Nuclear Reactor Safety: A Debate

Nuclear Reactor Safety: A Debate
Clearing the Air on Fast Breeder Reactors

This refers to "Problems with the Prototype Fast Breeder Reactor" by M.V. Ramana and Nidhi Sharma.

There are a few points mentioned in the section "Accident Risks" that need to be clarified as they are not correctly depicted and can mislead the public.

1) Sodium void coefficient: All large fast power reactors somewhat intrinsically have a positive though small sodium void coefficient. (A detailed discussion of this aspect is not possible in a short space and it may also not be appreciated by the lay public). Therefore this is not a special case for the PFBR. However, the author does not mention the strong negative and quick acting "Doppler coefficient" that is an important safety characteristic of fast reactors.

2) These aspects have been taken into account in the design such that in the case of a core disruptive accident (CDA), the energy release would be limited. This assessment compares well with figures reported for several plants designed earlier in Europe and elsewhere.

3) Furthermore, the primary containment, i.e. the main vessel housing the reactor core along with the sodium coolant pool, has been designed robustly to absorb the energy release in the case of an accident. Elaborate testing on the models of the primary containment has been conducted at the Terminal Ballistics Research Laboratory, Chandigarh.

(4) The containment building that houses the main vessel/reactor has also been designed to sustain any pressure build up as a result of a CDA and, therefore, there would not be any risk to public health and the environment as suggested in the article. Above all, it should be remembered that the probability of such a severe accident is extremely low and is statistically less than one in ten million.

The authors need to exercise caution in making sweeping statements that can make the general reader apprehensive about an important power plant for the country.

R.D. Kale, Group Director (Retd),

Fast Reactor Technology Group, IGCAR, Kalpakkam

R.D. Kale and M.V. Ramana
Downplaying the Risks of the Prototype Fast Breeder Reactor

I welcome the response from R.D. Kale (above) to the concerns we highlighted about the safety of the Prototype Fast Breeder Reactor (PFBR). I am aware of many of the arguments he has raised but still feel that there are serious risks, including from a Core Disruptive Accident (CDA), because even multiple safety systems can fail. That is one of the lessons of the multiple reactor meltdowns that took place ten years ago at the Fukushima Daiichi nuclear plant in Japan. Below I respond briefly to his points.

(1): Let us start with why a positive coolant void coefficient is a safety hazard. A positive coefficient means that an accident that begins via a heating of the coolant is more likely to spread to large parts of the core quickly. The dangers stemming from a positive coolant void coefficient have been recognized for long, especially after the 1986 Chernobyl accident. (The RBMK design also had a positive void coefficient.) It is possible to decrease this effect by interspersing fuel subassemblies within the depleted uranium blanket in what is termed a heterogeneous core design. The U.S. Clinch River Breeder Reactor, which was eventually cancelled, was designed with a heterogeneous core. However, designing the PFBR with a large and positive coolant void coefficient suggests that safety was not the highest priority.

(2) and (3): The question to ask is not whether the containment structure is robust but what level of energy release is the containment robust against. An analogy is the statement that a car is robust without any caveat; a car that might be robust in an accident sustained when traveling at 40 kilometres per hour might not be so robust if the car is traveling at 160 kilometres per hour. The energy release from an accident is not known a priori but is predicted by mathematical models, which are very sensitive to assumptions. The problem is that the PFBR safety calculations make many questionable assumptions. This results in severely underestimating the explosive energy that might be released in a CDA. The same models show that much larger energy releases are possible under other, plausible, assumptions. Larger energy releases will result in the containment structure being subject to pressures that are much higher than what it is designed for. If this were to occur, the containment’s integrity would be compromised leading to the escape of radioactivity into the surroundings.

(4): The statement about the probability of a severe accident being extremely low is not justified for two reasons. First, the PFBR is a new reactor design and so there is no statistical information on that specific design. Second, the known severe accidents at five commercial power reactors—three of them in Fukushima—shows that accidents occur much more frequently than the “one in ten million” figure. The Fukushima accident also demonstrated that risk estimates produced by nuclear establishments are not reliable, and do not account for unexpected failure modes during many accidents. 

M.V. Ramana
Further Comments

 With respect to M.V. Ramana’s rejoinder to my comments (above), I would like to point out:

(i) The author is obsessed with a ” large positive coolant void coefficient”. His statement ”However, designing the PFBR with a large and positive coolant void coefficient suggests that safety was not the highest priority” shows he is not aware of the facts. He ought to have known that the coolant void coefficient of the PFBR is lower than that in several other fast reactors( IAEA, IWGFR 1531), both operating /planned such as Superphenix 1,France, BN 600 Russia, DFBR Japan, CRBR USA and so on. It is also found that a strong negative Doppler coefficient together with a negative structural and fuel expansion coefficient dominate a positive coolant void coefficient. Besides, in India safety of the reactor whether in design or operation is supreme and there is no compromise. 

(ii) I would like to point out that the author quotes “Fukushima failures” out of context. It is well known that Fukushima’s multiple reactor accidents were not just because of the natural disaster resulting from a severe tsunami, but were aggravated by the operator’s mistakes— not acting in time to pump sea water into the reactor to prevent severe core melting. By the way, the author who is concerned with the positive void coefficient must know that this coefficient is always negative in light water moderated and cooled reactors such as in Fukushima where the accident occurred.

R.D. Kale, Group Director (Retd), Fast Reactor Technology Group, IGCAR, Kalpakkam
Nuclear reactors are hazardous

I would like to respond to R.D. Kale’s further comments with two points. 

(1) As I mentioned earlier, a positive coolant void coefficient is a safety hazard because it could lead to a core disassembly accident (CDA). Further, our article also briefly mentions the other element that is relevant to evaluating the safety of the PFBR, the containment structure. Building a strong reactor containment building is necessary because despite the best efforts of designers and operators, severe accidents might occur at any nuclear reactor, including the PFBR. 

One measure of the ability of a containment to withstand accidents is the ratio of two quantities: the product of the volume and the maximum design overpressure, and the thermal energy produced by the core. A larger volume or maximum design overpressure would allow the containment to remain intact for greater energy releases. Conversely, the more the thermal energy produced by the core, the larger the energy that could be released in an accident. Thus, the greater this ratio, the more capable the containment. For the Clinch River Breeder Reactor (CRBR) design in the United States that Kale offers as an example, this ratio is nearly 30,000 whereas the ratio for the PFBR is less than 2,000. The CRBR’s sodium void coefficient is 2.29, lower than the corresponding figure of 4.3 for the PFBR. If safety is indeed supreme, the PFBR should have been designed with a lower coolant void coefficient or a stronger and larger containment, or both.  

(2) A positive void coefficient is a risk factor. The negative void coefficient of the reactors that melted down at Fukushima reactors only means that there are other risk factors, such as operator mistakes. Such risk factors also exist in the case of the PFBR, as well as other reactors. Indeed, there is a long history of safety problems in other nuclear facilities. 

M.V. Ramana

(The debate on this topic is now closed –Editor, ‘The India Forum’)

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