1. Can All Research Reactors be Fueled only with LEU in the Future?
Nikolay Arkhangelsky
Rosatom (Russia)

2. Topics
Neutron-Physical and Economical Benefits of the Use of HEU in Research Reactors
Influence of the Utilization of Research Reactors to the Use of HEU
The Difference between Approaches to the Operating Reactors and Reactors under Construction
Conclusion

3. Aspects of the use of HEU in Research Reactors
There are many aspects of the use of HEU in research reactors
-        Neutron-Physical
-        Economical
-        Security
-        Proliferation
- Political
In these presentation only physical and partly economical aspects are discussed

4. The Definition of Research Reactor From the Code of Conduct on the Safety of Research Reactors, IAEA, 2004
“research reactor” means a nuclear reactor used mainly for the generation and utilization of neutron flux and ionising radiation for research and other purposes…. Facilities commonly known as critical assemblies are included.

5. Why in the past the enrichment of the uranium was increased?
According to the definition the main purpose of the operation of research reactor is to provide the scientists and engineers with neutron fluxes of different spectrum and intensity
It means that the quality of the experiment depends mainly on the level of neutron flux and also on the neutron spectrum
HEU has the neutron-physical and economical benefits in comparison with LEU and it explains constant increasing of the uranium enrichment in research reactors from the beginning till the middle of seventies years of the XX century
What kind of these benefits?

6. Neutron-Physical Aspects
The total amount of neutrons that can be used in the experimental facilities is proportional to
P*(K - 1)/ K
where P – reactor power,
K - infinite multiplication factor
For to increase K the concentration of U-235 shall be increased and the poisoning effect of absorbers in particularly U-238 shall be decreased
More easy way to increase the concentration of U-235 is to increase the enrichment of uranium
More difficult way is the development of fuel with the bigger concentration of uranium in the fuel meat

7. Example of the dependence of the infinite multiplication factor from the uranium-235 concentration in the active core (Calculations were made for the Russian type fuel assemblies WWR-M)

8. Neutron-Physical Aspects
Thus at low concentration of U-235 in the active core it is possible to keep K at a former level at reducing enrichment from HEU to LEU
It explains the relative simplicity of the conversion of low power reactors having as a rule low concentration of U-235 in an active core
On the contrary at high concentration of U-235 in the active core it is impossible to keep K at reducing enrichment
It means that in this case the conversion of reactor without penalties is impossible!

9. Neutron-Physical Aspects
Other neuron-physical aspect of the use of HEU deals with
the value of neutron flux
In many cases for experimenters it is very important to have neutron flux in experimental facilities as high as possible
Neutron flux is proportional to the specific power that is the power per volume unit; the demand for higher specific power created a need for lower core volume; it means that at the constant power of the reactor the neutron flux is inversely proportional to the core volume;
the core volume correspondingly decreases with increasing of the concentration of U-235 and consequently
the neutron flux will higher at the higher uranium enrichment!

10. Neutron-Physical Aspects
In any case at the same concentration of U-235 in the active core the neutron-physical parameters of the reactor:
The total amount of neutrons that can be used usefully in the experimental facilities
Neutron flux
will be higher in the case of higher enrichment!
But it is absolutely clear that to get the same concentration of U-235 is more easy in the case of HEU because at the same technological limitations
the concentration of U-235 is proportional to the enrichment

11. Economic Aspects But… The price of HEU is higher than LEU
On the other side for the economy of research reactor it is important not the price of total uranium but only the price of U-235
The price of the unit mass of U-235 is practically independent from the enrichment level (in the range from 20% to 90%)

12. Relative Price Values of Uranium-235 (These values are practically independent from the price of UF6, the price of SWU, conversion prices and tails assay)

13. Acceptability of the Reactor Conversion
For to establish acceptability of the use of LEU instead of of HEU several criteria shall be define so that after the reactor conversion the operators and experimenters could continue to operate and use the reactor practically with the same efficiency
Such criteria were established at the begin of the RERTR Program

14. The criteria which can be keep at the reactor conversion from HEU to LEU
In assessing the practical feasibility of utilizing lower enriched fuel in existing research reactors, the agreed criteria are:
that the safety margins and fuel reliability should not be lower than for the current design based on highly enriched uranium,
major reactor modifications should not be required,
and that preferably neither any loss in the overall reactor performance (e.g., flux-per-unit power) nor any increase in operation costs should be more than marginal.
What means “marginal”? It was not so easy question at the begin of the RERTR Program! It is not so easy question till now!

15. Problems of the Use of HEU Depending on Types of Research
Main tasks of research reactors
Fundamental Research
Material Testing
Neutron Physical Experiments
Generation of Intensive Neutron Pulses
Applied Tasks
For all of these tasks it is necessary to provide required neutron fluxes and neutron spectrum

16. Fundamental Research
For these research the neuron flux shall be high as much as possible
By these reason the fundamental research concentrates on high or very-high research reactors
The neutron spectrum is very important for these research but it depends slightly from the uranium enrichment

17. Material Testing
For these tasks the level of the uranium enrichment is not so important as for fundamental research
Usually the neutron flux in research reactors is higher than in power reactors and by this reason in many cases there is a sufficient margin for the reactor conversion
There is a specific problem of the use of HEU in fast research reactors

18. Neutron Physical Experiments
The most appropriate facilities for these tasks are critical assemblies, i.e. facilities that have very small power and neutron flux level
It means that the conversion of these facilities from HEU to LEU is relatively easy task from the point of view of keeping of the same level of the neutron flux
But because many of critical assemblies are mock-ups of the big reactors it is absolutely necessary to have the same enrichment as in model-based reactor

19. Generation of Intensive Neutron Pulses
Usually for these tasks the pulse type reactors are used
Practically in all of these reactors only HEU are used
May be it is only the result of the traditional approach
In any cases serious investigations of the possibility of the conversion of pulse type reactors were not made in previous years

20. Applied Tasks
There are a lot of applied tasks for which not necessary to have the high neutron flux
For the applied tasks there are no strong requirements for the neutron spectrum from the point of view of the uranium enrichment
Practically there is only one task that is very critical to the neutron flux level – it is the production of transuranium isotopes, mainly Cf-252

21. The amount of Cf-252 produced in one year from Pu-242 (a = 10-5 g Cf-252/g Pu-242; f - neutron flux)

22. Conclusion The Importance of the Use of HEU in Different Types of Research
Fundamental Research very important
Material Testing not so important
Neutron Physical important in some cases
Generation of Intensive it is necessary to Neutron Pulses carry out an additional investigations
Applied Tasks in some cases very important

23. Operating Reactors
Operating reactors usually have a plenty of complicated experimental facilities, and experimenters have adapted to an existing level and spectrum of neutron fluxes
By this reason in the start of the RERTR Program the criterion according to which providing of the achieved level of neutron fluxes was an important condition
If penalties in neutron fluxes will take place they should be minimal

24. Operating Reactors
At the same time it is obvious that among operating reactors there is a considerable number of the reactors that use HEU and that can be converted to LEU without any penalties for experimenters (mainly low power reactors)
Use of HEU in these reactors is only the result of the inertial approach

25. Operating Reactors
Conversion of the high-flux operating reactors inevitably will result in losses for experimenters;
The acceptability of these losses depends from very many reasons
political will of officials,
economic opportunities,
perseverance of experimenters
etc.

26. Reactors under Construction
Now the number of research reactors under construction is not so big; but in some cases there is an intention to use HEU in them
If these reactors are not unique reactors with record levels of neutron fluxes it is possible to reach the compromise with experimenters about the required neutron flux level
If the decision on use of the LEU is accepted on early design stages serious problems for experimenters do not arise, and they will receive such facility which is required for them
Therefore the problem can be rather easily solved if to discuss it on an early design stage; as experience shows, it is relatively easy to convince experimenters to agree to use of the LEU in the future reactor in spite of the fact that parameters of the reactor will be a little bit lower than at the use of HEU

27. Reactors under Construction
Nevertheless it is impossible to exclude that in the future the construction of very high flux reactors in which use HEU will be completely necessary can required
Probably, it should be very high-power reactors - not less than one hundred MW and intended for absolutely unique tasks – fundamental or applied; the number of such reactors all over the world will be very little, they will be used by the international collectives of users and to be under very strong physical protection
It seems that it will be right to have an opportunity for the use of HEU in such reactors in the future

28. Conclusion
The HEU has the neutron-physical advantages in comparison with LEU; by this reason till the middle of seventies years of last century the enrichments was increased
The use of LEU instead of HEU will require an additional expenses but for small power reactors it is possible to carry out the conversion of reactors from HEU to LEU without big penalties or maybe in general without some penalties

29. Conclusion
For different types of research the answer for the question about the possibility of the use of HEU can be different
More sensitive to the enrichment level are fundamental research; for other research the requirement of the use of HEU is not so strong except for the transuranium isotopes production

30. Conclusion
For limited number of very high power research reactors it is desirable to keep the capability to use HEU in the future because several applied scientific and applied research will require the neutron flux as big as possible and necessary results can not be get in the case of using of LEU
Full refusal of an opportunity of use of HEU would be probably wrong

31. General Conclusion As a whole
it is possible to say that the conversion of research reactors from HEU to LEU results in penalties in neutron fluxes and consequently it is desirable to keep an opportunity of use HEU in the limited number of very high-flux reactors in which the problem of the achievement of maximum possible neutron flux density is of fundamental importance