The Small Secure Transportable Autonomous Reactor (SSTAR, [IX-1]) is a 20 MW(e) (45 MW(th)) exportable, small, proliferation resistant, fissile self-sufficient, autonomous load following, and passively safe lead cooled fast reactor (LFR) concept for international deployment and deployment at remote sites. Potential users for the SSTAR include customers looking for energy security with small capital outlay; cities in developing countries, and deregulated power producers in developed countries. SSTAR makes extensive use of inherent and passive safety features, most notably, natural circulation heat transport, lead (Pb) coolant, and transuranic nitride fuel. The SSTAR nuclear power plant incorporates a supercritical carbon dioxide (S-CO2) Brayton cycle power converter for higher plant efficiency and lower balance of plant costs. The efficiency of the S-CO2 Brayton cycle increases as the reactor core outlet temperature increases; an efficiency of about 45% can be attained for a turbine inlet temperature of about 550°C. To take advantage of the economic benefits of such high plant efficiency, there has been interest in operating at higher Pb coolant temperatures. In particular, a peak cladding inner surface temperature of 650°C has been an objective. SSTAR is scalable to a higher power level of 181 MW(e) (400 MW(th)); this is the STAR-LM concept discussed in section IX-2. SSTAR is currently at a pre­conceptual level of development. The engineering design for manufacturing the components and systems has not yet been carried out. A probabilistic risk assessment has not been performed. Accident analyses of a set of design basis and beyond design basis accidents have not yet been carried out.

Figure IX-1 illustrates SSTAR, which is a pool type reactor. Lead coolant is contained inside a reactor vessel surrounded by a guard vessel. Lead is chosen as the coolant rather than lead-bismuth eutectic (LBE) to reduce the amount of alpha-emitting 210Po isotope formed in the coolant by two to three orders of magnitude relative to LBE, and to eliminate dependency upon bismuth, which might be a limited resource.

The Pb coolant flows through a perforated flow distributor head located beneath the core; this structure provides an essentially uniform pressure boundary condition at the inlet to the core. The Pb flows upward through the core and through a chimney above the core formed by a cylindrical shroud. SSTAR is a natural circulation reactor such that the vessel has a height to diameter ratio large enough to facilitate natural circulation heat removal at all power levels up to and exceeding 100% of the nominal. The coolant flows through flow openings near the top of the shroud and enters four modular Pb to CO2 heat exchangers located in the annulus between the reactor vessel and the cylindrical shroud. Inside each heat exchanger, the Pb flows downwards over the exterior of tubes through which the CO2 flows upwards. The CO2 enters each heat exchanger through a top entry nozzle, which delivers the CO2 to a lower plenum region in which the CO2 enters each of the vertical tubes. The CO2 is collected in an upper plenum and exits the heat exchanger through two smaller top diameter top entry nozzles. The Pb exits the heat exchangers and flows downward through the annular downcomer to enter the flow openings in the flow distributor head beneath the core.

A thermal baffle is provided near the Pb free surface. The baffle consists of a cylindrical shell welded to the reactor vessel and filled with argon cover gas providing thermal insulation to the reactor vessel. The insulating effect of the shroud is necessary to protect the vessel from thermal stresses that would result from exposure to

FIG. IX-1. General view of the SSTAR layout.

the heated Pb coolant during startup and shutdown transients. SSTAR does not incorporate an intermediate heat transport circuit. This is a simplification possible with Pb coolant which is calculated not to react chemically with working fluid below about 250°C (i. e., well below the 327°C Pb melting temperature). A passive pressure relief system is provided on the reactor system to vent CO2 from the reactor, in the event of a heat exchanger tube rupture.

STAINLESS STEEL PINS OF RADIAL REFLECTOR (SST AND Pb)

TWO INDEPENDENT GROUPS OF CONTROL RODS

LOW ENRICHMENT CENTRAL REGION (TWO ENRICHMENT ZONES)

DRIVER (THREE ENRICHMENT ZONES)

Figure IX-2 shows the 30-year lifetime core configuration. The core has an open lattice configuration of large diameter (2.5 cm) fuel pins arranged on a triangular pitch. This eliminates potential flow blockage accidents since crossflow paths are always available for cooling. The fuel consists of pellets of transuranic nitride fuel clad with a silicon enhanced ferritic/martensitic steel layer, providing protection against corrosion, co­extruded with a ferritic/martensitic base providing structural strength and irradiation stability. The fuel pellets are bonded to the cladding by molten Pb to reduce the temperature difference between the pellet outer surface and the cladding inner surface.

An active core diameter of 1.22 m is selected to minimize burnup reactivity swing over the 30-year core lifetime. The power level of 45 MW(th) is conservatively chosen to limit the peak fluence on the cladding to 4 x 1023 neutrons/cm2; this is the maximum exposure for which HT9 ferritic/martensitic cladding has been irradiated. The core has three enrichment zones to reduce power peaking and two central low enrichment zones which further reduce burnup reactivity swing. The core has strong reactivity feedback coefficients, which enable autonomous load following, whereby the reactor power adjusts itself to heat removal from the reactor as a result of reactivity feedbacks. Because heat transport is accomplished by natural circulation, the primary coolant flow rate and system temperatures also adjust themselves to transport heat from the core.

The core does not consist of individual removable fuel assemblies but is a single cassette/assembly. The fuel pins are permanently attached by welding or other means to a core support plate at the bottom of the core. This limits access to either fuel or neutrons. Normally, refuelling equipment is not present at the site. Refuelling equipment, including a crawler crane, is brought onsite only following the 30-year lifetime. The upper closure head for the guard and reactor vessels is removed, the spent core is removed from the vessel and placed inside of a shipping cask; it is then transported to a fuel cycle support centre for reprocessing and refabrication under international oversight. A fresh core is installed in the reactor vessel and the refuelling equipment is removed from the site.

Two sets of control rods are provided for independence and redundancy of the scram. Small adjustments of the control rods are carried out to compensate for small changes in the burnup reactivity swing. The control rod locations have been uniformly distributed throughout the core. Each control rod moves inside of a control rod guide tube occupying a position in the triangular lattice. Spacing between fuel pins is maintained by two levels of grid spacers. Each grid spacer is welded to a control rod guide tube; the grid spacer holds the surrounding fuel pins by means of spring clips allowing for thermal expansion of the fuel pins relative to the control rod guide tube. The active core is surrounded by a radial reflector, which is an annular ‘box’ containing stainless steel rods and Pb having approximately equal volume proportions. Stainless steel is needed to shield the reactor vessel from neutron fluxes. There is a small Pb flow through the reflector removing the power deposition that takes place there.

SSTAR incorporates a reactor vessel auxiliary cooling system (RVACS) for decay heat removal, should the normal heat removal path involving Pb to CO2 heat exchangers be unavailable. The RVACS involves heat removal from outside of the guard vessel due to natural circulation of air, which is always in effect. The RVACS is a safety grade system. To provide for greater reliability of emergency heat removal beyond that corresponding to the single RVACS system, it is planned to also incorporate safety grade direct reactor auxiliary cooling system (DRACS) heat exchangers into the reactor vessel.

Conditions, dimensions, and other parameters for SSTAR are included in Table IX-1. Notable achievements of the SSTAR development include:

Pb coolant;

30-year core lifetime;

Average (peak) discharge burnup of 81 (131) MW day/kg of heavy metal;

Burnup reactivity swing < 1 $;

Peak cladding temperature = 650°C;

Core outlet/inlet temperatures = 564/420°C;

Peak transuranic nitride fuel temperature = 882°C;

Small shippable reactor vessel (12 m height by 3.23 m diameter);

Autonomous load following;

Supercritical CO2 Brayton cycle energy conversion efficiency = 44.1%;

Plant efficiency = 43.8%;

Cost of energy generation < 5.5 US$ cents/kWh (55 US$/MWh).

Reactor name Power, MW(e) (MW(th))

Customer — Assume 4.0 tonnes of oil equivalent per capita per year = 167 GJ per capita per year = 5.3 KW(th)-year per capita per year, of which ~ 1/3 is used for electricity

Coolant

Fuel

Enrichment, %

Core lifetime, years

Core inlet/outlet temperatures, °C

Coolant flow rate, kg/s

Power density, W/cm3

Average (peak) discharge burnup,

MW day/Kg HM

Peak fuel temperature, °C

Cladding

Peak cladding temperature, °C

Fuel/coolant volume fractions

Core lifetime, years

Fuel pin diameter, cm

Fuel pin triangular pitch to diameter ratio

Active core dimensions; Height/Diameter, m

Core hydraulic diameter, cm

Pb to CO2 heat exchangers (HXs) type

Number of Pb to CO2 HXs

HX tube length, m

HX tube inner/outer diameters, cm

Number of tubes (all HXs)

HX tube pitch to diameter ratio

HX Pb hydraulic diameter, cm

HX-core thermal centres separation height, m

Reactor vessel dimensions; Height/Diameter, m

Reactor vessel thickness, cm

Gap between reactor vessel and guard vessel, cm

Gap filling material

Guard vessel thickness, cm

Air channel thickness, cm

Air ambient temperature, °C

Working fluid

CO2 turbine inlet temperature, °C Minimum CO2 temperature in cycle, °C Max./Min. CO2 pressure in cycle, MPa CO2 flow rate, kg/s Net generator output, MW(e)

Supercritical CO2 Brayton cycle efficiency, %

Net plant efficiency, %

19.7 (45)

Electricity for a town of ~ 25 400 Pb

Transuranic nitride (TRUN) enriched to N15 1.7/3.5/17.2/19.0/20.7 TRU/HM, 5 radial zones 30

420/564

2150

42

81 (131)

882

Si-enhanced ferritic/martensitic steel layer for corrosion protection co-extruded with a ferritic/martensitic substrate for structural strength and irradiation stability 650

0.45 / 0.35

30

2.50

1.185

0.976/1.22

1.371

Shell and tube 4

4.0

1.0 / 1.4 10 688 1.255 1.030 6.80

12.0 / 3.23

5.08

12.7 Air

5.8 15 36

Supercritical CO2

549

31.25

20/7.4

247

19.7

44.1

43.8

Characteristic/reactivity coefficient	BOC	Part of the cycle ~ 13 years	EOC
Delayed neutron fraction	0.0036	0.0035	0.0034
Prompt neutron lifetime, s	1.8 x 10 -07	1.8 x 10-07	1.8 x 10-07
Coolant density, cents/°C	-0.035	-0.001	-0.015
Core radial expansion, cents/°C	-0.16	-0.16	-0.16
Axial expansion, cents/°C	-0.08	-0.07	-0.07
Fuel Doppler, cents/°C	-0.07	-0.07	-0.06
Coolant void worth, $	-1.68	-1.63	-1.83

Table IX-2 presents reactivity feedback coefficients typical of SSTAR core configurations.