safety and environment studies for a european demo power plant meeting... · safety and environment...

Safety and Environment Studies for a

European DEMO Power Plant

Neill Taylor, SAE Project Leader

Outline

• Background

• Safety and environmental potential of fusion

• Aims and objectives of Safety and Environment project for EU DEMO

• Earlier studies

• Design and licensing requirements

• Safety approach and safety functions; minimizing inventories; confinement

• Licensing – what do we know?

• Integrated Safety Analyses / Source Terms / Models & Codes

• Experiments for code and model validation; neutronics; accident analysis

• Radioactive Waste Management

• Detritiation of solid waste

• Interaction with other Work Packages in the EU DEMO project

Research Units participating in the Safety and Environment project:

Neill Taylor | EU DEMO safety | 1st IAEA Technical meeting on Safety, Design and Technology of Fusion Power Plants | 3-5 May 2016 |

Potential for excellent safety performance of

fusion

• No climate-changing emissions

• Power excursions self-limited by

inherent processes

• Products of fusion reaction are not

radioactive

• Structures are activated by

neutrons but:

• Low power density (“decay

heat”) after termination of burn

• rapid decay of radiotoxicity

• No fissile or fertile material, no

actinides or fission-products

• Radioactive inventory

• Tritium

• Will require licensing like

any other nuclear facility

However…

A Demonstration Power

Plant should demonstrate

that these characteristics

lead to excellent Safety and

Environmental

performance.


EUROfusion Safety and Environment (SAE)

project: The aims

• To ensure that design choices take into account safety considerations

from the beginning

elaborate safety requirements

optimize safety provisions – iterative process with designers

define safety classification of systems, structures and components

• To ensure that DEMO will be licensable

understand the likely regulatory regime

• To resolve outstanding issues in safety and environment

perform R&D to resolve issues

develop and validate safety models and codes for DEMO

preliminary safety analyses, including accident consequences

• To minimize environmental impact of fusion

develop radioactive waste management techniques

identify and minimize contributions to routine releases


Eurofusion SAE Project Objectives (1)

1. To ensure that the safety approach for DEMO is well-founded

and takes maximum benefit from earlier work;

2. To ensure that the safety requirements for DEMO are soundly

specified and well understood, and that the design properly

takes into account these requirements and includes all

necessary safety provisions;

3. To ensure that design choices are made with due regard to

safety and environmental factors, so as to optimize safety

performance and to minimize the environmental impact;

4. To facilitate the eventual licensing of DEMO by understanding

the likely regulatory regime and discerning any requirements

that arise;


Eurofusion SAE Project Objectives (2)

5. To identify outstanding issues in safety and environment areas, and

to plan and perform R&D to resolve these issues;

6. To develop and validate safety models and codes needed for safety

analyses of DEMO, and to perform preliminary safety analyses,

including the evaluation of the consequences of a set of

representative accident scenarios;

7. To develop techniques to reduce the impact of radioactive waste

from fusion plant, in particular through the development of methods

for the detritiation of tritium-contaminated components and by

establishing the practical feasibility of methods for the recycling of

activated materials;

8. To minimize the environmental impact of the operation of DEMO by

identifying contributions to radioactive gaseous and liquid effluent

and proposing strategies to limit these releases.


Background – Previous studies

Earlier work that helps to support EU DEMO safety studies:

• Safety and Environmental Assessment of Fusion Power (SEAFP) • SEAFP 1992 – 95 • SEAFP2 1996 – 98 • SEAFP99 1999 • SEAL 2000 • All summarised in SEIF report (2001)

• Power Plant Conceptual Study (PPCS), 2001 – 05

• ITER Safety and Licensing • NSSR, 1996; NSSR2, 1998 • GSSR, 2001, 2004 • DOS, 2002 • RPrS, 2008, 2010, 2011


PPCS bounding accident analysis

Three areas of work in SAE project

1. Design and Licensing Requirements

2. Integrated Safety Analyses / Source Terms /

Models & Codes

3. Radioactive Waste Management


SAE Main Activities (1)

Design and Licensing Requirements

• Establish the safety approach and fundamental safety strategies (such as the confinement strategy) General Safety Principles

• Safety requirements are drafted and elaborated as the design concepts are developed Plant Safety Requirements Document

• Safety criteria are to be set and the safety impact of fundamental design choices (materials, coolant, etc.) are to be assessed.

• A review of the possible licensing regimes for DEMO is to be carried out, and implications for safety requirements determined.


Safety Approach for DEMO

• To protect workers, the public and the environment from harm;

• To ensure in normal operation that exposure to hazards within the

facility and due to release of hazardous material from the facility is

controlled, kept below prescribed limits and minimized to be as low as

reasonably achievable;

• To ensure that the likelihood of accidents is minimized and that their

consequences are bounded;

• To ensure that the consequences of more frequent incidents, if any,

are minor;

• To apply a safety approach that limits the hazards from accidents such

that in any event there is no need for public evacuation on technical

grounds;

• To minimize radioactive waste hazards and volumes and ensure that

they are as low as reasonably achievable.

ALARA Defence in depth Passive safety

Top-level safety objectives

Employ established safety principles


• To protect workers, the public and the environment from harm;

• To ensure in normal operation that exposure to hazards within the

facility and due to release of hazardous material from the facility is

controlled, kept below prescribed limits and minimized to be as low as

reasonably achievable;

• To ensure that the likelihood of accidents is minimized and that their

consequences are bounded;

• To ensure that the consequences of more frequent incidents, if any,

are minor;

• To apply a safety approach that limits the hazards from accidents such

that in any event there is no need for public evacuation on

technical grounds;

• To minimize radioactive waste hazards and volumes and ensure that

they are as low as reasonably achievable.

Main safety function is confinement of radioactivity,

achieved by multiple layers of protection:

Prevention of accident progression,

mitigation of consequences

Control of accidents within design basis

Control of abnormal operation and

detection of failures

“Defence in Depth” approach to safety

Prevention of abnormal

operation and failures

Fifth level: Mitigation of consequences of significant releases of radioactive material

Off-site emergency response (e.g. evacuation) – should not be necessary for fusion

• Natural shutdown

• Small inventories

• Conservative design

• High quality

construction

• Multiple barriers

(inherent in design)

• Extensive monitoring

• Redundant and

diverse safety

systems

• Safety systems

• Use of passive

means wherever

possible

• Multiple barriers

• Filtering and

detritiation systems


Adopted accident dose limits for DEMO

Anticipated

events /

Incidents

Unlikely

events

Extremely

unlikely events

Hypothetical

events

Accident

Frequency

/year

f > 10-2 10-2 > f > 10-4 10-4 > f > 10-6 f < 10-6

On-site Dose

5mSv/year

20mSv/event

Off-site Early

Dose 10mSv/event 50mSv/event

Off-site

Chronic

Dose

1mSv/year 5mSv/event 50mSv/event

No cliff-edge

effects.

Countermeasures

limited in time and

space.


Adopted normal operation dose limits

DEMO Dose

Design Target

DEMO Dose

Limit

Normal Operations

Off-Site Dose

(mSv/year)

0.1 1

Normal Operations

On-Site Dose

(mSv/year)

5 50

On-Site Dose

(mSv/5 years) 100

Meeting limits is not sufficient: all doses must be As Low As Reasonably Achievable


Limits are based on international guidelines, may be revised (downwards) later.

Safety Functions defined for European DEMO

• Confinement of radioactive and hazardous materials

• Limitation of exposure to ionizing and electromagnetic radiation

• Limitation of the non-radiological consequences of conventional

hazards

• Limitation of environmental legacy


Fundamental

safety

functions

Supporting

functions

Functions in support of confinement:

• Control of plasma energy

• Control of thermal energy

• Control of confinement pressure

• Control of chemical energy

• Control of magnetic energy

• Control of coolant energy

Functions to support personnel and the environmental protection:

• Limitation of radioactive and toxic material exposure to workers

• Limitation of airborne and liquid operating releases to the environment

• Limitation of electromagnetic field exposure to workers

• Limitation of other industrial hazards

Supporting functions to limit environmental legacy:

• Limitation of waste volume and hazard level

• Facilitation of clean-up and the removal of components

Location of radioactive material inventories

tritium

• in fuel cycle equipment (fuelling, pumping, processing)

• in breeder blankets and T extraction system

• retained in the vacuum vessel

adsorbed on surfaces

permeated into the structure of in-vessel components (IVCs)

absorbed in dust

• in RM equipment used to remove and transport IVCs

• in storage of IVCs awaiting maintenance or disposal

• in the Active Maintenance Facility

• in coolants, due to permeation

• in atmospheres of rooms containing contamination

products of neutron activation

• structure of plasma-facing components

• in-vessel dust from plasma-facing surface erosion

• activated corrosion products (ACP) in water or

lead-lithium coolant

• vessel and ex-vessel components (at lower level)


Minimizing inventories

In-vessel inventory limits for ITER

• Tritium: 1 kg

• Dust: 1000 kg

Can we reduce these for DEMO?


Tritium Higher throughput but: • No cryopumps (accounts for 180g

of inventory in ITER) • No plasma-facing Be, no Be dust.

W may have lower T retention. • Higher operating temperature

(>500°C compared with 140°C) • Reduced uncertainties?

Dust Higher fusion power, higher duty cycle but: • W instead of Be as plasma-facing

surface Lower erosion rate?

• Different plasma edge conditions? • Reduced uncertainties?

Confinement: in-vessel inventory

Confinement strategy:

• Two confinement systems

• each with one or more static

barriers and/or dynamic

systems


Tritium

(on plasma-facing surfaces,

permeated into components,

and in dust)

Active dust

(tungsten eroded from

plasma-facing surfaces)

Activated corrosion

products

(in accidents with in-vessel

loss of water coolant)

First confinement system

Vacuum vessel and its

extensions

Second confinement

system

Building walls and slabs

surrounding tokamak, rooms

served by ventilation with

filtering and detritiation

systems.

Other boundaries (e.g.

cryostat)

ITER experience: vacuum vessel and extensions

as first confinement


Penetrations

• neutral beams

• cooling pipes

• RF heating systems waveguides

• diagnostics systems

• vacuum pumping lines

• fuelling systems

• feeders for in-vessel coils

Confinement barrier includes

• seals

• bellows

• windows (including non-metallic)

• isolation valves

• pipes, ducts, waveguides

All must remain leak-tight in all

normal and accident situations, and

all are Safety Importance Class

ITER Vacuum Vessel:

• Robust, double-walled.

• Design loads include electromagnetic

loads in plasma events such as Vertical

Displacement Events

had to show that these loads are

enveloping

• Design pressure limit must be observed

pressure limited by relief system

with rupture discs

• Subject to nuclear pressure equipment

regulation (ESPN)

Proposed EU-DEMO confinement concept (HCPB)


In-vessel components of future Fusion Power

Plant

• High availability will be essential

• interruptions to electricity generation

unacceptable

• High reliability required of all components

• In-vessel components must not fail

• May be possible to give them full safety

credit for the confinement function

• This would simplify part of the

confinement strategy

• Can’t do this for DEMO.

But how far can we go?

Assessments and discussions are

ongoing.


Licensing of a future nuclear fusion facility

• ITER is licensed in France as a basic nuclear installation

(Installation Nucléaire de Base, INB)

under same law as all other nuclear facilities

• In other countries, and for a plant on the scale of DEMO, new

legislation may have to be created

An important regulatory principle:

Regulations should be

targeted

proportionate


Future trends in nuclear regulation

• Regulators are reluctant to voice

opinions until they have a firm proposal

in front of them

• How will nuclear regulation develop?

• In Europe, efforts towards harmonization

of regulatory approaches in different

countries, through the Western

European Nuclear Regulators

Association (WENRA).

• Although focussed on fission plant,

adaptation of approach to fusion is

possible

• WENRA emphasizes Defence in Depth

and independence of levels


Future trends in nuclear regulation

Changes may occur in reaction to unforeseen events

• WENRA specified “stress tests” applied to all European

nuclear plant after Fukushima accident

• In reaction to Fukushima, more emphasis on protection

against combinations of external aggressions

• Additional safety analysis of “design extension conditions”

featuring multiple independent failures

• focussed on conditions that could cause core melt in a

fission reactor, but still could apply to fusion facilities

• pay attention to common cause and common mode

failures


Nuclear regulation – what will not change

• Need to provide and defend a safety case that demonstrates

• acceptable safety objectives have been set and are achieved

• impact on public safety is minimized

• impact on personnel safety is minimized

• environmental impact is minimize

• Demonstration must be

• fully justified

• fully comprehensive (all conceivable

accident scenario are covered)

• where based on computer models,

that these are fully verified and

validated



Integrated Safety Analyses / Source Terms / Models & Codes

• Determine accident scenarios to be taken into account in the safety analyses, using Functional Failure Modes and Effects Analysis (FFMEA)

• Determine needs for code development for safety analyses and the validation experiments that are required for these

• Develop safety analysis tools, codes and models

• Perform tests as needed to validate the codes and models

• Perform full safety analyses including transient and accident analyses for Design Basis, Design Extension and selected Beyond Design Basis Events

• Assess the needs for source term development, dependent on fundamental design choices

• Perform R&D needed to improve quantification of source terms, evaluate inventories (e.g. ACPs)

• Assess environmental releases (liquid and gaseous) in normal operation, develop strategies for minimizing these

• Identify major contributions to Occupational Radiation Exposure, develop strategies for minimizing these, particularly in design choices

• Development of methods for computation of Shutdown Gamma Dose Rate, with the aim of establishing one common EU approach


Examples of experiments for code validation

• Simulations of LOFA and LOCA in blankets (KIT)

• Measurements of tritium permeation from beryllium pebbles and structural materials

(KIT)

• Measurements of tritium transport in ceramic breeder blanket (KIT)

• Chemical reactivity of Be with steam and air (ENEA)

• Measurement of liquid lithium-lead/steam reaction rates (ENEA)


27

T-release from pebbles after thermal loading

400 600 800 1000 1200

500

1000

1500

2000

2500

3000

3500

Rele

ase R

ate

(B

q/s

/g)

Temperature (K)

>100 mkm

30-60 mkm

10-30 mkm

IG

NGK 1mm

15000 20000 25000 30000

Time, s

400

600

800

1000

1200

1400

0 5000 10000 15000 20000 25000 30000

Time, s

Tem

pera

ture

(K

)

5K/min

7K/min

IG

>100µm

NGK

1mm

Bochvar

10-30 µm

Bochvar

30-60 µm

Bochvar

> 100 µm

Neutronics and activation analysis in support of

accident modelling

• Example: decay heat for DEMO based on HCPB blanket


HCPB Entire reactor

Cooling Time

1 s 1 h 1 day 1 week 1 month 1 year 10 years 100 years

Name of Zone length of zone

material fraction (*)

volume [cm3]

Nuclear heating

Decay heat

mm % MW/m3 MW/m3

First Wall (FW)

W 100% 2.45E+06 - 5.33E-01 4.03E-01 2.12E-01 1.39E-02 8.76E-03 5.00E-04 1.11E-08 2.33E-10

Eurofer 74.3% Eurofer + void

2.93E+07 - 2.00E-01 1.43E-01 5.47E-03 1.64E-03 1.35E-03 2.70E-04 1.46E-06 1.80E-10

Breeder module (BM)

- - - - - - - - - -

BM caps and lateral walls

74.3% Eurofer + void

4.66E+07 - 3.06E-02 2.25E-02 2.32E-03 5.60E-04 4.57E-04 7.24E-05 4.48E-07 5.60E-11

BM material mxiture

11.76% Eurofer + 37.9% Be +13.04% Li4SiO4 + void

7.65E+08 - 1.62E-02 4.27E-03 2.02E-04 4.87E-05 3.84E-05 8.14E-06 1.80E-07 6.87E-10

BM backwall 100%+ Eurofer

4.36E+07 - 3.08E-03 2.44E-03 6.14E-04 1.25E-04 1.01E-04 1.39E-05 9.84E-08 1.07E-11

BM back support /manifold

55.4% Eurofer + void

3.11E+08 - 7.97E-04 6.60E-04 2.28E-04 4.20E-05 3.37E-05 4.44E-06 2.89E-08 2.84E-12

Sum [MW] - - 2.13E+01 9.79E+00 1.04E+00 1.64E-01 1.26E-01 2.07E-02 2.15E-04 5.36E-07

Accident analyses

Initiating events

identification

Accident scenarios

Selection of representative

events

Modelling of accident

sequences


Predicted releases

Dispersion and dose modelling

Dose to Most Exposed

Individual

Must be comprehensive Use systematic techniques

e.g. FMEA, HAZOP

Postulate additional failures

Event trees, fault trees

Choose events with consequences that

will envelope others

Model all significant phenomena.

e.g. thermal-hydraulic

Calculate maximum environmental release

in worst case

Direct exposure plus ingestion and inhalation. All pathways considered.

Dose uptake for conservative

exposure scenario

Design information Failure rate data

Safety design information

Neutronics/activation data (source terms, decay heat etc.)

More detailed design info

Site characteristics Weather conditions

Consequences

Radioactive Waste Management

• A review of clearance indices for radioactive material to set an approach to

defining a fusion-specific set of limits, and to define these limits.

• A feasibility study of waste recycling to establish if viable and economic

recycling processes are possible; criteria to be defined

• Development of technologies for large-scale recycling

• Techniques for detritiation of solid waste have been reviewed

• A programme of R&D to develop techniques for the detritiation of solid

radioactive waste is starting

• Materials composition limits will be

established to minimize the

radiological impact of activation and

strategies developed to minimize

the quantity of waste. Simple recycle

36%

Cleared

49%

Hands-on

5%

Complex

recycle

10%

Permanent

waste

0%



PPCS results

H2020 Progress meeting 18 June 2015 31

Extract from Detritiation techniques review

Disposal ( 1 possibly suitable, 2 strong suitability):

Method In

vessel

IVC

transfer

area

Storage

facility

IVC

process

cells

Waste

and

recycling

RH

equipment

maintenance

Other

waste

treatment

3H plant

clean-up

Melting (3) 1 2 2 2 2

Thermal

treatment (7)

1 2 2 1 2 2

Cold crucible 1 1

Molten salt

oxidation

1 1

Interim

storage

1 2 2 1

Surface

abrasion

1 2

Evaporating

and

solidification

line

1 1

Incineration 2

Safety Requirements Document Safety Guidelines

Safety Importance Classification

Radioactive Waste Management

Materials composition limits

Work Packages concerned with design and materials

safety functions and required

safety provisions design optimisations

Safety analyses Accident analyses

Occupational Radiation Exposure studies

Study of effluents in normal operation


Interactions with other DEMO Work Packages – every WP has a Safety Liaison Officer

Summary

• The Safety and Environment project for EU DEMO is focussed on

• Setting a safety approach and developing requirements in cooperation

with the design teams

• Developing and validating safety models and codes, and applying these

to preliminary safety analyses

• Finding solutions to some key radwaste management issues

• Licensing requirements of the future are uncertain but

• Harmonisation of European regulatory approach is useful

• Defence in Depth is key

• Safety functions for DEMO have been defined

• Confinement of radioactive inventories is the most important

• Every opportunity must be taken to minimize inventories

• Safety considerations must be central to design activities from the beginning

• Dialogue is maintained between safety specialists and design teams


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