Study on the Neutronic Characteristics of Subcritical

Reactors Driven by an Accelerated Pulsed Proton Beam

Ali Ahmad

Outline

Motivation

The Simulation Set-up Results & Analysis

Conclusions

ADSR in question?

Three main components:– Accelerator– Spallation target– Subcritical core

Possible deployment of thorium fuel cycle

www.thorea.org

Motivation : Why pulsed beam operation?

LINAC is expensive

Classic cyclotron technology is mature and approaching its power limit

FFAG has the potential to be both affordable and technologically capable of doing the job

(Takahishi. 2001)

FFAG is a pulsed accelerator

Motivation : cont.Why is neutronic analysis required?

Pulsed proton beam means pulsed production of spallation neutrons

Oscillations in the power profile are inevitable

Frequent and rapid temperature transients

Thermal cyclic fatigue

Modeling of flux variation with time is needed

The Simulation Set-up

Simulations performed using the MCNPX neutron transport code.

En < 20 MeV : Nuclear data tables (ENDF/B-VI)

En > 20 MeV : Nuclear models– Bertini Model (Bertini 1969)

Delayed neutrons and thermal treatment are included

The Simulation Setup…cont

Reactor Materials:

- Target: Pb-208 - Fuel: Th-Pu MOX - Clad: Stainless steel - Coolant: Pb-208 / Water - Radiation shield: Lead

Core arrangement:

- Core geometry: hexagonal - Bundle geometry: hexagonal - Number of bundles: 125 - Number of fuel pins: 96 - Fuel active height = 202 cm

Results & Analysis (1) : Neutron spectrum evolution

Results & Analysis (1) : Neutron spectrum evolution

Very fast neutrons (En > 20 MeV) decay in less than 30 ns

This means that primary neutrons aren’t of interest to the core neutronics study

Hypothesis: Neutron generators can be used instead of a proton beam to study neutrons’ kinetic characteristics

Results & Analysis (1) …cont

14 MeV DT-n-source and beam-operated n-source have very similar core neutronic characteristics

A 14 MeV n-source can potentially replace the proton accelerator in an ADSR for research purposes

Similar results have been obtained by Yamamoto & Shiroya (2003)

Results & Analysis (2) : Spatial variations and diffusion

The diffusion of neutrons in a thermal ADSR is characterised by:

- The spallation neutrons are dominant for a period τ 10 µs

- After that time, the fission neutrons become dominant

Results & Analysis (2) : ... cont

A pulsed beam of frequency 1 kHz almost allows four orders of magnitude reduction in the neutron flux level in a fast ADSR

The sharp decrease in the neutron flux in a fast ADSR is instantaneous in all assemblies

The perturbation in the neutron flux due to a 1 kHz beam in a thermal ADSR is observed only in the assemblies close to the target

After a few pulses, the fission neutrons become dominant elsewhere

Thermal Fast

Results & Analysis (3) : Monitoring of ADSR reactivity

The ideal monitoring of core reactivity should be:– On-line– Accurate– Simple and robust measurement technique

Experiments done to measure the subcritical reactivity:– MUSE (Billeboud et al. 2003)– YALINA (Fernandez-Ordonez et al. 2003)

None of the experimental techniques meet all of the requirements

Results & Analysis (3) : Cont.

All of the proposed techniques rely on deliberate gaps in the beam to measure the subcritical reactivity

The diffusion equation in a thermal ADSR:

Using

At a certain spatial position in the reactor (detector position):

Results & Analysis (3) : Cont.

For the case when the beam is off the last equation has the solution:

Using the definition of neutron life time:

Then,

Define

α can be estimated from the flux decrease rate when the beam is off

Results & Analysis (3) : Cont.

Once the neutron life time of a certain core configuration is known, a reactivity measurement will be straightforward

To perform on-line measurement of Keff in a thermal ADSR, the separation time between pulses should be more than several tens of microseconds

Conclusions Experimental investigations of the neutronic transients in an

ADSR can be done through a relatively cheap neutron source In a thermal ADSR the diffusion time of spallation neutrons is

around 10 microseconds, while it is much quicker in a fast ADSR The flux fluctuations due to pulsed operation are almost

independent of the spatial position in a fast ADSR The reactivity of a thermal ADSR can be measured on-line if the

beam frequency is less than 10 kHz The next step in my research is to study the thermal issues

related to pulsed operation and their consequences

Thank you