Since the Great East Japan earthquake, nuclear power plant operators around the world have been reviewing installed safety systems against revised industry guidelines on ‘extended coping’ during station blackout scenarios.
For the Japanese nuclear industry which has been a national strategic priority since 1973, installing best-in-class safety technology is essential to getting the industry back on its feet. Currently 43 reactors are still operable, 24 of these are in the process of restart approvals.
Reactor 1 at the Sendai nuclear power plant was declared safe to operate in September 2014 and reached full capacity in August 2015. Reactor 2 was started soon after and started operation in November 2015. To the benefit of everyone in the industry, much has progressed since 2011. Globally, many power plants have installed additional safety equipment to ensure essential cooling water systems have further redundancy, improved reliability and are able to operate for extended times during blackout events.
There are a few different configurations that can be considered for essential core cooling on BWR (Boiling Water Reactor), RCIC (Reactor Core Isolation Cooling) service and PWR (Pressurised Water Reactor), AFW (Auxiliary Feed Water) service. The most important factor in any chosen system is that it has to be able to function with the minimum assistance of auxiliary service e.g. AC/DC power, cooling water, oil systems and pressurised air.
All such services could be lost during a blackout event potentially rendering safety systems inoperable.
The driver for any pumping system needs power. In most cases the driver is an electrical motor which relies on a managed electrical power supply. Diesel engines can be used as drivers which require fuel storage and delivery and starting systems often relying again on electrical power.
Turbine drives are also used for RCIC and AFW systems and others. Steam is available from the reactor or steam generator during a shutdown event and this steam is a good source of available energy for driving a pump.
Though the steam is readily available, the turbine drives still rely on auxiliary services such as DC power to provide turbine governing and control as well as oil lubrication.
Produced by SPX FLOW's Glasgow-based ClydeUnion business, the TWL™ pump (Turbine Water Lubricated) is a unique turbine driven pumping solution that does not require any supporting services making it an excellent choice for additional or replacement safety systems for RCIC and AFW services. Originally designed for boiler feed service on naval applications, it has been supplied as safety equipment in nuclear plants around the world since the 1970s. Since then the TWL pump has been used as an additional safety system (or ACIS – Alternative Cooling Injection System) going above existing guidelines by offering an additional safety system.
A TWL ready for despatch to a power plant in Asia.
The key features of the TWL are monoblock single casing construction with the turbine and pump rotating parts on a single shaft with no shaft seals or drive coupling. The pump incorporates a full mechanical governor and steam control mechanism requiring no electrical power or pneumatics as well as a product lubricated bearing system that eliminates the need for an oil tank and controls. Single casing eliminates couplings and leakage paths thus reducing seals wear, which in turn reduces maintenance requirements.
The TWL has an integral overspeed protection system – all in a single skid thereby saving space in the power plant. Recently the TWL has undergone a complete design review looking at enhancements and improved manufacturing techniques without detracting from the key features and benefits of the existing robust design.
Fundamental to the TWL operation is the mechanical governor and throttle mechanism, essential in controlling pump speed and product volume flow across the operating range.
The discharge branch of the pump incorporates a Venturi, a flow measurement device fitted in the discharge that is used to monitor flow and pressure. It communicates with the steam side to control pump and provides a pressure signal proportional to pump flow. The pressure governor converts this ‘signal’ into mechanical action which acts on the throttle mechanism which controls the steam flow to the turbine.
As there are no electrical speed control devices involved, the TWL starts rapidly on admission of steam and reaches its operating speed in under 10 seconds. The design can accommodate water slugs in the steam line, both at start up and while running with no detrimental effect on the equipment and only momentary interruption of pump performance.
Pump and system protection
For pump and system protection the TWL incorporates a mechanical overspeed trip system that shuts off the steam supply through mechanical and pressure balancing actions should there be any transient that forces the unit into an overspeed situation.
All the associated components for this system are within the confines of the skid with no additional valves required. In addition to the mechanical trip system an electrical trip system is provided for extra protection when electrical power is available. There is also a manual trip facility that can be used to stop the TWL at any time.
The pump/turbine shaft does not penetrate the casing boundary; rather it is completely contained and thus does not require sealing which is critical when considering contaminated water applications.
The rotor with its single turbine wheel at one end and impellers at the other is supported by bearings between the pump and turbine components. The bearings therefore are inside the casing boundary, they are water lubricated and are flushed with water taken from the pump side. The water taken from the first stage of the pump passes through a non-clogging cyclone separator to ensure no harmful debris can damage the bearings.
The single casing design of the TWL, without shaft protrusions is vital. It ensures that the equipment can run safely and efficiently while fully submerged. There are no leak paths for flood waters to enter the pump or turbine and no dependence on electrical systems.
In February 2013, ClydeUnion conducted a number of tests on a TWL pump whilst submerged demonstrating its capability to be started and run in a flooded environment. The test included starting the TWL whilst completely submerged.
It also entailed flooding the pump when running at operational conditions, followed by eight hours of continuous running in the flooded condition. All of this was achieved with no significant degradation in performance or detrimental damage.
Recently, GE Hitachi Nuclear Energy (GEH) delivered two TWL units to Tokyo Electric Power Company's Kashiwazaki-Kariwa Nuclear Power Plant. GEH and ClydeUnion Pumps have a teaming agreement to offer the TWL pumps and additional commitments have been made to support systems for Chugoku Electric Power Company's two Shimane plants, and Tohoku Electric Power Company's Onagawa and Higashidori plants. For the Japanese nuclear industry getting back on its feet after a difficult period, the enhanced safety features integrated in the TWL pump offers firm evidence of the focus on safety as well as a benchmark for the future.