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District heat from nuclear

Fortum Corporation
Combined heat and power (CHP) generation at Fortum’s nuclear power plant site in Loviisa could help cut Finland’s CO2 emissions by as much as 6%. Cogeneration in a nuclear plant on such a large scale is completely feasible and would be safe as well.

Fortum recently applied for permission to build a third reactor ranging in size from 1,000 to 1,800 MWe at its Loviisa nuclear site east of Helsinki. By combining heat and power into the design specification, the unit could generate some 1,000 MW of district heat. Intended for Greater Helsinki, this would be sent 77 kilometres west along pipes in a tunnel excavated into the bedrock.

The possibility of generating district heat for Helsinki using Loviisa’s existing reactors was investigated back in the 1980s, but the project proved unfeasible at the time. The concept looks much more attractive today, however, when one realises its potential for taking a significant chunk out of the country’s CO2 emissions, as much as 6% in fact.

Nevertheless, the project would be a challenge. Only relatively small volumes of district heat are generated by nuclear power worldwide at the moment, ranging from 10 to 200-300 MW. Set against this, the Loviisa 3 CHP concept is much more ambitious, not only because of the much larger output envisaged, but also because of the distance that the district heating water would have to be piped.

A completely closed circuit

In a conventional nuclear power plant design, all the steam generated is fed through a turbine to generate electricity, just as it is in condensing power plants fired on fossil fuels. Fortum’s concept for Loviisa 3 calls for the construction of a completely separate, closed district heat circuit. Steam taken from the turbine would be used to heat district heat water to around 120 °C, which would then be pumped to Helsinki, where the heat would be transferred through heat exchangers and sent for distribution via the local district heat network. The approximately 60 °C return water would be pumped back to Loviisa and reheated, and the cycle would start over again.

Graph 1: All district heat systems would be completely separate from the plant’s radioactive circuit.

Graph 2: District heat would be transferred from Loviisa to Helsinki along over 70 kilometres of tunnel.

An optimised turbine solution

Integrating district heat generation into the Loviisa site would have no direct implications for the new reactor itself. A number of changes to the design and configuration of the turbine island, compared to a conventional equivalent, would be required, however, to enable it to handle both electricity and district heat needs and the very large quantities of heat involved. It would be necessary, for example, to dimension the internals of the turbine so that the unit could also handle all the steam coming from the reactor to generate purely electricity at high efficiency.

Optimising turbine processes to be capable of generating as much electricity as possible would also be of central importance. When a proportion of the steam is used for district heating purposes, electrical output would drop by around a sixth in proportion to the district heat generated. In other words, generating 1,000 MW of district heat would reduce electrical output by 160-180 MW. CHP generation would increase the plant’s overall energy efficiency significantly, however.

The same CHP configuration would work equally well with both a pressurised water reactor (PWR) and a boiling water reactor (BWR). Both types of reactor would use two physical barriers to prevent district heating water becoming contaminated with radioactivity. A BWR plant would also need a separate intermediate circuit because of the radioactivity inherently present in the turbine processes in a BWR system. As the turbine processes in a PWR plant are isolated from the radioactive reactor circuit during normal use, a district heating circuit could be connected directly to the turbine via heat exchangers.

Regardless of the type of reactor, the district heating water produced would never come into contact with the radioactivity of the reactor circuit. Pressure flows would also be designed to ensure that, should a pipe fail, water would always flow back towards the turbine.

CHP generation at Loviisa would be highly energy-efficient and reduce the site’s environmental impact by cutting cooling water discharges into the Gulf of Finland.

Modelled using APROS technology

One of the biggest technical challenges in the concept is the district heat transfer system, as it would require two large pipes around 1.2 metres across in a tunnel excavated out of the bedrock extending over tens of kilometres. Between four and seven pumping stations would be needed along the route to maintain sufficient pressure.

Fortum has simulated how the system would behave using APROS (Advanced Process Simulation Environment) software, analysing safety issues that need to be taken into account to provide protection against unexpected incidents. This work has evaluated how a fracture in the district heat circuit would affect pressure in the system, for example, and the types of transients that could be expected in the event of pump failures. The results from these simulations will be used to develop safety systems to limit the impact of any transients and calculate things like structural strength requirements.

The district heat transfer system will be integrated into a broader model encompassing the entire plant for evaluating overall safety to ensure that any problems in the district heat system would not be a safety issue for Loviisa 3 as a whole.

Major cut in CO2 emissions

As the average level of district heat offtake in Greater Helsinki is in the order of 1,300 MW, with peak levels of 3,000-3,500 MW, Loviisa 3 would be capable of providing a very significant proportion of base load needs.

Current CHP capacity in Greater Helsinki is based on coal and natural gas, and produces 5-7 million tonnes of CO2 emissions annually. Generating district heat in Loviisa 3 would cut this figure by up to 4 million tonnes. In addition, CHP generation would also increase the overall efficiency of the new unit at Loviisa significantly and reduce the environmental impact on the local marine environment by cutting cooling water discharges into the Gulf of Finland.

> Nici Bergroth
(Published in HighTech Finland 2010)