POWER PLANT COOLING | UPRATING BLACKOUT RELIABILITY Proper operation of the AFW system in a PWR
traditionally requires DC power from a station battery, steam from the steam generators and an adequate supply of secondary plant water. The total amount of current draw on the station battery can be a limiting factor in determining the duration of SBO coping.
Coping without power Clyde Union Pumps (CUP), part of Celeros Flow Technology, has concentrated its efforts on improving the design of its turbine/pump set for RCIC applications. In addition, it has worked with Westinghouse to incorporate such turbine/ pump sets to provide an improved plant AFW system that addresses many of the concerns raised by the Fukushima accident.
Above: The CUP TWL™
pump
offers extended coping capability under SBO conditions without supporting services (AC or DC supply)
periods was a key reason why the Fukushima accident developed to a point where explosion was inevitable. The preliminary investigations and subsequent reports following the Fukushima incident considered several pumping solutions.
Pumps and nuclear safety The role of flow control equipment in the safe operation of nuclear power stations cannot be underestimated. Pumps form one of the most critical elements in a nuclear plant. Large-capacity pumps and high-pressure pumps used in domestic PWR nuclear power plants include reactor (primary) coolant pumps, charging pumps with safety functions, safety injection pumps and residual heat removal pumps in the nuclear island, as well as main feedwater pumps, condensate pumps and circulating water pumps in the turbine island. The most safety-significant systems in the case of a
station black-out are the RCIC systems for boiling water reactors (BWRs) and the auxiliary feedwater (AFW) system for pressurised water reactors (PWRs). The steam-driven water pump in these systems is typically the frontline component that addresses SBO and provides core heat removal.
The pump is a single wheel steam turbine, two-stage pump designed to ASME III Class 2 & 3 requirements. Both the turbine and impellers are mounted on a rigid shaft, supported by a central bearing assembly integral within a monobloc turbine/pump casing. It is one-third to one-half the size of a conventional pump and turbine unit, which makes it ideal for congested nuclear islands where space is at a premium. This compact design will also withstand water slugs in the steam line. The pump provides required performance pertaining to
discharge head, flow rates and required steam conditions. It has a capacity of up to 350m3
/hr (1550 USgpm) and a
delivery up to 1300m (4265 ft). This pump therefore exceeds existing PWR installation requirements (1200 psig and 900+ gpm), and the set is extendable for power uprates. These pumps are designed to operate with no external
lubrication, no external cooling water, no drive coupling, no barometric tank or vacuum pump and – more importantly – no electrical connections. These units also incorporate self-contained turbine governor/controls and overspeed trip mechanisms within one package. However, the main feature of the redesigned CUP pump
in the context of improved nuclear safety post-Fukushima is its extended coping capability under SBO conditions without supporting services (AC or DC supply). Even the control at partial flow is much simpler, since there is no need for repeated operator intervention to adjust the flow. This saves precious time and enables personnel to concentrate on other essential items in the event of a SBO or other incident. The pump has a single casing designed with no shaft
protrusions, enabling it to start and operate while fully submerged. Tsting has shown that it will maintain safety performance and continue to operate with no adverse effects or detrimental changes to performance for a minimum of eight hours. During this time, the pump can be started and stopped, with no detriments to its performance. Power plant owners, operators, original equipment
manufacturers (OEM), designers and regulators today are acutely aware of the potential plant vulnerabilities that can lead to a complete loss of a unit’s electrical power due to multiple natural phenomena. Climate change means that the likelihood of combined extreme weather events is growing, making the ability to ‘ride out the storm’ a safety- critical design consideration. The relationship between cooling systems and the
Above: Coolant pumps can operate with no external lubrication, cooling water, drive coupling, barometric tank or vacuum pump and no electrical connections
34 | May 2024 |
www.neimagazine.com
availability of uninterrupted power supply in case of SBO should be of primary importance to nuclear power plant design and operation. ■
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