Posted on: September 14, 2011 Posted by: Diane Swarts Comments: 0

SA is allowed by the International Atomic Energy Agency (IAEA) to conduct health, safety and environment stress tests on Koeberg nuclear power station in 2011.

SA Energy Minister Dipuo Peters said cabinet and his department had requested an integrity test in 2011 after concern over Japan’s recent Fukushima nuclear disaster. Koeberg, near Cape Town, was inspected in March for health, safety and environmental risks, as well as resistance to natural disasters and incidents of terror.

African ministers of energy met in September 2011 at a UN African Energy Ministers Conference in Johannesburg to discuss energy savings, alternative energy sources, and nuclear safety.

Several African and European suppliers nuclear plant stress tests could be due, as many African states have adopted policy to include uranium mining, beneficiation, and nuclear technology transfer in their energy mix.

Stress test results in Japan and France could shape policy at home, and in African and Middle East nucleaer client states like Egypt, Nigeria, South Africa and Turkey.

French nuclear waste treatment incident

Green politicians have called for Centraco nuclear waste treatment site in Marcoule to be shut down, as the French Nuclear Safety Authority (ASN) begins an inquiry into a blast there on 12 September that killed one man and injured four others, one with severe burns.

The site was slated for poor management in the past. An inspectorate team was sent to the waste facility. Regulatory questions will be asked in the investigation.

The European Commission said nuclear waste treatment facilities have not been subjected to a stress test exercise led by EC Energy Commissioner Günther Oettinger after the Japan, Fukushima disaster in 2010. However, ASN said the plant had been stress tested by ASN, and that incidents were noted.

ASN said its stress tests at the facility could be validated by the inquiry. “There is a technical accident, not a nuclear accident.”

Marcoule was opened in 1956 to aid France’s nuclear weapons industry, but in the 1990s it began recycling nuclear waste and creating mixed oxide fuel (MOX) for nuclear power stations.

Materials processed at the time of the accident were stable isotopes, a low level of radioactive waste.

Jan Haverkamp, Greenpeace nuclear energy spokesman, said the blast was most likely caused by a waste mixture, as metals, gloves and radioactive materials were melted down.
French nuclear stress tests

The majority of France’s electricity comes from its 58 nuclear reactors. Eric Besson, French minister of industry, energy and digital economy, said France will start a new stress test for its nuclear plants soon. The IAEA rules on nuclear safety, health and environmental management procedures worldwide.

France will tender for Turkey’s plans to construct two or three nuclear power plants. In July, Tepco, Japanese operator of the damaged Fukushima Daiichi nuclear power plant, withdrew from Japan’s bid to build Turkey’s second planned nuclear plant in the Black Sea region.

Japanese stress test results due
Japanese nuclear utilities will submit results of first stage stress tests for some reactors to the IAEA in September 2011.

Japan had ordered power utilities to carry out simulations of extreme events such as earthquakes and tsunamis. Reactors idled after regular maintenance, would seek approval from local authorities to restart before April 2012.

The Japanese radiation leak responses took nuclear power down to 15% of Japan’s electricity in July, from 30% before the radiation crisis, the worst radiation leak in 25 years.

IAEA to assess test results

The IAEA to would assess results of the stress tests, in addition to nuclear regulators. Japan meanwhile plans to ‘wean’ itself from reliance on nuclear power, depending on stress test results.

Nuclear safety shells

Nuclear plants have successive protective ‘shells’. Nuclear fuel pellets, in a ceramic oxide matrix, is the first barrier to retain most radioactive fission products.

Zircaloy casing is the second barrier between radioactive fuel and the reactor.

The core is placed in a pressure vessel of thick steel, at about 7MPa (1000psi), the third barrier to radioactive material release.

The primary loop of pressure vessel, pipes and pumps for water coolant, are in a containment structure, the fourth barrier to radioactive material release in the event of a meltdown. Containment is by air tight seal of very thick steel and concrete.

A large, thick concrete structure is poured around the containment structure, named ‘secondary containment’, forming a fifth barrier. The reactor building outer shell serves merely as weathering protection.

During normal, full-power operation, the neutron population in a core is stable and the reactor is in a ‘critical’ or working state. Nuclear fuel in a reactor can not cause a nuclear explosion as in a nuclear bomb.

Hydrogen caused Chernobyl disaster

The Chernobyl disaster was caused by excessive pressure buildup, a hydrogen explosion, and rupture of all structures, propelling molten core material into the environment. Chernobyl did not have a containment structure.

Control rods are made of boron that absorbs neutrons. During normal operation in some types of nuclear reactor, control rods maintain the chain reaction at a ‘critical’ state, or shut the reactor down from 100% power to about 7% power, at residual or ‘decay’ heat that must be removed by cooling systems to prevent fuel rods from overheating and failing.

Water cooling is critical

Maintaining enough cooling is the main challenge in damaged reactors. Many fission products decay (produce heat) extremely quickly, within seconds, others decay more slowly, like some cesium, iodine, strontium, and argon.

The earthquake that hit Japan was several times more powerful than the worst earthquake the Japanese nuclear power plant was built for.

When the earthquake hit, the nuclear reactors all automatically shut down. Control rods were inserted into the core and the nuclear chain reaction stopped. The cooling system has to carry away residual heat, about 7% of full power heat load.

The earthquake destroyed external power supply. The reactor and its backup systems are designed to handle this. For the first hour, the first set of multiple emergency diesel power generators had started, but thee tsunami flooded the diesel generators.

Reactor operators switched to emergency battery power lasting 8 hours. Residual heat remained and could lead to fuel failure or ‘core meltdown’ after several days.

Reactors have a number of independent and diverse cooling systems; water cleanup system, decay heat removal, reactor core isolating cooling, standby liquid cooling,  emergency core cooling. Some of theses had failed in the Japanese quake and tsunami.

Most nucler plants have to keep temperature below 1200°C, and keep pressure manageable. Steam and other gases in a damaged reactor have to be released from time to time.

Mobile generators were transported to the site and some power was restored, but more water was boiling off and being vented than was added, initiating a reaction between zircaloy and water, producing hydrogen gas which is extremely combustible in contact with air, and during venting, an explosion occurred.

Outer shell damage only

The explosion took place outside containment, but inside the thin ‘weather’ shell. A subsequent similar explosion occurred at Unit 3 reactor that destroyed the top and some of the sides of the reactor’s outer thin shell.

Some fuel rod cladding exceeded 1200°C and some fuel damage occurred. Engineers decided to inject sea water mixed with boric acid, a neutron absorber, to ensure the rods remained in water. Boric acid also traps some of the remaining iodine in the water.

Water used in the cooling system is purified and demineralised to limit corrosion. Sea water requires more cleanup after the event, but could be used in emergencies in this type of plant.

PHOTO; Boiling water reactor containment includes a pressure vessel of thick steel as thrid barrier, a containment structure of very thick steel and concrete as fourth barrier, and a large, thick concrete structure as fifth barrier.

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