ML18152A781

NRC ATF Pirt Public Meeting Slides

ML18152A781
Person / Time
Issue date:	06/12/2018
From:	Michelle Bales NRC/RES/DSA
To:
References
Download: ML18152A781 (27)

Text

Phenomena Identification and Ranking Tables for Accident Tolerant Fuel Michelle Bales Senior Reactor Systems Engineer Office of Nuclear Regulatory Research

Graphic from ATF Commission Meeting April 12, 2018 development of technical bases old paradigm development of regulatory infrastructure licensing activities phenomena ATF project plan identification PIRT maintenance and ranking exercise schedule efficiency refinement of regulatory enabled infrastructure by plan licensing activities 2

PIRTs provide basis for concept-specific licensing roadmaps 3

Assessment of ATF degradation and failure phenomenon is needed Assessment must be:

• Comprehensive

• Timely

• Independent

• Designed to support regulatory stability and predictability

• Designed to support regulatory efficiency 4

Calibrating PIRT Efforts

• PIRTS are one method to develop greater understanding

• Scope consistent with degree of departure from current state-of-practice

• Consider concept maturity and licensing schedule 5

Example: Fuel System Review

• Fuel system safety review provides assurance that:

- the fuel system is not damaged as a result of normal operation and anticipated operational occurrences (AOOs)

- fuel system damage is never so severe as to prevent control rod insertion when it is required

- the number of fuel rod failures is not underestimated for postulated accidents

- coolability is maintained during design-basis accidents

• To satisfy these objectives, acceptance criteria are needed for fuel system damage, fuel rod failure, and fuel coolability.

6

Example: Fuel System Review Continued Standard Review Plan (SRP) Section 4.2 identifies the known mechanisms for fuel system damage, fuel rod failure, and fuel coolability loss for zirconium clad uranium dioxide fuel.

Fuel System Damage Fuel Rod Failure Fuel Coolability

• Stress, strain, or loading limits

• Hydriding

• Cladding embrittlement for spacer grids, guide tubes,

• Cladding collapse

• Violent expulsion of fuel thimbles, fuel rods, control

• Overheating of the cladding

• Generalized cladding melting rods, channel boxes, and other fuel system structural members

• Overheating of the fuel pellets

• Fuel rod ballooning

• Fatigue of structural members

• Excessive fuel enthalpy

• Structural deformation mentioned above

• Pellet/cladding interaction

• Fretting wear at contact points

• Bursting

• Oxidation, hydriding and CRUD

• Mechanical fracturing buildup

• Dimensional changes and mechanical compatibility

• Rod internal gas pressure

• Worst case hydraulic loads

• Control rod reactivity and insertability 7

Example: Fuel System Review Continued Fuel System Damage Fuel Rod Failure

• Stress, strain, or loading limits for spacer grids, guide tubes, thimbles, fuel rods,

• Hydriding

• control rods, channel boxes, and other fuel system structural members

• Cladding collapse ATF designs may

• Fatigue of structural members mentioned

• Overheating of the cladding

• introduce new above

• Overheating of the fuel pellets mechanisms or result

• Fretting wear at contact points *

• Excessive fuel enthalpy in significantly

• *

• Pellet/cladding interaction*

• Oxidation , hydriding and CRUD

• Bursting (timing impacted?)

different limits for buildup

• Coating spallation with resulting hydride

• Mechanical fracturing

• existing mechanisms blister?

• Nodular corrosion?

Fuel Coolability * - mechanism is expected to be

• Dimensional changes and mechanical different for some ATF designs compatibility *

• Cladding embrittlement *

• Dissimilar metal interaction?

• Violent expulsion of fuel*

• Rod internal gas pressure *

• Generalized cladding melting

• Worst case hydraulic loads

• Fuel rod ballooning *

• Control rod reactivity and insertability

• Structural deformation

• Sump clogging source material?

8

Example: Source Term / Severe Accident Analysis

• Severe accident codes are repository of phenomenological understanding gained through NRC and international research since the TMI-2 accident

• Integrated models required for self-consistent analysis

- Accident initiation *

- Reactor coolant thermal hydraulics *

- Loss of core cooling

• ATF designs may

- Core melt progression

• introduce new

- Fission product release

• mechanisms or result

- Reactor vessel failure*

in significantly

- Transport of fission products in RCS and Containment

• different limits for

- Fission product aerosol dynamics

- Molten core/basement interactions existing mechanisms

- Containment thermal hydraulics

- Fission product removal process * - mechanism is expected to be

- Release of fission products to the environment different for some ATF designs

- Engineered safety systems - sprays, fan coolers, etc

- Iodine chemistry 9

Example: Source Term / Severe Accident Analysis

• New cladding and fuel materials may react differently with fission products

• Fission product release parameters may change

- Non-UO2 fuels may have different fission product release characteristics

• Core degradation may progress differently

- Changes in the relative melt temperature of cladding/fuel systems mean cladding could melt before fuel sinters, invalidating key assumptions about core geometry in severe accidents (for example candling)

- New cladding and fuel materials may have different eutectic interactions 10

What will NRC do with PIRT results?

Principle/General Design Criteria GDC X GDC Y GDC Z Regulations Reg A Reg B Reg C Reg D RG RG RG RG Regulatory Guidance 1 2 RG 3 RG 4 5 6 S S S S S S S R R R R R R R Standard Review Plan P P P P P P P a b c d d c d PIRTS will help identify if there are issues not contemplated or covered by the current regulatory infrastructure.

11

Pre-PIRT Activities

• NRC concept-lead would begin with pre-PIRT investigation to determine:

- if a PIRT is needed

- if there is sufficient information to start a PIRT

- to inform the objectives and scope of the PIRT

• Envisioned to be a literature review and discussion with experts on nuclear and non-nuclear applications of ATF features 12

Relationship of concept maturity to PIRT activities KL - High No need for a PIRT, high confidence that all significant phenomenon are identified and well understood KL - Medium Knowledge level Pre-PIRT There is sufficient information to engage in a PIRT. PIRT activities can help gain activities insight into new phenomenon and identify areas were more information is assess KL needed KL - Low Not enough information for a PIRT 13

PIRT Proposal

• Three types of PIRTS

1. Concept-specific PIRTS for normal operation, transients and design basis accidents in reactor

2. Severe accident PIRT that covers a wide range of ATF concepts

3. Storage and transportation PIRT that covers a wide range of ATF concepts 14

Generic ATF Severe Accident PIRT

• Obtain greater understandings of the design basis accident (DBA) source term and evaluation of whether the existing technical basis applies to ATF concepts

• Two key considerations, how concept changes:

- fission product release quantities

- melt progression and fuel/cladding interaction and thus release timing 15

Generic ATF Spent Fuel Storage and Transportation PIRT

• Key considerations, how ATF changes:

- mechanical integrity of cladding and cladding failure modes

- neutronics/criticality

- Long term storage degradation and failure modes 16

Structured Discussion 17

When should potential operational flexibility for ATF be defined?

• Is it useful to do a PIRT without this information?

• What delays might arise if we conduct a PIRT or conduct testing without knowledge of the use?

• What delays might arise if the use requires rulemaking?

18

What is the best separation and categorization for PIRTs?

• By technical issue/panel expertise needs

• Work scope/schedule drivers

• Concept maturity

• Proposal:

1. Normal and DBA performance

2. Severe accident and source term

3. Storage and transportation 19

What is the best timing and sequencing for PIRTs?

• Pre-PIRT activities

• Order of concepts

• Order of categories (i.e. DBA, SA, Storage, etc.)

• Lead time for resulting investigations 20

Who are the experts?

• What are the appropriate credentials for panel members?

- Publication/citation index

- Years of experience

• How are experts solicited?

- Public notice

• How is alignment reached on panel membership?

- Stakeholder feedback

- Public comment 21

What is the product?

• Content

- Documentation of the PIRT process

- Discussion of expert selection

- Training/orientation of experts, elicitation process, (2) scope of areas considered, (3) results of PIRT,

• Characteristics

- Must be objective database

- Significance level and knowledge level supported by references

- May have provisions for both proprietary and public information 22

Who leads the PIRT?

• Transparency is key

• Lead may differ for each PIRT

• Considerations of NRC lead

- NRC is experienced in conducting PIRTs

- NRCs schedule requirements may drive PIRT schedule

- NRC has the ability to handle proprietary information

• Considerations of non-NRC lead

- Access to information

- Potential streamlined contracting process for obtaining experts 23

Other Subjects?

24

Next Steps

• To be completed during the meeting 25 2

5

Backup 26

The PIRT Process

1. Define the issue that is driving the need for a PIRT

2. Define the specific objectives for the PIRT

3. Define the hardware and the scenario for the PIRT

4. Define the evaluation criterion

5. Identify, compile, and review the current knowledge base

6. Identify plausible phenomena, that is, PIRT elements

7. Develop importance ranking for phenomena

8. Assess knowledge level for phenomena

9. Document PIRT results 27