Central Research Institute of Electric Power Industry and Toshiba Corporation,

Japan

VIII — 1. DESCRIPTION OF THE 4S-LMR CONCEPT

The Super-Safe Small and Simple Liquid Metal cooled Reactor (4S-LMR) is a small sodium cooled fast reactor concept under development in Japan by the Central Research Institute of Electric Power Industry (CRIEPI) and Toshiba Corporation features of which include long operation without on-site refuelling. This concept is described in detail in Annex XV of [VIII-1].

The 4S-LMR is being developed to meet the needs of certain segments of the diverse global energy market [VIII-1]. An economic disadvantage is pointed out as the principal obstacle to realizing small reactors. Higher safety levels are also needed, because the number of nuclear power plants would increase in case small reactors are deployed around the world. Improved economic performance tends to be incompatible with enhanced safety levels, as shown by the experience of nuclear power reactors of previous generations. Stronger reliance on passive safety design options is expected to establish a certain synergy between economic performance and safety. To facilitate such a synergy, the 4S-LMR is being designed to ensure simple operation, simplified maintenance, including refuelling, a high safety level, and improved economic performance. A specific design policy for the 4S-LMR could be summarized in the following nine design objectives:

(1) No refuelling over 10 — 30 years;

(2) Simple core burnup control without control rods and without control rod driving mechanisms;

(3) Reactor control and regulation executed by systems and components not belonging to the reactor system;

(4) Quality assurance and short construction period based on factory fabrication of the reactor unit;

(5) Minimum maintenance and inspection of reactor components;

(6) Negative reactivity coefficients on temperature; negative sodium void reactivity;

(7) No core damage in any conceivable initiating events without the reactor scram;

(8) Safety system independent of emergency power systems and not incorporating active decay heat removal systems;

(9) Complete confinement of radioactivity under any operational conditions and in decommissioning.

Items 1 through 5 are related to simplification of the systems and maintenance. Items 6 through 9 are related to safety design.

Based on the abovementioned design objectives, the 4S-LMR concept supplies multiple passive safety design features. Such an approach could help realize a high safety level and simultaneously reduce the number of auxiliary systems otherwise required to support safety functions of the safety system. The resulting reduction in the number of systems and system simplification may, in turn, reduce the required scope of maintenance work.

Small reactors are meant to be installed closer to end users. In order to allay public fears, a ‘sense of security’ is essential, which means that a transparent safety concept, a proven or easily demonstrable technology, and a small number of systems are cumulatively preferable. A fully passive heat removal system is employed in the 4S-LMR so that auxiliary support systems can be eliminated. 4S-LMR safety can easily be demonstrated in full scale tests, because of its small size. Design status and passive safety features of the 4S-LMR are described in reference [VIII-1]. This reference also presents safety performance of the reactor in anticipated transients without scram and combinations thereof, based on completed safety analyses.

The 4S-LMR incorporates a load following capability provided by a simple control of the feed water rate in the power circuit. Analyses have shown that the reactivity of core thermal expansion, which is one of the passive reactivity feedbacks, is important to realize this option. Core thermal expansion feedback also helps to secure reactor safety. Specifically, analytical results predict that the presently selected cladding material, HT-9, is compatible with the mechanism of core expansion reactivity feedback. It is also shown that flow rate control of the secondary pump would enhance the power range of reliable reactor operation due to improved stability of the steam generator at the steam-water site. As the irregular load following operation affects schedule pre­programming, the plant control systems of the 4S-LMR would be reconsidered in case the reactor is assumed to operate at partial power.

The 4S-LMR is a pool type sodium cooled fast reactor with a steam-water power circuit. The power output is 50 MW(e), which corresponds to 135 MW(th). The refuelling interval for the variant considered in this description is 10 years. Major specifications of the 4S-LMR are listed in Tables VIII-1 and VIII-2.

Figure VIII-1 shows the vertical layout of the reactor, including the primary heat transport system (PHTS). The PHTS consists of the containment vessel (guard vessel), the reactor vessel, the intermediate heat exchanger (IHX), the electromagnetic (EM) pumps, the reflectors, the internal structures, the core, and the shielding.

The reactor vessel is 3 m in diameter and 18 m in height and is divided into the inner part of a coolant riser plenum and the outer part of a coolant down-comer by an inner cylinder of 1.8 m diameter. The inner cylinder accommodates the core and the reflector. It also accommodates the reflector drivelines and the ultimate shutdown driveline. In the outer part, there are the direct heat exchanger (DHX) of the primary reactor auxiliary cooling system (PRACS), the intermediate heat exchanger (IHX), the electromagnetic (EM) pumps, and the radial shield assemblies, from top to bottom. As a design option, PRACS can be replaced by intermediate reactor auxiliary cooling systems (IRACS), which removes shutdown heat via secondary sodium in active normal operation) or the passive (postulated initiating events) mode. The primary coolant travels from the riser into the down-comer and then returns into the coolant plenum underneath the core. There are no moving parts inside of the reactor vessel except for the reflector, which moves very slowly at 1~2 mm per week.

The guard vessel covers the reactor vessel to prevent a loss of the primary coolant. The guard vessel also forms the containment boundary, together with the top dome. A natural draught air cooling system between the guard vessel and the cavity wall, the so-called reactor vessel auxiliary cooling system (RVACS), is designed as a passive decay heat removal system. The PRACS (or IRACS) mentioned above is then the second passive decay heat removal system. These two systems are redundant and diverse.

TABLE VIII-1. MAJOR DESIGN PARAMETERS OF THE 4S-LMR Items Specifications Reactor: Diameter [m] 3.0 Height [m] 18.0* Reactor vessel thickness [mm] 25 Guard vessel thickness [mm] 15 Inner cylinder: Inner diameter [m] 1.84 Thickness [mm] 15 Reflector: Material Graphite Height [m] 2.1 Thickness [mm] 300 Core barrel: Inner diameter [m] 1.33 Thickness [mm] 10 Primary electromagnetic (EM) pump Rated flow [m3/min.] 50 Head [MPa] 0.08 x 2 * from bottom to coolant free surface

TABLE VIII-2. MAJOR DESIGN SPECIFICATIONS OF THE 4S-LMR Items Specifications Thermal output [MW] 135 Electrical output [MW] 50 Primary coolant condition [°C] (outlet/inlet) 510/355 Secondary coolant condition [°C] (outlet/inlet) 475/310 Steam condition [°C/MPa] 453/10.8 Core diameter [m] 1.2 Core height [m] (inner/outer) 1.0/1.5 Number of fuel sub-assemblies (inner/outer) 6/12 Number of reflector units 6 Reflector thickness [m] 0.3 Core lifetime [years] 10 Plant lifetime [years] 30 Number of fuel pins 469 Fuel pin diameter [mm] 10.0 Cladding thickness [mm] 0.59 Smear density [%TD] 75 Pitch/Diameter 1.15 Duct thickness [mm] 2 Duct gap [mm] 2 Bundle pitch [mm] 258 Assembly length [mm] 4800 Average burnup [GW day/t] 70 Pu enrichment [weight %] (inner/outer) 17.5/20.0 Maximum linear heat rate [kW/m] 25 Conversion ratio (middle of cycle) 0.71 Coolant void reactivity (end of cycle) [%] ~0 Burnup reactivity swing [%] ~9 Core pressure drop [MPa] ~0.1

The primary pump system consists of two EM pumps arranged in series. Each EM pump is a sodium immersed self-cooled type pump with an annular single stator coil. The total rated flow is 50 m3/min, and each pump has a 0.08 MPa head. Such a system of pumps arranged in series provides a favourable inherent response in the case of single pump seizure, when it is necessary to mitigate a decrease of core flow through a pump that is still working, ‘using’ its Q-H (flow-head) curve. At the same time, reverse flow may occur at a failed pump in a parallel arranged pump system.

The annular reflector, divided into six segments, controls reactivity in the reactor core and compensates the burnup reactivity swing. Any stuck event or malfunction of the reflector driving systems will eventually result in a reactor subcritical state, when negative reactivity due to fuel burnup will not be compensated by a slow upward movement of the reflector. Dropping the reflector down will make the reactor subcritical from any operational state, due to the resulting increase in neutron leakage from the core.

The intermediate heat transport system (IHTS) consists of one EM pump, one steam generator (SG), the piping, and a dump tank. The EM pump is integrated in the SG.

The 4S-LMR core is designed for lifetime operation without on-site refuelling and provides for negative reactivity coefficients and a reduced pressure drop at a relatively large core height. The requirement of a 10-year core lifetime could reduce maintenance work and contribute to non-proliferation [VIII-1]. Negative reactivity coefficients and a reduced pressure drop could enhance safety by providing intrinsic protection against loss of

flow (LOF) events. The selection of core height was also limited by the available choices for performing full-core irradiation tests, in view of the existing facilities.

Fig. VIII-2 shows the 4S-LMR core configuration. There are 6 inner sub-assemblies and 12 outer sub­assemblies. The ultimate shutdown rod is arranged at the centre of the core. It is a backup shutdown system; the primary shutdown system provides for dropping down the reflector. The active height of the inner core is shorter than that of the outer core. This 0.5 m sodium region above the inner core helps to decrease the coolant density reactivity coefficient over the entire core. Coolant void reactivity is kept below zero during the core lifetime and is nearly zero at the end of core life.

The average core outlet temperature was selected based on the condition of not exceeding the minimum liquefaction temperature of 650°C, at which a (metallic) fuel-steel eutectic starts to be formed. The hottest interface temperature between the outer fuel surface and the inner cladding surface was evaluated using the hot channel factor of ~1.9 (including the engineering safety factor), which is a conservative assumption. Safety design criteria for the cladding were also evaluated taking into consideration cladding thinning due to this metallurgical effect.

Reactivity feedback coefficients on temperature integrated over the core region are summarized in Table VIII-3. Reactivity feedback coefficients on fuel density, the coolant and the structures (cladding and duct) were derived from a diffusion calculation in R-Z geometry based on the perturbation theory. Density coefficients multiplied by thermal expansion rates of the fuel and structures make up the temperature coefficients. The thermal expansion rate of the cladding was used to describe fuel axial expansion. Because the

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FIG. VIII-2. Core configuration of the 4S-LMR (Annex XV [VIII-1]).

TABLE VIII-3. REACTIVITY FEEDBACK COEFFICIENTS ON TEMPERATURE INTEGRATED OVER THE CORE VOLUME

Doppler T— I -2.80 x 10-3

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Fuel ^Dkkk — j -7.29 x 10-6

Coolant ^Akkk’ j -3.23 x 10-6

Structure |^Ak/kk’ j -0.50 x 10-6

expansion rate of the cladding is smaller than that of the fuel, such an approach produced conservative results. The safety analyses performed considered spatial distributions of reactivity coefficients and expansion effects.