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Enhancing Nuclear Safety


Research Programmes

Temperature Effect Experimental Programme

An experimental programme of the temperature effect in diluted plutonium solutions


Various theoretical studies performed in the past have shown that highly diluted plutonium solutions could have a positive temperature effect. This is in contrast with uranium solutions for which a temperature increase will always lead to a negative reactivity effect. If such a positive effect would occur (for instance in a reprocessing facility where solutions of fissile materials are manipulated on a daily basis), an accidental super-criticality in a diluted plutonium solution could result in an unforeseen increase of reactivity from the start of the incident. Nevertheless, up to now no experimental programme has confirmed this effect.
To demonstrate the existence of this positive effect, the Temperature-Pu programme has been set up in the French Apparatus B at the CEA research center in Valduc .
The possibility for an experimental determination of the temperature effect in diluted plutonium solutions has already been studied in early ‘90s (IPSN/CEA). These experiments were however postponed and re-examined in 2002. The actual experiments started in late 2006 and were concluded in July 2007.

The challenges and the objectives

The main goal of the French Plutonium Temperature Effect Experimental Programme is to effectively show that such a positive temperature effect exists for diluted plutonium solutions. The experiments were conducted in the “Apparatus B” facility at the CEA Valduc research center (France) and involved several sub-critical approach type of experiments using plutonium nitrate solutions with concentrations of 14.3, 15 and 20 g/l at temperatures ranging from 22 to 40 °C.

All of the experiments will also contribute to the qualification forms criticality calculations for solutions of plutonium.

Description of the projet, methodology

During the course of the experimental program, a number of independent sub-critical approach experiments (designated as phase I experiments) were performed for three plutonium concentrations and various different temperatures. These experiments consisted of determining the height of the plutonium solution that is required to make the system critical. During the subcritical approach, the reactivity of the system is slowly increased through the sequential addition of small amounts of the plutonium solution into the core. This process continues as long as the effective multiplication factor keff is lower than or equal to 1-ß/10 (where ß is the effective delayed neutron fraction, estimated to be equal to 210 pcm). The critical height of the solution is accurately estimated using the neutron amplification method. At the end of each approach, both the plutonium solution and water reflector are transferred to their respective storage tanks.
A total number of 14 phase I experiments have been performed (5 at 20 g/l, 4 at 15 g/l and 5 at 14.3 g/l). Three of these phase I experiments were terminated prematurely due to technical difficulties.

 Plutonium concentration

 T [°C]


 20 g/l



 End 2006


 15 g/l




Early 2007

 14.3 g/l

2228 (phase II)





May-july 2007

Because of the independent nature of the phase I experiments, the uncertainty on the plutonium concentration and the acidity of the solution in every experiment must be taken into account to determine the overall uncertainty on the critical height. Even though these uncertainties are relatively low (of the order of 25 mg/l for the plutonium concentration and 1.5% on the acidity), they do lead to a significant uncertainty on the critical height.
It is therefore not easy to clearly demonstrate the positive temperature effect in these independent phase I experiments (an exhaustive study on these uncertainties is ongoing). An experiment in which these uncertainties could be eliminated was therefore necessary to clearly demonstrate the positive temperature effect.
The idea was to use the same solution at multiple temperatures (without draining the plutonium solution from the vessel) as follows:
first, a standard sub-critical approach at the initial temperature is performed;
for safety reasons, a small amount of the solution is drained to reduce the plutonium mass because the positive temperature effect will lead to a reactivity increase (roughly 1% of the initial solution will have to be drained);
the temperature of the solution is increased slowly by heating the water reflector;
and finally, a standard sub-critical approach is performed at the final temperature.
The advantage of this type of experiment is that it will completely eliminate the uncertainties associated with the acidity and the plutonium concentration. There will be perfect correlation between both temperatures. Unfortunately, this type of experiment also takes a long time to perform (55 hours were required to perform this experiment with a temperature increase from 22 to 28 °C). Furthermore, if the plutonium solution is drained from the vessel due to technical or safety reasons (for instance an interruption of the electrical supply) then the experiment must be started over again. Because of these issues and the associated safety constraints this experiment (referred to as phase II) was only performed once from 22 to 28 °C, but it did clearly demonstrate the existence of the effect.

Expected results

All of the experiments will also contribute to the qualification calculations of criticality forms  for solutions of plutonium. Notably, these experiments will be integrated into the basic qualification form CRYSTAL, developed in collaboration between IRSN, AREVA and CEA.

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> Start : 2002
> End: 2007

> Project manager : IRSN
> Owner : CEA Valduc
> Associated themes :  criticality, neutronique
> IRSN unit involved :
Le laboratoire d’études, de recherches, de développement et de qualification des codes (LERD)



International PHYSOR 2008 congress  at Interlaken:
1. “The Plutonium Temperature Effect Experimental Program”, Wim HAECK, Nicolas LECLAIRE, Eric LETANG (IRSN), Emmanuel GIRAULT, Patrick FOUILLAUD (CEA); PHYSOR 2008 September 14 - 19, 2008, Interlaken, Switzerland.
2. “Thermal scattering data and criticality safety”, Wim HAECK, Nicolas LECLAIRE (IRSN); PHYSOR 2008 September 14 - 19, 2008, Interlaken, Switzerland.


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