NSSS and component design

A close review of the design aspects of major components is in progress.

Significant conceptual changes have been made in a few of the major components based on the design and manufacturing experience with the PFBR. For example, the changes in the arrangement of shielding around the core, reactor assembly, sodium circuits, fuel handling, construction materials, in-service inspection, etc., are highlighted briefly below.

4.3.1. Radial shielding around the core

Shields around the core and blankets form a major part of the reactor assembly in fast reactors. Boron carbide and stainless steel have been the main choice shield materials for shields in fast reactors. The PFBR core is configured with 609 stainless steel (in six rows) and the radial shielding with 417 boron carbide (in three rows). For the CFBR, it is proposed to use ferroboron as a shield material for economic considerations. Commercially available ferroboron in the form of lumps, granules and powder has 15–18 wt% boron and a bulk density of around 4 g/cm³. Detailed calculations indicate that eight rows of shielding

CFBR PFBR

FIG. 4. Primary sodium purification.

PLENARY SESSION 1

99 assemblies with ferroboron meet the radial shield requirements satisfying the radiological safety criteria. The effectiveness of ferroboron stems from the fact that boron is spread throughout the shield region, though in lower atomic densities, and iron present in the shield regions also contributes significantly to attenuation. Measurements of thermal, epithermal and fast neutron attenuation in ferroboron have been carried out in the KAMINI reactor for comparison with boron carbide, which have indicated the effectiveness of ferroboron as a shield material. As part of the development work on ferroboron, diffusion experiments under accelerated test conditions are in progress.

4.3.2. Reactor assembly

Significant design changes are being contemplated in the design of major reactor assembly components with a view to optimizing the design and reducing capital cost. Further, the manufacturing experience with components for the PFBR has highlighted the focus areas that need simplification. Table 2 broadly TABLE 2. SUMMARY OF OPTIONS FOR THE CFBR

Component/feature PFBR Options being studied for CFBR

Top shield Welded box structure filled with high density concrete

Welded box structure filled with higher density concrete — reduced height Thick plate — reduced height Formed dished head with external shielding

Grid plate Large diameter grid plate

(botted) •Smaller grid plate (welded)

•Shell enveloping core subassemblies that are cooled

•Support for peripheral shielding subassemblies that are not cooled through spikes

Support for reactor

assembly •Through an extended shell (~1.5 m) in tension

•Increased component height over top shield

•Through shell in compression

•Reduced component height over top shield

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summarizes the options being studied for the CFBR. For example, the adoption of a fully welded design concept for the grid plate (Fig. 5) with a reduced number of sleeves that support only the core subassemblies requiring coolant flow and a spike supporting arrangement for other peripheral subassemblies has offered considerable size and economic advantages, reflected in overall manufacturing time as well. These changes have resulted in a 55% reduction in the overall weight of the grid plate. For reducing the shielding in annular gaps between rotatable plugs and the roof slab in the top shield, thick plates with machined gaps offer a potential solution. The other changes that are being actively pursued include options for a roof slab, change of configuration in the support of the reactor assembly, etc.

4.3.3. Heat transport and auxiliary systems

Optimization of piping layout with a view to minimizing the overall length and number of welds, closer assessment of margins in system capacities, review of the number of valves and piping supports in the sodium and auxiliary systems and optimization of steam generator design for a reduced number of tube to tube sheet welds, etc., are some of the measures being thoroughly reviewed for the CFBR. The PFBR steam generator was designed with a tube length of 23 m.

However, for the CFBR, the longer length of 30 m is preferred in order to reduce the number of tube to tube sheet joints by ~35%, thus minimizing the possibility of sodium–water reaction throughout the design life, reducing manufacturing time and improving economics (Fig. 6). Detailed studies, taking into consider-ation the overall effect of capital cost, outage cost and construction schedule, have indicated that a design with 3 modules per loop each is optimum.

FIG. 5. Grid plate arrangement in the CFBR.

PLENARY SESSION 1

101 4.3.4. Fuel handling system

The fuel handling system in the PFBR uses a combination of two rotatable plugs, an offset arm type in-vessel handling machine (transfer arm), an inclined fuel transfer machine for ex-vessel handling and a water pool for ex-vessel storage of subassemblies. For the CFBR (Fig. 7), the single ex-vessel water pool storage is retained with the number of storage locations optimized to meet the normal storage requirements of both units and emergency full core unloading of one unit. Further simplification of the in-vessel and ex-vessel handling schemes is also being attempted by the use of a flask type transfer in place of the inclined fuel transfer machine; the handling flask being common to both the units. The offset arm type concept is retained for in-vessel handling and two such machines will be utilized.

The sodium cleaning and decontamination systems are housed inside the reactor containment building in the PFBR. For the CFBR, significant reduction in the cost of the fuel handling system is envisaged by sharing the fuel handling and decontamination facilities between both the units. The decontamination system is also located outside the reactor containment building in a separate building common to both the units. The use of water inside the reactor containment building is thus minimized, resulting in enhanced safety.

Handling Flask PFBR

CFBR

FIG. 6. Steam generator.

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4.3.5. Other design measures

The options considered for reducing the size and cost of the reactor assembly are:

(a) Three thermocouples at each subassembly outlet against two in the PFBR;

(b) Study of a ‘bean shaped’ intermediate heat exchanger (IHX) in order to reduce the radial width of the roof slab required to accommodate the IHX support flange (Fig. 8);

(c) Capability to handle short cooled fuel (with higher decay heats) for faster recycling of spent fuel.