# Specific hazards that are of concern for a reactor line

Explain how these hazards are addressed in a SMR

1 Prevent unacceptable reactivity transients

—Low burnup reactivity swing over long core lifetime/refuelling interval reduces the need for reactivity investment in control rods —Large inherent reactivity feedbacks of a fast spectrum core provide negative reactivity contribution upon rise in coolant and fuel temperatures, compensating positive reactivity insertion, reducing reactivity to zero, and stabilizing power and system temperatures

2 Avoid loss of coolant

—Vessel pool configuration with surrounding guard vessel

—Ambient pressure Pb coolant with high boiling temperature (1740°C) eliminates flashing of primary coolant

3 Assure heat removal from core

—Natural circulation heat transport with ambient pressure single phase Pb coolant to remove core power

—Provision of natural circulation driven air cooling of guard vessel enables removal of reactor power at decay heat levels in the event of loss of heat removal through the in-vessel heat exchangers

4 Avoid loss of flow

—Natural circulation heat transport at power level > 100% of the nominal. —Open lattice core configuration prevents flow blockage

5 Avoid overcooling of reactor system

To be defined

6 Avoid combustible gas generation or exothermic chemical reactions

—Pb primary coolant and CO2 working fluid do not react chemically —Pb coolant does not react vigorously with air or water/steam

7 Prevent consequences of in-vessel heat exchanger tube rupture

—High inertia/density of Pb coolant retards transient bubble/void growth during blowdown of CO2 working fluid into the coolant; formation of small bubbles that could be transported to core region does not occur —Escape path for gas/void to pool free surface, provided by design, avoids potential for transport of void to the core

—Passive pressure relief from primary coolant system precludes over­pressurization of coolant pressure boundary

8 Maintain integrity of fuel pin cladding

Heat removal from the core by single phase natural circulation and large reactivity feedbacks of fast spectrum core limit system temperatures during operational transients and postulated accidents to values well below those at which cladding strength is significantly reduced or nitride fuel decomposition occurs

9 Maintain coolant pressure boundary

—Heat removal from core by single phase natural circulation —Large reactivity feedbacks of a fast spectrum core, and emergency decay heat removal by vessel air cooling of the guard vessel limit system temperatures during postulated accidents to values well below those at which vessel steel strength is significantly reduced

—Passive pressure relief from primary coolant system precludes over­pressurization of coolant pressure boundary

10 Limit radiation exposure to public and plant personnel

—Progression to core melt is deterministically eliminated by passive safety features —Containment consisting of guard vessel and upper closure head is provided for defence in depth

—Additional containment structure provides additional mitigation of radioactivity release

#

Safety design features

Category: A-D (for passive systems only), according to IAEA-TECDOC-626 [IX-6]

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

1

Selection of Pb as a coolant

A, B

1,3

2

Selection of transuranic nitride as a fuel

A

1,3

3

Natural circulation heat transport

B

1,3

4

Vessel pool configuration with surrounding guard vessel

A

1,3,4

5

Open lattice core configuration

A

1

6

Large reactivity feedbacks from fast spectrum core enabling passive load following and passive shutdown

A

1,3

7

Low burnup reactivity swing over long core lifetime/ refuelling interval, reducing reactivity investment in each control rod

A

1

8

Vessel air cooling by natural circulation

B

3

9

Escape path for gas/void to reach free surface, provided by design

A

3

10

Passive pressure relief from primary coolant system

C

3

11

Supercritical carbon dioxide Brayton cycle energy conversion — CO2 working fluid does not react chemically with Pb primary coolant

A

1

12

Containment

A

3, 4

Passive safety design features

Positive effects on economics, physical protection, etc.

Negative effects on economics, physical protection, etc.

Pb coolant

Lack of chemical interaction with working fluid enables elimination of intermediate heat transport circuit reducing capital and operating costs

-Weight resulting from high Pb density may require greater vessel thicknesses, increasing capital costs — Coolant chemistry control/filtering systems needed to prevent corrosion/corrosion effects contribute to increased cost

Transuranic nitride fuel

-Transuranics are self-protective in safeguards sense

-Transuranic nitride fuel together with fast spectrum core and closed fuel cycle reduces fuel costs

Natural circulation heat transport

Natural circulation cooling, enabled by Pb coolant properties, eliminates main coolant pumps, contributing to reduced plant cost

Need for height separation of thermal centres between heat exchangers and core may require taller reactor and guard vessels, increasing capital costs

Large reactivity feedbacks from fast spectrum core enabling passive load following and passive shutdown

Enhances reliability and reduces operator requirements potentially reducing operating costs

Low burnup reactivity swing over long core lifetime/ refuelling interval, reducing reactivity investment in each control rod

Core is fissile self-sufficient with conversion ratio near unity such that the spent core can be reprocessed to further utilize its energy content, influencing positively upon fuel economics

Escape path for gas/void to reach free surface in primary coolant system, provided by design

Requires slightly greater reactor and guard vessel diameters, increasing capital costs

Supercritical carbon dioxide Brayton cycle energy conversion; CO2 working fluid does not react chemically with Pb primary coolant

-Lack of chemical reaction between primary Pb and CO2 working fluids enables elimination of intermediate coolant circuit, reducing capital and operating costs — Use of supercritical carbon dioxide Brayton cycle with smaller turbo-machinery components than Rankine saturated steam cycle reduces plant capital and operating costs

-Research and development costs will be required for supercritical CO2 Brayton cycle

-Need to contain CO2 with potential activity entrained from Pb coolant released from the reactor system following in-vessel heat exchanger tube rupture impacts upon containment requirements, potentially increasing the containment building costs — Need to preclude radiolytic decomposition of CO2 may require additional shielding of in-vessel Pb to CO2 heat exchangers, potentially increasing reactor system costs