Nuclear Fuel Cycle

Problem: A MOX fuel assembly contains 4.3 wt% PuO$_2$ in UO$_2$. Using the data from Table 3.4, estimate the total annual dose to a worker who spends an average of 30 minutes per day at 1 m from such an assembly over a working year (250 days).

Solution:

Step 1: Identify the dose rate at 1 m

From Table 3.4, the total dose rate at 1 m from the MOX assembly is approximately:

$\dot{D}_{1\text{m}} \approx 7 \text{ } \mu\text{Sv/h}$

(This comprises approximately 5 $\mu$Sv/h gamma + 2 $\mu$Sv/h neutron.)

Step 2: Calculate the total exposure time per year

$t = 0.5 \text{ h/day} \times 250 \text{ days/year} = 125 \text{ hours/year}$

Step 3: Calculate the annual dose

$D = \dot{D} \times t = 7 \text{ } \mu\text{Sv/h} \times 125 \text{ h}$

$\boxed{D \approx 875 \text{ } \mu\text{Sv} \approx 0.9 \text{ mSv per year}}$

Step 4: Compare with dose limits

The UK annual dose limit for classified radiation workers is 20 mSv. The calculated dose of 0.9 mSv is well within this limit. However, applying the ALARP (As Low As Reasonably Practicable) principle, exposure time should still be minimised and distance maximised where practical.

Tip: In a real scenario, dose rates from multiple sources and multiple assemblies would need to be considered. The inverse square law ($\dot{D} \propto 1/r^2$) can be used to estimate dose rates at other distances, though this is an approximation for extended sources.