First Workshop on Challenges for Coolant in Fast Spectrum Systems:

Chemistry and Materials Session 4: Safety and operational aspects

G.B. Bruna

Vienna, Austria 5 - 7 July 2017

Safety topics have been widely addressed in the

Fast Reactors and Related Fuel Cycles: Next Generation Nuclear Systems for

Sustainable Development FR17

26–29 June 2017 Yekaterinburg, Russian Federation

They included:

General safety approach, Safety programs, Safety analysis, Accident prevention , CDA, PSA, Aspects of sodium leaks, ….

Two main safety related sub-topics are addressed in the present Workshop:

- Inventory control, accountancy & Qualification procedures, - Enveloping system analysis.

The coolant choice affects nuclear system architecture & performance (from Robert’s Introduction)

In the GIF choice 4 out of 6 concepts are fast reactors • Sodium Fast Reactor (SFR) • Lead cooled Fast Reactor (LFR) • Gas cooled Fast Reactor (GFR) • Molten Salt Reactor (MSR) (Fast

Version)

2017-07-14 5

Selection criteria Sustainability (fuel utilization/ transmutation/ waste

reduction Economy (long cycles, life >60y, compactness) Safety (increased safety/operational reliability /low

probability of core accidents/elimination for off-site emergency response)

Proliferation resistance What about integration in the energy mix?

Conventional PWR loop type SFR GFR LFR

The coolant activation

In NPPs the coolant activation strongly depends on the system features and the coolant nature. It can be a either a minor or a major treat to the safe and secure operation, depending on their design and characteristics. In the Metal Cooled Reactors, the coolant can be poisoned by its-own by-products of activation, such as the Sodium isotopes engendered by the neutron capture in the Sodium Cooled Fast Reactors and the Polonium in the case of the eutectic Lead-Bismuth Cooled Fast Reactors. In the Gas Cooled Fast Reactors the activation of the coolant depends both on the abrasion of the graphite structures and the presence of oxidants in the gas (mainly helium).

The coolant activation In the ADS (Accelerator Driven Systems) as well as in NPF (Neutron Production Facilities) with proton incident energies up to GeV-range, the coolant is contaminated by lots of Tritium and activated target material which are stripped-out the spallation source. This holds for both gas cooled systems with solid targets (example ESS with tungsten helium cooled target) or MYRRHA (with self-cooled liquid metal concept). The problem holds for thermal systems too. E.g., in the High Temperature (HTR) and Very High Temperature (VHTR) Reactors, it is considered introducing traces of an oxidant (water?) in the helium to control the coolant chemistry. The main purpose of these oxidants is to maintain a protective film on the surface of the metallic materials.

The coolant activation

Ahead from the activation itself, the problem of the dispersion of contaminants within the coolant from the fuel is to be accounted for. These contaminants have to be detected and cleaned-out more or less continuously depending on the operational scenarios and the design constraints (e. g. the assembly plugging in boxed sub-assembly designs). A very specific case is the MSR (Molten Salt Reactor) where the mixing of the fuel and the coolant engenders specific problems of filtering and cleaning-out of by-products of fission and capture to allow the system keep on operating efficiently with a reactivity swing close to zero. Higher efficiency is achieved for MSR fast neutron systems due to a lower impact of FP capture on reactivity swing vs. time.

Nuclear component qualification

The qualification of nuclear components is aimed at guaranteeing the safe and economical operation of electrical, electronic, electromechanical and mechanical equipment of the NPPs for postulated service conditions. From the regulatory viewpoint, the main objective of the component qualification is the identification of the measures and practices to be adopted in the fabrication, installation, operation and control to enable the NPP components to operate in a safe way for the entire duration of the plant operation. Whenever that is allowed, these practices and measures should be generalized and harmonized to allow an extended opportunity for licencing at national and international level.

Nuclear component qualification These measures have to account not only for the construction and operation aspects, but also for the decommissioning and dismantling ones, in a whole. Piping, structures and other passive NPP components which, because of their own design and operational use, cannot undergo conventional qualification procedures, merit specific attention. . Accordingly their qualification is to be achieved directly by design, construction, testing and inspection, in compliance to suitable codes.

Nuclear component qualification

The reliability and operation domain of the passive safety systems are extremely hard to define and to extrapolate, because the parameters in the testing phase can poorly & hardly feature the actual operating conditions. Among them, we mention: - the laboratory testing constraints, - the scaling factors, - the unknown side factors, - ….

• Topics addressed in the Session 4 Inventory control, accountancy & Qualification procedures sub-section papers:

– B. Babu, B.K. Nashine, P. Selveraj: Twenty Years Experience in Handling Sodium Experimental Sodium Facilities (Safety and operational experience gained over 20-Y of handling sodium facilities in the Indira Gandhi Centre for Atomic Research in Kalpakkam – India - and improvements proposed for new ones);

– W. Hering, X.Z. Jin, E. Buelis, S. Perez-Martin: Operation in the Helium cooled DEMO Fusion Power plant and Related Safety Aspects (Description of the heat transfer process and the heat transport and storage system for extraction of the plasma generated pulsed thermal power and its conversion to electricity);

– N. Kharitonova: Some aspects of Coolant Chemistry Safety regumation at Russia’s NPP with Fast Breeder Reactors (Main elements in regulation of the FBR coolant chemistry and chemistry control in Russian Federation);

– B. Annop, S. Athmalingam, S. Raghupathy, P. Puthiyavinayagam: Design Provisions for Sodium Inventory Control in FBR (Monitoring of the sodium inventory in the primary and secondary circuit as well as the decay heat removal system in pool-type sodium-cooled FBR, detection and handling of leaks).

Enveloping system analysis allows providing evidence for safety demonstration, which relies upon regulation & international standards

The safety demonstration has to show that the NNPs and any other nuclear installation - including cycle ones - are designed and operated in any circumstance, including incidental & accidental conditions, shut-down, decommissioning and dismantling, in compliance with safety rules established and shared by the national end / or international Regulators and translated into practice through application of acknowledged suitable codes, standards and practices.

Enveloping system analysis allows providing evidence for safety demonstration, which relies upon regulation & international standards.

The safety demonstration relies upon the definition of the safety case of the system, which is to cover a given set of initiators and transients included in the so called Design Basis or DBA, the remainder, or Beyond Design Basis BDB, being addressed as an overall load. Currently, according to the more and more demanding new trends and requirements in regulation (e.g., non-evacuation of neighbouring population, practical elimination of transients with early releases) the DBA is expanding more and more to practically include all internal and external initiators.

Enveloping system analysis allows providing evidence for safety demonstration, which relies upon regulation & international standards.

The safety demonstration has to rely upon both probabilistic and deterministic analyses, which - respectively - allow evaluating the contribution of the initiators to the risk and quantifying it. In view of attaining public acceptance, a credible validated safety demonstration is mandatory, which requires substantial effort for installations and reactors. That can rely only upon a sparse operational experience. An accurate, verified and validated approach to codes and standards are key elements towards a plant certification.

Enveloping system analysis allows providing evidence for safety demonstration, which relies upon regulation & international standards. The SARGEN-IV program sponsored by Euratom was carried-out in the aim at identifying fast neutron systems transient-initiators to harmonize the safety analysis and demonstration. Fast reactor systems were classified vs. their own features (including coolant properties). Asymptotic contribution to the risk originating from different initiators was investigated through a simplified risk-informed approach. The study demonstrated that, due to system features, similar imitators can lead to very different contributions to the overall risk.

Enveloping system analysis allows providing evidence for safety demonstration, which relies

upon regulation & international standards.

E.g. for Na cooled FR, among the main initiator, we mention:

- RIA initiated by inadvertent / uncontrolled CR removal, - Sodium void (gas bubble crossing the core, any other

origin), - Subassembly plugging (migrant body),

- Reactivity swing (differential dilatation effects), .... .

Enveloping system analysis allows providing evidence for safety demonstration, which relies upon regulation & international standards.

An extended evaluation and propagation of uncertainties (of systematic and epistemic nature, as well) must be performed to evaluate the compliance with safety rules and a suitable estimation of the residual risk is also to be made. The definition of the safety case is to be achieved adopting adequate computation tools and computation chains, the verification of which can be achieved through comparison with analytical solutions and benchmarking, and the validation of which should of which rely upon wide and internationally acknowledged data base resulting from comprehensive and representative mock-up experiments. Whenever possible, the safety demonstration must rely also upon operational experience, as well.

Enveloping system analysis allows providing evidence for safety demonstration, which relies upon regulation & international standards. Due to the generalized lack of experimental facilities availability and the easily understandable absence of operating experience for new systems, any evolutionary and / or advanced design must be accompanied by the launching od a wide-range multi-scope experimental program, suitable for validation. Moreover, in the design phase, comfortable margins are to be enforced on the onset parameters, adopted in the safety case.

• Topics addressed in the Session 4 Enveloping System Analysis sub-section papers: – MSR: unfortunately missing;

– B.V. Kuteev, I.V. Daniliv, A.V. Razmerov, Yu. S. Shpanski: Choice of Coolants for DEMO-FNS Fusion-Fission Hybrid Facility (selection of the coolant and materials for the facility for the main coolant-using components such as the divertor, the first wall, the active core, the breeding blankets);

– R. Augusto, P. Bricault, M. Delonca. M. Dierckx, L. Egoriti, A. Gottberg, D. Houngbo, T. Mendonca, L. Popescu, J.-P. Ramos, S. Rothe, T. Stora, S. Warren: High Power Molten Target for the Production of Radioactive Ion Beams (description of advanced project of high power targets for production of radioactive ion beams).

• But safety issues are also adressed in the poster session. E.g.: – A. Bojanowska-Czajka, E. Chajduk: Study of Processes Occuring under

Reguar Operation of Water Circulation Systems in Nuclear Power Plants with Suggested Actions Aimed at Upgrade of Nuclear Safety,

– I. Critescu: Tritium Transfert and Extraction from Water, He, and PbLi Coolants,

– E.D. Gugiu: Evaluation and Analysis of the Lead-Coolant of ALFRED Demonstrator – Challenges, Needs and Gaps.

Thank-You for your attention