# Thermal Energetic Reactor with High Reproduction of Fission Materials

Authors: Vladimir M. Kotov
Journal: Science and Technology of Nuclear Installations
 (2012)

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## Abstract

Existing thermal reactors are energy production scale limited because of low portion of raw uranium usage. Fast reactors are limited by reprocessing need of huge mass of raw uranium at the initial stage of development. The possibility of development of thermal reactors with high fission materials reproduction, which solves the problem, is discussed here. Neutron losses are decreased, uranium-thorium fuel with artificial fission materials equilibrium regime is used, additional in-core and out-core neutron sources are used for supplying of high fission materials reproduction. Liquid salt reactors can use dynamic loading regime for this purpose. Preferable construction is channel type reactor with heavy water moderator. Good materials for fuel element shells and channel walls are zirconium alloys enriched by90Zr. Water cooled reactors with usage 12% of raw uranium and liquid metal cooled reactors with usage 25% of raw uranium are discussed. Reactors with additional neutron sources obtain full usage of raw uranium with small additional energy expenses.
On the base of thermal reactors with high fission materials reproduction world atomic power engineering development supplying higher power and requiring smaller speed of raw uranium mining, than in the variant with fast reactors, is possible.


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## Body

## 1. Introduction

According to the forecast for the end of XXI century, NPP can produce not more than 30% of total energy [1]. It is considered that  the main role will  be played fast reactors, which have high reproduction of fission materials and, correspondingly, less raw uranium requirement. Fast reactors need huge start expenses for fuel producing. Thermal reactors with enrichment plants will play the role of the producer of initial fission material for fast reactors [2]. These prognoses are based on characteristics of modern light water reactors, which have base niche in nuclear energy production. Advantages of these reactors are based on high-level burn-up possibilities by means of 235U enrichment increasing in fuel. But simultaneously, fission materials reproduction is decreasing.There are thermal reactors with higher fission materials reproduction [3, 4]. Works at IAE NNC RK are devoted to possibilities of its development. Features of “dynamic loading” technology, in which fuel periodically works strictly defined time (e.g., 5 hours) and after that is replaced with another fuel portion and is sustained out of core (50 hours, e.g.), were researched [5]. This technology decreases neutron losses in 135Xe due to its decay during sustaining and can be easily applied in liquid salt reactors with additional simplification of fuel reprocessing [6].For solid fuel reactors “dynamic loading” technology is less applicable. Other approaches have more sense in this case. In this paper, some tasks of thermal reactor modernization with target to achieve high levels of burn-up and reproduction are discussed including the question of possible nuclear energy engineering scale on its base.


## 2. About  Fission  Materials  Reproduction Increasing in Thermal Reactors

For fission materials reproduction in thermal reactor, neutrons are used, which formed in fission, remaining after absorption without fission in fission material, absorption in construction materials, actinides, fission products, and neutron leakage from the core. For uranium-fuelled reactors, which have 2.07 secondary neutrons, full reproduction can be achieved if all losses are 7% from fission acts. For thorium-fuelled reactors, it is ~28%. So, in the reactors with full reproduction of fission materials, absorption cross-section has to be equivalent to fission cross-section. For a hypothetical uranium reactor with238U and 235U and homoenergetic neutron spectrum, this equivalence is achieved at 235U portion of ~0.474%, that is less than its counter port in raw uranium.It should be noted that full reproduction in thermal reactor is a complex task, which  can not always  be accomplished or justified. But approach to full reproduction increases level of raw uranium usage in reactor campaign. Accordingly, raw uranium requirement for reactor work is decreasing.


### 2.1. Losses Decreasing

The main way to reproduction increasing is neutron losses decreasing in construction materials, fission products, and leakage.Loss decreasing in constructive materials is achieved by use of materials with low neutron absorption cross-section. The best material for moderator is heavy water. Its use defines channel type of reactor. The best constructive material for channel walls and fuel shells is alloys on zirconium base. A possible way for neutron absorption decreasing is use of enriched by90Zr zirconium.A significant part of neutron losses is losses in control elements. In campaign with single-phase fuel, loading initial reactivity is maximal. Losses of neutrons in control elements are also maximal. These losses can be decreased in zone superposition regime, when portions of fuel are repeatedly replaced during campaign. Per se, at every moment of reactor work, it contains fuel with different burn-up, work-time in its campaign. Reactivity after replacements is less, than at beginning of single-phase loading campaign. So neutron losses in control elements are also less.Neutron absorption in fission products is increasing during a campaign because of stable and long-lived nuclides creation with high absorption cross-section. This process can limit maximal burn-up of some types of reactors (e.g., CANDU).Less absorption in135Xe is possible with use of “dynamic loading” technology [4]. Less absorption in long-lived fission products is possible with use of interim fuel cleaning in campaign from accumulated fission products. This cleaning is foreseen for liquid salt reactors, but large effect of it is also possible in solid fuel reactors [6].Maximal effect of leakage decreasing is reactor size increasing, which is typical for large power reactors.


### 2.2.  Secondary Fission Neutrons Number Increasing

In majority of modern thermal reactors, uranium fuel is used. It is connected with presence of fission nuclides in raw uranium. Use of thorium fuel with more secondary neutrons needs to carry out preparation stage for fission nuclide production—233U. But these expenses are significantly less than preparation expenses for work of fast reactor.Uranium and thorium fuels have another differences, which not always are advantages of thorium fuel. Thus, predecessor of233U-233Pa has relatively large half-time (27.4 day) and large neutron absorption cross-section (66 barn), which decreases allowable losses of neutrons in construction materials at reactor work with flux larger than 1013 n*cm-2*s−1.In real reactor campaign besides main components (235U and 238U in uranium fuel and 233U and 232Th in thorium fuel), there are other heavy nuclides, which can be fissionable or weak fissionable. In uranium fuel during campaign is an increasing amount of nuclides with more secondary neutrons from fission than 235U (239Pu-2.09 and 241Pu-2.2).


### 2.3.  Campaign Conducting in Regime Similar to Equilibrium Regime

The maximal effect of nuclides with more secondary neutrons from fission than235U is achieved in campaign conducting with equilibrium of these nuclides during campaign. That means the amount of these nuclides (233U, 239Pu, and 241Pu) is constant in any moment of campaign. This constant is broken just due to influence of accumulation from its predecessors in fuel replacements.Contents of235U is significantly changing during campaign. Contents of raw nuclides 232Th and 238U are slightly changing. 235U supplies work reliability as compensator of not full reproduction of fission materials in core.It should be noted that this campaign conducting regime has slight reactivity fluctuations, and fuel replacements for decreasing neutron loss in control elements can be minimal.


### 2.4.  Role of Neutron Spectrum in Fuel

Neutron absorption in238U leads to campaign duration decreasing in reactors with single-phase fuel loading, for example, in CANDU. A channel lattice with large size is needed in these heavy-water reactors. This requirement is obsolete for reactors with equilibrium regime. Fuel with a little more contents of fission materials and more resonance absorption in raw nuclides (233Th and 238U) can be used for realization of equilibrium regime. It leads to requirement decreasing of expensive heavy water, the reactor become more compact, its power is increased at the same neutron flux.


### 2.5. Additional Neutron Sources in Reactor

If fission neutrons formed as a  result of fission on thermal neutrons in fission materials are considered as main neutron source, then neutrons formed as a result of n-2n reactions in heavy water and beryllium, fission on fast neutrons in fission materials, and external devices are considered as additional neutron sources.Neutron portion of n-2n reaction in heavy water reactors is small. Increasing it is not an actual task in CANDU reactors. The task becomes actual in terms of equilibrium campaign development, when even small step to full equilibrium plays, a great role in raw uranium usage. In this case beryllium inserts installation near fuel is also an effective step.Task of neutron amount increasing by means of fission on fast neutrons is quite known. It is solved by means of fuel rod diameter increasing, material amount decreasing with small atomic weight in the rod, and coolant usage with low moderating efficiency.External neutron sources can be made with use of proton accelerators directed to target made from material with high charge of nuclei. Such sources can be used separately from reactor for fission materials accumulation. Neutron source on the base of thermonuclear reaction is used in this regime in paper [7].It is considered that all additional neutrons are used for fission materials reproduction increasing.


## 2.1. Losses Decreasing

The main way to reproduction increasing is neutron losses decreasing in construction materials, fission products, and leakage.Loss decreasing in constructive materials is achieved by use of materials with low neutron absorption cross-section. The best material for moderator is heavy water. Its use defines channel type of reactor. The best constructive material for channel walls and fuel shells is alloys on zirconium base. A possible way for neutron absorption decreasing is use of enriched by90Zr zirconium.A significant part of neutron losses is losses in control elements. In campaign with single-phase fuel, loading initial reactivity is maximal. Losses of neutrons in control elements are also maximal. These losses can be decreased in zone superposition regime, when portions of fuel are repeatedly replaced during campaign. Per se, at every moment of reactor work, it contains fuel with different burn-up, work-time in its campaign. Reactivity after replacements is less, than at beginning of single-phase loading campaign. So neutron losses in control elements are also less.Neutron absorption in fission products is increasing during a campaign because of stable and long-lived nuclides creation with high absorption cross-section. This process can limit maximal burn-up of some types of reactors (e.g., CANDU).Less absorption in135Xe is possible with use of “dynamic loading” technology [4]. Less absorption in long-lived fission products is possible with use of interim fuel cleaning in campaign from accumulated fission products. This cleaning is foreseen for liquid salt reactors, but large effect of it is also possible in solid fuel reactors [6].Maximal effect of leakage decreasing is reactor size increasing, which is typical for large power reactors.


## 2.2.  Secondary Fission Neutrons Number Increasing

In majority of modern thermal reactors, uranium fuel is used. It is connected with presence of fission nuclides in raw uranium. Use of thorium fuel with more secondary neutrons needs to carry out preparation stage for fission nuclide production—233U. But these expenses are significantly less than preparation expenses for work of fast reactor.Uranium and thorium fuels have another differences, which not always are advantages of thorium fuel. Thus, predecessor of233U-233Pa has relatively large half-time (27.4 day) and large neutron absorption cross-section (66 barn), which decreases allowable losses of neutrons in construction materials at reactor work with flux larger than 1013 n*cm-2*s−1.In real reactor campaign besides main components (235U and 238U in uranium fuel and 233U and 232Th in thorium fuel), there are other heavy nuclides, which can be fissionable or weak fissionable. In uranium fuel during campaign is an increasing amount of nuclides with more secondary neutrons from fission than 235U (239Pu-2.09 and 241Pu-2.2).


## 2.3.  Campaign Conducting in Regime Similar to Equilibrium Regime

The maximal effect of nuclides with more secondary neutrons from fission than235U is achieved in campaign conducting with equilibrium of these nuclides during campaign. That means the amount of these nuclides (233U, 239Pu, and 241Pu) is constant in any moment of campaign. This constant is broken just due to influence of accumulation from its predecessors in fuel replacements.Contents of235U is significantly changing during campaign. Contents of raw nuclides 232Th and 238U are slightly changing. 235U supplies work reliability as compensator of not full reproduction of fission materials in core.It should be noted that this campaign conducting regime has slight reactivity fluctuations, and fuel replacements for decreasing neutron loss in control elements can be minimal.


## 2.4.  Role of Neutron Spectrum in Fuel

Neutron absorption in238U leads to campaign duration decreasing in reactors with single-phase fuel loading, for example, in CANDU. A channel lattice with large size is needed in these heavy-water reactors. This requirement is obsolete for reactors with equilibrium regime. Fuel with a little more contents of fission materials and more resonance absorption in raw nuclides (233Th and 238U) can be used for realization of equilibrium regime. It leads to requirement decreasing of expensive heavy water, the reactor become more compact, its power is increased at the same neutron flux.


## 2.5. Additional Neutron Sources in Reactor

If fission neutrons formed as a  result of fission on thermal neutrons in fission materials are considered as main neutron source, then neutrons formed as a result of n-2n reactions in heavy water and beryllium, fission on fast neutrons in fission materials, and external devices are considered as additional neutron sources.Neutron portion of n-2n reaction in heavy water reactors is small. Increasing it is not an actual task in CANDU reactors. The task becomes actual in terms of equilibrium campaign development, when even small step to full equilibrium plays, a great role in raw uranium usage. In this case beryllium inserts installation near fuel is also an effective step.Task of neutron amount increasing by means of fission on fast neutrons is quite known. It is solved by means of fuel rod diameter increasing, material amount decreasing with small atomic weight in the rod, and coolant usage with low moderating efficiency.External neutron sources can be made with use of proton accelerators directed to target made from material with high charge of nuclei. Such sources can be used separately from reactor for fission materials accumulation. Neutron source on the base of thermonuclear reaction is used in this regime in paper [7].It is considered that all additional neutrons are used for fission materials reproduction increasing.


## 3. Relationship between Reproduction and Raw Uranium Usage

Contents of fission materials is changing in thermal reactor campaign with reproduction coefficient not equal to unity. The reproduction term means fission material replacement with equivalent material. Real difference of fission material contents in campaign gives some difficulties in reproduction determination that was noted for fast reactors [8].In thermal reactors, where reproduction just approaches unity, the determining factor of reproduction, in contrast to fast reactors, is not the possibility of fuel base widening by means of reproduction, but the degree (fullness) of raw uranium usage as a supplier of initial fission materials. It is understandable that thorium is always fully used whenever fuel cycle is perfect or not.


## 4. Reactor Design Examples

On the base of design of reactor, its core, reflector, and fuel assembly neutron characteristics are determined in the program [9]. Fuel contents change during campaign, fission products accumulation, and accordingly multiplication constant are determined by means of using the program [10].The most worked out reactor in technical details is water cooled channel reactor [11]. Among these reactors, the most perspective is usage of boiling coolant. Positive feature of reactors with high reproduction of fission materials and high usage of natural uranium is decreasing water mass in coolant channels. But even in this case usage of heavy water is preferable. It allows decreasing neutron absorption in constructive materials down to 1%. It supplies fission materials reproduction during campaign at which usage of raw uranium is up to 5% in pure uranium reactors and 12% in uranium-thorium reactors.In fuel rod bundles with water coolant (Figure1) increasing of reproduction is achieved due to reaction “n-2n,” which is in beryllium placed nearby fuel rods. Neutron birth in this reaction exceeds the absorption up to 1.5% from total absorptions in the core.Figure 1
Design variant of fuel rod bundle of thermal reactor with water coolant.Development problems of this reactor type are concerned with optimization of neutron and thermal characteristics of fuel rod bundles. It is possible to decrease thickness of channel walls in channel variant of the reactor if coolant pressure is compensated by moderator pressure.Large effect at reproduction can be achieved in reactors with liquid metal coolant. In it fission neutrons can interact with fuel in adjacent fuel rods. It increases number of fissions in232Th, 234U, 238U, 240Pu, and number of neutrons in fission of fission materials and decreases neutron absorption in it.High uniformity of loading in fuel rod assembly with liquid metal coolant (Figure2) is achieved by means of seven fuel rods deletion from central part of the assembly. Filling this area by low pressure gas allows fast neutron exchange between all fuel rods of the assembly. Usage of 25% of a raw uranium is achievable in the reactor with such uranium-thorium assemblies.Figure 2
Design variant of fuel rod assembly for liquid metal thermal reactor.Alloy on the base of zirconium must be wall and fuel cladding material in liquid metal thermal reactor. Alloy Bi + Pb + Sn with low melt temperature should be used as a coolant. Tin should be enriched by120Sn isotope. Tin increases heat capacity of alloy and decreases energy losses for pumping. Tin is compatible with zirconium as a constructive material. Used as a coolant, it forms high-temperature coating.Calculations show that external neutron sources allow obtaining fission materials reproduction close to unity if reproduction coefficient of the reactor is good enough. Energy expenses for external neutron sources are about few percents of reactor power [7].


## 5. About Neutron Balance

Figures3 and 4 show neutron balances of reactors in equilibrium regime with different designs of fuel assemblies. A reactor calculation in program [9] and campaign calculation in program [10] on the base of the results of the reactor calculation were used for it. Campaign calculation is made on condition of integral reactivity keeping above zero.Figure 3
Heavy water-cooled reactor neutron balance in campaign with equilibrium by fuel nuclides regime.Figure 4
Liquid metal-cooled reactor neutron balance in campaign with equilibrium by fuel nuclides regime.On the figures are shown the amount of neutron absorption (black piles) and neutron fissions (blue piles) in different fuel nuclides. There are two types of235U—natural, specified as U235N, and formed in result of thorium transformation chain (232Th-233Pa-233U-234U-235U), specified as U235S.In fissile nuclides, margins between amounts of secondary neutrons and total absorbed neutrons (red piles) are shown. In raw nuclides232Th and 238U, margins between amounts of secondary fission neutrons of these nuclides and neutron absorptions with fission (yellow piles) are shown.The columns 1 and 2 show neutron absorptions in construction materials and fission products, respectively. Total height of red and yellow piles must be equal to total height of black piles of232Th, 238U, 234U, 240Pu nuclides, construction materials, and fission products.It can be seen that total neutron absorption in fission products (8%) for heavy water cooled reactor is less than one for liquid metal cooled reactor (13%).


## 6. High Power Nuclear Energy Creation Possibility

Application of above-mentioned steps in sufficient extent for fission materials reproduction increase in thermal reactor supplies high fullness of raw uranium use (25% versus ~1% of in thermal reactors with enriched fuel) besides low requirement in fuel for reactor’s core loading. These effects allow creating high power world nuclear energy.Demands in raw are determined by the following formulas:(1)mU=n*t*(ms*YU*+mg*Ki*KU2),mTh=n*t*(ms*(1-YU)*+mg*(1-KU)2),
where n is  number of these reactors; t development time of nuclear power engineering, years; ms natural uranium mass needed for core creation; YU portion of natural uranium in raw; mg annual fuel mass for inflow of   reactor; Ki value of uranium usage in fuel cycle; KU portion of natural uranium in inflow fuel.Figure5 shows requirement in uranium and thorium for different reactor types with initial zero power and its even development during 80 years till 4000 GW. This development variant is similar to possible development of nuclear energy with fast reactors.Figure 5
Dependencies of raw uranium and thorium requirement for different reactor types at the stage of even power growth up to 4000 GW.It can be seen that to the full development moment in variant with 25%, use of raw uranium only is applied. At full use of raw uranium (variant with external neutron sources), ~2.5% of cheap uranium reserve is used. For comparison, fast reactors to that moment use all cheap uranium.So, there is significant reserve for power increasing of world nuclear energy. Figure6 shows energy development variant with even development up to 8000 GW in 80 years with further stabilization.Figure 6
Dependencies of raw uranium requirement for different reactors at the stage of even power growth of nuclear power engineering to level of 8000 GW and further stabilization on this level.Duration of NPP work with thermal reactors on this power level with full usage of uranium will run up to ~2500 years. It is understandable that in case of such low raw uranium requirement, use of more expensive deposits, where uranium reserve is quite bigger than in cheap ones, is rational.


## 7. Conclusion

Perspective thermal reactors with high fission materials reproduction can supply sufficient decreasing of natural uranium requirement. For high reproduction, solutions for neutron losses decreasing are needed and it is preferable to use uranium-thorium fuel in equilibrium of artificial fission nuclides regime and additional internal and external neutron sources.Most competitive reactors in that are channel reactors with heavy-water moderator. Preferable materials for cladding and channel walls are alloys on zirconium basis enriched by90Zr isotope.Usage of raw uranium in water coolant reactors is up to 12% and in liquid metal reactors can exceed 25%; in reactors with external neutron sources, full use of raw uranium is possible.On the base of thermal reactors with high reproduction of fission materials, creating high power world nuclear energy, which demands use of raw uranium with less speed of its extraction than in fast reactors variant is possible.



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