My views on the UCS has varied over time, from viewing their nuclear safety concerns as being helpful, to seeing them as obstructionist, who are making bad problems created for the nuclear industry even worse. The UCS complains that the NRC is in bed with the reactor operators, that it cooperates with them to the extent that the NRC allows operators to operate reactors in unsafe conditions. Now it should be noted, that reactors are designed to shutdown if they are in unsafe conditions, and there is no problem of the NRC allowing operators to override safety shutdowns. Furthermore, the NRC does seem to be taking actions against nuclear safety violators.
For example, The Des Monies Register reported that the NRC is enforcing safety regulations at area reactors,
Nebraska's Fort Calhoun Station, 19 miles north of Omaha, is one of three plants in the United States facing the highest level of regulatory scrutiny. That's because the plant's safety systems were found last year to be in danger of flooding, records show.And,
Inspectors found the plant, located on the Missouri River, didn't have enough sand to fill bags that operators planned to place on a flood wall to protect buildings and equipment.
Other plants in or near Iowa have been cited for less serious problems, records show. They include:Yet the Des Moins Register points out that while the USC complains about the safety problems of American reactors, David Lochbaum of the UCS acknowledges that
• NextEra Energy's Duane Arnold Energy Center near Palo. Iowa's only nuclear plant has received four notices of violation in the past five years, including two stemming from incidents dating back to 2003 and 2004.
• Nebraska Public Power District's Cooper Station near Brownville. The plant located along the Missouri River about 15 miles from the Iowa border had three violations.
• Exelon Corp.'s Quad Cities plant at Cordova, Ill. The plant located across the Mississippi River from Davenport has been cited once in the past five years.
the NRC greatly improved the frequency and thoroughness of its assessments over the past decade. But it is not perfect, . . .This is the UCS at its most reasonable, and if the UCS were always this reasonable, I would have no complaint. Unfortunately the UCS is not always reasonable, and at times appears to be ignorant of nuclear safety concepts, and makes unjustified attacks on the safety of reactors which have outstanding safety potential. I have already pointed out Ed Lyman's failure to attend to discussions of the flammability of nuclear graphite, with respect to Pebble Bed Modular Reactor safety.
One of the fundamental safety mechanisms in the PBMR is Doppler Broadening. Kontent Konsult explains,
As a PBR reactor gets hotter, the rapid motion of atoms in the fuel decreases probability of neutron capture by U-235 atoms. This effect is known as Doppler Broadening. Nuclei of heated uranium move more rapidly in random directions generating a wider range of neutron speeds. U-238, the isotope which makes up most of the uranium in the reactor, is much more likely to absorb the faster moving neutrons. This reduces the number of neutrons available to spark U-235 fission. This, in turn, lowers heat output. This built-in negative feedback places a temperature limit on the fuel without operator intervention.The PBMR is often mentioned as an inherently safe reactor that will automatically shut down if it starts to overheat. While it is fair to ask, as Lyman does, if the PBMR is inherently safe. It is dishonest to ignore PBMR safety features in answering the question. Yet Lyman claims,
The most significant unresolved issue involves how the PBMR’s fuel would hold up during an accident, which is the key to the reactor’s safety. The coating of PBMR fuel can maintain its integ- rity to temperatures of about 1,600°C—several hundred degrees higher than the temperature at which conventional reactor fuel would begin to degrade in a loss-of-coolant accident.134 The claim that the reactor is meltdown-proof rests on the assertion that fuel temperatures would not exceed 1,600°C, even if the reactor loses coolant. (When reactor fuel degrades, it releases highly radioactive fission products.)Yet when I attempted to verify Lyman's 1600 C claim I found that Lyman had referenced an article he had himself written, "The Pebble-Bed Modular Reactor (PBMR): Safety Issues," in which Lyman acknowledges,
However, computer models are used to predict peak fuel temperatures during an accident.
Modeling the movement of the fuel pebbles in the reactor—and hence accurately predicting the peak temperature—has proven extremely difficult. This is significant; as the fuel temperature exceeds 1,600°C, the ability of the fuel to retain fission products rapidly diminishes.135 Thus the safety case for the PBMR depends largely on an ability that does not yet exist—namely, to accurately predict peak fuel temperatures during accidents.
Experiments measuring the He coolant temperature in the AVR found numerous "hot spots" in the coolant that exceeded 1280 CNow 1280 C is still 300 C below the the 1600 C temperature at which the integrity of the graphite pebbles start to break down. What Lyman demonstrates is that it is desirable to have comprehensive research on core temperatures of PBMRs.
A South African PBMR web page explains,
The peak temperature that can be reached in the core of the reactor (1 600 C or 2912 F under the most severe conditions) is well below the temperature that may cause damage to the fuel. This is because the radionuclides, which are the 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.Thus Lymans claim that PBR pebbles cannot withstand heat above 1600 C, is directly contradicted by a South African claim that it can. According to the South Africans,
This inherently safe design of the PBMR renders obsolete the need for safety backup systems and most aspects of the off-site emergency plans required for conventional nuclear reactors. It is also fundamental to the cost reduction achieved over other nuclear designs. 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.South African research showed that in a loss of coolant accident in a PBMR, the maximum fuel temperature would reach a little over 1400 c, nearly 200 C below the 1600 C which Lyman claims would cause problems with graphite pebbles. It should be noted that Eskom, the South African firm that was developing a PBMR prototype claimed,
The reactor core concept is based on the well-tried and proven German AVR power plant which ran for 21 years. This safe design was proven during a public and filmed plant safety test, when the flow of coolant through the reactor core was stopped and the control rods were left withdrawn just as if the plant was in normal power generation mode.
It was demonstrated that the nuclear reactor core shut itself within a few minutes. It was subsequently proven that there was no deterioration over and above the normal design failure fraction of the nuclear fuel. This proved that a reactor core meltdown was not credible and that an inherently safe nuclear reactor design had been achieved.
The peak temperature that can be reached in the reactor core (1 6000 degrees Celsius under the most severe conditions) is far below any sustained temperature (2 000 degrees Celsius) that will damage the fuel. The reason for this is that the ceramic materials in the fuel such as graphite and silicone carbide - are tougher than diamonds.Lyman did not indicate any source that backed up his 1600 C maximum claim for pebble integrity. It is clear that Lyman has not done his homework on Pebble Bed Reactor safety, but passed judgement on the basis of assertions that do not appear to be adequately researched. The weak research that serves as a basis for Lyman's judgement about safety issues concerning the BPMR extends to his assertions about the flammability of graphite. These problems raise questions about the accuracy and judgements of the entire UCS Report, Nuclear Power in a Warming World, and about the quality of UCS work products generally.