs (e.g., 239 Pu or 241 Am in liver and bone). If a radionuclide that enters blood is an isotope <strong>of</strong> an essential element (e.g., 45 Ca or 55 Fe), it is expected to follow the normal metabolic pathways for that element. If it is chemically similar to an essential element (e.g., 137 Cs as a chemical analogue <strong>of</strong> potassium, and 90 Sr as a chemical analogue <strong>of</strong> calcium), it may follow the movement <strong>of</strong> the essential element in a qualitative manner but may show different rates <strong>of</strong> transfer across membranes. The behaviour <strong>of</strong> a radioisotope <strong>of</strong> a non-essential element after its uptake to blood (e.g., 106 Ru, 125 Sb, 232 239 241 Th, Pu, or Am) depends on such factors as the extent to which it can be sequestered by the reticuloendothelial (RE) system, its affinity for specific biological ligands, its filterability by the kidneys, and the ability <strong>of</strong> the body to eliminate it in liver bile or other secretions into the gastrointestinal tract. In some cases, the biokinetics <strong>of</strong> an isotope <strong>of</strong> a non-essential element may resemble that <strong>of</strong> an essential element to some extent due to common affinities for some but not all components <strong>of</strong> tissues and fluids. For example, the behaviour <strong>of</strong> plutonium in blood and liver is related to that <strong>of</strong> iron due to an affinity <strong>of</strong> plutonium for certain proteins that transport or store iron (e.g. transferrin), but as a whole the biokinetic behaviour <strong>of</strong> plutonium in the body differs greatly from that <strong>of</strong> iron. Also, the behaviours <strong>of</strong> lead and uranium in the skeleton bear some resemblance to that <strong>of</strong> calcium because these elements can replace calcium to some extent in bone crystal, although the biokinetic behaviours <strong>of</strong> lead and uranium in other parts <strong>of</strong> the body show greater differences compared with calcium. Nevertheless, it is important to emphasise that the use <strong>of</strong> chemical or biological anologues has its limits (Ansoborlo et al, 2006). 87