Nuclear power plants must:

• be economically competitive;

• have superior safety; and

• satisfactorily manage their waste.

These are the three imperatives, which are shared by all power plants, but have a particular importance, especially the last two, in the case of nuclear plants.

Economic competiveness is an obvious given. It is always a necessary condition, but in the case of nuclear power it is not sufficient, as it must be accompanied by the other two. Also, it must be substantially better than the competition and bring in additional considerations to compensate for the mere fact of being nuclear.

Safety must be far superior to conventional power plants because the consequences of a nuclear accident might impact a much larger population because of the dispersion of radioactive effluents. How critical safety is for acceptance of nuclear plants has been underscored by the aftermath of the three major accidents which have occurred. Objectively speaking, Three Mile Island (1979) was actually a minor accident, with very moderate radiation release, but it became the catalyst to halt nuclear growth for more than a decade. In the meantime came Chernobyl (1986) which was eventually overcome because of the atypical conditions of this Soviet reactor. When nuclear was on the rebound, Fukushima (2011) happened and several countries recoiled from nuclear, the most striking example being Germany, which is very willingly accepting the economic penalty of forgoing its nuclear plants.

Disposal of the waste is on a completely different scale for nuclear power than for any other power source. For the latter, handling of the waste is a minor, or at least manageable, issue. For nuclear plants, even though various technical solutions are available to deal with their waste, the mantra is that nuclear waste ‘will poison the earth for millennia’.

So, where does nuclear power stand now in fulfilling the three imperatives?

• Economics: green light, mostly. It is a highly competitive field, but the bottom line is that hundreds of nuclear power plants are operating and no utility would be willing to spend billions of dollars to build a plant which is not economically competitive.

• Safety: yellow light. Nuclear plant safety has proven to be fundamentally sound; older plants have been improved or shut down and new designs have much improved safety. The key issue is the eventual occurrence of ‘big’ accidents.

• Waste disposal: flashing red light. Resolution of the waste problem has not made any substantial progress from the early days of nuclear power and disposal at site. Technical solutions do exist, but the problem is political.

Small modular reactors (SMRs) can satisfactorily address the three imperatives. Moving in reverse, in regards to the waste disposal imperative, fast spectrum reactors run in a burner mode will dispose of the plutonium and minor actinides. Large and smaller fast and thermal reactors using both uranium and thorium cycles can significantly reduce the present waste legacy as well as avoid future additions.

To turn this third imperative from red to green requires the political will to move ahead with available technological solutions.

The scenario regarding the safety imperative is completely different. The iPWR is the very type of plant uniquely capable of reaching the ultimate safety in a reasonably short time using extensively proven technology.

The basic tenet of nuclear safety is very simple. The plant must be capable, through its intrinsic design and auxiliary safety systems, to survive any conceivable accident without releasing excessive radiation. Of course the rub is in the definition of ‘conceivable accident’ and ‘excessive radiation’, as well as the adopted design and the choice of auxiliary safety systems.

Major accident probabilities are defined through the core damage frequency (CDF), that is the probability that a postulated accident results in core damage, which is automatically considered as cause for radiation release to the environment. The CDF target for the early reactor designs was of the order of E-4/yr (0.0001 events/ yr), that is the probability for core damage and radiation release, accounting for all hypothetical accidents, was once in 10 000 years; very small indeed, especially considering that the design lifetime was 30 years. However, the number of plants kept increasing, as well as their operating life. If the probability of core damage remained at E-4/yr for every plant and there are, say, 400 plants around the world with an average lifetime of 30 years, the probability of a major accident at any plant in the world would be 400 E-4 or 0.04 every single year, 1.2 over a 30-year lifetime, which is approximately the Three Mile Island/Chernobyl time frame. After approximately another 30 years, Fukushima. The pattern is there.

It is almost certain that another major accident in the next 15-30 years will be the end of the line for nuclear power. It is therefore necessary to decrease the CDF value. After Three Mile Island, a very significant amount of effort was spent to increase the reliability and redundancy of the safety systems. This improved the safety, but also significantly increased the reactors’ price tag. No wonder that no new nuclear plants were ordered for quite some time. A breakthrough occurred with the adoption of passive safety systems in lieu of the previously active systems. The CDF for the new designs dropped to the order of E-6; also, existing designs are being retrofitted to decrease their CDF. However, the lifetime of the new designs as well as of the retrofitted ones is now projected to be of the order of 60 years. Finally, the new passive designs with a CDF of E-6 have a very hefty price tag and thus the market tendency is to prop up the older designs for as long as possible.

A clean breakthrough is necessary, i. e. to introduce reactors having a very low CDF as well as a low price tag to easily replace the old plants.

First, the CDF. If the CDF probability is decreased to E-8, which is the frequency of a severe event known as ‘act of God’, 1000 plants with a lifetime of 60 years will yield a failure probability over their 60 year lifetime of 6E-4, or 6 major accidents over 10 000 years. Even with 10 000 nuclear plants worldwide, we would have major accidents at an average interval of 170 years. The safety issue has disappeared.

Is it possible to have nuclear plants with such an infinitesimal CDF value? For the current generations of plants operating, in construction or offered now, the answer is no. With a new generation of plants, specifically SMRs of the iPWR type presented in Section 3.3, the answer is possibly yes. It becomes a definite yes if the iPWR design is safety-driven, as will be discussed in Section 3.4. Finally, this safety driven iPWR must be economically competitive to cover a majority of the market; this will be discussed in Section 3.5.