Filtration in nuclear power

Until recently the nuclear industry in the United States and parts of Western Europe has been dormant or in decline. At the same time, nuclear capacity has been expanding in Eastern Europe and Asia. We examine the part that filtration technology can play in the nuclear industry, and the future of this market.

Nuclear power plants rely on efficient filtration of air, water, and process fluids to operate reliabily. Older units are being upgraded with the latest reverse osmosis systems to improve reliability. Improvements in absolute filters for capture of radioactive particles insure greater safety.

Nuclear industry trends 

Until recently the nuclear industry in the United States and parts of Western Europe has been dormant or in decline. At the same time, nuclear capacity has been expanding in Eastern Europe and Asia so that globally the share of nuclear power in world electricity has remained constant at around 17 percent since the mid 1980s, with output from nuclear reactors actually increasing to match the growth in global electricity consumption.

Today nuclear energy is back on the policy agendas of many countries, with projections for new nuclear plants that are similar to or exceeding those of the early years of nuclear power. There are more than 30 plants under construction, 94 planned reactors and 230 proposed reactors. This activity is driven by the following:

  • The growth in the demand for energy: global population growth in combination with industrial development will lead to a doubling of electricity consumption by 2030. Besides this incremental growth, there will be a need to renew much of the generating stock in the US and the EU over the same period. An increasing shortage of fresh water calls for energy-intensive desalination plants, and in the longer term, hydrogen production for transport purposes will need large amounts of electricity and/or high temperature heat.
  • Global warming: Increased awareness of the dangers and effects of global warming and climate change has led decision makers, media and the public to believe that the use of fossil fuels must be reduced and replaced by low-emission sources of energy. Nuclear power is the only readily available large-scale alternative to fossil fuels for production of continuous, reliable supply of electricity.
  • Economics: increasing fossil fuel prices have greatly improved the economics of using nuclear power for generating electricity. Several studies show that nuclear energy will be a cost-effective base-load technology. As the nuclear industry moves away from small national programs towards global cooperative schemes and standardised designs, serial production of new plants and off-site fabrication techniques will drive construction costs down, shorten construction times and further increase the competitiveness of nuclear energy. In addition, as carbon emission reductions are encouraged through various forms of government incentives and trading schemes, the economic benefits of nuclear power will increase further.
  • Insurance against future price exposure: a longer-term advantage of uranium over fossil fuels is the low impact that increased fuel prices will have on the final electricity production costs, since a large proportion of those costs are in the capital cost of the plant. This insensitivity to fuel price fluctuations offers a way to stabilise power prices in deregulated markets.
  • Security of supply: a re-emerging topic on many political agendas is security of supply, as countries realise how vulnerable they are to interrupted deliveries of oil and gas. The abundance of naturally occurring uranium makes nuclear power attractive from an energy security standpoint.
  • Public acceptance: during the early years of nuclear power, there was a greater tendency amongst the public to respect the decisions of authorities licensing the plants, but this changed for a variety of reasons. No revival of nuclear power is possible without the acceptance of communities living next to facilities and the public at large as well as the politicians they elect.

The Chernobyl disaster marked the nadir of public support for nuclear power. However, this tragedy underscored the reason for high standards of design and construction required in the United States and Western Europe. It could never have been licensed outside the Soviet Union, incompetent plant operators exacerbated the problem, and partly through Cold War isolation, there was no real safety culture. The global cooperation in sharing operating experience and best practices in safety culture as a result of the accident has been of benefit worldwide. The nuclear industry’s safety record over the last 20 years is unrivalled and has helped restore public faith in nuclear power. Over this period, operating experience has tripled, from about 4,000 reactor-years to more than 12,500 reactor years.

Perhaps the most significant factor behind the shifting political and public acceptance of the need for building new nuclear power plants is the concern of global warming.

Types of nuclear power reactors

A nuclear reactor produces and controls the release of energy from splitting the atoms of certain elements. The principles for using nuclear power to produce electricity are the same for most types of reactors. The energy released from continuous fission of the atoms of the fuel is harnessed as heat in either a gas or water, and is used to produce steam. The steam is used to drive turbines which in turn drive generators to produce electricity (as in most fossil fuel plants).

There are several components common to most types of reactors:

  • Fuel: usually pellets of uranium oxide (UO2) arranged in tubes to form fuel rods. The rods are arranged into fuel assemblies in the reactor core.
  • Moderator: this is material which slows down the neutrons released from fission so that they cause more fission. It is usually water, but may be heavy water or graphite.
  • Control rods: these are made with neutron-absorbing material such as cadmium, hafnium or boron, and are inserted or withdrawn from the core to control the rate of reaction, or to halt it. (Secondary shutdown systems involve adding other neutron absorbers, usually as a fluid, to the system.)
  • Coolant: a liquid or gas circulating through the core so as to transfer the heat from it. In light water reactors the water moderator functions also as primary coolant. In PWRs there is a secondary coolant which makes the steam in steam generators. In BWRs the steam is made directly in the reactor vessel.
  • Pressure vessel: in a PWR type of reactor the pressure vessel is a robust steel vessel containing the reactor core, fuel rods, control rods and moderator/coolant. In a BWR it also includes a section to generate steam.
  • Steam Generator: in a PWR type reactor where the heat from the reactor is used to make steam for the turbine. In a BWR reactor the steam is made in the reactor pressure vessel.
  • Containment vessel: in a PWR type reactor the concrete and steel structure around the nuclear reactor, pressuriser, pumps and steam generators designed to protect it from outside intrusion and to protect those outside from the effects of radiation in case of any malfunction. In a BWR it contains the nuclear reactor and water pumps.
  • The Pressurised Water Reactor (PWR): this is the most common type, with over 230 in use for power generation and a further several hundred in naval propulsion. The design originated as a submarine power plant. It uses ordinary water as both coolant and moderator. The design is distinguished by having a primary cooling circuit which flows through the core of the reactor under very high pressure, and a secondary circuit in which steam is generated to drive the turbine.

A PWR has fuel assemblies of 200-300 rods each, arranged vertically in the core, and a large reactor would have about 150-250 fuel assemblies with 80-100 tons of uranium.

Water in the reactor core reaches about 325°C, hence it must be kept under about 150 times atmospheric pressure to prevent it boiling. Pressure is maintained by steam in a pressuriser. In the primary cooling circuit the water is also the moderator, and if any of it turned to steam the fission reaction would slow down. This negative feedback effect is one of the safety features of the type. The secondary shutdown system involves adding boron to the primary circuit.

The secondary circuit is under less pressure and the water here boils in the heat exchangers which are thus steam generators. The steam drives the turbine to produce electricity, and is then condensed and returned to the heat exchangers in contact with the primary circuit.

The Boiling Water Reactor (BWR): this design has many similarities to the PWR, except that there is only a single circuit in which the water is at lower pressure (about 75 times atmospheric pressure) so that it boils in the core at about 285°C. The reactor is designed to operate with 12-15 percent of the water in the top part of the core as steam, and hence with less moderating effect and thus efficiency there.

The steam passes through drier plates (steam separators) above the core and then directly to the turbines, which are thus part of the reactor circuit. Since the water around the core of a reactor is always contaminated with traces of radionuclides, it means that the turbine must be shielded and radiological protection provided during maintenance. The cost of this tends to balance the savings due to the simpler design. Most of the radioactivity in the water is very short-lived, so the turbine hall can be entered soon after the reactor is shut down.

Filter needs in nuclear power plants

Air filtration

The air filtration system at a nuclear plant has to cater for widely varying conditions and may include a multitude of elements, sub-systems and ancillary equipment. The strictest safety and protection regulations are enforced. Everything is designed for fail-safe operations.

The prevention of even extremely low concentrations of airborne contamination is fundamental to the safe operation of a nuclear facility and an important factor for cost efficiency. This is why the nuclear industry has made major contributions to the development of new and improved air cleaning equipment.

All air and other gaseous effluents are exhausted through a ventilation system to remove radioactive particulates and gases. This is where High Efficiency Particulate Air (HEPA) and carbon filters play a major role.

A nuclear air cleaning system protects the public and plant operating personnel from airborne radioactive particles and gases which might be generated or released by the nuclear reactor, during fuel fabrication or during radiochemical or laboratory operations.

Absolute and carbon filters are almost universally included in nuclear air cleaning systems. Protecting the health and safety of the public and plant personnel is the primary task. A secondary task is to avoid the high cost of decontamination and possible shutdown of a facility because of an accidental airborne release of radioactive material.

HEPA and ULPA (Ultra Low Penetration Air) filters are the primary air filters used for safeguarding the handling of nuclear products. Applications in nuclear power plants include control room emergency air supply systems and exhaust systems connected to containment vessels. Processing or manufacturing plants also control radioactive exhaust from prepared materials.

Because of the sensitive nature of the application, rigid guidelines establish Department of Defense Approval of HEPA filters manufacture, and may include testing by the Department of Energy.

The Nuclear Regulatory Commission (NRC) posts "Information Notices" to help guide the plants. One example is in regards to the deterioration of HEPA filters in a pressurised water reactor containment fan cooler unit. A low-flow alarm triggered an inspection of one of the five filter banks, each containing 64 2 X 2 ft. HEPA filters. Inspection of the unit revealed damaged filters which had failed due to lack of surveillance or review, collapsed because there were no internal stiffeners or corrugated separators, and had changed shape because of failure of seal between media and mounting frame. Corrective measures were taken, establishing a maximum six year life on replacement filters which were altered to include stiffeners. Additional review of filter efficiency was made to determine effectiveness of HEPA filtration when media remains wet.

Nuclear facilities have been a customer for Camfil’s Absolute filters ever since the early 1960s, when Camfil started to supply the Studsvik nuclear research facility, located not far from Camfil’s international head office in Trosa, Sweden. Over the years Camfil has become the leading supplier to the European nuclear power industry, notably in Belgium, Finland, France, Germany and Sweden.

In Scandinavia, Camfil supplies filters to four nuclear power plants in Sweden and two in Finland. The Finnish plants (TVO) have won a prize for being among the world’s safest and most efficient nuclear facilities (close to 40 percent of Finland’s power needs come from nuclear energy).

The Group’s French company, Camfil SA, manufacturer of Absolute filters for nuclear plants since 1967, supplies air filters, activated carbon cells and filter housings for virtually all nuclear facilities in France, a total of 59 reactors producing 60 GW of electricity (close to 80 percent of France’s power needs).

Filters and housings are also supplied to seen reactors in China, South Africa, South Korea and Spain. The French subsidiary supplied filters for two nuclear plants at the Lung Ao site in China.

Each country has its own design specifications, requirements and safety regulations for air cleaning systems for the nuclear power industry.

In Sweden, Camfil supplies Kombifilter, a combination Absolute/carbon filter, and Trippelfilter, a three-stage filter consisting of a special impregnated carbon filter that is sandwiched between tow HEPA filters of different efficiencies. These filters remove gases and radioactive iodine.

France is Camfil’s largest market for nuclear plant filters. Key products include Acticarb, Sofilair and special filters for glove-boxes. Camfil GmbH is also the main air filter supplier to nuclear plants in Germany and has developed Camcount, a brand new computerised device for testing nuclear filters.

AAF has provided HEPA filtration products, filter houses, and cooling coils to the worldwide nuclear industry’s power plants and fuel processing plants for over 30 years. Many of the technologies and systems which are used to provide cleaned and tempered air for environmental safety systems were developed by AAF. The Three Mile Island Reactor Containment Coolers were designed and made by AAF and operated successfully during and after the accident.

AAF specialises in air cleaning systems that handle toxic, hazardous, and radioactive gas streams safely. One example is the Bag in/Bag Out system which ensures removal of the filter without exposure to personnel.

Water filtration

Power plants have conventional and unique water filtration requirements. The make up water for the steam generation system is identical to fossil power plants. Pre-treatment is followed by reverse osmosis systems. Filters for the cooling tower system and filters for plant drinking water would also be conventional. The steam turbine requires conventional lube oil filers. Filters are required to purify the condensate. Ion exchange resins are also utilised.

Dow just announced that its DOWEX™ MONOSPHERE™ ion exchange resins 650C and 550A for condensate polishing, along with DOWEX™ MARATHON™ C, DOWEX™ MARATHON™ A and DOWEX™ MARATHON™ WBA series for brine cleaning will be installed at Ling Ao Nuclear Power Plant (Phase II), which is located adjacent to the Daya Bay Nuclear Power Plant, and together comprise one of the largest, total installed capacity nuclear power bases in China.

The deep-bed condensate polishing systems in pressurised water reactor (PWR) nuclear power plants filter corrosion products transported from the main condenser to help control secondary cycle steam generator chemistry. It requires complete separation of the mixed resin to improve effluent water quality. Excellent mechanical strength and oxidative stability are also desirable features for less release of high megawatt (MW) > 1000 total organic carbon (TOC) species from cation resin to help minimise downstream sulfate excursions.

There are a number of special applications as well. They include the fuel pool, radwaste, reactor water cleanup, protection for radioactive exposure in the steam system, and removal of irradiated particulate in the reactor cooling pump.

Pall recommends its Ultipor GF Plus filters to optimise the visual clarity in the fuel pool. Pall radwaste filtration systems provide decontamination of liquid radwaste. Complete backwashable systems provide continuous automated service for small- and large-flow applications. Disposable systems are equipped with filter elements that are highly permeable, sturdy and efficient.

For reactor water cleanup (RWCU), Pall offers PMM septa and Rigimesh septa as alternatives to spiral welded mesh, wedge wire, and coarse metal elements commonly found in RWCU systems.

In addition to replacement activity there is some upgrade activity of the older plants. For example the WE Energies Point Beach nuclear plant has been operating since 1970.

The original water treatment design consisted of intake screens; lime-softening solids contact clarifier, one multimedia filter, two cation exchangers, one vacuum deaerator, two anion exchangers, and a polishing mixed-bed demineraliser.

In 1999, water treatment system improvements were made with the addition of three 100-gpm reverse osmosis (RO) units. The organic traps were converted to cation resin softeners and the RO units were installed at the effluent of the softeners. The purpose of these changes was to reduce the level of organic compounds reaching the steam generators.

The RO systems are spiral-wound, five-stage units that operate at 75 percent recovery employing low pressure Dow FILMTEC LE-440i membranes.

In 2004, Point Beach installed a Memcor® microfiltration system to replace the chemical feed, clarifiers and multimedia filters. The microfiltration system employs PVDF membranes with a 0.1 micron rating. By eliminating the chemical feeds, the clarifier, and the multimedia filters, the unadulterated backwash wastewater from the membrane filtration system is routinely discharged directly back to Lake Michigan.

Membrane filtration has greatly improved the water quality at Point Beach Nuclear Power Plant. The plant has significantly reduced its operating costs, increased the reliability of the water treatment plant, and reduced the waste associated with the water treatment system.