Innovative Steam Generator for Sodium-Cooled Fast Reactors

1.1. European Reactors
1.1 European Reactors
1.1.1 Rapsodie
The Rapsodie experimental sodium cooled reactor was the first French fast neutron
reactor located at the Cadarache Research Centre and was operated by the CEA. The
construction started in 1962 within an association of CEA and EURATOM. The reactor
went critical on 28 January 1967, reaching 20 MW(th) power on 17 March 1967. The core
and equipment were modified in 1970 to increase the thermal power level to 40 MW with
a peak neutron flux of 3.2×1015 ncm-2s-1. In operation, the primary sodium entered the
core at 400◦C and flowed out at an average temperature of 550◦C.
The operating parameters were similar to those in large commercial size reactors.
The reactor core was cooled by two identical loops each comprising a primary sodium
circuit from which thermal power is transferred to a secondary sodium circuit through an
intermediate (sodium/sodium) heat exchanger (IHX) by means of a primary pump.
The system lines are enclosed in concrete cells inside a double containment barrier.
The principal geometric specifications of the primary piping system are the following:
ˆ Core to intermediate heat exchanger (IHX): D = 300/314 mm, 16 m long;
ˆ IHX to pump: D = 300/314 mm, 8.5 m long;
ˆ Pump to Y junction: D = 200/208 mm, 18 m long;
ˆ IHX vessel dimensions: D = 884 mm, 5.2 m high;
ˆ Pump vessel dimensions: D = 850 mm, 4.5 m high;
ˆ Expansion tank: 36 m2 surface area.
The installation consists of six main buildings, access to three of them is restricted.
These are:
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1.1. European Reactors
ˆ The reactor building or secondary containment including the reactor vessel and its
upper closures as well as the two primary loops, each of which was equipped with a
mechanical pump and an intermediate heat exchanger. All these components were
enclosed in concrete cells to provide radiation shielding. The secondary, nonradioac-
tive sodium, is piped to a conventional building containing the components of the
two secondary loops including a sodium/air heat exchanger in each;
ˆ The active building comprising interim storage facilities for both fresh and used fuel,
and various other facilities such as the washing cell for decontaminating components
polluted with primary sodium, and a dismantling hot cell used for conditioning used
irradiated equipment for long term storage as waste;
ˆ The fuel assembly dismantling building comprising hot cells for non-destructive
examination of fuel pins, and the assembly of experimental sub-assemblies. All
the circuits and components are made of austenitic stainless steel, the main pipes
and vessels have a double wall. The reactor vessel is immediately surrounded by
special high density concrete containing rare earth oxides, called Sercoter. This was
protected externally by a steel liner which was considered to constitute the second
barrier.
Rapsodie was designed, built and operated to obtain data on the physical behaviour of
a fast neutron reactor under static and dynamic conditions, to offer information of direct
use for the design of future LMFRs, and to supply a fast neutron flux for irradiation tests
of fuels and materials. Mixed oxides were used as reactor fuel. During its 15 years of
operation, more than 30 000 fuel pins of the driver core were irradiated, of which about
10 000 reached a burn-up beyond 10%, and 300 irradiation experiments and more than
1000 tests were performed.
In 1971, the irradiations performed in the core revealed a phenomenon of irradiation
swelling in the stainless steel of the wrapper and the fuel cladding in the high neutron flax.
The Rapsodie results have been extrapolated in the Phe´nix reactor. The operating history
of Rapsodie has been reviewed in journals and conference papers, and have regularly been
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1.1. European Reactors
published by the TWG-FR. The decision to stop running the reactor was taken after two
successive defects were detected in the primary system containment.
The first defect, which appeared in 1978, consisted of a sodium micro leak: radioactive
sodium aerosols were found in the double wall reactor vessel. Investigations did not
find any liquid sodium in the gap nor locate the defect. The reactor was subsequently
operated at a reduced power level (˜ 0.6 PN), which was high enough for irradiation
needs but did not cause the leak to reappear. The second defect appeared in 1982 and
consisted of a small leak from the nitrogen blanket surrounding the primary system.
Before the final shutdown of the reactor, a series of end-of-life tests were conducted in
April 1983. The LMFR Rapsodie was shutdown in April 1983. Pre-decommissioning
operations were then conducted until 1986. They consisted essentially in unloading the
fuel and fertile assemblies, and in draining the sodium from the primary and secondary
circuits. Decommissioning operations started in 1987.
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1.1. European Reactors
Figure 1.1: Rapsodie reactor cross section
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1.1. European Reactors
1.1.2 Phe´nix
The reactor plant Phe´nix (see [16] for details) with a nominal ˜ 255 MW(e) power
rating (565 MW(th)), was firstly connected to the electricity grid on 13 December 1973;
the nominal power was reached on 12 March 1974, 18 days ahead of plan.
Figure 1.2: Elevation through Penix primary circuit
The nuclear power plant (NPP) construction cost was 800 million franc (approximately
512 million Euro). The planned budget was exceeded by less than 10%. The NPP was
generally operated at the power tolerated by reactor and equipment, with comparatively
high load factor. Phe´nix has currently provided about 100000 hours of grid-connected
operation representing 3900 equivalent full power days at operating temperatures of 560◦C
for the reactor hot structures. The plant has achieved the objectives of demonstration of
fast breeder reactor technology which were set at the time of construction, including the
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1.1. European Reactors
following significant achievements:
ˆ The measurement of the breeding ratio made at the time of dissolution of the fuel
evacuated from the plant, gave a true value of 1.16 (projected 1.13); — The fuel
cycle, based on mixed oxide fuel and PUREX reprocessing, has been closed and the
first fuel subassembly made with reprocessed plutonium was loaded in the reactor
in January 1980. About five Phe´nix cores (˜ 25 tons) were reprocessed.
ˆ The successful and regular operation at the highest temperatures and nominal power
until 1990 resulted in validation of the pool concept option and did much knowledge
regarding the high temperature design and structural material of fast reactors;
ˆ The highest in the nuclear power engineering practice gross/net plant’s thermal
efficiency of 45.3–42.3% during the periods of stabilized operations with nominal
parameters. On the average, gross/net plant’s thermal efficiency is equal to 40–38%
owing to operations at 2/3 the rated power now and then.
The Phe´nix reactor block is of an integrated (pool) design except for a few auxiliary
circuits (Fig. 1.2).
The entire primary sodium system, containing 800 tons of radioactive sodium, is en-
closed in the main reactor vessel. The reactor block is suspended to the slab via 21
hangers. These hangers have three welds, which are difficult to access.
The reactor vessel dimensions: inside diameter × height: 11.82×12 m, wall thickness:
15 mm is under the upper slab, 15 m in diameter and 1.5 m high, on which the equipment
is installed.
The slab (˜ 800 tons: 200 tons metal and 600 tons concrete) is installed on 22 ball-
and-socket type pads.
The slab bearing plate operates under low temperature because between the plate and
the reactor cover there is insulation and besides this zone is cooled. The reactor vessel
was manufactured on site workshop using 15 to 25 mm sheets after prefabricated elements
were examined in detail.
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1.1. European Reactors
The pumps and heat exchangers are located on movable sliding support, sealing of the
pump and heat exchanger penetrations is carried out by means of the bellows. A peculiar
feature of this reactor top design is a massive cover plate, of 60 mm thick and 12000 mm
in diameter having significant thermal inertia. It restraints a power growth rate or the
reactor starting up and may be a cause of some thermal stresses occurring in the junction
of the vessel cylindrical part and the horizontal cover.
The sodium in the vessel is separated into two zones:
ˆ Hot pool at the core outlet where the hot sodium flows into the intermediate heat
exchangers;
ˆ Cold pool taken from a peripheral annular space between the primary tank and the
wall of the main reactor vessel, which contains the three main circuit circulating
pumps and six heat exchangers, suspended from the upper slab;
ˆ The leak tightness of the penetrations of the reactor vessel by the IHX is provided
by an argon seal.
A number of other devices are located in the main vessel: the fuel transfer arm, the
six control rods, neutron flux detectors, thermocouples, failed fuel detection and location
devices, the core acoustic detection system components, etc. An argon gas atmosphere is
maintained above the sodium surface to prevent any contact with air.
The main vessel is closed at the top by a flat roof with openings for pump and heat
exchanger pipes. It is associated with the cylindrical seating of a rotating plug in the slab
penetrations forming the top of the reactor block. An outer guard vessel surrounds the
main vessel. It has the double function of containing any sodium escaping by leakage, and
preventing a drop in the sodium level of the main vessel which might affect core cooling.
Owing to an intermediate circuit between the reactor and the steam generator, it is
very likely to prevent an accidental interaction between the radioactive primary sodium
and the water/steam in the electricity generating system. In such nuclear steam sup-
ply system design, the incident-secondary sodium-water reaction could be classified as a
chemical incident in the non nuclear components.
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1.1. European Reactors
Figure 1.3: Phenix reactor block: isometric view
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1.1. European Reactors
The three primary sodium pumps are variable speed units (150 to 970 rpm) deliv-
ering about 950 kg/s at 825 rpm, which is their normal service speed. The circulating
sodium enters the core at 400◦C and moves from there, at 560◦C, to six intermediate heat
exchangers which are connected in pairs with three independent secondary loops.
Sodium must be kept very pure to prevent corrosion of the steel piping and plugging of
circuit components. It is purified by cold traps operating on the principle of precipitation
of any oxide in the sodium at low temperature.
Secondary sodium, which is not radioactive, is circulated by a mechanical pump with
a flow delivery of 700 kg/s. It enters the intermediate heat exchangers (IHX) at 350◦C
and leaves at 550◦C. Each secondary loop is connected to a steam generator consisting of
an evaporator, superheater, and reheater, in 12 modules for each stage.
Figure 1.4: Phenix steam generator design
The intermediate heat exchanger (IHX) transfers heat from the primary radioactive
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