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

SUMMARY OF PASSIVE SAFETY DESIGN FEATURES FOR THE 4S-LMR

Tables VIII-5 to VIII-9 below provide the designer’s response to the questionnaires developed at the 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 the provisions of IAEA Safety Standards [VIII-2] and other IAEA publications [VIII-3, VIII-7]. The information presented in Tables VIII-5 to VIII-9 provided a basis for the conclusions and recommendations in the main part of this report.

TABLE VIII-5. QUESTIONNAIRE 1 — LIST OF SAFETY DESIGN FEATURES CONSIDERED FOR/ INCORPORATED INTO THE 4S-LMR DESIGN

# Safety design features What is targeted?

1. Low linear heat rate of fuel

2. Metallic fuel with high thermal conductivity

3. Double boundaries for primary and secondary sodium

4. Secondary sodium coolant loop (intermediate heat transport system)

5. Increased reliability of sodium leakage prevention systems, achieved by the use of double wall SG tubes with detection systems for both inner and outer tubes

6. All temperature reactivity feedback coefficients are negative

7. Negative whole core sodium void reactivity

8. Effective radial expansion of the core (with negative feedback on reactivity)

9. Simple flow path of coolant in the primary loop

10. Low pressure loss in the core area

11. Electro-magnetic pump

12. Two electro-magnetic pumps in series

13. Two redundant and diverse passive auxiliary cooling systems (RVACS and IRACS or PRACS) with natural draught of environmental air acting as a heat sink

14. Two diverse passive shutdown systems with each having enough reactivity for a reactor shutdown

15. No control rods used in core; power control executed via feedwater flow rate control in the power circuit [55]

A large margin to fuel melting

Decrease of fuel centreline temperature and temperature gradients in a fuel pin

Prevention of loss of coolant

Prevent sodium-water reaction from affecting the core Prevention of sodium-water reaction

Accomplish passive shutdown and prevent accidents with core disruption

Accomplish passive shutdown and prevent DBE from progressing into severe accidents

Passive insertion of negative reactivity in transients with temperature rise; simple reactor control in load following mode

Enhance natural convection of the primary sodium coolant Enhance natural convection of the primary sodium coolant Prevent immediate pump trips due to a stuck pump shaft Prevent loss of flow or limit its consequences Assure reliable removal of decay heat

Assure reliable reactor shutdown in normal operation and in accidents

Enhanced power range of reliable reactor operation; elimination of accidents with control rod ejection; simplified reactor design and operation

Prevention of transient over-power accidents

# Specific hazards that are of concern for a reactor line

Explain how these hazards are addressed in SMR

1. Prevent unacceptable reactivity transients

-No control rods in the core, reactor power control via feedwater flow rate in the power circuit

-All negative temperature reactivity feedbacks — Negative whole core sodium worth — Prevention system of reflector insertion accident

2. Avoid loss of coolant

-Vessel pool configuration with a surrounding guard vessel — Double boundaries for primary and secondary sodium — Double wall SG tubes with detection systems for both inner and outer tubes — Because all temperature reactivity feedback coefficients are negative, coolant boiling will not occur

3. Avoid loss of heat removal

-Decay heat transport by natural circulation with diverse IRACS and RVACS using environmental air as an ultimate heat sink

-Relatively large volume of sodium in the interconnected primary and secondary coolant systems of a pool type reactor

4. Avoid loss of flow

-The flow rate of natural convection sufficient to remove decay heat, boosted by simple flow path of the primary sodium and low pressure drop in the core — Local blockage of flow pass in the core is prevented by inlet geometry of a fuel assembly, providing an axial and a radial barrier to the debris — Two primary electromagnetic pumps arranged in series

5. Avoid exothermic chemical reactions (sodium-water and sodium-air reactions)

-Secondary sodium coolant loop (intermediate heat transport system)

-Double wall SG tubes with detection systems for both inner and outer tubes — Because all temperature reactivity feedback coefficients are negative, coolant boiling and consequent high pressure generation, which may lead to a disruption of the coolant pressure boundary, will not occur

6. Prevent radiation exposure of public and plant personnel

-Low linear heat rate of fuel

-Because all temperature reactivity feedback coefficients are negative, temperature of the cladding inner surface will not increase up to eutectic temperature — Progression to core melt is prevented by the inherent and passive safety features

List of initiating events for # AOO/DBA/BDBA typical for a reactor line (sodium cooled fast reactors)

Design features of the 4S-LMR 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

1.Loss of flow

-Two primary electromagnetic pumps arranged in series with each capable of handling 05 of the nominal coolant flow rate — Passive reduction of reactor power by all negative temperature reactivity coefficients

-Heat transport by the flow rate of natural convection sufficient to remove decay heat, boosted by simple flow path of the primary sodium and low pressure drop in the core

2.Transient over-power

-All temperature reactivity feedback coefficients are negative — Whole-core sodium void reactivity is negative — No feedback control of a moveable reflector

-No control rods in the core (power control via pump flow rate in the power circuit)

-Limitation of high speed reactivity insertion by adopting electromagnetic impulsive force (EMI) as a reflector driving system — Limitation of reactivity insertion at the startup of reactor operation — High thermal conductivity of metallic fuel

-Failure in insertion of the ultimate shutdown rod

— Failure in the operation of a pre-programmed moveable reflector

3.Loss of heat sink

-Environmental air draught is used as an ultimate heat think, with two redundant and diverse passive decay heat removal systems (RVACS and IRACS) being provided

-Relatively large volume of sodium in the interconnected primary and secondary coolant systems of a pool type reactor

-Passive reduction of reactor power by all negative temperature reactivity coefficients

-Whole-core sodium void reactivity is negative

4.Local fault

-High thermal conductivity and low centreline temperature of metallic fuel

— Local blockage of flow pass in the core is prevented by inlet geometry of a fuel assembly, providing an axial and a radial barrier to debris

5.Loss of on-site power

-Gravity driven insertion of ultimate shutdown rod — Gravity driven drop of reflector parts to shut down the reactor — With a stuck moveable reflector, the reactor would operate for some time and then become subcritical because burnup reactivity loss will not be compensated by slow upward movement of the reflector — All temperature reactivity feedback coefficients are negative — Whole-core sodium void reactivity is negative — Natural convection in the primary circuit sufficient to remove decay heat

-Environmental air draught is used as an ultimate heat think, with two redundant and diverse passive decay heat removal systems (RVACS and IRACS) being provided

6.Sodium leak

— Secondary sodium coolant loop (intermediate heat transport system) — Double-wall SG tubes with detection systems for both inner and outer tubes

* The analyses performed have shown that all postulated designs basis and beyond design basis accidents can be terminated without core melting, relying only on the inherent and passive safety features of the plant [VIII-1].

#

Safety design features

Category: A-D (for passive systems only), according to IAEA-TECDOC-626 [VIII-5]

Relevant DID level, according to NS-R-1 [VIII-2] and INSAG-10 [VIII-3]

1.

Secondary sodium coolant loop (intermediate heat transport system)

A

1, 4

2.

Double wall SG tubes with (active) Na leak detection system for each wall

A

2

3.

Electromagnetic pump

B

1

4.

Two electromagnetic pumps in series

A

2

5.

Simple flow path in the primary loop

A

2, 3

6.

Low pressure loss in the core

A

2, 3

7.

Reactor vessel auxiliary cooling system (RVACS, IRACS or PRACS) with the environmental air as an ultimate heat sink

B

3, 4

8.

Two redundant and diverse passive decay heat removal systems (PRACS or IRACS and RVACS)

A

2, 3

9.

Metallic fuel (high thermal conductivity)

A

1, 3

10.

Low linear heat rate

A

1, 3

11.

Relatively large volume of sodium in the interconnected primary and secondary coolant systems of a pool type reactor

A

3, 4

12.

A whole core sodium void worth is negative

A

1, 3

13.

All temperature reactivity feedback coefficients are negative

A

1, 3

14.

Fuel assembly inlet geometry providing axial and radial barriers to the debris

A

1, 2

15.

Radial expansion of the core

B

2, 3

16.

Two redundant and diverse gravity driven reactor shutdown systems (drop of the reflector and ultimate control rod insertion)

C

1, 2, 3

17.

No feedback control of the reflector movement

A

1

18.

No control rods in the core

A

1

TABLE VIII-9. QUESTIONNAIRE 5 — POSITIVE/NEGATIVE EFFECTS OF PASSIVE SAFETY DESIGN FEATURES IN AREAS OTHER THAN SAFETY

Positive/negative effects of passive safety design features on economics, physical protection, etc. have not been investigated yet.

REFERENCES TO ANNEX VIII

[VIII-1] INTERNATIONAL ATOMIC ENERGY AGENCY, Status of Small Reactor Designs Without On-site Refuelling, IAEA-TECDOC-1536, IAEA, Vienna (2007).

[VIII-2] INTERNATIONAL ATOMIC ENERGY AGENCY, Safety of Nuclear Power Plants: Design, IAEA Safety Standards Series No. NS-R-1, IAEA, Vienna (2000).

[VIII-3] INTERNATIONAL NUCLEAR SAFETY ADVISORY GROUP, Defence in Depth in Nuclear Safety, INSAG-10, IAEA, Vienna (1996).

[VIII-4] CLINCH RIVER BREEDER REACTOR PLANT PROJECT OFFICE, Clinch River Breeder Reactor Project Preliminary Safety Report, Clinch River Breeder Reactor Plant Project, Clinch River, USA (1978).

[VIII-5] AMERICAN NATIONAL STANDARDS INSTITUTE/AMERICAN NUCLEAR SOCIETY STANDARD, Nuclear Safety Criteria for the Design of Stationary Pressurized Water Reactor Power Plants, ANSI/ANS-51.1- 1983 (1983).

[VIII-6] AMERICAN NATIONAL STANDARDS INSTITUTE/AMERICAN NUCLEAR SOCIETY STANDARD, Nuclear Safety Criteria for the Design of Stationary Boiling Water Reactor Power Plants, ANSI/ANS-52.1-1983 (1983).

[VIII-7] INTERNATIONAL ATOMIC ENERGY AGENCY, Safety Related Terms for Advanced Nuclear Plants, IAEA-TECDOC-626, IAEA, Vienna (1991).

Annex IX