Nuclear Fuel Cycle

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

(b) State the four IAEA Type B package test conditions. (2 marks)

(c) A transport flask has 250 mm of steel walls (HVL for fission product gammas in steel = 25 mm). The unshielded dose rate at 1 m from the fuel (treated as a point source) is 800 Gy/h. The flask surface is 1.2 m from the fuel centre. Calculate the dose rate at the flask surface. The IAEA limit for a Type B package surface is 2 mSv/h --- does this flask meet the limit? (4 marks)

Solution

(a) Four safety functions:

1. Containment --- prevent release of radioactive material under normal and accident conditions.
2. Shielding --- reduce external dose rates to acceptable levels for transport workers and the public.
3. Criticality safety --- maintain subcriticality under all conditions, including water ingress.
4. Heat dissipation --- conduct decay heat away from the fuel to prevent overheating.

(b) IAEA Type B package test conditions:

The flask must survive the following sequential tests without loss of containment or significant increase in external dose rate:

1. 9 m drop onto an unyielding surface
2. 1 m drop onto a punch bar (a steel bar of 15 cm diameter)
3. 800 °C fire for 30 minutes (fully engulfing)
4. 15 m immersion in water

(c) Dose rate calculation at the flask surface:

Step 1: Apply the inverse square law to find the unshielded dose rate at 1.2 m.

The unshielded dose rate at 1 m is 800 Gy/h. At 1.2 m:

$\dot{D}_{1.2\text{m}} = 800 \times \left(\frac{1}{1.2}\right)^2 = 800 \times \frac{1}{1.44} = 800 \times 0.694 = 555.6 \text{ Gy/h}$

Step 2: Calculate the number of HVLs in 250 mm of steel.

$n = \frac{250 \text{ mm}}{25 \text{ mm}} = 10 \text{ HVLs}$

Step 3: Calculate the attenuation factor.

$\text{Attenuation} = 2^{10} = 1{,}024$

Step 4: Calculate the shielded dose rate at the flask surface.

$\dot{D}_{\text{surface}} = \frac{555.6}{1{,}024} = 0.543 \text{ Gy/h} = 543 \text{ mSv/h}$

$\boxed{\dot{D}_{\text{surface}} \approx 543 \text{ mSv/h}}$

Step 5: Compare with the IAEA limit.

The IAEA limit for a Type B package surface is 2 mSv/h. The calculated dose rate of 543 mSv/h exceeds this limit by a factor of approximately 270. This flask does NOT meet the regulatory limit with steel shielding alone.

In practice, real transport flasks use combined shielding materials --- steel for structural integrity, lead for gamma attenuation, and sometimes water or resin for neutron moderation. The design is optimised using Monte Carlo radiation transport calculations (not simple HVL estimates) to ensure compliance with regulatory limits under all conditions.

End of Chapter 4