Turning MSRs into high-performance items30 May 2000
The role of moisture separator reheaters as passive components has changed radically. They are no longer attended to only when they become a maintenance concern, an operational problem, or poor performers that lose potential megawatts.
The moisture separator reheater (MSR) is employed in a nuclear steam cycle in conjunction with its saturated steam turbines. Since the expansion of the steam takes place largely within the wet steam range, MSRs are used to dry the exhaust steam leaving the high pressure turbine, and then to reheat the dry steam before it enters the low pressure turbine. In most cases, MSR vessels comprising both the moisture separation system and the reheater are used. They can contain one or two stages of reheat – using main steam as heating steam for the former, and
main steam as well as extraction steam for the latter.
The rise of power uprating and plant life extension have given MSRs a more prominent role in generation and operational flexibility. MSRs are now called upon to actively contribute to the power uprate process. They must also contribute to generation and operational flexibility.
Nuclear power plant uprating emerged as a viable consideration in the early 1990s, when steam generators began to fail and the idea to redesign them to produce increased steam output was prompted by design advances and the advent of competition in power generation. On the other hand, MSR redesign and reconstruction have benefited from over 25 years of experience and technological advances.
MSRs were not originally given much attention in older nuclear plant designs; they were considered a relatively basic adjunct, and hence MSR problems arose early in plant life. Recently MSR technology has improved to make it a component uniquely able to contribute to plant uprating and life extension.
Levels of design confidence, gained through experience, have overcome some of the excessive conservatism that previously inhibited the full utilisation of MSR benefits. For example, older designs provided for actual terminal temperature differences (TTDs) from as low as 22°C to as high as 45°C, which could still produce acceptable superheat levels at LP turbine inlets. These TTDs have now been reduced to as low as 5°C, producing a much higher LP inlet steam superheat. The beneficial effects here on MWe gain and LP turbine blade life are clear. Another example is that modern reheaters can be placed in service earlier, during load ramping. Their ability to endure transients and large thermal gradients comes from their flexible structural design.
In considering the MSRs’ role in a power uprate programme, there are several approaches to evaluate. Each approach is viable, but the choice depends on ownership circumstances and the particular nuclear power plant’s type, age, specific needs, and uprate targets. They are:
Existing MSR refurbishment
Here, the functioning internals – reheaters and moisture separation systems – are selectively replaced by advanced-technology components within the existing internal MSR structure. While this is probably the least expensive approach, it produces
less than full utilisation of modern MSR technology.
The recent Hatch 8% power uprate programme was an interesting case in point. Here, the cost of extensive HP turbine modification was reduced by routing added uprate cycle steam flow directly to the MSRs’ HP reheaters. This, of course, greatly increased LP turbine superheat, increasing MWe output and minimising LP turbine moisture-induced blade erosion.
Full MSR internals
redesign and reconstruction
Where the reconstruction approach is indicated, the existing MSR shells are completely gutted. Advanced MSR component technology and improved structural design can be used to produce maximum MWe gain through near-100% moisture separation, minimum TTD (resulting in maximum superheat to the LP turbine) and lowest attainable pressure drop. The recent project at Korea Electric Power’s Kori 3 and 4 is an example. Here, TTDs have been reduced from 31.8°C to 4.5°C and 8MWe gains were achieved in each unit.
Complete MSR vessel replacement
As well as using modern MSR technology, entirely replacing the MSR vessel allows the MSR shell size to be optimised. This approach is particularly attractive where excessive internal steam velocities have produced flow-assisted corrosion (FAC) damage. This was the case at the recent MSR replacement project for Virginia Power’s North Anna 2. In such a case while cross-around cycle-steam piping centrelines can be maintained, as they were at North Anna 2, other, smaller-bore piping connections have to be relocated.
UPGRADING TO UPRATE
Upgrading MSRs to meet plant uprate needs breaks down into two basic classifications: rectifying existing MSR performance and reliability problems; and enhancing MSR thermohydraulic performance.
Rectification includes eliminating mechanical and/or structural failures due to thermally–induced cracks and distortions in key internal support elements and reheater tubing. These past failures can now be overcome through the use of a flexible tube-support system that allows for controlled, intermittent relief of mismatched thermal expansions between hotter reheater tubes and progressively cooler shrouding side plates. This design concept employs a slide-plate so that the sealed sideplates can automatically adjust – in plane – to tube and structural thermal expansions without restraining the tubes or damaging the plates themselves.
The current trend is to use ferritic stainless steel type 439 tubing in reheaters instead of carbon steel or CuNi alloy, as was often used in the past. This produces a chemistry that is more compatible with the fluids to which the tubing is exposed and the type 439 material is more formable and weldable than many alternatives. Also, it is immune to stress corrosion cracking. The tubing effective fins per inch count can be increased to optimise heat transfer.
When cycle steam is permitted to bypass the reheater tubes, a significant portion of the superheating effect of the reheaters is lost. Thus a careful examination and an effective design of the reheater tube field configuration within the shroud plates and the bundle’s support system, to limit steam bypass, is very important.
A critical re-examination of all materials used, their chemistry and their thermal expansion characteristics, can go a long way toward eliminating flow-assisted corrosion and structural failures due to stress concentrations. So can controlling fluid velocities, and streamlining flows throughout the MSR.
Replacing older 4-pass reheaters, a rearrangement of the flow passes is now possible where heating steam condensate flows downward by gravity, greatly reducing the amount of excess steam required to purge the condensate into its drain system. Also, manually operated throttling valves can be installed in the fourth pass condensate drain lines of all HP and LP reheaters instead of the fixed-flow orifices previously used. These valves, in conjunction with integral temperature sensors in the outlet ends of selected reheater tubes, permit occasional manual trimming to optimise, and usually minimise, excess steam flow through the reheaters, and maximise overall MSR thermal performance. A case in point is Almaraz in Spain where unit 1 MSRs with the older flow-pass arrangement are inferior to unit 2 MSRs with the advanced flow pass design.
A review of the original design criteria will frequently reveal other areas where meaningful operating and performance improvements can be made, such as higher vibration endurance limits for MSRs which are subject to power uprate with higher steam flows. Modern designs also allow higher heating rates during hot starts.
As regards enhancing performance, achieving higher steam superheat levels through drastically reduced TTDs has already been mentioned. This reduced cycle-steam pressure drop can be attained by eliminating the tortuous cycle steam paths frequently encountered in older MSR designs.
In some older designs, moisture separation was often assumed to be 100% efficient. The actual separation in those designs was never attained – it rarely exceeded about 85% – so it adversely altered calculated cycle heat balances. High performance double-pocket, chevron-type moisture separators, protected by a perforated plate to provide good incoming steam distribution and counteract aggressive steam flow, now produce virtually complete moisture separation. This prevents wasting throttle-steam thermal energy needed to reboil moisture on reheater tubes. In fact, there have been instances in some older plants that used only moisture separators (no reheaters) where the installation of these modern moisture separation systems have added nearly 1% to the plant’s MWe output.
In addition to the three approaches – refurbishment, internals redesign and reconstruction, and complete replacement – there are other considerations to be taken into account.
Required plant outage time to complete an MSR upgrade project is in most cases critical. Obviously, a complete MSR replacement option involves the least time. For example, the replacement of the MSR at North Anna 2 was accomplished in about 16 days. This is less time than has been experienced in other MSR refurbishment and redesign/reconstruction projects which typically last 25 to 32 days.
Another consideration is to protect new refurbished LP turbine sections. The investment in an improved MSR is modest in comparison to the cost of new LP rotors, and an efficient MSR contributes significantly to the longevity and performance of the machine by extending the dry state of the steam and reducing susceptibility to stress corrosion cracking.
The expected payback on the MSR portion of an uprate programme – typically two to three years – requires it to be totally integrated with the overall plant uprate programme. In the Hatch uprate/upgrade programme, increased throttle steam bypassed the HP turbine and was routed directly to the MSRs. This eliminated the need for costly major HP turbine modification and is a prime example of the benefits of total plant-uprate/MSR-upgrade integration. In fact, without such integration the uprating may not be fully realised.