The French Institute for Radiological Protection and Nuclear Safety (IRSN) has produced an important critique of Generation IV nuclear power concepts.1 IRSN is a government authority with 1,790 staff under the joint authority of the Ministries of Defense, the Environment, Industry, Research, and Health.
There are numerous critical analyses of Generation IV concepts by independent experts2, but the IRSN critique is the first from the government of a country with an extensive nuclear industry.
The IRSN report focuses on the six Generation IV concepts prioritised by the Generation IV International Forum (GIF), which brings together 12 countries with an interest in new reactor types, plus Euratom. France is itself one of the countries involved in the GIF.
The six concepts prioritised by the GIF are:
- Sodium cooled Fast Reactors (SFR);
- Very High Temperature Reactors, with thermal neutron spectrum (VHTR);
- Gas-cooled Fast Reactors (GFR);
- Lead-cooled Fast Reactors (LFR) or Lead-Bismuth (LB) cooled Fast Reactors;
- Molten Salt Reactors (MSR), with fast or thermal neutron spectrum; and
- SuperCritical Water Reactors (SCWR), with fast or thermal neutron spectrum.
The report states: "There is still much R&D to be done to develop the Generation IV nuclear reactors, as well as for the fuel cycle and the associated waste management which depends on the system chosen."
IRSN considers the SFR system to be the only one to have reached a degree of maturity compatible with the construction of a reactor prototype during the first half of this century − and even the development of an SFR prototype would require further preliminary studies and technological developments.
Only SFR and VHTR systems can boast operating experience. IRSN states: "No operating experience feedback from the other four systems studied can be put to direct use. The technological difficulties involved rule out any industrial deployment of these systems within the time frame considered [mid century]."
The report says that for LFR and GFR systems, small prototypes might be built by mid-century. For MSR and SCWR systems, there "is no likelihood of even an experimental or prototype MSR or SCWR being built during the first half of this century" and "it seems hard to imagine any reactor being built before the end of the century".
IRSN notes that it is difficult to thoroughly evaluate safety and radiation protection standards of Generation IV systems as some concepts have already been partially tried and tested, while others are still in the early stages of development.
IRSN is sceptical about safety claims: "At the present stage of development, IRSN does not notice evidence that leads to conclude that the systems under review are likely to offer a significantly improved level of safety compared with Generation III reactors, except perhaps for the VHTR ..." Moreover the VHTR system could bring about significant safety improvements "but only by significantly limiting unit power".
The report notes that the safety of fast reactors can be problematic because of high operating temperatures and the toxicity and corrosive nature of most coolants considered. It says that issues arising from the Fukushima disaster require detailed examination, such as: choice of coolant; operating temperatures and power densities (which are generally higher for Generation IV concepts); and in some cases, fuel reprocessing facilities that present the risk of toxic releases.
The report is unenthusiastic about research into transmutation of minor actinides (long-lived waste products in spent fuel), saying that "this option offers only a very slight advantage in terms of inventory reduction and geological waste repository volume when set against the induced safety and radiation protection constraints for fuel cycle facilities, reactors and transport." It notes that ASN, the French nuclear safety authority, has recently announced that minor actinide transmutation would not be a deciding factor in the choice of a future reactor system.
The reports findings on the six GIF concepts are briefly summarised here:
Sodium-cooled Fast Reactors (SFR)
The main safety advantage is the use of low-pressure liquid coolant. The normal operating temperature of this coolant is significantly lower than its boiling point, allowing a grace period of several hours during loss-of-cooling events. The advantage gained from the high boiling point of sodium, however, must be weighed against the fact that the structural integrity of the reactor cannot be guaranteed near this temperature.
The use of sodium also comes with a number of drawbacks due to its high reactivity not only with water and air, but also with MOX fuel.
It seems possible for SFR technology to reach a safety level at least equivalent to that of Generation III pressurised water reactors, but IRSN is unable to determine whether it could significantly exceed this level, in view of design differences and the current state of knowledge and research.
Very High Temperature Reactors (VHTR)
The VHTR benefits from the operating experience feedback obtained from High Temperature Reactors (HTR).
This technology is intrinsically safe with respect to loss of cooling, which means that it could be used to design a reactor that does not require an active decay heat removal system. The VHTR system could therefore bring about significant safety improvements compared with Generation III reactors, especially regarding core melt prevention.
VHTR safety performance can only be guaranteed by significantly limiting unit power.
The feasibility of the system has yet to be determined and will chiefly depend on the development of fuels and materials capable of withstanding high temperatures; the currently considered operating temperature of around 1000°C is close to the transformation temperature of materials commonly used in the nuclear industry.
Lead-cooled Fast Reactors (LFR)
Unlike sodium, lead does not react violently with water or air.
The thermal inertia associated with the large volume of lead used and its very high density results in long grace periods in the event of loss of cooling.
In addition, the high boiling point at atmospheric pressure is a guarantee of high margins under normal operating conditions and rules out the risk of coolant boiling.
The main drawback of lead-cooled (or lead-bismuth cooled) reactors is that the coolant tends to corrode and erode stainless steel structures.
LFR safety is reliant on operating procedures, which does not seem desirable in a Generation IV reactor.
The highly toxic nature of lead and its related products, especially polonium-210, produced when lead-bismuth is used, raises the problem of potential environmental impact.
IRSN is unable to determine whether the LFR system could guarantee a significantly higher safety level than Generation III reactors.
Various technical hurdles need to be overcome before a reactor of this type could be considered.
Gas-cooled Fast Reactors (GFR)
Given the current state of GFR development, construction of an industrial prototype reactor would not be technically feasible. GFR specifications are highly ambitious and raise a number of technological problems that are still a long way from being solved.
From the safety point of view, the GFR does not display any intrinsic quality likely to lead to a significant improvement over Generation III reactors.
Molten Salt Reactors (MSR)
The MSR differs considerably from the other systems proposed by the GIF. The main differences are that the coolant and fuel are mixed in some models and that liquid fuel is used.
The MSR has several advantages, including its burning, breeding and actinide-recycling capabilities.
Its intrinsic neutron properties could be put to good use as, in theory, they should allow highly stable reactor operation. The very low thermal inertia of salt and very high operating temperatures of the system, however, call for the use of fuel salt drainage devices. System safety depends mainly on the reliability and performance of these devices.
Salt has some drawbacks − it is corrosive and has a relatively high crystallisation temperature.
The reactor must also be coupled to a salt processing unit and the system safety analysis must take into account the coupling of the two facilities.
Consideration must be given to the high toxicity of some salts and substances generated by the processes used in the salt processing unit.
The feasibility of fuel salt processing remains to be demonstrated.
SuperCritical-Water-cooled Reactors (SCWR)
The SCWR is the only system selected by GIF that uses water as a coolant. The SCWR is seen as a further development of existing water reactors and thus benefits from operating experience feedback, especially from boiling water reactors. Its chief advantage is economic.
While the use of supercritical water avoids problems relating to the phase change from liquid to vapour, it does not present any intrinsic advantage in terms of safety.
Thermal inertia is very low, for example, when the reactor is shut down.
The use of supercritical water in a nuclear reactor raises many questions, in particular its behaviour under neutron flux.
At the current stage of development, it is impossible to ascertain whether the system will eventually become significantly safer than Generation III reactors.
1. IRSN, 2015, 'Review of Generation IV Nuclear Energy Systems', www.irsn.fr/EN/newsroom/News/Pages/20150427_Generation-IV-nuclear-energy...
Direct download: www.irsn.fr/EN/newsroom/News/Documents/IRSN_Report-GenIV_04-2015.pdf
2. See for example:
International Panel on Fissile Materials, 2010, 'Fast Breeder Reactor Programs: History and Status', www.ipfmlibrary.org/rr08.pdf
Helmut Hirsch, Oda Becker, Mycle Schneider and Antony Froggatt, April 2005, 'Nuclear Reactor Hazards: Ongoing Dangers of Operating Nuclear Technology in the 21st Century', www.greenpeace.org/international/press/reports/nuclearreactorhazards