Plutonium

January 2001

This issue is devoted to plutonium, an artificial radioactive element. This dangerous radiotoxic element is, moreover, known for its first military use in 1945.

As AC Lacoste underlines in his foreword:
     ” This material is today continuously generated as an inevitable by-product of irradiation. fuels in nuclear power plants ……. The quantities of plutonium to come to France, in the order of 8 to 9 tonnes per year, come mainly from the operation of nuclear power plants . “
     He concludes : “Plutonium therefore presents particular risks, which call for adequate management at all stages: storage of quantities of unseparated plutonium, safety of operations linked to the transformation of separated plutonium, environmental monitoring, immobilization of quantities not reused and recovery. old waste, without forgetting the mastery of aspects relating to non-proliferation . “

 I would add that interpretations on the end of the cycle and on the need to enhance the value of Pu are also mixed. Here is also what we have published in Gazette 165/166 (1998) about the Mandil-Vesseron report.

 EXTRACTS FROM THE GAZETTE 165/166
     Page 4 and page 5 of the report two items are in contradiction. The first is the statement “downstream of the cycle must continue, without a priori, to be the subject of a watch in various aspects:
     -economic aspects, with the follow-up of the evolution of energy raw material resources,
     -technical aspects, in establishing cooperation with our foreign partners as necessary, whether in the industrial field or in the field of safety,
     – international aspects …,
     – regulatory aspects …,
     – safety and assessment aspects of the impact on the environment. “

continued )

continuation:
     The second is ” It is appropriate that the current strategy while awaiting the decisions which will be taken at the deadline envisaged by the law, can continue to develop until 2006 in a harmonious context and by optimizing the performances of the park This optimization involves the authorization to load the 12 900 MWe reactors with MOX fuel that are not yet authorized to do so, the extension of storage capacities for the uranium resulting from reprocessing and the increase in fuel irradiation rates subject to a prior safety analysis.

 Explain to me how we can make a watch without a priori of all the aspects of the cycle and at the same time moxer, reprocess and increase the rate of irradiation. Finally, why do an expensive watch, useless since we already know that we are going to mox and reprocess the cash …

 It should be noted that this document constantly refers to the law of December 30, 1991, the reading of which must be rather difficult because each person interprets the various articles in their own way. Let us repeat that this law obliges the nation and the various nuclear players to take the waste problem seriously. Unfortunately, it is limited to medium and high level long-lived waste. It does not force us to have solutions in 2006 but it does not say either that we must have gotten stuck in nuclear power to the point of not being able to do anything else for centuries !!

 Finally, all hopes are allowed ” this document constitutes a stage and in no way an end in the reflection “
     Except that the reflection having started badly and with only those of nuclear players, it is difficult to see how it could change on its own.

p.11


Plutonium
Thierry Charles

Presentation of plutonium and its different isotopes (15 in total, all radioactive and with a period varying between 87.7 years for Pu 238, 24390 years for Pu 239, 387000 years for Pu 242, and a few minutes for Pu 232 , 233).

Plutonium is produced in reactors. What we were looking for was the 239 isotope which is used in bombs. To obtain it we built what we called plutonigenic batteries. The Graphites Gaz reactors (UNGG) and the Russian RMBKs could be used to obtain 239. If the plutonium of PWR type reactors does not have the right isotopic composition to make very efficient bombs, it can still be used.

 It is stressed that its nuclear properties make it a product inducing risks: “in order to fix an order of magnitude, it can be noted that, on the basis of an assessment using average or conventional parameters, both for the composition of the plutonium and the atmospheric dispersion and the dose assessment, the achievement of an effective dose of 1 mSv, in the event of inhalation of plutoniferous aerosols by an adult at a distance of 500 m downwind, corresponds to a release of less than 100 mg of plutonium resulting from the reprocessing of irradiated fuel originating from a pressurized water reactor . “

 Plutonium is a substance whose characteristics require appropriate sizing of storage:
     – partial self-absorption of radiation -> heating, thermal power released 0.57 Watts / g (Pu 238) and 0.002 Watts / g for 239,
     -radiolysis => production of gas for example hydrogen, explosive,
     -pyrophore. In the form of metallic powder, Pu ignites in a humid atmosphere. We must work under an inert atmosphere (eg nitrogen)
     – PuO2 oxide, used in MOX. stable but ” American researchers have demonstrated a reaction of this compound with oxygen in the presence of water, which slowly forms a more oxidized compound, hitherto unidentified, which can in particular lead to greater mobility of Pu in a humid environment oxidant. “

Note an important point the neutron emission of Pu due to the spontaneous fission of certain isotopes (238, 239, 240, 242): 162g x 2500 n / s / g + 5723g x 0.022n / s / g + 2215g x 910n / s / g + 491g x 1700n / s / g = 6.8 million neutrons / s for 1 ton of fuel enriched to 3.5% in uranium 235 and irradiated at 33,000 MWd / t. As can be seen, a container for transporting irradiated fuel emits a copious ration of neutrons. The dose received in neutrons represents approximately 3 times that coming from gamma rays.


MOX fuel in electricity production at EDF
Michel Debès and Gilles Zask

and
The safety of MOX fuel in a reactor
Nicolas Tricot and P. Tran Dai

These 2 articles complement each other too much for me to separate them. The first tells us that EDF’s strategy is to use MOX to allow ” the recovery of separated materials while respecting the economy of nuclear kWh. It is also in line with the prospect of increasing the average combustion rates of UO2 and MOX fuels, up to 57 GWd / t for UO2 on average by 2010.

This strategy also allows:
     – safe, sustainable and confining conditioning of fission products from the nuclear reaction thanks to vitrification carried out at the end of reprocessing operations at La Hague,
     -stabilization, or even reduction, of the quantity of UO2 assemblies, irradiated pending reprocessing, thanks to the smaller quantity of irradiated fuel produced when the fuel combustion rates are increased,

continued )

continuation:
     -the concentration of plutonium produced in UO2 fuels, in MOX assemblies and consequently in a reduced volume (one MOX assembly irradiated for 7 UO2). These assemblies are then stored for cooling for a few decades, which leaves open the choices as to their future.
     This involves improving the MOX performance in order to maintain economic UO2 with which it substitutes for electricity production equivalence “.

The statements quoted above are simply false. In fact:
     – If we do not retreat, we avoid rejections and especially the big black spot represented by the La Hague plant. We leave the fuel as it is and we avoid all technological waste which represents large volumes,
     -As to stabilize or reduce, this is another story. In any case, only 20 units out of 28 have been loaded with MOX and are authorized to use this fuel,
     -Why do you want to extract plutonium if it is to confine it in another assembly. Apart from the high costs of this MOX, the entire chain of rejects is avoided.
     The first article absolutely does not take into account the technical problems linked to the use of MOX, which are as follows:
     “With regard to core physics, the behavior of MOX fuel differs from that of all uranium on the following main points:
     1) The different isotopes of plutonium have absorption cross sections markedly higher than those of uranium in the core. thermal domain, which causes a shift of the neutron spectrum towards high energies and a decrease in the efficiency of all thermal absorbers. In particular:
     -The clusters are less efficient, which made it necessary to increase the number of stop clusters available in the reactor (53 in UO2, 57 in MOX),
     -soluble boron has a lower efficiency, which has need an increase in the boron concentrations required in operation,
     -the poisoning due to xenon will be reduced and the heart will be more stable due to the reduction in the amplitude of any xenon oscillations,
     2) Certain reactivity coefficients (Doppler coefficient, vacuum effect, temperature coefficient of the moderator) are increased in absolute value. This characteristic is unfavorable on the kinetics of certain accidental transients such as cooling accidents.
     3) The effective fraction of delayed neutrons is lower for plutonium than for uranium; the mixed core will consequently react with an increased neutron kinetics in the event of insertion of reactivity and the risk of prompt criticality will be greater.
     4) The difference in neutron behavior between mixed assemblies and any uranium leads to power peaks at the interfaces between UO2 and MOX. In order to reduce these power peaks in the peripheral assemblies, the mixed assembly had to be zoned. The management of power peaks at the interfaces also introduces greater constraints in the preparation of loading plans.
     5) Finally, the residual power of the core is greater than that of the all UO2 core, which is unfavorable during the management of the long-term phases of accidents (evacuation of the residual power) . “

COMMENT
     Like what the comparison between the 2 articles makes it possible to better understand the difference. Indeed, the use of MOX poses safety problems which are still being studied. In particular, ” the introduction of MOX is likely, depending on the type of management adopted (low neutron leaks or not), to increase the fluence received by the vessel .” However, the life of the tank is linked to this fluence but also the evolution of the defects.

p.12


Consequences of the development of MOX fuels on the downstream side of the cycle
Jean-Pierre Goumondy

2 reminders:
     1- Per year 1200 tonnes of fuel are unloaded from French reactors, 850 tonnes are reprocessed providing between 8 and 9 tonnes of plutonium. A part is used at MELOX (COGEMA-Marcoule) for the 20 reactors loaded with MOX.
     2- 3 types of UO2 fuels exist:
     -UOX1 at 3.5% in U 235 and with a combustion rate of 33 GWj / t,
     -UOX2 at 3.7% in U 235 and with a combustion rate of 45 GWj / t,
     – UOX3 at 4.5% in U 235 and at a combustion rate of 60 GWj / t.
     If the rate of UO2 irradiation is increased, the concentration of neutrophagous isotopes (Pu 240 and 242) increases. This therefore decreases the 239 isotope content and therefore ” the fissile properties of the plutonium produced. “

 As the objective is to have the same combustion rate for MOX and UO2 in order to ” obtain identical residence times in the reactor (…) and facilitate their management. “, This leads to initial Pu contents in strong increase (from 5.5% to 33 GWd / t we would consider 11% to 55 GWd / t and even 17.7% to 65 GWd / t). According to studies carried out at EDF (among others) and published in Gazette 163/164 page 21: ” There is a limit value for the Pu content beyond which the neutron balance tilts in favor of fast fissions: the coefficient vacuum then becomes positive. Numerous studies have made it possible to locate this limit content between 12 and 14%. “

In these conditions, wanting to reach 17.7% means that we accept the risk of having a reactor that can get carried away and in which, for certain accidental sequences, we can have the start of an explosive reaction.

With regard to the radiochemical characteristics of fuels and UO2:

 MOXUO2
irradiation rate in GWd / t
cooling time year
45
 5
45
 4
Fuel characteristics
alpha activity Bq / tmli * Bq
activity beta
/ tlmi *
power W / tmli
neutron emission
n / s / tmli *
Am241 g / tmli *
Cm244 g / tmli *

 
 27200
4150
3870
8.8 billion
in 2893
796

 
 5550
546
505
1.1 billion
355
82