22.08.2015   DESIGN FEATURES TO ACHIEVE. DEFENCE IN DEPTH IN SMALL AND. MEDIUM SIZED REACTORS

SUMMARY OF PASSIVE SAFETY DESIGN FEATURES FOR THE GT-MHR

Tables VII-5 to VII-9 below provide the designer’s response to questionnaires developed at an IAEA technical meeting Review of Passive Safety Design Options for SMRs, held in Vienna on 13-17 June 2005. These questionnaires were developed to summarize passive safety design options for different SMRs according to a common format, based on provisions of IAEA Safety Standards [VII-3] and other IAEA publications [VII-4, VII-2]. The information presented in Tables VII-5 to VII-9 provided a basis for the conclusions and recommendations of the main part of this report.

TABLE VII-5. QUESTIONNAIRE 1 — LIST OF SAFETY DESIGN FEATURES CONSIDERED FOR/ INCORPORATED INTO THE GT-MHR DESIGN

#

Safety design features

What is targeted?

1.

Helium coolant

-Reliable cooling of the reactor core without phase changes of the coolant — Chemical inertness

2.

Graphite as structural material of the reactor core

Retaining of the reactor core configuration under various mechanical, thermal, radiation, and chemical impacts

3.

Large temperature margin between the operation limit and the safe operation limit

Prevention of the progression of abnormal operation occurrences to accidents

4.1

Negative reactivity coefficient on temperature

Passive shutdown of the reactor accomplished even in

4.2

Stop of reactor core cooling by helium as a safety action

ATWS

4.3

Limited reactivity margin in reactor operation

4.4

Neutronic properties of helium prevents reactor power growth at coolant density variation

5.1

5.2

Low power density of the core

Annular reactor core with a high surface to volume ratio to facilitate core cooling

Passive decay heat removal accomplished with a long grace period

5.3

Central reflector

5.4

High heat capacity of the reactor core and the reactor internals

5.5

Heat resistant steel used for the reactor vessel and the reactor internals

#

Safety design features

What is targeted?

6.1

TRISO coated particle fuel capable of reliable operation at high temperatures and burnups

Reliable retention of fission products within a fuel particle by passive means

6.2

Safe operation limits for fuel are not exceeded in passive shutdown and aftercooling of the reactor

7.

No large diameter pipelines and no steam generator in the primary circuit

Limitation of the scope and consequences of accidents with air and water ingress

8.

Containment designed to retain helium-air fluid and to withstand external loads

Limitation of a release of fission products by passive means

TABLE VII-6. QUESTIONNAIRE 2 — LIST OF INTERNAL HAZARDS

Specific hazards that are of concern

# for a reactor line Explain how these hazards are addressed in a SMR

(high temperature gas cooled reactors)

1. Transient overpower

-Any possible changes of reactivity do not lead to an excess of the safe operation limits (high temperature margin to fuel failure; negative reactivity coefficient on temperature)

-Ingress of water to the core is limited by design features (primary circuit pressure in operation modes is higher then pressure in the SCS and PCU water circuits)

2. Loss of coolant

-Decay heat removal is accomplished by passive systems relying on radiation, conduction and convection in all reactor structures and media; loss of coolant does not lead to an excess of the design limits for design basis accidents — The activity is localized within the containment

3. Loss of heat removal

4. Loss of flow

Any possible disruptions of core cooling conditions does not lead to an excess of the safe operation limit (high temperature margin to fuel failure; negative reactivity coefficient on temperature; effective passive decay heat removal even in the event of a complete loss of coolant; primary system depressurization as a safety action)

5. Loss of external power sources

With the operation of passive safety systems (passive reactor shutdown on de­energization, passive decay heat removal), station blackout does not lead to an excess of safe operation limits

6. Exothermic chemical reactions: Air ingress to the core

Oxidation of fuel compacts is precluded by design features limiting air and water ingress to the core (the containment and a limited size of possible breaks) and by an option to restart active normal operation heat removal systems during a long process of passive decay heat removal via the RCCS (which effectively limits the time of the mode with possible oxidation of fuel compacts)

7. Violation of refuelling and fuel handling conditions

Corrective actions of normal operation systems or use of safety systems ensures that such a violation does not lead to an excess of safe operation limits

8. Combinations of hazards 1-7 for BDBA

With the operation of passive safety systems, such combinations do not lead to an excess of established radiation criteria

List of initiating events for AOO/DBA/BDBA
typical for a reactor line
(high temperature gas cooled reactors)

Design features of the GT-MHR used to
prevent progression of the initiating events
to AOO/DBA/BDBA, to control DBA,
to mitigate BDBA consequences, etc.

A. Events for abnormal operation and pre-accidental conditions

1. Events associated with changes of reactivity and power distribution

1.1 Inadvertent removal of one or several of the most effective control rods from the reactor core

1.2 Inadvertent insertion of one or several of the most effective control rods into the reactor core

1.3 Inadvertent insertion of absorbing elements from the RSS hoppers into the reactor core

1.4 Incorrect fuel assembly loading into the reactor core and then its operation

2. Events associated with failures of heat removal from the primary circuit

2.1 Complete stop of water circulation through the PCU heat exchangers

2.2 Ruptures of the PCU cooling water system pipelines within and beyond the containment

2.3 SCS failures in standby modes (stop of water circulation and ruptures of the SCS cooling water system pipelines within the containment)

3. Events associated with a decrease of coolant flow rate through the reactor core

3.1 Failures of the turbomachine or of individual turbomachine components, which require an emergency shutdown of the turbomachine

3.2 Inadvertent opening of the bypass shut-off and control valves of the turbomachine control and protection system

3.3 Increase of bypass flows in the primary coolant circulation path due to inadvertent opening of valves or due to depressurization of in-vessel components

#

List of initiating events for AOO/DBA/BDBA typical for a reactor line (high temperature gas cooled reactors)

Design features of the GT-MHR used to prevent progression of the initiating events to AOO/DBA/BDBA, to control DBA, to mitigate BDBA consequences, etc.

Initiating events specific to this particular SMR

B. Events for design basis accidents

3.5 Fuel assembly damage during refuelling

C. Events for beyond design basis accidents (taking into account additional failures)

1. Events associated with loss of power supply sources

1.1 Blackout

1.2 Blackout with a complete failure of the RCCS

-Effective reactor shutdown system (RSS)

1.3 Blackout with a failure of actuation of the reactor with spherical absorbing elements emergency protection system (ATWS)

2. Events associated with reactivity variation (taking into account additional failures)

2.1 Inadvertent withdrawal of several most effective — Passive localization of radioactivity in the

control rods from the reactor core with actuation containment

failure of the reactor emergency protection system (ATWS)

List of initiating events for AOO/DBA/BDBA
typical for a reactor line
(high temperature gas cooled reactors)

Design features of the GT-MHR used to
prevent progression of the initiating events
to AOO/DBA/BDBA, to control DBA,
to mitigate BDBA consequences, etc.

3. Events associated with a decrease of the coolant flow rate through the reactor core (taking into account additional failures)

3.1 Turbomachine failure or failure of individual turbomachine components, which require an emergency shutdown of the turbomachine, with actuation failure of the reactor emergency protection system (ATWS)

4. Events associated with primary circuit leakage (taking into account additional failures)

4.1 Primary circuit depressurization with a blackout and ingress of a considerable amount of air into the primary circuit (CPS standpipe guillotine break)

4.2 Primary circuit depressurization with actuation failure of the reactor emergency protection system (ATWS), a blackout and ingress of a considerable amount of air into the primary circuit (CPS standpipe guillotine break)

4.3 Rupture of the helium transportation pipelines and storage system beyond the containment, followed by a failure of the system for activity localization within the primary circuit, and a blackout

4.4 Inter-circuit depressurization of the primary circuit and of the PCU or SCS cooling water circuits, followed by a failure of the isolation systems, a blackout, and ingress of a considerable amount of water into the primary circuit

TABLE VII-8. QUESTIONNAIRE 4 — SAFETY DESIGN FEATURES ATTRIBUTED TO DEFENSE IN DEPTH LEVELS

#

Safety design features

Category: A-D (for passive systems only), according to IAEA-TECDOC-626 [VII-2]

Relevant DID level, according to NS-R-1 [VII-3] and INSAG-10 [VII-4]

1.

Helium coolant properties

AOO (A)

Level 1, 2

2.

TRISO coated particle fuel capable of effective operation at high temperatures and fuel burnups

AOO, DBA, BDBA (A)

Level 1, 2, 3, 4

3.

Graphite as structural material of the reactor core

DBA, BDBA (A)

Level 3, 4

4.

Large margin between operation and safety limit temperature

AOO

Level 1, 2

5.

Negative temperature reactivity coefficient

AOO, DBA, BDBA

Level 1, 2, 3, 4

6.1

Limited excess reactivity during operation

AOO, DBA, BDBA

Level 1, 2, 3, 4

6.2

Helium neutronic properties preventing reactor power growth at coolant density variation

7.

No large diameter pipelines in the primary circuit, and no steam generator

AOO, DBA, BDBA (A)

Level 1, 3, 4

8.

Stop of reactor core cooling for protective purposes

BDBA (active)

Level 4

9.

Passive decay heat removal from the reactor core accomplished in the absence of the primary helium, relying on conduction, convection, and radiation in all structures and media and assisted by passive operation of the RCCS

DBA, BDBA (B)

Level 3, 4

10.1

10.2

Low core power density

Annular reactor core with a high surface to volume ratio

DBA, Facilitate RCCS BDBA (A) operation (A)

Level 3, 4

10.3

Central reflector

10.4

High heat capacity of the reactor core and the reactor internals

10.5

Heat resistant steel used for the reactor internals and vessel

11.

Fuel safe operation limits met in the case of reactor passive shutdown and cooling

DBA

Level 3

12.

Containment designed to retain helium-air fluid and to withstand external loads

DBA, BDBA (A)

Level 3, 4

# Passive safety design features

Positive effects on economics, physical

Negative effects on economics, physical protection, etc.

protection, etc.

1. Helium coolant properties

Primary circuit and coolant costs are increased, taking into account helium volatility

2. Graphite as a structural material for the

-Facilities should be constructed to produce

reactor core

graphite of specified properties — Increase of reactor core cost — Need to dispose of large volumes of graphite

3. Low core power density

-Decrease of specific economic indices — Increase of reactor cost

4. Annular reactor core with a high surface to

volume ratio to facilitate core cooling

Increase in reactor vessel dimensions and cost

5. Central reflector

6. Heat resistant steel used for the reactor internals and the reactor vessel

-Increase in reactor cost

7. TRISO coated particle fuel capable of

reliable operation at high temperatures and burnups

-Increase in fuel cost

-Fuel production facilities need to be constructed

8. No large diameter pipelines in the primary

Decrease of

circuit and no steam generators

reactor plant cost

9. Containment designed to retain the helium — air fluid and to withstand external loads

Increase of NPP cost

REFERENCES TO ANNEX VII

[VII-1] INTERNATIONAL ATOMIC ENERGY AGENCY, Status of Innovative Small and Medium Sized Reactor Designs 2005: Reactors with Conventional Refuelling Schemes, IAEA-TECDOC-1485, IAEA, Vienna (2006). [VII-2] INTERNATIONAL ATOMIC ENERGY AGENCY, Safety Related Terms for Advanced Nuclear Plants, IAEA-TECDOC-626, IAEA, Vienna (1991).

[VII-3] INTERNATIONAL ATOMIC ENERGY AGENCY, Safety of Nuclear Power Plants: Design, IAEA Safety Standards Series No. NS-R-1, IAEA, Vienna (2000).

[VII-4] INTERNATIONAL NUCLEAR SAFETY ADVISORY GROUP, Defence in Depth in Nuclear Safety, INSAG-10, IAEA, Vienna (1996).

Annex VIII

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