NuGas: adding a CCGT to a nuclear power plant

In the course of two years' development, power consultancy PB Power (PB) has created a new thermal cycle. Linking the saturated steam cycle of a nuclear plant to a CCGT (combined cycle gas turbine) steam cycle, this innovative NuGasTM cycle achieves an exceptional gas to power efficiency. For example, using prudent design practice, an F Class gas turbine in the NuGas cycle would deliver an efficiency of around 62%, compared with a CCGT plant at around 57%.

The business implications of achieving such an efficiency gain at capital costs comparable with a conventional CCGT are substantial.

Continuing a progression

From a historical perspective the development of the NuGas cycle continues a progression from the earliest combined cycles: the basic single pressure concept yields a typical steam cycle efficiency of about 23%, whereas modern three pressure reheat cycles produce about 30% on a like-for-like comparison. NuGas goes further, lifting the efficiency of the steam cycle to 43%, yielding significantly increased overall capacity for the same energy input.

The evolution of the CCGT cycle illustrates how thermal cycle engineers have developed the integration of the gas turbine Brayton cycle with the steam plant Rankine cycle to extract the maximum energy from the gas turbine exhaust gases. However, each step of enhanced performance has increased steam cycle complexity and cost. NuGas maximises the work derived from the high temperature energy in the gas turbine exhaust gases by avoiding boiling in the HRSG, considerably simplifying the steam and water cycle compared with a modern CCGT.

NuGas is thermodynamically equivalent to a supercritical cycle, which has similar steam cycle efficiency, but avoids the extreme conditions, exotic materials and operational limitations associated with supercritical plant.

How does NuGas work?

NuGas links the two steam cycles to 'borrow' steam from the nuclear plant. The most widely used reactor type is the pressurised water reactor (PWR), which generates dry saturated steam isolated from the reactor. NuGas takes a small part of this steam and passes it through an oversized superheater in the gas turbine HRSG. The steam, at typically 540ºC and 62 bara, is expanded through a conventional steam turbine to its condenser. Part of the condensate is returned to the PWR condensate system while the balance is heated in the HRSG economiser and returned as PWR feedwater, maintaining the mass balance of the overall cycle.

The steam borrowed from the PWR slightly de-loads the nuclear cycle steam turbine. However, the increased output of the CCGT steam turbine more than compensates for this offset and the overall installation shows increased total electrical capacity and improved input energy utilisation. The overall PWR and CCGT installation has a positive environmental impact thanks to the minimisation of additional heat and exhaust emissions rejected to the environment.

Figure 1 shows an outline of the NuGas cycle. The flows of energy around the cycle differ somewhat from those in a conventional CCGT.

Figure 2 shows a simplified Sankey diagram for the NuGas cycle, including the energy exchanges between the CCGT and PWR cycles.

Integration of the PWR cycle requires that the PWR energy balance should be maintained. Thus the CCGT returns power to the PWR to compensate for the reduction in output due to the 'borrowed' steam and returns rejected heat in the CCGT cooling water to the PWR to account for the reduced heat rejection from the nuclear turbine condenser. The diagram therefore shows the additional energy input, the additional losses and the additional power generated by the cycle, demonstrating its high efficiency.

The Sankey diagram illustrates one of the key features of the NuGas concept – the exceptional efficiency achieved in converting gas to power. The other important aspect of the NuGas concept is achieving a simple, safe and effective interface between the cycles.

Co-operation between the cycles

The integration of the CCGT and PWR steam cycles must avoid introducing new operating interactions which could adversely affect the pressurised water reactor. NuGas goes much further than merely mitigating the interactions, it provides mutual independence of the cycles. Thus upsets in either cycle do not affect the safe operation of the other. There are five operating modes of the two cycles:

The two thermal cycles operate independently in four of these operating modes. The NuGas plant includes elements within the HRSG to allow unlimited CCGT operation where steam is raised in the HRSG to drive the high temperature steam turbine independently of the PWR cycle. This would be the normal operating mode for start-up of the CCGT plant.

Start of NuGas operation would always begin from parallel and independent operation of the two cycles. NuGas operation reverts to parallel independent operation on planned shutdown or to CCGT-only or PWR-only operation in the event of a breakdown.

Hazard analysis of the operating modes and their transitions has demonstrated the robustness of the concept, with conditions remaining within conventional limits under all the transient conditions, including following a reactor trip event when steam from the PWR cycle is lost while the gas turbine remains at full output.

By maintaining the independence of the cycles the NuGas concept delivers key benefits including:

• elimination of impact on the nuclear safety case of CCGT faults or breakdowns;

• limiting the lost capacity to the grid to less than 104% of PWR rating in the case of a reactor trip;

• sufficient flexibility to enable despatch of the CCGT capacity independently of the PWR.

Without these intrinsic strengths the NuGas cycle would be unacceptable operationally.

Safely linking the cycles

As Figure 1 shows, the cycles require linkage by three pipes. In other respects the additional plant required for the NuGas cycle has only minor interconnections, eg make-up water for start-up. The electrical auxiliary systems are only linked at the high voltage substation and the control systems are independent and do not depend on the exchange of signals for safe or reliable operation.

While the interconnections are limited, new or undesirable upsets to the nuclear plant must not be introduced, even in case of major failures. Limiting the steam flow taken from the nuclear cycle to close to 10% of the steam generator evaporation reflects this objective. PWR plants are already designed to tolerate a 10% instantaneous flow change without exceeding normal operational conditions. Thus even severe failures of the NuGas plant would not cause disturbances outside the normal operational provisions of the PWR safety case.

The impact of failures downstream of the interfaces is managed by the provision of suitably qualified and redundant shut-off valves and associated sensors. These will give a very high level of assurance that any adverse event will be quickly controlled. The introduction of new pipe connections, subject to proper QA procedures, should introduce no new failure conditions, while the pipework through the nuclear facility can be readily designed to ensure that there should be no increase in risks from a pipe break. Beyond the range of damage from pipe failure, the interconnections and plant can follow conventional CCGT standards and practice, achieving highly competitive cost levels.

The direct effects of the linkage of the cycles are not the only concern; the adjacency of the two plants requires risk analysis. Location risks are managed by providing an adequate separation distance, typically over 100m, between the new plant and any safety-critical nuclear plant.

Fire or explosion hazards arising from the fuel gas can be safely addressed. The gas supply capacity is relatively small and the stored volumes of gas between the transmission connection point and the gas turbine are limited. For the scale of plant envisaged, the lethal range of the worst case jet fire from a gas line break is significantly less than 100m. The risk from gas explosion from the small gas connection is lower than has been proven safe for several existing nuclear stations where larger gas transmission pipelines run nearby.

There are known risks of turbo-machinery failure generating projectiles that can damage other plant. These risks are managed in the conventional way by aligning the turbines so that sensitive plant lies outside the likely impact area for any projectiles. It can be seen therefore, that safe linkage of the two cycles and location of a CCGT adjacent to an operational PWR plant is feasible, with low costs incurred since the application of nuclear standards is limited to the interface components.

The robust simplicity of the interconnection enables existing or new PWR plants to benefit from the NuGas cycle.

Future developments

The NuGas cycle can be independent of the heat source used, indeed the HRSG could use forced draft fans and directly fired fuel or any generation of gas turbine technology. Since the efficiency of the NuGas cycle results from both the GT and steam cycle efficiency, use of a more advanced gas turbine will offer a higher NuGas efficiency. Hence in future there is real potential for NuGas cycle efficiencies of over 65% as GT technology progresses.

The other potential enhancement offered by NuGas is for the PWR cycle where developments in fuel design offer a progressive thermal uprating of the reactors. Up to the capability of the steam turbine and generator the additional heat can be used within the PWR cycle itself. However, beyond this point the only options are to increase capacity by plant modification or replacement, involving extended outages that are often not commercially viable. The alternative, where a NuGas cycle is installed, is to use the capacity in the de-loaded PWR steam turbine, enabling the additional reactor heat to be exploited with neither additional investment nor extended shutdown, releasing the value of the reactor uprating.

Less risk, more efficiency

In summary, the NuGas technology enables plant owners and developers to benefit from a larger increase in fuel efficiency in a CCGT cycle using proven technology than offered by the technically riskier advanced gas turbines or supercritical cycles. The benefit of this efficiency gain enables power to be generated at a cost 10% below that of competitive plant at the same fuel price.

The links between the CCGT and nuclear plant turn out to be relatively simple, with benign operating modes and simple operation on upset in either plant. The NuGas cycle has minimal impact on the PWR safety case and only marginally increases the grid single contingency capacity loss above that of the PWR itself, while offering flexibility of despatch for the additional capacity.

Since NuGas is a thermodynamic improvement that can be applied to any generation of GT, a NuGas cycle will always offer a significant gain in efficiency over a conventional CCGT. Its creation marks the start of the realistic pursuit of cycle fuel efficiencies of over 65%.