The University of Rome ‘La Sapienza’
Italy

V — 1. DESCRIPTION OF THE MARS DESIGN

The Multipurpose Advanced Reactor, inherently Safe (MARS) is a 600 MW(th), single loop, pressurized light water reactor (PWR); its design was developed at the Department of Nuclear Engineering and Energy Conversion of the University of Rome “La Sapienza”. The design was conceived in 1984 as a nuclear power plant able to conciliate well proven PWR nuclear technology with special safety features intended to facilitate plant location in the immediate proximity of highly populated areas in fast growing countries, to meet their energy and potable water needs. The plant has to guarantee a high and easily understandable safety level, has to be inexpensive and easy to build, operate, maintain and, eventually, repair; and has to ensure low production of radioactive wastes. The objective of the design effort was to find those (suitably supported by tests) plant solutions that could keep the features of a ‘traditional’ PWR in an essentially simplified design. A detailed description of the MARS design is presented in [V-1].

The core cooling system includes only one loop with a recirculation type steam generator. During normal operation, forced circulation of the primary coolant is applied, based on the use of a pump while, in emergency conditions, the necessary coolant flow rate in the core is maintained by an independent cooling system, which transfers heat to the external atmosphere through natural convection and relies only on static components and on one non-static, direct action component (a check valve, 400% redundant).

The MARS reactor module is enclosed in a pressurized containment filled with cold water; the complete nuclear power plant (NPP) also incorporates a containment building needed to cope with external events (such as aircraft crash) in accordance with Italian and European regulations. The containment building is able to withstand any internal pressurization, also in the hypothetical event of complete destruction of the core coolant boundary.

Loss of coolant accidents (LOCAs), loss of flow accidents (LOFAs), and anticipated transients without scram (ATWSs) are eliminated in the MARS concept by design, which is intended to make the plant reliable, safe, and easy to operate. With major accidents being eliminated, the plant incorporates a substantially reduced number of safety related structures, systems, and components, and provides for maximum possible pre­fabrication and easy assembly/disassembly, particularly by allowing easy component substitution in the case of a failure or consumption, instead of requiring a local repair.

Major design specifications of the MARS are shown in Table V-1. The reactor cooling system and some of its components are shown in Fig. V-1, V-2 and V-3.

Some features of the MARS concept are similar to well known features of standard PWRs (loop type primary circuit design, similar core geometry and materials, similar means of reactor control, etc.) [V-1, V-2]. For example, the core is cooled and moderated by pressurized light water containing a boron solution. Boron and burnable poisons compensate for excess reactivity during the irradiation cycle[49].

Different from many other PWR designs, the MARS primary coolant system includes only one loop with 25 inch internal diameter pipes, one vertical axis U-tube type steam generator, and one canned rotor pump connected to the steam generator outlet nozzle (see Fig. V-1).

The safety core cooling system (SCCS) is connected to the reactor vessel. A vapour-bubble type pressurizer controls the pressure inside the primary coolant system.

On/off valves in the primary loop main isolation system (MIS) are installed in the primary cooling loop to isolate, if necessary, the steam generator and the primary pump (i. e., in the event of a steam generator tube rupture).

TABLE V-1. MAJOR SPECIFICATIONS OF MARS NPP [V-1]

Characteristic Value

Power rating

Reactor rated thermal power, MW 600

Rated electric power (one module), MW 150

Rated electric power (suggested cluster of 3 modules), MW 450

Suggested rated electric power in cogeneration configuration 300

(electricity + desalinated water/district heating)

Core average volumetric power density (kW/litre) 56.5

Thermal-hydraulic characteristics

Primary coolant flow rate (forced flow) (kg/s) 3327

Operating pressure, bar 75

Total RCS internal volume (m3) 130

Pressurizer heaters power (kW) 800

Steam flow rate (kg/s) 277

SG steam pressure (bar) 18.8

Temperatures (°C)

Reactor vessel outlet 254

Reactor vessel inlet 214

Steam generator steam outlet 209

Steam generator feedwater inlet 150

Reactor vessel data

Internal diameter of the shell, mm 3000

Internal design pressure, bar 83

Length of the cylindrical shell, mm 8056

Upper head thickness, mm 80

Bottom head thickness, mm 80

Overall length of the assembled vessel, mm 11 091

Shell thickness, mm 120

Total weight (approximate; dry), kg 88 000

FIG. V-1. Reactor cooling system (RCS) and main auxiliaries [V-1].

When any of the four check valves is opened, after a short transient phase, flow in the PSC is assured by a difference in level of about 7 m between the vessel outlet nozzle and the primary heat exchanger and by the difference between vessel inlet and outlet temperatures. A horizontal axis, U-tube type heat exchanger transfers heat from the PSC to the ISC.

Pressure in the ISC loop is slightly higher than 75 bar (thanks to a dedicated pressurizer); this value guarantees sub-cooled water conditions of the fluid during any accidental situation or transient; the difference in level for natural circulation in the ISC loop is about 10 m. The second heat exchanger transfers heat from the ISC circuit to the water of a reservoir; see Fig. V-3.

Steam produced in the reservoir is mixed with air initially present in the dome over the pool; pressure in the dome rises and this causes a flow of the air-steam mixture towards a small connection path with the atmosphere. An inclined tube heat exchanger is placed between the pool dome and the connection path to the atmosphere, where steam is partially condensed due to passive draught of external air, drawn by a chimney.

The above listed design features introduced some constraints to plant design. In particular, with the selected safety core cooling system (SCCS) and its functional requirements, reactor thermal power cannot exceed approximately 1000 MW(th). The MARS design version described in this paper has a thermal output of about 600 MW(th). Another characterizing parameter is primary system pressure. It was selected to equal 75 bar, which is different from the pressure values typical of standard PWRs (150-170 bar) [V-2]. Such selection leads to a loss in the thermodynamic efficiency of the plant because of the resulting limitation of a higher isotherm in the steam cycle. At the same time, it allows for adoption of a pressurized primary containment for protection of the primary loop (CPP, the pressurized boundary that envelopes the primary coolant system and

FIG. V-2. Pressurized containment for primary loop protection (CPP) [V-1].

FIG. V-3. Scheme of the safety core cooling system (SCCS) [V-1].

emergency core cooling system, see Fig. V-2), substantially eliminating the possibility of LOCA of any type and of a control rod ejection.

Inclusion of the primary coolant system (with an average operating temperature of 234°C) inside the low enthalpy water filled pressurized containment (CPP, at a temperature of 70°C) requires thermal insulation to reduce heat losses from the primary coolant system. An insulating system has been designed on the external side of the primary coolant boundary, with only the lower head of the reactor vessel being thermally insulated in the internal part through the use of matrices of stainless steel wiring that cause the presence of semi-stagnant water which resists high pressure and fast pressure gradients with acceptable flow shape modifications. This system limits heat losses to about 0.3% of the reactor’s thermal power.

It should be noted that the special design of the passive emergency decay heat removal system (SCCS) avoids thermal stratifications of any type, contributing to increased reliability of this system.