Exploring a potential high energy solution to the problem of long-lived waste

Waste from commercial nuclear plants contains large quantities of plutonium, other fissionable actinides, and long-lived fission products that represent potential proliferation concerns and create challenges for long-term storage.

The current US policy is to store unprocessed spent fuel in a geological repository, but long-term uncertainties are hampering the acceptability and eventual licensing of the repository and driving up its cost.

The greatest concerns over the repository are the potential – which stretches over tens of thousands of years – for radiation release and exposure from the spent fuel, and the possible diversion and use of the actinides contained in the waste for weapons construction.

Accelerator-driven transmutation of waste (ATW) systems are designed to destroy long-lived fission products by reducing the time required for the waste to decay naturally from 10,000 to less than 1000 years. Computer simulations have greatly assisted the design process for the ATW concept.

One of the key components in the ATW concept is a target that is bombarded with protons from a linear accelerator, which in turn produces neutrons that sustain the transmutation of the waste to a stable or less radioactive material.

The ATW concept

In the ATW concept, spent fuel would be shipped to a site where the plutonium, transuranics, and selected long-lived fission products would be destroyed by fission or transmutation using an accelerator-driven subcritical burner. This process generates a huge amount of heat, which can be removed with a liquid lead-bismuth eutectic (LBE) and limited pyrotechnical treatment of the spent fuel and residual waste.

This approach can be contrasted with the current reprocessing practices used in Europe and Japan in which high-purity plutonium is produced and used in the fabrication of fresh mixed oxide fuel that is then shipped off-site for use in light water reactors.

THE ATW PROCESS

An ATW facility has three major elements as indicated in the figure below.

•A high-power proton linear accelerator.

•A pyrochemical spent fuel treatment system and liquid lead-bismuth eutectic target that produces the high intensity neutron source.

•The surrounding subcritical blanket containing the transmutation assemblies.

One of the greatest design challenges in the development of a working ATW system is designing the target and planning its operating conditions in order to maintain the proper temperature. ATW takes advantage of the exceptional properties of liquid LBE as a nuclear coolant and as a spallation neutron source. Spallation neutron sources exploit the thermal excitation of heavy nuclei with energetic (GeV) protons and the subsequent decay of these nuclei – mainly by evaporation of neutrons with energies of a few MeV.

Significant neotron multiplication and heat production occurs from the actinides in the surrounding transmutation assemblies, and therefore adequate means for removing the heat are required.

Only a small fraction of the proton’s kinetic energy is actually dissipated into thermal excitation. A larger fraction of it is carried off during the initial intra-nuclear cascade by particles (mainly nucleons) that have energies that are in the MeV range. In a thick target, these pre-equilibrium particles initiate secondary reactions (through an inter-nuclear cascade) which produce additional neutrons.

Because it operates in subcritical mode, ATW is well suited to incinerate nuclear waste material that is not well characterised, that transmutes either poorly or not at all in reactors, that has potentially unstable and hazardous reactivity responses, and that cannot be isolated and placed in reactors.

PRODUCING THE TARGET

The International Science and Technology Center (ISTC) is an intergovernmental organisation that is dedicated to the nonproliferation of weapons and the technologies of mass destruction. The ISTC has funded the Institute of Physics and Power Engineering and the Experiment (IPPE) and Design Organisation-Gidropress (EDO-GP) in Russia to design and manufacture a pilot target (which is known as Target Circuit One, or TC1) that incorporates Russian LBE technology into the ATW concept. Since 1994, the ISTC has provided $267 million of funding to projects at nearly 400 institutes.

Researchers at the Los Alamos National Laboratory are using finite element analysis-based computational fluid dynamics software in order to analyse the designs, and the results have been used to significantly improve succeeding versions of the target.

Target design

The target is due to be tested in the 800MeV, 1.25mA proton beam at the Los Alamos National Laboratory located in Los Alamos, New Mexico, in two years time. These target experiments will provide valuable information on the performance of LBE as a spallation target and as a coolant. They will also help in the design target/blanket systems for future ATW facilities.

As a part of the preparation for the beam-on test, the researchers at LANL have carried out a thermal hydraulic analysis for the TC1 target (geometry shown in the figure opposite) that works as follows.

The proton beam from an accelerator is first injected into the target through a steel window. Liquid LBE then flows in from an outer annulus channel, sweeps over the target window, and flows out through an inner channel. A diffuser plate is placed near the window in order to enhance the flow around the window centre, where the heat deposition from the proton beam is the greatest.

IPPE’s neutronics calculations predict heat deposition in the target.

The researchers at Los Alamos performed the analysis using FIDAP computational fluid dynamics (CFD) software from Fluent, based in Lebanon, New Hampshire. The finite element method is an ideal on for generating the complex and irregular geometries required to model ATW targets. FIDAP has been validated by Los Alamos researchers and has been found to provide accurate predictions of target thermal hydraulics.

A CFD analysis provides fluid velocity, pressure, and the temperature values throughout the region of interest for problems with complex geometries and boundary conditions. As part of the analysis, a researcher may change the geometry of the system or the boundary conditions such as inlet velocity and flow rate and view the effect on fluid flow patterns or temperature distributions.

CFD can also provide the detailed parametric studies that are able to significantly reduce the amount of experimentation that is necessary to develop a prototype device, and thus reduce design cycle times and costs.

SIMULATION SCENARIOS

LANL researchers have simulated two scenarios, one for nominal beam power of 1MWe with an inlet temperature of 242°C and the other for 80% of the nominal power with an inlet temperature of 235°C. The flow rate of liquid LBE is fixed at 14.2m3/h. To date, they have carried out simulations with a 2D axisymmetric model, and studies with a 3D model are under way.

The results from the 2D simulations show that the coolant flows through the target smoothly, with no recirculation zones. This is an important factor because recirculation may lead to unwanted temperature buildup.

Because of the diffuser plate, the majority of the coolant is forced to sweep over the window centre and then pass through the centre hole of the diffuser plate at high speed (2.0m/sec). This swift flow of coolant plays a key role in cooling the centre of the target window, where the energy deposition from the proton beam is the highest.

The calculated temperature ranges from 242°C to 462°C. This temperature range is within the working range for both the LBE coolant and the construction materials. The highest temperature occurs outside the centre of the target window, while the average temperature at the outlet is 360°C.

This temperature increase between the inlet and outlet is consistent with the total energy deposition in the target. There is a 30-40°C temperature drop through the window near its centre, and the temperature variation in the target window is negligible 5 cm away from the centreline. The temperature in the diffuser plate varies from 250°C to 280°C: a relatively small variation, compared to that in the window.

The temperature distribution in the target with 80% nominal power is similar to that with nominal beam condition. The highest temperature still occurs outside the centre of the window but it is only 412°C, and the temperature drop in the window reduces to 25°C.

Design iterations

Each time that the Russians develop a new design iteration, they send it to Los Alamos, where it is then analysed from a thermal hydraulic standpoint. The results that are calaculated at Los Alamos are then sent back to Russia and are then used to improve the existing design, and over the last few years the ATW design has been dramatically improved through the use of this iterative process. Analysis shows that the latest designs are suitable for beam-on testing.

The diffuser plate successfully enhances the coolant flow around the window centre but still avoids generating recirculation zones downstream. The temperature range is within the proper operation range for both the LBE coolant and the structural materials.

Additional work

Researchers at Los Alamos are now proceeding with additional work that needs to be carried out prior to testing, including development of 3D models and analysis of accident scenarios.