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A new methodology is outlined and demonstrated on the improvement of uncertainty and sensitivity analysis based on the random sampling method

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Abstract

In several hours of a calm meteorological situation, a relatively significant level of radioactivity may accumulate around the source. When the calm situation expires, a wind-induced convective movement of the air immediately begins. Random realisations of the input atmospheric dispersion model parameters for this CALM scenario are generated using Latin Hypercube Sampling scheme. The resultant complex random radiological trajectories, passing through both calm and convective stages of the release scenario, represent the necessary prerequisite for the prospective uncertainty analysis (UA) and the sensitivity analysis (SA). The novel approximation-based (AB) solution replaces the non-Gaussian sum of individual puffs at the end of the calm period with one Gaussian “super-puff” distribution. This substantially accelerates generation of a sufficiently large number of random realisations for the radiological trajectories, thus facilitating the subsequent UA and SA. Both of these procedures exploit a common mapping between the pairs of calculated output fields on the one hand and the realisation vectors of the associated random input parameters on the other hand. This paper presents the necessary technical background, as well as the idea of the AB solution and its use. Examples of 2-D random trajectories of deposited 137Cs are presented in a graphical form. Global sensitivity analysis based on random sampling methods is outlined and improved feasibility o f the originally long-running computation is demonstrated.

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Notes

  1. As said in the Introduction, the used algorithm for a Gaussian puff model, Adriaensen (2002), with the plume-segmented modification, was selected here for demonstration purposes only. In reality, more sophisticated dispersion codes are assumed to be applied. Their complexity and computational load could be diminished by the positive effect of the approximation-based (AB) procedure.

  2. Someone may object why the input parameter of the total inventory of radionuclide was not considered as random. This factor is assumed to enter separately at a higher level procedure of inverse modelling for estimation of the source term. The problem was addressed, e.g. in Pecha and Šmídl (2016), and requires real measurements from the monitoring networks.

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Acknowledgements

The authors are grateful to the IT department of the National Radiation Protection Institute in Prague for free access to the archives of the historical meteorological data. The research of MK was partially supported by MŠMT ČR LTC18075 and EU-COST Action CA1622. Dr. T.V. Guy provided us a useful feedback on the presentation way.

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Appendix

Appendix

Demonstration of the “super-puff” concept (AB solution), see Pecha and Kárný (2021).

The CALM scenario ranks among the worst-case episodes of Weather Variability Assessment. Let total inventory \(Q_{TOT}^{n}\)  = 6.0 E + 07 [Bq] of radionuclide 137Cs be discharged into the motionless ambient air during the calm conditions lasting TCALM = 5 [h], \(T^{CALM} = \left( {T_{END}^{CALM} - T_{START}^{CALM} } \right)\) . The release is modelled as a sequence of M instantaneous discrete discharges, the first (oldest puff) for m = 1 at time\(T_{START}^{LEAK}\) , the last for m = M at time \(T_{END}^{LEAK}\) . The release propagates from the elevated point source of pollution at a height of H (x = 0; y = 0; z = H) over the terrain. The radioactivity progresses during the calm episode time interval \(\left\langle {T_{START}^{LEAK} ;\;T_{END}^{CALM} } \right\rangle\). The chain of consecutive discrete puffs \(Q_{m}^{n}\) of 137Cs, m ∈ {1,…,M}, are ejected stepwise with the time periods Δtm. The release-source strength \(Q_{TOT}^{n}\) /M is initially assumed to be constant within the entire calm episode. After five hours of the calm episode, the wind begins to blow. The convective transport of the radioactivity clew immediately arises. We trace the drifting over the terrain in the next four hours. Meteorological records are extracted from stepwise forecast series for the given point of radioactive release. Hourly meteorological data of the convective transport immediately following the five hour calm episode are shown in Table

Table 1 Convective transport in the subsequent four hours (immediate continuation of the calm episode). Archived meteorological data at point (49° 05′ 00.73″ N, 16° 07′ 26.95″ E) of the Dukovany nuclear power plant: start at Dec 4, 2019, 01.00 CET (time_stamp 2,019,120,401). Rain in the 4th convective hour was chosen deliberately for demonstration purposes

1.

Each puff reflects a partial discharge of the radioactivity \(Q_{m}^{n}\), m ∈ {1,…,M}, which dissipates into the motionless ambient atmosphere just until to the calm-period termination. The results of current realistic calm scenario in STAGE I (related to \(T_{END}^{CALM}\)) are displayed in Fig. 

Fig. 9
figure 9

Distribution of 137Cs concentration in air for separate puffs m having its own age. Red line: procedures of handling the further convective superposition of all puffs m; m ∈ {1,…,M}

9. Radioactivity concentrations of 137Cs in air (in the height H) for each individual puff m are shown here. The sharpest shape for the “youngest” puff m = 99, the flattened shape for the “oldest” m = 30, has been expected. The red curve shows, clearly non-Gaussian, shape of the puffs’ superposition.

The radiological results transformed into radioactivity concentration values of 137Cs deposited on the ground at the end of the STAGE II (just after 9 h from the same beginning of the CALM start) are shown in Fig. 

Fig. 10
figure 10

Comparison of two alternative procedures of handling the further convective transport

10. It represents the peripheral distribution (around the angular beam of the computational grid containing maximum values of deposited radioactivity) on the concentric circle c* (~ 25 [km] around the release point (of the computational grid). The results confirm a good agreement of both compared solutions in the domains of interest (with potential high values of harmful effect on the population). Several variants have been successfully tested including nonlinear releasing, serrated form, the dependency of AB approximation on the number of puffs M, etc.

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Pecha, P., Kárný, M. A new methodology is outlined and demonstrated on the improvement of uncertainty and sensitivity analysis based on the random sampling method. Stoch Environ Res Risk Assess 36, 1703–1719 (2022). https://doi.org/10.1007/s00477-021-02110-0

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