Advantages of a safe nuclear energy

Nuclear energy is a concentrated energy source (adapted to mega-cities), not very dependent of weather or climate (if nuclear reactors are sited on the sea)

A well proved technology, which can be still largely improved (life of reactor up to 60 years, load ratio up to 90%, BU up to 60 GWjt-1)

Important and diversified sources of natural and man-made fissile nuclides (Pu isotopes). U ores are widespread and abundant without competing uses and it is easy to store. More than 50 years of production at the present level of production can be achieved

A low cost of kWhe (“full life cycle cost” analysis).

A very low radiological impact for the environment and no release of CO2 (but 6% of the world energy consumption).

A worldwide system of protection for workers

Extension of the present technology to produce electricity (with reactors of 300 MWe) and high temperature heat (metallurgical processes, H2 production). New technology using all U isotopes (and Th) could provide fission energy over a very long time (see later).

Disadvantages (drawbacks) of safe nuclear energy

A high investment costs with a long payback of money (10 years for construction of a reactor and a large amount of money in case of accident)

A difficulty to follow variable electricity demand (base load generation only)

An accumulation of civil Pu. In average the world-wide production is 75 tons a year. The status of Pu is peculiar according of its use as a fuel or its management as waste. As Pu239 has a lower % of delayed neutron than U235 % the quantity to be introduced in reactor is limited to 12 % (30 % of MOX sub-assemblies in a 900 MWe PWR initially loaded with UOX). The isotopic composition Pu MOX SF does not allow a second recycle in PWR (too much even-even nuclides which decrease the quantity of fissile nuclides by thermal neutrons).  

Attached to the accumulation and separation of Pu from SF is the question of dissemination of fissile material (nuclear weapons proliferation)

A production of nuclear wastes difficult to manage in the long term whatever they are, SF or packages of wastes from reprocessing. LL-SLW packages (PF and AP), are disposed of in ground or underground repositories. The HL-LLW and MLL-LW packages, which contain LL radionuclides, are put in interim storage waiting for a final destination, which is the matter of hard discussions

A controversial appreciation by citizen (anti-nuclear people against nuclear technocrats, very low risk can have tremendous consequences, effects of radioactivity difficult to understand). Real efforts, but too recent, have been done to give an objective information on nuclear problems, but in general the dialogue is difficult to be established (except in some countries)

Nuclear energy, environment and society. Is this energy compatible with sustainable development?

Releases

The radioactivity of the gas or the aqueous solutions (called ultimate effluents) which are released in atmosphere and hydrosphere during operating reactors or facilities of the fuel cycle are controlled according to the safety rules for radiation protection. The authorisations of releases are calculated on the basis of the dose for the most exposed people at the limit of the site according to scenarios (and others regulations). The true releases are only several per cent of the authorisations (T, volatile FP, emitters).

The case of accidents is special

The release of heat and chemicals by reactors and facilities of the fuel cycle have no special status versus industrial rules.

Society

The links between nuclear energy and society are complex. The concerns are all linked to the concept of radiological dose and associated risk.

In short the hazard of nuclear energy comes from an exposure to ionising radiations, appreciated by the calculated “dose” The dose depend on scenarios of exposure. The risks depend on the dose. A reference point is the natural dose.

The low doses (less than immediate lethal doses) are given in Sievert (Sv). Natural dose is around 2.5 mSv/year.

The radiological risk, RR, associated to a dose is RR (t-1) = p (Sv-1) i Pi (t-1) Ei (Sv-1) where p is the probability of occurrence of the effects, Pi is the probability of the occurrence of an event i, and Ei is the calculated dose given by the event i. For low dose the unit of time is the year.

The basic hypothesis supporting the RR for low doses is called the LNT (linear no threshold). The value of p = 0.073 Sv-1 (0.06 for a cancer leading to death and 0. 013 for hereditary effect according to IRCP 60) means that 6 cancers per year can appear in a population of 100 000 inhabitants, each having got 1mSv. The value 0.073 is on debate. There are also debates on LNT and the mechanisms of induced cancer by ionising radiations

IRCP considers that an “acceptable risk” correspond to a dose added to the natural dose of 1 mSv/year for any individual of the public. This dose is considered to have any effect on the “expected life”

IRCP considers that a “trivial added dose” is 1 microSievert (1/100 of the natural dose).

For a given amount of radioactive matter (1 ton of SF for instance or quantity of SF to produce 1 TWhe) the “inventory of radiotoxicity”, in short the” radiotoxicity”, is defined as i RRi calculated supposing that all radionuclides, i, are incorporated. It represents a potential risk.

The “residual risk” is i RRi calculated for a given case of exposure . It takes into account the management of the radioactive matter (like in geological disposal for instance).

Doses due to the use of nuclear energy are low compared to the natural doses and those given by medical diagnostic (1.5 mSv/year in average per person)

Sustainable development

Sustainable development requires to meet technical and societal criteria. Here they are essentially discussed in the context of the pursuit of the present technology of reactors according to a moderate increase of the use of nuclear energy. Up to 2050 nuclear energy will lie on U/Pu fissile nuclides (impossibility to launch many fast neutron reactor to use U238, lack of Pu and reprocessing SF facilities). Phase out of nuclear energy is not considered (but is not a simple problem)

Forecasts are difficult (visibility on the demands of the open market, safety rules) due to the complexity of the economy and the changing feeling of the society on the nuclear energy.

Technological criteria

At least 4 technical criteria have to be fulfilled by nuclear energy (and any source of energy) : providing adapted power to the energy needs, safe technology, durability of resources (Unat), no collateral damage (proliferation of nuclear material). Management of nuclear wastes for the coming years rise some technical problems but the main problems are of societal type (see later).

As discussed the 3 first criteria are fulfilled (30 to 80% of electricity depending countries, Generation III reactors will have improved systems to reinforce safety, Unat resources are secure for the next decades at known prices, Pu can be recycled once in PWR without difficulty).

The proliferation is a question of technical and political means controls. It is more easy to have fissile nuclides with high level enriched Unat in U235 (ultra-centrifugation) than to produce Pu239. Both techniques are very heavy.

Social criteria

There are 2 important criteria : cost and wastes management, the radiological impact being low (except in the case of accidents)

Price is around 3 to 4 cents of Euro per kWhe (including provisions for waste management and decommissioning) whatever would be the interest of money (5 to 10 %) on the next 40 years

Waste management of LLW is a major concern (U ores mining, ML-LLW and HL-LLW). High isolation and confinement or radioactivity over long periods of time is necessary (105 time compared to toxic industrial wastes).

Actually 10 000 tons of SF are yearly unloaded in the world. Nobody knows exactly if it will be possible to transmute on an industrial scale the LL radionuclides, indefinite storage raise the problem of the stability of society, sitting of deep geologic disposal is difficult.

A geologic repository could be designed to dispose of around 80 000 tons of SF (or equivalent reprocessing packages). In 2020, 200 000 tons of SF will have to be managed. If geological disposal is chosen, that will need 3 disposal sites (an increase of the use of nuclear energy must be considered with respect to the need of more repositories)

There are 4 characteristic periods of time in LL nuclear wastes

- 5 (to 10 ?) decades during which SF or HL-LLW must be cooled (heat released by FP and SL actinides, Pu241, Cm244, Cm243 for UOX and additionally Pu238 for MOX). During this time either the way to change the type of wastes or to dispose of the wastes is decided.

- several decades to implement the choice (no need of cooling)

The problems during these two periods are of national relevance and solved by man-made technique (for geological disposal heat released by Am241 and Pu238 lay down the size)a

- the third period extends to 100 000 years during which radionuclides must be isolated/confined, for instance in canisters and engineered barriers of high performances (Pu239, Pu240, Pu242, Am241 and Np237). Concentration of emitters must be less than 10-10 M in environment, much less than chemical pollutants (linked to 1mSv/year got by drinking water).

- over 100 000 years the radionuclides (U isotopes, Np237 and Pu242 and also long-lived FP) must be confined by natural rocks

During these two last periods the problems can only be solved by “geology”

Is nuclear energy renewable?

The question is a matter for the next half of this century and later

The present yield for the utilisation of the energy of fission contained in Unat is less than 1%. But when U238 is used as fissile and fertile nuclide (fast neutrons) the period of time during which energy can be produced is measured, as least in theory, in thousands of years (renewable energy?)

In the case of lack of U, or in parallel for technical or social reasons, the use of Th (thermal neutrons) can also be considered on the same scale.

But the massive use of “new reactors” and new “nuclear systems” is mandatory.

Resources in U

The stockpile of Udep is enormous (230 000 tons in France) and will increase using Uenr as nuclear fuel. Udep is easy to manipulate. The quantities of Urep are less (20 000 tons in France) but will increase using MOX fuel. Declassified military high enriched U (up to 90 % in U235) or fissile Pu239 could be used.

According to some forecasts the need of Unat could be important. For instance a power of 250 GWe in 2020 would require 100 kt per year of Unat. So extraction of Unat both from pure U ores and as by-product (industry of Cu, Au or P) should be boosted

Reactors for the future (2050), valorisation of resources, optimisation of waste management.

The nuclear energy for the future will be developed in the direction of mixing this source of energy with other sources in an “energetic mix” whatever the other objectives are.

The future “nuclear systems energy” including Generation IV reactors and associated cycle facilities should have the objective to valorise the resources in fissile nuclides and to optimise the management of wastes.

Along the long way to launch these systems the most advanced new reactors are HTR and FNR

Two international projects of HTR of low power (100 and 300 MWe) are developed (derived from experimental reactors operated in USA and Germany in the sixties-seventies). Two HTR are presently operated in China-10 MWth- and Japan -30 MWth)

The coming HTR will be fuelled with Uenr (8 to 10 %) or with MOX made with military Pu239, moderated with C, cooled with He and operated following an open cycle (75 % of loaded Pu could be burnt). With He at 600°C directly associated with a gas turbine a yield of 50 % is expected (Brayton cycle). Their fuel will be based on micro-spheres of ceramic oxides (or carbides) coated by several layers of C and by SiC and embedded in C. This is a new fuel.

The layers will isolate FP and actinides from He (like cladding of pins in PWR) up to 1600 °C. SF at high BU (100 to 150 GWjt-1) will be a waste because its reprocessing will be very difficult (but not impossible).

FNR.

Fast neutrons allow the use of all isotopes of U, Pu and heavier actinides (f/c of fast neutrons > f/c of thermal neutrons). The resource of fissile nuclides is practically increased by a factor of 50 (twice in theory). The technology of FNR cooled by Na is known. They are fuelled with MOX of high content in Pu (up to 20 %) and need to be launched 10 to 15 tons of Pu/GWe (in fact with 9.6 tons of fissile Pu isotopes)

They can be operated giving as much Pu as they burn (regeneration) or more (over-generation or breeding). But it takes 2 to 3 decades to have sufficient Pu to launch a new FNR (50 years for a PWR!).

Worldwide Pu production is around 75 tons (and 7.5 tons of other actinides) which could allow to launch 5 to 7 GWe/year. In 2030 the stockpile of Pu in SF will be around 3000 tons.

Transmutation of actinides (FNR and ADS)

FNR can burn actinides (Np, Pu, Am, Cm) if they are included in U based fuel but in limited quantities (due to safety problems raised by the decrease of available delayed neutrons). Special fuel (metallic alloys, oxides, other compounds) must be used of which preparation and certification have to be implemented.

ADS (Accelerator driven system).

ADS are based, like FNR, on fission induced by fast neutrons. But the core (U) is sub-critical (keff around 0.98 for instance) and the fast neutrons needed to have = 0 are given by a spallation source.

In this device (cooled molten Pb-Bi alloy, diameter 0.5 m, height 1 m) a beam of high energetic and high flux of proton (1 GeV, 20 mA at the limits of present accelerators) is transformed in fast neutrons (1 to 2 MeV) by nuclear spallation reactions

The power of the reactor, Pr, is controlled by the power of the accelerator, Pa (Pr/Pa = 6 / (keff/1-keff)).

The use of ADS is foreseen to transmute Am (and/or Pu) embedded in inert target (without U). Transmutation with ADS allows a load in actinide higher than in critical FNR because operating the reactor does not need delayed neutrons.

Other reserves of fertile nuclides

It is possible to launch thermal neutrons based reactors using the fertile monoisotopic Th232 and fissile nuclides (U235 or Pu isotopes) as a match. Fissile U233 is formed, which can be recycled. U233 has very attractive nuclear properties. U233 can be, or could be, produced in PWR by irradiation of Th232

A molten salts reactor (MSR) in USA (7.5 MWth, U235 and U233 fuelled) has shown, in the sixties, the possibility of breeding. A modern version is under evaluation. MSR are high temperature reactors (600 to 700 °C) where molten salts (Li and Be fluorides) are both fuel and coolant and the moderator is C. It needs 1.2 tons of fissile nuclide per GWe, around 1/10 of a FNR-Na load. It does not produce heavy actinides (U238 is not present)

Such reactor can transmute actinide in line.

Conclusion

Technology and economy make fission nuclear energy a “sustainable energy”: Unat resources for decades (and possible use of made-man fertile nuclides), safe technology and possible improvement, low price of kWhe compared to other sources (without considering CO2 emission tax), environment friendless and no health impact by additional low dose of radiation. A renew of nuclear energy could be possible.

But problems remain: public antipathy (difficult to change), waste management (problems identified, but not solved), policy for international licensing (visibility for development), ethical (intergeneration relationships).

It is reasonable to forecast that during the next 15-20 years (say up to 2025) no drastic change will occur in nuclear energy production.

This period will be for each country a period of” thinking” on the use of nuclear energy, confirmation of phase out, maintaining present level or increasing it in the “energetic mix”.

This period will be also a test period for the implementation of programmes set up to renew world-wide nuclear energy as for instance the GNEP (Global Nuclear Energy Partnerships) leaded by USA.

Finally this period will be devoted to test the will of countries in developing international research to prepare possible launching of reactors of “Generation IV”, and associated nuclear fuel cycles, in the second half of the century according to the objectives of GIF.

GIF (Generation IV International Forum, 11 partners) was initiated in 2002. Several organisational steps have been implemented up to 2005. It is aimed at developing 6 new types of reactors based on fast and thermal neutrons for optimising the use of fertile nuclides, producing less waste and opening new uses of nuclear energy (high temperature heat production).

The GNEP organisational programme, launched in 2005 by USA, proposes to complete the objectives of GIF as follows. In the short term: encourage launching of new reactors particularly for developing countries (low power reactor), in the long term: develop new technologies proliferation resistant, for recycling Pu and other actinides and develop advanced burners of Pu and actinides. For both actions it is proposed to set up an international system of nuclear services (enrichment, reprocessing) under international control

What will happen after 2025 for nuclear energy is relevant to prospective because choices on energy are subject to too many parameters

In the countries where nuclear option will remain open one can think that coexistence of Generation II and III of reactors will exist. Indeed the Generation III reactors are designed for a 60 years lifetime. These reactors, and associated fuel cycle facilities, could finally dominate the nuclear landscape up to the end of the century. This will not lead to a great change in nuclear industry

In the case of positive tests for a possible development of nuclear energy, which will mean that a drastic increase in nuclear energy will have been accepted, research for “nuclear energy for the future” will be boosted to prepare the use of fertile nuclides (U238 and Th232) and to implement the objectives of GIF.