MSR upgrade adds megawatts at Cofrentes

In November 1997, the MSR reheater bundles, which had been completely redesigned and reconstructed, were installed at the BWR plant at Cofrentes (originally 975 MWe) on an extremely short schedule.

The decision to replace the MSRs followed a balance-of-plant (BOP) evaluation which was conducted as a part of an across-the-board power uprating programme to produce a net plant power increase of up to 11% (from 975 MWe to 1081 MWe). This revealed that the MSRs should be reviewed in detail to assess their ability to safely handle the possible flow increases and other loading requirements that this power uprating would impose upon them.

The subsequent MSR review process, in turn, revealed the possibility of gaining additional MWe over and above the 11% uprating goal through a complete reheater redesign and reconstruction as well as the employment of state-of-the-art moisture separation elements, all taking full advantage of modern MSR technology. As a direct result, a further 7 MWe gain was realised over and above the 11% unit uprating. Feasibility studies to further stretch power to 115% in the near future, have shown that the redesigned MSRs will perform adequately at this higher power level.

THE ORIGINAL MSRs

The two original Cofrentes MSRs installed about fifteen years ago were approximately 16 m (52 ft) long by 3.5 m (12 ft) in diameter. They were located on each side of the LP turbine and slightly below its centreline. Figures 1 and 1a depict the general layout of these MSRs with respect to the turbine-generator and piping interferences impacting MSR reconstruction. In this connection, note that, with the exception of the LP turbine steam inlet piping, all MSR interconnecting piping penetrations are located through the welded-on head that had to be removed, as shown in Figure 2. Also, in order to provide a long, clear path to remove the old reheaters and install the new ones, a tremendous amount of external piping had to be removed and later reinstalled.

Figures 3 and 3a clearly show that even after the welded-on head was removed, a considerable amount of internal piping had to be removed before the reheaters could be replaced. The picture also shows the crosswise cylindrical heads of the LP and HP reheaters.

Since no overhead cranes existed above the MSRs, all piping and head handling operations, involving the removal of external and internal piping and the welded-on MSR head, had to depend upon hanging the necessary hoists on suitably braced, overhead piping.

In order to cut off the MSR head on a bevel and minimise grinding before rewelding, a special, automatic cutting torch mounted on a flexible track that could be curved to match MSR circumference was used.

To physically remove these relatively flexible reheaters, each weighing 27216 kg, special rolling racks and tracks had to be pre-constructed and a shielding wall had to be breached.

Obviously a tremendous amount of pre-planning as well as the installation of rigging components and modification of the shieldwall to minimise physical interferences had to be accomplished. These, and other measures, had to be completed during the 1996 scheduled outage in order to comply with the specified 32-day project completion window during the 1997 scheduled outage.

The individual, rectangular, 2.1 m by 0.73 m, 2-pass LP and HP reheaters, each having 4355 m2 of heat transfer surface, employed horizontally oriented, 25.4 mm finned U-tubing. This configuration meant that the inlet and outlet of each individual tube was at the same elevation. Therefore, since gravity could not play a role in condensate drainage, and the lower tubes were subject in their entirety to very high heat loads, excessive excess steam of nearly 10% was required, reducing MSR effectiveness.

REDESIGNED LP & HP REHEATERS

The optimised redesign of the LP and HP reheaters took full advantage of modern MSR technology and maximised heat transfer capabilities. Figure 4 shows one of these modern MSR reheaters being unloaded at the seaport in Valencia, ready to be transported to the Cofrentes station.

This included the use of 19 mm, Type 439 stainless steel finned tubing (1 fin per mm or 27 fins per inch) replacing the original larger 25 mm tubing size. Also, the LP reheater width was increased by approximately 180 mm through internal structural redesign in this area. These steps permitted the use of many more tubes within the same HP reheater envelope and within the expanded LP reheater envelope. This LP reheater heat transfer surface was increased by 47% to 6385 m2 and the HP reheater heat transfer surface was increased by 36% to 5940 m2.

To further increase this heat transfer capability, tube configuration was updated and U-bend orientation of the horizontal tubes was changed from horizontal to vertical. This redesign permitted an enhanced gravity condensate flow, thus reducing excess steam requirements. It also resulted in more even thermal loading among the rows of tubes. In addition, the inlets of individual U-tubes were fitted with restrictors to fine-tune the steam flows and produce nearly equal thermal loading among the U-tubes.

All of these steps served to drastically reduce the Terminal Temperature Difference (TTD) which directly led to the additional MWe gain achieved over their 11% overall uprating programme.

To further take advantage of modern MSR technology, the redesign of these reheaters incorporated a flexible tube support system that allows for controlled, intermittent relief of mis-matched thermal expansions between hotter tubes and progressively cooler shrouding side plates. This concept employs a “slide plate” design, so that the scaled side plates can automatically adjust – in plane – to tube and structural thermal expansions without restraining the tubes or damaging the plates themselves.

Tube holes in the support plates are “radiused” (avoiding straight edges) to further facilitate unobstructed tube movement during pressurisation and decompression.

Instead of an orifice restrictor in the discharge lines previously used to control a fixed excess steam/condensate flow from the LP and HP reheaters, remote, manually operated throttling valves were installed. These valves, in conjunction with integral thermocouples installed in the outlet ends of selected reheater tubes, permit occasional manual flow trimming to optimise MSR operation and maximise MWe output gain.

Since the new LP and HP reheaters were quite differently configured, new carriages had to be constructed for their installation. Figures 5 and 5a show the installation of the widened LP reheaters while Figures 6 and 6a show the installation of the HP reheaters (Figure 6a also shows the opening through the shield-wall that had to be pre-prepared during the 1996 outage). In addition to this reheater optimisation, the moisture separation sections of the MSRs below the reheaters were fitted with up-to-date double pocket chevron vanes, shown in Figure 7, to increase moisture separation efficiency to nearly 100%.

These high performance vane elements were arranged in several extended banks in variance with the numerous original framed panels. The bank design and the stacking of the vanes along the bank, assure a long term fit and do not allow loosening of the individual vane elements, while permitting easy maintenance.

OPERATIONAL ENHANCEMENT ACHIEVED

Through extensive preplanning and preparation during the 1996 scheduled outage, the MSR upgrade project, which added significantly to the power capability of the Cofrentes station, was accomplished in the 32 day specified period during the scheduled 1997 outage.

After the MSR reheater upgrade was completed, the several measurements taken during a 100-hour continuous run for certification purposes confirmed that the predicted additional sevenmegawatt gain over the uprating target had indeed been achieved through optimisation of the MSRs. Furthermore, no problems have been identified as a result of the MSR upgrade.

Finally, in view of the severe time constraint and the unique MSR disassembly and reassembly complexities, this project has undoubtedly established new standards for the upgrading and optimisation of moisture separator reheaters in nuclear power plants worldwide.