WP4_Work&Future

IDPSA: WP4PSA2 scenario of containment

over-pressurization due to hydrogen combustions.

Iván FernándezIndizen Technologies S.L.

José M. IzquierdoConsejo de Seguridad Nuclear

Calle Tarrragona 30, Madrid Indizen Technologies

Justo Dorado 11, MadridConsejo de Seguridad Nuclear

2Indizen Technologies ®

H2 Background

The basic goal of severe accident management is the protection of the containment integrity against the release of fision products to the environment.

● For most NPPs, severe accident lead to hydrogen release rates that exceed the capacity of hydrogen control at DBA.

● An understanding of the phenomena is crucial for planning hydrogen management.

● No strict regulatory requirements on the implementation of hydrogen mitiggation measures for existing plants, but the new ones have to incorporate these measures into its design.

● PSA2 has shown that hydrogen combustion can be one of the major contributors to early containment failure.


Hydrogen Phenomenology

● Metallic components as Zirconium at high temperatures in presence of steam will be oxidised, and then hydrogen gas will be released.

● In a melt accident, the heat generation rate exceeds the nuclear decay heat.

● In addition to the hydrogen production, the hydrogen distribution inside the plant, potential ignition sources and the combustion process are of high significance.

● Hydrogen combustion can take place in a variety of forms: mild deflagration, fast or accelerated flames, deflagration to-detonation transition (DDT) and detonation.


Hydrogen Safety significance

●The heat produced increases the core degradation rate.

●Mixed with oxygen, hydrogen may ignite and create combustion pressure and the containment may be damaged.

●Hydrogen combustion is one of the most significant threats to the containment and it could lead to large early releases.

●In small containments the hydrogen could contribute significantly to the static containment pressure.

●The metallic oxides produced by the reaction have different properties than the metal components. This affects the following phases of the accident.


WP4. Overview

● Study of containment over-pressurization due to H2 deflagrations.

● Significant contributor to containment failure probabilities in PSA 2

● PSA/DSA methods alone limitations.

● Special focus on;

a) Different in-vessel core degradation and possible reflooding sequences.

b)Modelling of mutual interactions between time dependent physical phenomena and sequences of stochastic events.

c) Occurrence of stochastic events conditioned by process variables.

d)Hydrogen release, depending on timing of operator actions and core degradation state.


WP4. Objectives

✔ Study scenarios where hydrogen production and combustion are risk significant phenomena.

✔ State-of-the-art PSA/DSA and IDPSA for integration into a single platform.

✔ Using data produced for the considered scenarios with IDPSA platforms in the PSA/DSA analysis.

✔ Help in the decision making process.


WP4. Proposed Tasks.

Task 1.- Specification for an IDPSA application to the analysis of containment over pressurization due to hydrogen combustion.

✔ Preparation and finalization of a general specification for the IDPSA application.

✔ Development of detailed planning for analysis by each partner according to the general specification.

The specifications of the problem will include:

a) Identification of potential damage sequences; b) Identification of header models, including FT; c) Selection of available tools.



Task 2.- Analysis of hydrogen combustion and containment over pressurization scenarios.

✔ Detailed specification of the analysis goals.✔ Listing of the aleatory and epistemic uncertainties addressed in the

analysis. ✔ Preliminary results and conclusions of the analysis.

Analysis will be specifically concerned with:

a) Identification of damage/failure parametric domains;b) Identification of damage/failure safeguards action initiating times;c) Quantification of safeguard action probabilities;d) Evaluation of safety limit: Exceedance frequencies.



Task 3.- Synthesis of results and development of recommendations of platform for joint applications of IDPSA in the decision making process.

✔ Advantages and present limitations of DSA, PSA and IDPSA. ✔ Further development of the methods and tools.✔ Lessons learned report.✔ Specific problems of the applied methods and tools.✔ Recommendations for future applications.

The synthesis will be specifically concerned with:

a) Features of the DSA/PSA/IDPSA tools;b) Methodological aspects;c) Potential IDPSA platform.


Present Work. CSN-Indizen.Futu

Currently CSN-INDIZEN is working in the specification of the WP4 problem.

● The starting point are the results of the prior pioneering work made in the context of SARNET WP5.3 dynamic reliability project. The scenario will be the same, but the dynamic model will be made more realistic.

● CSN used its SARNET WP5.3 to develop the TSD theory and methodology, and to develop a prototype for testing the risk assessment and path assessment modules of the SCAIS-TSD approach.

● The prototype is an off-line, academic oriented platform to test different strategies and approaches in small size problems with solutions able to be incorporated into full-scale SCAIS-TSD problem.


Current Work. TSD PrototypeTo assess the containment failure risk due to hydrogen combustion in case of water injection during in vessel core degradation.

● The dynamic model is encapsulated in a module emulating the evolution of the process variables describing the accident progression in a single transient, as decided at the path generation module.

● The SARNET WP5.3 over-simplified model is used as an extreme case to test features independent on the physical fidelity of the scenario evolution.

● A better model has been incorporated that emulates better the features of the expected nuclear SCAIS codes without being too time consuming.

Prototype Scheme


Current Work. TSD Prototype StagesThe strategy used currently by the prototype to find the damage domain follows the stages shown in the figure.

A new and more general algorithm is being studied and tested to be incorporated to SCAIS.


Current Work. TSD Prototype ResultsThe results found by the prototype are coherent with the ones achieved during SARNET benchmark.

The table shows the sequences that lead to damage conditions and its total frequency.


Next Work. FT/ET

✔ Testing the incorporation of FT/ET at transient level

● PSA models are based on grouping features by sequences.

● ISA Methodological framework incorporates FT/ET

● TSD requires the same models to be modified at the transient level, avoiding sequence grouping.

● New methods to discriminate transients versus sequences and tool modifications to help the conversion will be investigated and tested.


WP4. On the road ...

✔ The generation of large amounts of hydrogen is a typical phenomenon associated with severe accidents in NPPs.

✔ There is no unique answer to the question how to mitigate the risk.

✔ Work is necessary to fully incorporate TSD to the system of codes SCAIS.

✔ SARNET enlarged benchmark will be the starting point to define the WP4 of IDPSA project.

✔ TSD will test the FT/ET integration at transient level. s.l.) for being applied to real cases

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WP4_Work&Future

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