Innovative Concepts and Transmutation New Ways to Reduce Radioactivity of Nuclear Waste

In his presentation, Dominique Vignon, Chairman and CEO of Framatome, introduced three innovative reactors and their application in reducing and managing nuclear waste. There is indeed a strong interaction between waste management and reactor concepts, as various types of waste can be transmuted, by irradiation in adequate reactors, into non (or less) radioactive materials. As these various waste (see annex) behave differently, for transmutation purposes, they have to be sorted. This process, in the present state of the art, requires major technical breakthrough, needing adequate R & D.

Problems to be solved, advantages and disadvantages of the three innovative reactors, were then discussed.

The need to initiate demonstration projects resulted as essential conclusion.
New concepts of reactors

The EPR – European Pressurised-water Reactor -, the most recent generation of PWRs, developed jointly by the French and Germans. This is a proven, safe technology based on active heat removal and redundant safeguard systems. Among other advantages, it offers increased safety vis-ˆ-vis major accidents. The EPR can be operated with 100% MOX fuel, which makes it a consumer of plutonium.

The EPR is at a development stage where it is ready to be built. It will produce electricity at a price that is competitive with other sources.

The ADS – Accelerator Driven System – or hybrid system (the Rubbia concept or Rubbiatron belongs to this family). The concept is not new, since the principle has been known since the 1950s. A particle accelerator (normally a proton accelerator) is combined with a subcritical reactor. The energy is removed passively, but the reactivity is controlled actively by the accelerator.

This system will never be competitive for energy generation, since an essentially conventional reactor is combined with an accelerator (high cost). On the other hand, operating with fast neutrons, it is capable of burning all the actinides without prior separation. One would need one ADS unit to burn the production from four 1000 MWe PWRs (order of magnitude).

The MHR – Modular Helium Reactor -, the latest modular evolution of the high-temperature reactor (HTR) now would operate according to a direct-cycle operation, with a gas turbine driven directly by the helium used to cool the core, which permits obtaining efficiencies approaching 50%. Each module has a capacity of 280 MWe. HTRs use passive energy removal and have remarkable inherent safety, since the coated ceramic fuel assemblies cannot melt, even in the absence of cooling. In addition, the spent fuel can be stored as is, since the ceramic envelope exhibits remarkable resistance with time.

This reactor is by far the best consumer of plutonium (more than 90%) and, subject to confirmation by a prototype, it should provide electricity at a price that would be competitive in its power category.

Problems to be resolved

Practically none as far as the EPR is concerned, and it is now available for construction.
An large amount of basic R&D is required concerning the hybrid systems (ADS): Construction of an accelerator of the required capacity; development of the material for the window, which will be subjected to heavy neutron bombardment; corrosion of the surrounding structures, caused by the subcritical assembly; and residual heat removal.

Slightly fewer but nevertheless difficult problems with the MHR, since the difficulty concerns building and assembling components that are better understood individually.
Essentially technological R&D: helium leaktightness; compactness of the turbine-generator-compressor assembly and maintenance of this assembly; metal creep.

Advantages and disadvantages associated with the various technologies

ADS: Good transmutation of the actinides; some questions concerning the fission products. Will never be a good producer of electricity compared, for example, with a PWR. Will have very high cost. The number of units required to handle PWR waste must be determined.
MHR: A good plutonium incinerator and good producer of electricity; modular construction. Spent fuel can be stored as is, without risk of dangerous products escaping from its ceramic matrix.

Whatever methods for incineration and separation will be developed, it will always be necessary to store a small portion of long-lived waste. However, due to the sensitivity of nuclear waste management in developed countries, and considering the potential of innovative concepts to transmute the waste, such concepts have to be developed.

Demonstration projects

Nuclear reactors, regardless of the type, have a large margin for improvement with respect to safety and waste storage. One can consider that the plutonium stock can be controlled by using fast breeder reactors or HTRs (MHRs). Long-lived waste can be transmuted.
Good international co-operation must be established with respect to storage, separation, and incineration of nuclear waste.

A strategy must be chosen. One could go on forever carrying out laboratory research to improve each of the basic” building blocks” comprising these new systems, or one could decide to go ahead immediately and build a first facility, freezing its characteristics, without being certain that the best choices have been made.

It would seem, however, more reasonable to initiate a project that has been duly analysed, based on international review and decision. The European Community provides an appropriate framework for this.

Demonstration projects must be initiated, for without them theoretical work will continue without ever having tangible proof of the feasibility of any combined solution, or of its viability.

Annex

Sorting the waste

Each year, a current 1000 MWe PWR produces:
230 to 250 kg of plutonium (Pu), which can be used as fuel in certain reactors or classed as waste.
Approximately 30 kg of “minor actinides” (elements having an atomic weight greater than that of uranium and plutonium), all having long half-lives, but which can be transmuted into short-lived or stable elements.
60 kg of long-lived fission products transmutable by neutron capture in appropriate reactors.
If they were not sorted, the general toxicity would be broken down after 10.000 years, as follows: 94% for the plutonium, 5% for the other transuranic elements, and less than 1% for the fission products