May 24th, 2011
Nuclear Q&A: Answers on Nuclear Energy’s Future and Plant Design and Safety
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The questions below come from the Webinar Fukushima Daiichi – How We Got Here and Where We're Going
. Since there wasn't time to answer all questions during the webinar, we've decided to post responses to unanswered questions here.
Nuclear Plant Design – Technical
Is there a reason that they had to build these plants next to the ocean?
Nuclear plants need a source of water for cooling. Thus reactors are typically sited near rivers and oceans.
Was all of the H2
from reaction between H2
O and Zr? Is there any other source?
I understand the most significant source of hydrogen was the reaction between steam and zirconium metal in this situation. Another source generally considered by nuclear safety analysts is from water radiolysis, whereby radiation splits water into hydrogen and oxygen gas. However, this source was insignificant compared to that generated from the steam-zirconium reaction.
Is Fukushima Daiichi a typical power plant design in terms of size compared to US nuclear power stations?
Yes, the basic design of the Daiichi plant is a boiling water reactor (BWR), which is a type of light water nuclear reactor used for the generation of electrical power. It is the second most common type of electricity-generating nuclear reactor after the pressurized water reactor (PWR), also a type of light water nuclear reactor. The BWR was developed by the Idaho National Laboratory and General Electric in the mid-1950s. The main present manufacturer is GE Hitachi Nuclear Energy, which specializes in the design and construction of this type of reactor. There are six power plants located at the Fukushima Daiichi site ranging in size. They are fairly typical, in electricity generating terms, of those operating in the US. The Daiichi plant is a Mark 1 containment structure. Here is a list of BWR currently operating.
Nuclear Plant Design – Safety and Preparedness
I've heard that thorium is a safer fuel source than uranium. Would you please comment?
Uranium naturally contains the fissile isotope with mass number 235. It is the uranium-235 that fissions and produces heat in a nuclear power plant. In contrast, thorium has no naturally fissile isotopes but thorium-232 is known as ‘fertile’ because it transmutes to fissile uranium-233 when it captures a neutron in a nuclear reactor. The neutrons used to generate uranium-233 must still be generated by a fissile isotope such as uranium-235 or plutonium-239 in a nuclear reactor. So a conventional nuclear reactor could be used to generate fissile uranium-233 from thorium. The fissile uranium-233 can then be separated, purified, and formed into fuel to power a nuclear reactor. Uranium-233 is significantly more hazardous, from a radiological standpoint, than other uranium isotopes and many other radioisotopes. The main advantage with thorium is that it is more abundant than uranium with commensurate economic and energy security advantages. Additionally, a thorium fuel does not generate significant quantities of very long-lived transuranic elements (e.g. plutonium and neptunium) that makes spent nuclear fuel radioactive for millions of years. It is the technical and safety issues associated with processing and handling uranium-233 that makes a thorium-based nuclear fuel uneconomical.
How often does the nuclear industry test their backup systems?
The US nuclear industry is driven by NRC regulations, which require an extensive safety analysis. Inside this safety analysis is a set of proscribed technical safety requirements, which include backup safety system testing. For example, a backup power system might be tested once a month. Other systems maybe tested weekly. The frequency of testing is driven by a safety analysis and approved by the NRC. Each type of reactor system may have different requirements.
Are there any new designs that use passive cooling techniques that do not need pumped water to cool the reactors? How could new “passive” technologies have helped prevent such an accident?
Yes, Westinghouse AP1000 system uses a passive backup cooling system that provides 72 hours of cooling in the event of a total loss of power. Other more advanced designs offer completely passive cooling systems (Pebble Bed Reactor).
Did Japan Nuclear authorities consider earthquake and tsunami scenarios separately? Seems like if they were not considered independent events, then more layers of protection would have been added. What has this incident taught us about how to prepare for the unexpected?
Question is somewhat outside my area of technical expertise. However, my understanding is that they were considered independent events from a consequence standpoint. It is important to note that the damage to the reactors was caused by the tsunami, not by the earthquake. Power plants at Onagawa automatically shut down and were maintained in cold shutdown despite being closer to the earthquake’s epicenter than those at Fukushima. Additionally, the diesel generators of unit 6 at Fukushima were unaffected by the tsunami so that both it and unit 5 could be maintained in cold shutdown. The number of directly comparative situations associated with the earthquake and tsunami would seem to indicate that some relatively definitive lessons can be learnt.
US Plans/Future Regulations
How will this event impact the design of backup systems and spent fuel cooling ponds to protect against similar incidents in the future?
This question is best answered by the NRC reviews. However, one needs to remember that the US instituted major upgrades to its nuclear power plants post 9-11 to provide additional protection against the loss of backup power supplies.
Are the NRC reviews including expert input from outside the agency?
I only know the reviews will be conducted by current senior managers and former agency experts.
What sort of impact will this have on other experimental nuclear efforts like ITER?
None, the ITER and other existing DOE nuclear research programs are unaffected. However, new research programs may result that investigate reactor systems that offer additional safety features.