PHOTON DOSE RATE

Calculated dose rates for MTR-type fuel assemblies are shown in Table 8. These dose rates are from Ref. 3 and are for fuel assemblies with up to 80% U-235 burnup, specific power densities between 0.089 and 2.857 MW/kg²³⁵U, and fission product decay times of up to 20 years.

The data in Table 8 are photon dose rates in air that are averaged over a 60-cm long cylindrical surface, located at a radius of 1 m from the fuel assembly axial center line. For MTR-type fuel assemblies, these average dose rates are independent of the assembly rotational orientation and the number of fuel plates in the assembly. These data also can be interpolated for specific decay time, burnup and assembly power density. In all cases, the dose rates must be multiplied by the mass of U-235 burned in the fuel assembly to estimate the fuel assembly dose rate. The mass of U-235 burned per fuel assembly that is necessary for an unshielded, 100 rem/h self-protecting dose rate at 1 m, is shown in Fig. 1.

Additional analyses have shown that the photon dose rates of MTR, TRIGA and DIDO-type fuel assemblies are similar, given the same fuel assembly characteristics of U-235 burnup, fission product decay time, and specific fuel assembly power density. The average dose rates at 1 m in air for TRIGA (25-rod) and DIDO (4-tube) fuel assemblies are respectively, 1.04 and 1.05 times the dose rates given in Table 8 for MTR fuel assemblies. The dose rates of all three fuel assembly types are for fuel assembly models (nominally 8cm by 8cm by 60cm) containing spent fuel in the form of either rods (TRIGA fuel), annuli (DIDO fuel) or plates (MTR fuel). The small difference in the dose rates are due to the different shielding effects of the fuel elements in the fuel assemblies.

Table 8. Photon Dose Rates At 1 M In Air, rem/h per g²³⁵U burned

Decay Time, y	Burnup, %235U	Assembly Power Density, MW/kg235U
		2.857	1.429	0.714	0.357	0.179	0.089
2 3 4	1%	1.84+0 1.13+0 9.01-1	1.84+0 1.13+0 9.01-1	1.83+0 1.13+0 9.01-1	1.80+0 1.13+0 9.01-1	1.77+0 1.11+0 9.01-1	1.70+0 1.11+0 8.92-1
2 3 4	10%	1.89+0 1.19+0 9.52-1	1.87+0 1.20+0 9.61-1	1.80+0 1.20+0 9.61-1	1.64+0 1.16+0 9.44-1	1.50+0 1.09+0 9.10-1	1.28+0 9.95-1 8.59-1
2 3 4 5 10 15 20	20%	2.01+0 1.31+0 1.04+0 8.97-1 6.67-1 5.78-1 5.10-1	1.98+0 1.32+0 1.05+0 9.10-1 6.67-1 5.78-1 5.10-1	1.86+0 1.28+0 1.04+0 9.05-1 6.67-1 5.74-1 5.10-1	1.66+0 1.21+0 9.99-1 8.80-1 6.59-1 5.70-1 5.06-1	1.42+0 1.11+0 9.44-1 8.46-1 6.50-1 5.61-1 4.97-1	1.19+0 9.78-1 8.63-1 7.95-1 6.25-1 5.44-1 4.85-1
2 3 4 5 10 15 20	40%	2.40+0 1.62+0 1.27+0 1.07+0 7.03-1 5.87-1 5.14-1	2.30+0 1.60+0 1.27+0 1.07+0 7.03-1 5.84-1 5.12-1	2.09+0 1.53+0 1.22+0 1.04+0 6.95-1 5.80-1 5.08-1	1.82+0 1.39+0 1.14+0 9.90-1 6.80-1 5.70-1 5.02-1	1.52+0 1.22+0 1.03+0 9.20-1 6.55-1 5.53-1 4.87-1	1.21+0 1.02+0 8.99-1 8.12-1 6.10-1 5.23-1 4.59-1
2 3 4 5 10 15 20	60%	2.95+0 2.05+0 1.59+0 1.30+0 7.55-1 5.96-1 5.17-1	2.79+0 2.00+0 1.56+0 1.29+0 7.51-1 5.96-1 5.17-1	2.52+0 1.87+0 1.49+0 1.24+0 7.37-1 5.88-1 5.13-1	2.15+0 1.66+0 1.35+0 1.15+0 7.07-1 5.72-1 4.99-1	1.74+0 1.40+0 1.17+0 1.02+0 6.70-1 5.50-1 4.76-1	1.34+0 1.12+0 9.63-1 8.54-1 6.02-1 5.04-1 4.39-1
2 3 4 5 10 15 20	80%	3.85+0 2.73+0 2.08+0 1.66+0 8.28-1 6.18-1 5.27-1	3.62+0 2.64+0 2.03+0 1.63+0 8.21-1 6.15-1 5.20-1	3.26+0 2.43+0 1.90+0 1.54+0 8.00-1 6.05-1 5.13-1	2.76+0 2.11+0 1.69+0 1.39+0 7.59-1 5.82-1 4.97-1	2.21+0 1.74+0 1.41+0 1.19+0 6.97-1 5.44-1 4.66-1	1.64+0 1.33+0 1.12+0 9.57-1 6.04-1 4.87-1 4.20-1