Severe Accident Research Network

This network was born in 2004 in the Euratom framework program, and continues today even with more force after Fukushima accident that highlighted the need for further study of severe accident.

The CSN was represented in this project, specifically in the work package 5.3 where a comparison and improvement of methodologies for the assessment of uncertainties encountered in PSA level 2 was performed.

The objectives of the work package were:

  • Study uncertainties propagated from level 1
  • Uncertainties of sequences that had already been harmed in level 1
  • Study incompleteness of APET
  • Stochastic events
  • Uncertainties in the cutoff frequencies of APET
  • Uncertainties not considered in the APET.

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ISA methodology was used in this project, where a MATLAB code was previously developed as a prototype where the equations of SDTPD where included and was also the precursor of TSD. This prototype simulated transients in which several typical events of a severe accident of hydrogen  production occurred.

From the new theory TSD, was designed and developed the current SCAIS module that performs calculations of the Damage Frequency of Exceedance of a given sequence. Among the advantages of SCAIS programming module versus the MATLAB  one is that SCAIS module is general and can be used to any simulation with events. Proof of this is that it  has been validated against the results of SARNET project and also SM2A project in addition to be being used in other applications of the ISA that are sent to international conferences and journals.

SARNET currently is of great interest and as told in the passage devoted to IDPSA project is very similar to one of the work packages, which could be selected as a project for the HORIZON 2020 framework and can be coordinated by NFQ Solutions.

SARNET exercise

We propose to accomplish a benchmark exercise in order to test the behaviour of the SDTPD as a dynamic risk assessment methodology. This application concerns containment failure risk assessment due to hydrogen combustion in case of water injection during in vessel core degradation for a French 900MWe PWR. This exercise is based on the results of two transitories computed by ASTEC in which there is no water injection, the first one without spray system activation and the second one with spray system activation. The description of the transitories is the following:

  • There is a LOCA initiating event with a 3’’ break size in a cold leg of RCS.
  • Core uncovery starts at Tcu = 4080s (1h08min).
  • Total core uncovery occurs at Ttcu = 5875s (1h38min).
  • In the second transitory, the spray system starts at Tspr = 7000s (1h57min).
  • Vessel rupture occurs at Tvr =14220s (3h57min).

The benchmark exercise introduces a stochastic character in the three main processes changing the dynamics: water injection, spray system and H2 combustion. Recombiners act as a passive system being always available during the transient.

To simplify the analysis of result, the containment will be supposed to fail if overpressurization due to hydrogen combustion exceeds value Pmax_cont. The value 0.5 MPa will be retain as the reference case.

Simulation results