Ferreira says design changes to the PBMR reactor, which allowed for an increase in the nominal thermal output of the demonstration reactor from 302 MW to 400 MW (165 MWe), prompted Eskom to enter into a new environmental-impact assessment (EIA) process for the demonstration reactor project. To this end, a revised final scoping report was made available to the public in January 2007.
A new round of public participation meetings will be held later this year, to be followed by the submission the EIA reports to the Department of Environmental Affairs and Tourism (Deat). “Construction cannot start unless a positive record of decision on the EIA is obtained from the Deat, which is only expected some time in 2008,” says Ferreira.
He says Eskom and the National Nuclear Regulator (NNR) are also awaiting the PBMR’s safety analysis report. “The report has to take every possible scenario into account. It is an extremely comprehensive and complicated process, but a good one, since you have to convince the NNR of the safety of the technology.”
Ferreira points out that, while safety objectives in all current nuclear power stations are achieved by means of custom-engineered, active safety systems, the PBMR is characterised by inherently safe features. “This is as a result of the design, materials used, the fuel and the physics involved. This means that, should a worst-case scenario occur, no human intervention would be required in the short or medium term.”
Ferreira explains that nuclear accidents are principally driven by the residual power generated by the fuel, after the chain reaction has been stopped. This is caused by radioactive decay of fission products. If this decay heat is not removed, it will heat up the nuclear fuel until its fission product retention capability is degraded, and its radioactivity is released.
In conventional reactors, the heat removal is achieved by active cooling systems such as pumps, which rely on the presence of the heat transfer fluid. Because of the potential for failure in these systems, they are duplicated to provide for redundancy. Other systems, such as a containment building, are provided to mitigate the consequences of failure and to act as a further barrier to radioactive release.
In the PBMR, explains Ferreira, the removal of the decay heat is independent of the reactor coolant conditions. The combination of the very low power density of the core and its resistance to the high temper-ature of fuel in billions of particles, underpins the superior safety characteristics of this type of reactor.
Ferreira states that the helium, which is used to transfer heat from the core to the power-generating gas turbines, is chemically inert. It cannot combine with other chemicals and is noncombustible. “Since air cannot enter the primary circuit, oxygen cannot get into the high temperature core to corrode the graphite used in the reactor.”
The peak temperature that can be reached in the core of the reactor is well below the temperature that may cause damage to the fuel, reports Ferreira. This is because the radionuclides, which are potentially harmful products of the nuclear reaction, are contained by two layers of pyrocarbon and a layer of silicon carbide, which are extremely good at withstanding high temperatures.
“Even if there is a failure of the active systems that are designed to shut down the nuclear reaction and remove the core decay heat, the reactor itself will stop any nuclear fission and eventually cool down naturally.” Ferreira adds that, unlike the Chernobyl-type of reactor, which, during the accident in 1986, produced more energy the hotter it became, the PBMR reactor has a strong negative temperature coefficient of reactivity, which halts the chain reaction. “It also cools down naturally by heat transport to the environment.”
The size of the PBMR core ensures a high surface area to volume ratio, states Ferreira. This means that the heat that it loses through its surface is greater than the heat generated by the decay of fission products in the core. “The reactor, therefore, never reaches a temperature at which significant degradation of the fuel can occur. The plant can never be hot enough for long enough to cause damage to the fuel.”
This inherently safe design of the PBMR renders the need for safety backup systems and most aspects of the off-site emer- gency plans required for conventional nuclear reactors, obsolete. “Although plans related to aspects such as the transport of fuel will still be required, they will be modified to suit the specific characteristics of the fuel and the transport mode.”
Ferreira states that construction of the first demonstration plant will take up to four years to complete, since this is the first of its kind in the world. “Since the fuel is the only component which has not been adapted from the original German design, very few of the components are available off the shelf. Once we have the licence for construction in place, we can place the orders for the long-lead items.” He adds that, once the commercial phase is in full swing, construction time could be reduced to about 24 months.