Small change, powerful gains

One way to improve the economics of nuclear power plant upgrades or lifetime extensions is to increase the net output of the plants. However, because most nuclear power plants have a restricted thermal power, the electrical output can only be increased by improving efficiency of the water-steam-cycle, the turbine/generator set, and improving the cold end; or with the reduction of house load and a more precise measurement of the reactor power.

This article looks at four alternatives for increasing the efficiency of a power plant with a boiling water reactor through improvements to the water-steam cycle. See below for a generic schematic diagram, this page; changes are mapped on a second schematic on p. 38.

Improvements are possible because existing components have been designed based on conventional plant and thanks to modern design tools and software (like computational fluid dynamics, or CFD), new technologies (for example, ribbed tubes) and new materials.

Efficiency can be increased by:

¦ Improving steam conditions through replacement of the main steam isolation valves

¦ Replacing a single-stage moisture separator reheater (MSR) with a double-stage one

¦ Increasing the feedwater temperature by replacing the existing preheater with one with a larger heat transfer surface

¦ Optimizing preheater degassing and degassing route changes.

This article focuses on BWRs, but similar improvements are also possible in pressurized water reactors (PWRs), since in general the water-steam cycle of a PWR is the same.

Replacement of MSIVs

Improved steam conditions can be achieved by decreasing the pressure drop in the main steam line. One way to do this is to replace the main steam isolation valves. Gate valves usually have a lower zeta-value (pressure drop) than globe valves. In a 1300 MW BWR, replacing globe-based main steam isolation valves (MSIVs) with gate valves can will reduce the pressure drop by about 1.5 bar, resulting in a higher steam temperature at the HP turbine inlet and an increased electrical output of around 3.5 MWe. Studies on a 900 MW BWR in 2002 showed similar results: the pressure drop was decreased by about 1.9 bar and the electrical power rose about 3.2 MWe. AREVA is currently replacing the inner MSIVs in the 600 MW Oskarshamn 2 boiling water reactor for a 106% to 135% power uprate as part of Project PLEX.

Replacement of MSIV is typically carried out with adaptation of HP turbine and the turbine control valves, or in conjunction with a power uprate. These gate valves have been replaced in at least five Swedish and two Finnish BWRs.

Double-stage MSR

The Moisture Separator Reheater (MSR) is usually installed between the HP turbine outlet and the LP turbine inlets to remove the moisture from the HP turbine exhaust steam and to reheat the steam before being admitted to the LP turbines. It plays a key role in preventing erosion and corrosion in the low-pressure turbine.

With the implementation of a double-stage moisture separator reheater, the first stage of the MSR will be fed by the first extraction of the HP turbine rather than from the main steam system. This means that more steam flows to the HP turbine (around 40 kg/s) that can be converted into electrical energy. A study for a 900 MW BWR plant in 2002 showed that this optimization would lead to a 5.1 MWe electrical power increase.

Preheater replacement

Investigations conducted at a 500 MW BWR plant have shown that it is possible to increase the heat transfer surface in the preheater up to 15% in comparison with the existing design. Studies have shown that new technologies, optimized design and new materials can produce more effective preheaters.

While the geometric design of the preheater is kept largely the same (1.5m shell diameter reduced to 1.3m), the number of the 2mm-thick tubes can be increased from 620 to 672, and their size can be increased too, from 15mm to 16mm outer diameter. These changes increase the heat transfer area from 627 m2 to 730 m2, increasing the heat transfer power by 15% with the same pressure drop as before. This change could enable an increase of electrical power by up to 1.5 MWe per unit.

A study for the optimization of a preheater found that a higher heat-transfer area, which decreases the thermal temperature difference (TTD), enables an increase of power. In this example, a reduction of TTD from 5 K to 3 K yielded a 1.1 MWe increase in electrical power.

Preheater degassing

In BWR nuclear power plants, non- condensable gases are found in all systems which are supplied by the main steam system. These gases can accumulate in equipment such as feedwater heaters, reducing their heat-transfer performance. Although the issue was taken into account during the design of the condenser and feedwater tank, components such as feedwater heaters have historically been designed on the basis of experience from fossil-fired power plants.

In the course of investigations on a 1300 MW BWR it was found that within the preheater only 50% of the total heat- exchange surface was actually being used for heat transfer. The poor efficiency of the heat-transfer process was shown by the temperature difference between the auxiliary condensate and the saturated steam: 12 K. Inert gas content was calculated to be approximately 43%.

AREVA was instructed by the plant operator to perform a study to determine how to eliminate gas that accumulated inside the heaters. The probable positions of the gas bubbles were calculated by pressure drop calculations in different levels of the preheater. With the known positions of the gas accumulations, a concept for preheater venting during plant operation was developed together with the operator and a hardware supplier. This work included pipe cut-off, pipe elimination, introduction of degassing pipes and connection to the condenser system.

After installation of the optimized preheater degassing main and auxiliary, heating steam flow increased by 11 kg/s and condensate temperatures increased by about 7 K from 88°C to 95°C. In addition, auxiliary condensate flow in preheater A3 decreased by 13 kg/s. This means that an averaged amount of 12 kg/s steam is routed to the A2 extraction line. Due to the better heat transfer of preheater A2 the main condensate at the inlet of preheater A3 has a higher temperature; therefore the temperature difference at A3 decreases. With an enthalpy difference of approximately 160 kJ/kg between both extractions, the electrical output has been increased by almost 2 MWe per preheater. Also, since the inert gas content decreases from 43% to 15%, the effectively utilized heat transfer surface rises from 50% to 89%. With that enlarged surface area, the heat-transfer capacity of preheater A2 was raised from 75 MW to 95 MW.

Through optimization of all six low-pressure preheaters, plant efficiency was improved and the electrical output was increased by 9.1 MWe. The justification period for this investment was less than one year.

Degassing route changes

Traditionally, preheater degassing lines in a BWR run to the condenser. AREVA investigated several measures to increase the efficiency of the plant, including optimizing the routing of the auxiliary condensate and degassing. First, AREVA investigated the efficiency of routing the degassing line of preheater 5 to the feedwater tank instead of to the condenser. AREVA carried out this investigation with help of software package KRAWAL to model the entire water-steam cycle. Analysis of this model showed that switching the destination of the degassing line would feed an extra 7.7 kg/s steam to the feedwater tank. Therefore the feedwater tank would draw 8.1 kg/s less steam from turbine extraction 4. Consequently, the steam mass flow from the HP turbine to the LP turbine increases by 8 kg/s; at the outlet of the low pressure turbine the steam mass flow is about 7 kg/s greater. With this higher mass flow, the power of the low pressure turbine will be increased, and electrical power rises by about 4.4 MWe.

Furthermore a forward pumping of the condensate of the preheater A1 and A2 would increase the electrical power by 0.3 MW. In addition, operating the steam ejector with 5.3 kg/s feedwater tank degassing steam instead of main steam would increase power by 4.3 MW.

Outlook

The measures mentioned above show that it is possible to increase the efficiency of a boiling water reactor, potentially by 30 MWe, with comparably little technical and financial effort. At planned component replacement intervals (for example, during lifetime extension), components with improved design, and/or improved system design, should be considered instead of a like-to-like replacement. For new- builds, a complete optimization of the water- steam cycle should be considered.

Author notes

Sören Künne and Wolfgang Schuch, AREVA GmbH, PESB-G BWR Systems Engineering, Offenbach, Sweden
This paper is based the presentation 'Efficiency increase by optimization of water steam cycle' at Jahrestagung Kerntechnik 2014, 6-8 May 2014, Frankfurt am Main, Germany.