RD&D-Programme 2004 - SKB

• A subcritical nuclear reactor with fuel that contains the long-lived radionuclides to be transmuted.
Since transmutation is caused by nuclear fission, additional neutrons are released.
For each neutron from the spallation source, about 20 new neutrons are released, which
cause further nuclear fissions. The reactor is designed so that a self-sustaining chain reaction
cannot occur.
• Equipment to remove the heat that is generated and make appropriate use of the energy
(for example for electricity generation).
• Equipment to control and monitor the entire system.
Proton accelerators can be of two different basic types: a linear accelerator (linac) or a cyclotron
(with a circular particle track). Cyclotrons have certain technical limitations with regard to
proton energy, beam current and flexibility. As a result, interest has focused increasingly on
linacs. A linac for the required proton energy is very long – several hundred metres – and
requires advanced technology and advanced radiation protection.
So far accelerators have mainly been built for pure research purposes where the requirements on
availability have been modest. However, a plant for transmutation contains large systems with
high temperatures and built-in inertia to changes in the states of the systems. This, along with
requirements on efficiency and reasonable economy, imposes high demands on the availability
of accelerators for ADS.
The spallation source is a central component in an ADS situated between the accelerator and
the subcritical reactor. It is exposed to a demanding environment with high radiation, high
temperature and highly varying pressure and requires very effective cooling. Some heavy
materials can in theory be used for the source (lead, bismuth, tungsten, tantalum etc), but
considering the cooling requirements and other factors the foremost candidate at present is a
eutectic mixture of liquid lead and bismuth. An important question is the design of the window
between the source and the vacuum tube that conducts the proton beam from the accelerator to
the lead-bismuth target. The alternatives are a “hot” window consisting of a radiation-resistant
special metal close to the target or a windowless version, by which is meant a “cold” window
located some distance from spallation source. The latter design requires a sufficiently good
vacuum to be maintained in the access tube between the window and the spallation target so
that the protons will not lose energy on the way. A “hot” window, on the other hand, makes very
high demands on radiation resistance and strength so that window replacements can be limited
to no more than one per year. Currently, two major spallation sources with liquid lead-bismuth
are being built or are planned in Europe: Megapie at PSI in Switzerland, where a hot window
design has been chosen, and Myrrha at Mol in Belgium, where a windowless version will be
tested.
Different designs are being considered for the subcritical reactor. Fuel as well as coolant are still
open for choice. Water is not a feasible coolant for a fast flux reactor. Possible options are gas
(particularly helium) or liquid metal. Gas cooling requires high pressure and is therefore often
rejected due to the risk of loss of cooling. Considerable experience exists with liquid sodium
from previous fast reactor programmes. The problems encountered are clearly discouraging, at
least for the present time, and interest is increasingly focusing on a eutectic mixture of liquid
lead and bismuth. This mixture has a relatively low melting point and a high boiling point and is
also the prime candidate for the material in the spallation source. The latter naturally simplifies
the design of the systems. Experience from reactors cooled with liquid lead-bismuth exists only
in Russia, where seven submarine reactors have been built and operated with this coolant.
The research on fuel for ADS is being pursued along several parallel lines. Nitrides and alloys
with inert metals, such as zirconium, are attracting considerable interest. The requirements on
the fuel include good thermal conductivity for high power density, good irradiation resistance
and an ability to achieve high burn-up – tens of percent compared with a few percent in today’s
light-water reactor fuel. Fuel rods with plutonium nitride will be irradiated in the R2 reactor in
Studsvik during the next few years within the framework of an EU-funded project.
312 RD&D-Programme 2004