Nuclear Fuel Cycle

(a) List the four safety functions that an irradiated fuel transport flask must provide. (2 marks)

(b) A transport flask has walls made of 360 mm of steel. The half-value layer of steel for the average gamma energy of the fuel is 25 mm. The unshielded dose rate from the fuel at the flask wall position would be 2000 Gy/h. Calculate the dose rate on the outer surface of the flask. (3 marks)

(c) State whether the flask dose rate would be acceptable for public transport (the regulatory limit for dose rate at the surface of a Type B package is 2 mSv/h). (1 mark)

Solution

(a) Four safety functions:

1. Containment --- protect the radioactive contents from release following impact or other damage
2. Shielding --- reduce external dose rates to acceptable levels for transport workers and the public
3. Criticality safety --- prevent the fuel from achieving a critical configuration under any credible conditions
4. Heat removal --- allow adequate dissipation of decay heat to prevent overheating

(b) Dose rate calculation:

Step 1: Calculate the number of HVLs.

$n = \frac{360 \text{ mm}}{25 \text{ mm}} = 14.4 \text{ HVLs}$

Step 2: Calculate the attenuation factor.

$\text{Attenuation} = 2^{14.4} = 2^{14} \times 2^{0.4} = 16384 \times 1.32 = 21,627$

Step 3: Calculate the shielded dose rate.

$\dot{D}_{\text{surface}} = \frac{2000}{21,627} = 0.0925 \text{ Gy/h}$

$\boxed{\dot{D}_{\text{surface}} \approx 92.5 \text{ mGy/h} \approx 92.5 \text{ mSv/h (for gamma)}}$

Note: For gamma radiation, the radiation weighting factor is 1, so Gy $\approx$ Sv numerically for dose equivalent purposes.

(c) Acceptability assessment:

The calculated surface dose rate of approximately 92.5 mSv/h significantly exceeds the regulatory limit of 2 mSv/h for the surface of a Type B package. This simplified calculation using narrow-beam attenuation only (no build-up factor) already gives a result far above the limit. In practice, the flask would also contain water (internal shielding), and would be designed with additional features to ensure compliance. The calculation demonstrates that extremely thick shielding is needed and that flask design is a precise engineering task.

Important note: This simplified calculation uses only the exponential attenuation formula and does not include build-up factors (which account for scattered radiation reaching the dose point). In a full calculation, the build-up factor would increase the dose rate further, making the shielding requirement even more demanding. Real flask designs use detailed Monte Carlo radiation transport calculations.