This is a summary of: Hirsch, H., et al. April 2005. Nuclear Reactor Hazards Ongoing Dangers of Operating Nuclear Technology in the 21st Century. A report prepared for Greenpeace International.
Nuclear power plants (NPP) age in four ways at a microscopic level, often impossible to detect before a component breaks down:
- Thermal loads
- Mechanical loads
- Corrosive, abrasive, erosive, and embrittlement processes
The combinations and interactions of the processes above increase the danger of nuclear accidents, which grow significantly every year after a plant is 20 years old.
Electric Grid Failure
If the power lines or a regional electric grid collapse occurs, onsite emergency generators will start. If the emergency diesel generators fail, there are batteries, but they can’t supply electricity for the larger components such as pumps and only last 2 hours. After they fail, the situation at the plant becomes critical (“station blackout”). This could lead to a core meltdown, the spent fuel pool overheating, leading to radioactive releases.
If the grid fails for a long time, traffic and communication infrastructures will be greatly impaired, making it difficult to tell people to evacuate. Monitoring and alarm systems might not be operational.
Natural disasters and Climate change
Natural disasters such as earthquakes, tsunamis, flooding, and extreme weather can lead to failure of the electric grid or destruction of a Nuclear Power plant. The 2007 IPCC report stated that extreme weather events may become the norm –more frequent heat waves; intense storms (hurricanes and tornadoes); increased intensity of floods and droughts; landslides, mudslides, and soil erosion from increased rain, rising seal levels, etc. Many of these issues affect the ability of NPP to operate.
Economic: Just at the point where plants have reached serious ages, maintenance is being cut back and NPP are being pushed to their limits to produce power.
Staffing: For decades new power plants weren’t built, so now there aren’t enough young, well-trained workers to take over old and poorly-documented plants. Many existing workers are about to retire and will be hard to replace, since they’re used to the quirks and problems of the plant they work at.
Terrorism and war
- Radioactive contamination with long-lived radio-nuclides could lead to economic damage for decades.
- Large areas of cities will have to be evacuated for an indefinite period, which could destabilize entire regions.
- A sudden shutdown of such a large source of electricity could lead to a collapse of the local electricity grid
- After such an attack has demonstrated the vulnerability of a Nuclear Power Plant (NPP), it’s possible that other NPPs will be shut down in the country affected as well as other countries.
- The destruction of an NPP could significantly increase the radioactive contamination produced by a nuclear fission weapon — the fission product inventory of a commercial nuclear power plant is in the order of magnitude of 1000 times that released by a fission weapon.
- It’s likely that the reactor building will be the main target in an attack. If the reactor is operating when the attack occurs and cooling is interrupted, a core melt can result within a very short time (about 1 hour). A core melt can result in radioactive releases that are very high.
- The spent fuel storage pool is another vulnerable component with considerable radioactive inventory. In some plants, it can contain several times more fuel than the reactor core itself. In many cases the pool is installed in a separate building with less protection.
- If there’s a core melt, the consequences could affect a region up to 10,000 km2 or 3800 square miles. Contamination requiring relocation of people can measure up to 100,000 km2 or 38,000 square miles.
- La Hague has 370 times the amount of caesium-137 released at Chernobyl.
Plutonium. The UK has the most: 90 tons, France 80 tons, Russia 37 tons. It’s bad news that most of the plutonium is stored at the same sites where there are the largest concentrations of spent fuel. Plutonium is of high strategic value as a weapon ingredient and is high radiotoxic. A few kilograms are enough to make a fission weapon, a few micrograms are sufficient to develop cancer.
As you can see from above, there are many reasons besides earthquakes and tsunamis that may cause nuclear power plants to fail in the future.
Details of “Nuclear Reactor Hazards”
Below are my notes. If you find this topic interesting, read the 128 page original document, and if you don’t have the time to do that, skip to the conclusion at the end of each chapter in the original (which also explains the different types of reactors and lots of other interesting information).
Fifty years ago, fusion proponents were convinced that it would be best to skip fission reactors because they were too dangerous, and go straight for fusion reactors. In the early 70s, it was expected that there would be “competitive fusion” by the year 2000. At the turn of the century, it was clear that an enormous development effort is still needed before a fusion power station can go into operation. Engineers are aiming for an economical fusion reactor by the middle of the 21st century.
It is highly questionable whether fusion will be worth all the efforts planned for the next decades. Already, it is foreseeable that it will be neither safe, nor clean, nor proliferation resistant (p 59).
The radioactive inventory of a fusion reactor is expected to be high, comparable to that of a fission reactor of the same size. All in all, long-term radioactive waste management will be required for fusion plants. They give rise to a waste problem comparable to that of fission power plants (p 60).
Aging processes are difficult to detect because they usually occur on the microscopic level of the inner structure of materials. They frequently become apparent only after a component failure, for example, break of a pipe has occurred.
For a nuclear power plant, whatever the reactor type, the aging phase will begin after about 20 years of operation. This is a rule-of-thumb number — aging can begin earlier.
As the world’s nuclear power plants gets older, there are efforts to play down the role of aging (p 63).
The most important influences leading to ageing processes in a nuclear plant are (p 64):
Irradiation Thermal loads Embrittlement Mechanical loads
Corrosion Abrasion Erosion
Combinations and interactions of the processes above
Changes of mechanical properties frequently cannot be recognized by non-destructive examinations, so it’s difficult to get a reliable, conservative assessment of the actual state of materials. Because of limited accessibility due to the layout of components and/or high radiation levels, not all components can be examined 100%.
With increasing age, damage mechanisms might occur which have not been foreseen, or which were excluded (for example, stress corrosion cracking in titanium-stabilized austenitic steels), exacerbating ageing problems (p 65).
Aging Effects at Specific Components
Reactor pressure vessel
Materials close to the core: embrittlement (reduction of toughness, shift of the ductile-to-brittle-transition temperature) through neutron irradiation. This effect is particularly relevant if impurities are present.
Welds: crack growth because of changing thermal and mechanical loads.
Vessel head penetrations: crack formation and growth due to corrosion mechanisms
Penetrations of vessel bottom: damages due to corrosion, abrasion, and thermo-mechanical fatigue. BWRs
Inner edge of nozzles…
Core internals, core shroud…
Bolts and nuts under pressure: localized leakages of borated coolant can lead to corrosive damage at flange surfaces and screws’ materials.
Pipelines: have had stress corrosion cracking, strain-induced corrosion, wall thinning and material fatigue from resonance vibrations, water hammer, etc are very difficult to keep under surveillance, damage becomes more likely with the aging of materials.
Main coolant pumps: crack formation and crack growth can occur due to thermal and high-frequency fatigue processes, supported by corrosive influences.
Concrete structures: structural components like the concrete parts of the containment, protective outer hulls of buildings, biological shields, basis structures and cooling towers are subject to thermo-mechanical loads, but also to effects of weather, chemical attacks, and high radiation doses.
The consequences of ageing can roughly be described as:
1) the number of incidents and reportable events at an NPP will increase – small leakages, cracks, short-circuits due to cable failure etc. In Germany the ten oldest plants (of 19) were responsible for 64% of all reportable events in the time span 1999 – 2003.
2) There are effects leading to a gradual weakening of materials which may never have any consequences until the reactor is shut down, but which could also lead to catastrophic failures of components with subsequent severe radioactive releases. Most notable among those is the embrittlement of the reactor pressure vessel, increasing the hazard of vessel bursting. Failure of the pressure vessel constitutes an accident beyond the design basis. Safety systems are not designed to cope with this emergency. Hence, there is no chance that it can be controlled. Furthermore, pressure vessel failure can lead to immediate containment failure as well, for example through the pressure peak after vessel bursting, or the formation of high-energy fragments. Catastrophic radioactive releases are the consequence. Pressure tube embrittlement of RBMK or CANDU reactors also falls into the category of ageing processes with potentially catastrophic consequences. In case of failure of a single or a small number of tubes, there is a chance that the accident can be controlled – but not with large numbers failing. Another example are corrosion processes which may be overlooked for years – as a recent event at the U.S. pressurized water reactor Davis Besse illustrates.
In probabilistic risk assessment studies (PRAs), which are increasingly used as a tool by nuclear regulators, aging is usually not taken into account. Thus, it is clear that the risk of a nuclear accident grows significantly with each year, once a nuclear power plant has been in operation for about 2 decades. But it is not possible to quantitatively describe this continuous increase of risk. Increased vigilance during operation and increased efforts for maintenance and repairs have the potential to counteract this tendency. However, in the age of liberalization and growing economic pressure on plant operators, the trend rather goes in the opposite direction, even as the reactor fleet is ageing.
All components crucial for safety can be replaced except two:
The reactor pressure vessel (RPV), and the containment structure.
Under present circumstances, economic pressure is severe to the extent that even inspections are being reduced – the opposite of what would be required for ageing control. This is combined with general cost-reduction strategies of nuclear utilities because of the liberalization of the electricity markets, accompanied by deregulation and increased competition. It is claimed that intensification of plant monitoring can be a sufficient replacement for inspections; however, this claim rather appears as an attempt to mask the reduction of safety margins, and is by no means reassuring.
Another difficult task for regulators is to contribute to ensuring that there is a continuing supply of competent personnel to operate and maintain older plants where design details, technical limits etc. may be less well documented than for modern ones. This problem can be exacerbated by the gradual retirement of plant designers as well as operators that were working at the plant from start-up.
Examples of Age Related Problems
In this appendix, some examples for nuclear plants with ageing problems which led to decommissioning, or to prolonged shutdown periods are presented.
Yankee Rowe PWR (USA, 185 MWe) was permanently shut down in Feb 1992, after about 31 years of operation. Ironically, Yankee Rowe was to be the U.S. lead plant for license renewal to obtain life extension from 40-60 years. The license renewal procedure was started early because it was the first attempt of this kind. Review of the safety case for the reactor pressure vessel showed that embrittlement of the weld near the core had already reached a critical stage, mainly because of high content of copper impurities.
Würgassen BWR (Germany, 670 MWe) was permanently shut down in May 1995, after less than 24 years of operation. During the overall maintenance inspections beginning August 1994, special inspections of the core shroud were performed in addition to routine inspections, because cracks had been found in this component in 13 boiling water reactors in other countries. The Würgassen core shroud was found to be severely cracked. Repairing the core shroud plus the necessary accompanying backfits and modernization measures would have cost DM 350 – 400 million (about € 220 million at 2004 value). The plant owner, PreussenElektra, decided against performing the repairs and decommissioned the plant [JATW 1995; NNI 1995].
Davis Besse PWR (USA, 925 MWe) was temporarily shut down after a hole was discovered in the reactor pressure vessel head on March 6, 2002. The reactor was off the grid for more than two years and restarted on April 8, 2004. The hole went through the whole thickness of the vessel head; only the stainless steel liner welded to its inner surface was still intact. This liner (less than 5 mm thick) was the last remaining barrier to prevent a severe loss-of-coolant accident. It had already bulged by about 3 mm under the high pressure in the RPV. Total costs for the shut-down, including replacement power costs, were about US$ 600 million.
Stade PWR (Germany, 662 MWe) was permanently shut down in November 2003, after less than 32 years of operation, although the amount of electricity assigned to this plant by the German Atomic Law (revision of 2001) had not been produced yet. Similar to the situation at Yankee Rowe, critical welds in the Stade reactor pressure vessel were particularly prone to embrittlement due to a high copper content.
Terrorism and war
A nuclear power plant (NPP) might be selected for these reasons:
1) symbolic character – epitome of technical development
2) long-term effects — radioactive contamination with long-lived radio-nuclides leading to economic damage for decades. Large areas of cities will have to be evacuated for an indefinite period, which could destabilize entire regions.
3) A sudden shutdown of such a large source of electricity could lead to a collapse of the local electricity grid
4) After such an attack has demonstrated the vulnerability of an NPP, it’s possible that other NPPs will be shut down in the country affected as well as other countries.
ACTS OF WAR
Military action against nuclear installations constitutes another danger deserving special attention in the present global situation.
NPP could be damaged unintentionally, or deliberately destroyed, since some serve military purposes or are perceived that way. The destruction of a nuclear power plant could significantly increase the radioactive contamination produced by a nuclear fission weapon — the fission product inventory of a commercial nuclear power plant is in the order of magnitude of 1000 times that released by a fission weapon.
NPP are probably the most “attractive” targets for terrorist or military attacks, since they have a lot of radioactive inventory and are an important component of the electricity supply system.
It’s likely that the reactor building will be the main target in an attack. If the reactor is operating when the attack occurs and cooling is interrupted, a core melt can result within a very short time (about 1 hour). A core melt is the worst case – rapid melting results in radioactive releases that are very high.
The spent fuel storage pool is another vulnerable component with considerable radioactive inventory. In some plants, it can contain several times the amount of fuel than the reactor core itself. In many cases the pool is installed in a separate building with less protection.
- Air: commercial/freight/business/military planes or unmanned drone helicopter with explosives or bombs
- Water: crash of a boat laden with explosives into the cooling water intake, explosion of a gast tanker close to an NPP
- Firing on a Nuclear power plant from a distance. Shelling with field howitzer, with explosive grenades, armor-piercing weapons (rockets)
- Intrusion of attackers into NPP area: car bombs, armed attackers with explosives
- Insiders: they support attack from outside, i.e. with creation of confusion, obstruction of counter measures, or simultaneous attack from inside. Smuggle in explosives, intervene in the op of the plant to trigger an accident, sabotage during repair and maintenance
- Attack other installations – the cooling water intake or grid connections and on-site emergency power supply
If there’s a core melt, the consequences could affect a region up to 10,000 km2 or 3800 square miles. Contamination requiring relocation of people can measure up to 100,000 km2 or 38,000 square miles.
Every NPP has a spent fuel pool where the nuclear fuel is cooling off at least for a few years. The largest quantities of spent fuel are stored at reprocessing plants.
La Hague has 370 times the amount of caesium-137 released at Chernobyl.
Climate Change & Nuclear Safety
Extreme weather events may become the norm. Of particular importance are storms and floods, which may occur in combination.
Extreme weather conditions can lead to failure of the electricity grid. Emergency power systems are required which may not be reliable and the failure of which can lead to a severe accident. Therefore, blackout situations are especially worth paying attention to.
IPCC forecasts call for more frequent heat waves; more intense storms (hurricanes, tropical cyclones, etc); increased intensity of floods and droughts; warmer surface temperatures, especially at high latitudes, rising seal levels, which could inundate coastal areas and small island nations.
Precipitation: total atmospheric water vapor has increased several percent per decade over many regions of the northern hemisphere (where 99% of all NPPs are located).
More intense and more frequent precipitation events increase flood, landslide, avalanche, and mudslide damage, and also soil erosion.
Precipitation extremes are projected to increase more than the mean failure. The frequency of extreme precipitation events is projected to increase almost everywhere.
An increase of hot days and heat waves is very likely over nearly all land areas.
Very small-scale phenomena such as thunderstorms, tornadoes, hail, and lightning are not simulated in global models, but an increase is also feared.
Together with 2002 and 1998, 2003 was one of the warmest years ever recorded. About 700 natural hazardous events were registered in 2003, 300 of these were storms and severe weather, and 200 were major flood events.
Consequence of climate change for Nuclear Power Plant Hazards
USA July 1993: Cooper NPP on the Missouri River in Nebraska was forced to shut down the reactor as dykes and levees collapsed around the site, closing many emergency escape routes in the region. Below grade rooms in the reactor and turbine buildings had extensive in-leakage with rising water levels.
France 2003: two NPPs shut in response to torrential rainfall along the lower Rhone river
India 2004: a tsunami sent seawater into the pump house. Operators brought the unit to a safe shut-down. Nuclear authorities are now talking about factoring tsunamis into the design of any new NPP located near the sea coast.
Hurricane Andrew went over turkey point NPP with a sustained wind speed of 145-175 mph. The plant lost all offsite power during the storm and next 5 days. Fortunately, the backup power system worked.
USA 1998 Davis-besse NPP in Ohio was hit with a tornado, lightning caused a loss-of-offsite power, automatically shutting the reactor down.
Vulnerabilityof Atomic Power Plants After Grid Failure
Should the power lines to the NPP be cut-off or a regional electrical grid collapse occur, onsite emergency generators are designed to automatically start. Every NPP has emergency power supplies, which are often diesel-driven [another place where remaining oil will go before delivered to the People]. These generators provide power to special electrical safety distribution panels. These panels in turn supply power to those emergency pumps, valves, fans, and other components that are required to operate to keep the plant in a safe state.
If the emergency diesel generators (EDG) fail, the situation at the plant becomes critical (“station blackout”). A natural disaster that disables the incoming power lines to a nuclear power station coupled with the failure of on-site emergency generators can result in severe accident. Apart from the diesel generators, there are also batteries that supply direct current in case of an emergency; however, the batteries cannot provide electricity for large components such as pumps and have only very limited capacity (typically for about 2 hours).
NRC reviews in recent years have shown that a station blackout at a nuclear power station is a major contributor to severe core damage frequency. According to NRC studies, over 50% of all postulated accidents leading to a core melt accident begin with a station blackout. Without electricity the operator looses instrumentation and control power leading to an inability to cool the reactor core.
Counter measures (accident management) are practically impossible. If the blackout lasts for a long time, not only the reactor, but also the fuel in the spent fuel pool can overheat, contributing to radioactive releases.
Every nuclear power plant has at least two emergency diesel generators. These generators are typically tested one or two times per month, when they are run for about one to four hours. Several times per year, the diesels may be run for up to 24 hours to ensure that the equipment would function during a loss of offsite power.
However, emergency power systems with diesel generator are notoriously trouble-prone.
Disturbances in the emergency power system are responsible for a considerable number of reportable events in nuclear power plants. Emergency diesel generator defects and problems at US nuclear plants as reported to occur on a weekly basis. In 1999, there were 32 reports affecting virtually half (49.5%) of all US nuclear plants. Over 40% of U.S. nuclear power plant emergency diesel generators (EDG) are obsolete. EDG voltage regulators, typically of 1950-60 vintage, have recently experienced ageing and obsolescence problems that have created a heightened awareness among nuclear utilities because of the threat to overall EDG performance.
Furthermore, in case of a grid failure of longer duration, traffic and communication infrastructures will be massively impaired. Thus, emergency measures that might be required in case of an accident with radioactive releases (for example, information of the population, evacuation) will be hindered. Monitoring and alarm systems might not be operational.
Vulnerability of Atomic Power Plants in the Case of Flooding
Cooling needs of nuclear reactors dictate a location at the sea or at a large river.
All reactors on sea coasts are endangered by sea level rise. Over the next hundred years there will be significant rises, while many sea coasts, for example in England, are gradually sinking. Many closed nuclear reactor sites could be flooded, including the stored nuclear waste. That could contaminate the coast lines for decades. Back in 1992 a study was performed in the U.K. on flood threats to U.K. nuclear reactors. All but one of the U.K. reactors are located on sea coasts at or near sea level. By 2025, several nuclear sites are predicted to be under water. Until now, no protective measures around nuclear sites in the U.K. or anywhere else have been taken. It also seems likely that natural land movements along the south-eastern coastline of China (currently sinking), where the Chinese NPPs are located, would exacerbate the effects of sea level rise.
Vulnerability of Nuclear Power Plants to Other Natural Hazards
In the summer of 2003, the highest temperatures occurring so far were recorded in France. The heat was exceptional in both intensity and duration. A potential impact on the safety level of French NPP units was seen through high air temperature, high cooling water temperature and low cooling water level. Long-lasting and repeated heat waves can also lead to unexpected acceleration of ageing processes, increasing the probability of safety system failure in case of an accident.
Heat and dry weather have led to an increased occurrence of forest fires in the last years. According to the NRC, severe tornadoes can produce winds and tornado missiles that can badly damage steel reinforced concrete structures. (aging mechanisms can aggravate such effects).