Cooling considerations29 March 2017
Cooling is king in nuclear power. Marcus Saul discusses how modular solutions can be leveraged during plant maintenance, outage and decommissioning projects.
Nuclear is arguably the most important element of the modern electricity mix – it is a reliable, low-carbon form of electricity that is not reliant on the weather or an abundance of natural resources. Currently, nuclear power contributes about 11% of global power; about 21% in OECD countries.
As it is not possible to convert all the heat generated in a nuclear reactor into electricity, the safe operation of a nuclear power station depends on the efficiency of its cooling systems. To ensure safe and smooth operation, heat has to be continually transferred from the reactor, so cooling is a key issue for plant owners.
The cooling method used varies with location. Water-based cooling systems are used in all boiling water reactor (BWR) and pressurised water reactor (PWR) plants.
In the USA, Aggreko also provides the nuclear industry with oil-free air compressors, which dry-cool equipment, for things such as redundancy in instrument air and valve operation, steam generator sludge lancing, and integrated leak-rate testing. In the UK, carbon dioxide or water-based cooling systems tend to be more common.
Maintaining equipment throughout the plant lifecycle is vital to ensuring the safe running of a plant. The most common need for modular cooling equipment during this process is to cool the containment building where the reactor is housed.
Workers have limited time to spend on essential tasks due to heat stress limitations.
Our work during plant maintenance ensures comfortable working conditions. Our turnkey solution comprises two generators of 350kVA. Each feeds a 750kW chiller, which contains glycol water cooled to about 3°C before being pumped through a specially developed air battery and distributed through an air ventilator. This method of cooling the containment building decreases downtime for maintenance by six days and saves an average of £5 million per maintenance cycle.
On occasions we have also offered a total substitute for the on-site cooling tower; on these occasions speed of response has been essential to ensure that equipment does not get too warm and to prevent the plant shutting down. We have worked to integrate heat exchangers, chillers and pumps into the plant’s existing systems, which eliminates downtime completely.
In the USA, the majority of our nuclear projects relate to outage management. Our cooling projects, as well as some power and compressed air projects, aim to reduce outage days incurred. During a refuelling outage at a nuclear station in Michigan, the turbine building was due to undergo modifications that required its air conditioning system within the building to be shut down. This meant the turbine building, which was already uncomfortably warm, would become hotter and create a working environment restricted by heat stress, prolonging the outage.
Our engineering group, Aggreko Process Services (APS) and the nuclear station’s outage project team worked together to develop and install a temporary cooling system consisting of modular mechanical refrigeration units and skid-mounted air handlers, diesel-powered generators and over half a mile of hose and ducting. This achieved the desired temperatures throughout the building.
Radwaste and decommissioning
Spent fuel continues to generate heat as it decays and must be continually cooled to make it safe. The decay heat is removed from the spent fuel pool (SFP) via a series of heat exchangers and the plant component cooling water system. By keeping the temperature down, evaporation is minimised and the water remains deep enough for proper radiation protection. The spent fuel is stored until it cools enough to be transferred to an Independent Spent Fuel Storage Installation.
Maintaining a consistent ambient temperature is equally important during the three to five years it takes to decommission a nuclear plant. Where a reactor is being decommissioned cooling solutions can be provided using heat exchangers, chillers or cooling towers that will reduce, or eliminate, the need to run costly onsite pumps for the service water.
We worked closely with EDF to ensure the safe decommissioning of the Dungeness A plant in Kent, UK. We specified and installed two 800kVA generators, along with a bulk fuel tank and an isolating transformer. These generators and chillers ensured stable and suitable conditions for the service teams, so the power station could be decommissioned in a safe and timely manner.
During the cooldown of the plant to cold shutdown state, the dissipated heat transported by the nuclear service water reaches a maximum, as the stored heat and the residual heat from the primary circuit will be transferred to the ultimate heat sink.
Author information: Marcus Saul is Aggreko business development manager for Marine Renewables & Nuclear