Incineration of Unwanted Transactinides from HTR's in DT

INFCE International Xfl?c-o7/3^ Nuclear WCE/0EP./WG.8/74 Fuel Cycle Evaluation

Incineration of unwanted transactinides from HTR's in UT r.icro-explosion fusion reactors July 1978 Contributed by Switzerland

INFCE WORKING GROUP 8 ADVANCED FUEL CYCLE AND REACTOR CONCEPTS

INCINERATION OF UNWANTED TRANSACTINIDES FROM HTR'S IN

DT MICRO-EXPLOSION FUSION REACTORS

W. Seifritz Eidg. Institut fttr Reaktorforschung Wlirenlingen, Switzerland

Contribution to Summary Report on the HTR and its Fuel Cycle Options Chapter V - 2 -

It is a well known fact that the fission to capture cross- anv section ratio, °f/°c» °f fissionable (even or odd neutron) nuclei increases very strongly with the energy of the incident neutrons. Therefore, a high flux of energetic 14 MeV neutrons originating from the DT fusion reaction is particularly well suited to "burn" unwanted actinides directly.

In fusion reactors, based on the magnetic confinement principle, neutron wall loadings in the blanket region of about 1-3 MW/m^ will probably be available immediately behind the first wall to burn or transmute unwanted nuclei. The corresponding neutron 14 flux is not very high. The fluxes are in the range of only 10 14 MeV neutrons/(cm2sec).

However, in a DT-micro explosion fusion reactor, based on the inertial confinement principle, the available flux of 14 MeV neutrons can be many orders of magnitudes larger than in a magnetically confined system.

The concept springs from work over the past years which has indicated thet it is preferable to surround the hollow sphere of frozen DT fuel of the fuel pellet by a shell of high-Z material. This shell is surrounded in turn by an outermost ablator shell of a low-Z material which is blown away by beam heating to generate the needed implosion pressure. The high-Z material shell has several functions:

1. it serves to shield the fuel from hot electrons and x-rays which could preheat it and could thus prevent isentropic compression,

2. it works as a pusher and tamper to confine the OT-fuel and to increase Its burn efficiency, - 3 -

3. it can serve as a target for 14 MeV fusion neutrons released during thermonuclear burn to increase the energy yield by fast fission events or to fission very effectively unwanted fissionable nuclei.

It is the last advantage which is of interest here. In a DT- micro explosion reactor the yield of one pellet explosicr is typically in the realm of 100 MJ and more. The idea is now to fabricate the high-Z material shell with unwanted actinides from fission reactors in order to fission that material in the compressed pusher region during thermonuclear burn by virtue of 14 MeV neutrons. More details are given in ATOMKERNENERGIE, Bd.29 (1977) p.300-301.

Aside from burning U-238 (i.e., depleted uranium from the isotope separation plants), unwanted actinides built up in fission reactors, particularly from high-temperature reactors, are of interest in this application.

The neutron poison U-236 is the first obvious candidate. 15 to 20 % of all neutron absorptions in U-235 in thermal fission reactors lead to U-236 instead of fission and are thus lost. The high-temperature reactor (HTR) operating with highly enriched uranium fuel is of particular interest. Two examples may illuminate the U-236 problem in HTRs (see also Nucl.Technol.28, May 1978, p.378-383):

1. The discharged uranium in the so-called "Recycling Separate Elements (RSE)" fuel cycle has the following typical isotopic vector U3/U4/U5/U6/U8 » 3/19/7/51/20, i.e. more than half of the residue uranium is U-236.

In the RSE-cycle the recovered uranium from mixed-oxide elements is recycled once in separate elements without thorium. Subsequently it is removed from the cycle, which would achieve the removal of U-236. - 4 -

2. The discharged uranium in the so-called "Separated Feed- Breed (SFB)" fuel cycle has the following isotopic vector 0/0/1/73/26, i.e. almost three quarters of the rest uranium is in the form of L-236.

In the SFB cycle the highly enriched uranium and thorium are loaded into different fuel elements. Bred U-233 is separately recycled as mixed oxide; the driver fuel is recycled only once in separate elements.

Except for the near- and prebreeder HTR designs, the inventory of U-236 for all of the various fuel cycles which are taken into consideration, is roughly between 300 and 1000 kg/GW. In other words it would be worthwhile to burn that U-236 gaining another 200 MeV energy and about 4.3 neutrons per fission. In this way, the conversion ratio of HTRs could substantially be improved.

Another candidate to be used as the tamper material of DT-fusion pellets is Np-237 which is produced in a HTR by neutron capture reactions in U-236.

Conclusion:

The main point of this note is to stress that in a future fusion/HTR symbiotic system there is the possibility that the fusion hybrids (here the DT-micro explosion reactors) can not only breed fuel for the HTRs but can also burn unwanted actinides from HTRs - thereby improving the neutron and energy economy cf the total system.

WUrenlingen, Juli 1978

Dr. W. Seifritz