LMi of the lymph (TLNM in Figure 4) reaches equilibrium values by about 7,000 days. However, the bone burden is still increasing after 20,000 days since the time for bone to reach equilibrium is on the order of 700 years (255,500 days). When a compartment is in equilibrium, the time derivative of the burden is zero (dy/dt = 0) and the equilibrium burdens of the lung compartments can be calculated from the algebraic system of equations formed by setting all the time derivatives to zero in Equations (1) through (17). The compartments of the pulmonary region are the most significant for computing the radiation dose to the lungs. Since , is so small in comparison to the other A values, it can be set to zero in Equations (1) through (18) and the equilibrium *39Pu burden of the pulmonary region is: (20) Yp = ADs (£/, + Ep/he + £ IX, + fA, ) . The equilibrium rate that 239py reaches the blood is: "RB = ACE, + fF 5) D3 + (f. + faf,) D, + (f, + fet, + fifi + tify Ds) + fH (21) The *22Pu burden in bone is never in equilibrium during the time period of interest (less than 70 years) but its burden after 20 years can be easily calculated since T is at equilibrium and the solution to Equation (18) for constant T is: Yn = [fen rp, J] [1.0 - exp (-A,t)]. (22) Equations (19) through (22), along with expressions for estimating the inhalation and ingestion of 233py, form the simplified model used to examine the effects of variations in source term and parameter values. The model previously used to estimate inhalation of 239pu (Martin and Bloom, 1976; 1977) is already simple in form and is: A TA =BLC (23) mas where Ba is man's inhalation rate (about 20 m?/day). L, is the mass loading factor for soil in the air (about 100 ug/m?), C_is the average concentration of 239pu in the soil of a contaminated area (uCi/g). The model previously used to estimate ingestion is considerably more complicated but, as indicated in Table 1, ingestion of 239py is not nearly as significant as inhalation. Therefore, the only variation 524