Calculation and Modeling

The red rectangle models an intervention agent, with a thin layer representing their skin. The pipe is divided into several layers to adjust their significance, thereby enhancing statistical analysis by controlling the particle population. The code of the geometry is shown below

Calculations

The pipe is uniformly contaminated on its internal surface with an activity of 1.5 GBq/m². Through β decay, two photons (1.1732 MeV and 1.3325 MeV) are emitted. Given the steel thickness of the pipe, electron dose consideration is unnecessary, as they are rapidly stopped. Only photon transport will be analyzed.

First, the actual source must be defined: this involves multiplying by the internal surface area of the pipe and by 2, as two photons are emitted. Thus, 1.5 GBq/m² = 11.304 × 10⁹ γ/s. The F6 tally is used for the initial simulation:

R.E = 0.015 | FOM = 833.25

Results are deemed acceptable.

ϕₘ𝚌ₙₚ = 3.04 × 10⁻⁹ ± 4.66 × 10⁻¹¹ MeV/g, converted as per Equation (2):

Dγ = 19.81 ± 0.30 µGy/h

However, these doses do not reflect reality, as the equivalent dose is required. A table is referenced using two F4 tally options: DE to specify photon energy ranges with logarithmic interpolation and DF to provide conversion coefficients for equivalent dose. Results:

R.E = 0.047 | FOM = 852.38

The F4 tally now provides results in pSv/history:

ϕₓ₄ × source = 5.59964 × 10⁻⁷ × 11.304 × 10⁹ = 6329.833 pSv/s = 22.79 µSv/h

HF₄ = 22.79 ± 1.06 µSv/h

The professional dose limit is 20 mSv for 12 months. Agents intervening on the NEUNEU reactor or in the piping room can remain for the following durations, respectively:

tTUYAU = 877.2 hours = 36.55 days

Tese results are valid at the calculated distance: a few meters from the pipe. Therefore, there is no danger for agents intervening under these conditions. However, the ALARA principle applies. Any intervention must therefore be carried out as quickly as possible to ensure that the work is carried out in the best possible conditions.