The possibility is examined of developing a vessel-type fast reactor cooled by water with supercritical

parameters (BR-VSP) and a channel-type fast plutonium–water reactor cooled by boiling water, using the

RBMK scheme, or water with supercritical parameters for burning weapons or power-production

plutonium (RBMK-Pu).

A reactor with construction similar to that of an RBMK reactor but without the graphite moderator and

zirconium can be used for burning plutonium. Removing the graphite masonry while retaining a 25-cm

spacing in the square lattice gives a large free volume in the neutron field for holding, for example, cobalt.

A neutron-physical substantiation is given for the serviceability of such a reactor with the maximum

burnup of removed fuel 10% h.a. and the possibility of obtaining a negative reactivity effect in the case of

water loss.

Water with supercritical parameters, low density [1], and low-moderating power at high temperature (Fig. 1) has a

definite place among many substances which have been and are being studied as coolants for fast reactors (sodium, lead, heli-

um, water vapor, and others). These coolants were conceived in the midst of nuclear power at a time when water with super-

critical parameters was used primarily in heat generation. The contribution of nuclear power to overall power production in

the future can be increased by using the equipment which is now widely used at heat and electricity production plants for

operating with water with supercritical parameters. Thus the transition from heat and electricity production plants to nuclear

power plants could be smooth from the standpoint of equipment; this is attracting attention to the study of the advantages and

disadvantages of this path [2]. An obvious and basic drawback of water with supercritical parameters is that reactor systems

with a high internal pressure (25 MPa) need to be developed. However, a gradual increase in water-coolant pressure is the

main path at heat and electric power plants and it is helpful, though less significant, at nuclear power plants, where the fuel

component of the cost of electricity is much lower and high efficiency is not the main goal.

As an example of the use of water with supercritical parameters we shall consider two fast reactors. One uses

BREST reactor-type fuel elements but with closer fuel-element packing in a triangular instead of a square lattice to decrease

neutron moderation. The neutron spectrum of this reactor (BR-VSP) is compared with that of the lead-cooled BREST reac-

tor in Fig. 2. An adequate internal breeding ratio (BRC = 1.02) can be achieved by increasing the height of the core and sub-

stantially decreasing the large core applanation, commonly used in fast reactors, from d/h = 3–5 to 1. Such a core can be pro-

duced, since the high temperature drop of water with supercritical parameters makes it possible to lower substantially the flow

rate (by a factor of 7–8) and the pressure drop of the water. In addition, a close-spaced lattice of fuel elements has a large

hydraulic resistance and is hydraulically stable.

Atomic Energy, Vol. 95, No. 4, 2003

VESSEL AND CHANNEL FAST REACTORS

COOLED BY BOILING WATER OR WATER

WITH SUPERCRITICAL PARAMETERS

B. A. Gabaraev, I. Kh. Ganev,V. K. Davydov, Yu. N. Kuznetsov,V. A. Reshetov, and A. V. Smirnov

UDC 621.039.519

Federal State Unitary Enterprise, N. A. Dollezhal’ Scientific-Research and Design Institute of Power Engineering.

Translated from Atomnaya Énergiya, Vol. 95, No. 4, pp. 243–251, October, 2003. Original article submitted September 15, 2003.

1063-4258/03/9504-0655$25.00 ©2003 Plenum Publishing Corporation 655

For the second reactor, the RBMK structure was used in the initial state. In this reactor, gas replaces graphite to

increase safety and create a volume for producing radionuclides in the core while preserving the channel spacing; the

steam–water mixture with average density 0.52 g/cm3 can be retained or replaced with water with average density 0.1 or

0.18g/cm3 (the input temperature is 670 or 540 K and the output temperature is 820 K). To produce a power reactor, the prob-

lem of burning power or weapons plutonium and producing isotopes,for example, cobalt,can be added. If the main purpose

is to utilize, i.e., denature and burn, weapons plutonium,then the increase of the load of fissioning material in the core due

to the large neutron leakage changes from a drawback to an advantage. In an open cycle, the increase of the content of

high- and low-order isotopes (denaturization) in 239Pu and of the radioactivity as a result of the accumulation of Np,Am, and

Cm and fission products results in the accumulation of irradiated fuel with denatured plutonium and high activity in the reac-

tor core and in storage.

In this paper, certain neutron-physical indicators of fast reactors cooled by boiling water or water with supercritical

parameters are examined – a fast vessel reactor with fuel elements and BREST-type reactor core structure (BR-VSP) and an

RBMK-type fast channel reactor without graphite masonry for burning weapons or power-production plutonium

656

Water density, g/cm3

570 770 970 T, K

0.8

0.6

0.4

0.2

Fig. 1. Water density versus temperature at 25 MPa pressure.

Fig. 2. Neutron energy spectrum in BREST-300 (1) and BR-VSP (2)

reactors cooled with water with supercritical parameters.

Spectrum, arb. units

10–6 10–2 1

12

10–4 102

E, MeV

6

4

2

0

(RBMK-Pu). In addition to the composition for which criticality is achieved, the breeding ratios and reactivity effects with

loss of coolant (water loss) and flooding of the reactor with cold water were determined. For comparison,plutonium utiliza-

tion with RBMK coolant parameters – 7 MPa with average steam-water coolant density 0.52 g/cm3 – was also examined.

In contrast to thermal reactors operating in an open cycle, for which the goal is to maximize the number of fissions

per fissioning atom, fast reactors operate in a closed cycle with maximum burnup up to 10% h.a. It is technically feasible to

increase the content of fission products in steel-cladded fuel elements up to 0.8 g/cm3 fuel, i.e., average burnup 5% h.a. with

mixed uranium–plutonium dioxide fuel density 8 g/cm3. To increase the BR in a fast reactor, the content of fissioning pluto-

nium must be decreased. For RBMK-Pu,the problem is operating with a high content of weapons plutonium in the initial

and irradiated fuel. As burnup increases,denaturation increases as a result of an increase in the content of high-rder plutoni-

um isotopes,which is proportional to the content of fission products in the irradiated fuel. At the same time, decreasing the

burnup increases the rate of denaturation. Then the optimal number of fissions per fissioning atom will become less than 1.

BR-VSP Reactor. To shape the energy release, the diameter of the fuel elements in three radial zones is increased

in the direction from the center to the periphery (9.1,9.6,10.4 mm) in a triangular lattice with constant spacing 13.6 mm

and core height 110 cm. The diameters of the radial zones are 250,332,and 378 cm,the uranium screen is 30 cm thick,

and the steel reflector is 50 cm thick. A 0.25 mm thick lead sublayer and nitride fuel with density 13.5 g/cm3 from weapons

plutonium (239/240 = 95/5%) and depleted uranium (238/235 = 99.7/0.3%) is placed in 0.5 mm thick steel fuel-element

cladding. In the working state, at fuel temperature 900 K Keff = 1.03 was achieved with plutonium content 9.2% and ura-

nium content 90.8%. The calculations performed with the MCNP computer code [3] and the ENDF B6 nuclear data library

[4] showed the following:

• in a fast reactor cooled with water with supercritical parameters,BR = 1.17 and BRC = 0.94 with a uranium screen;

• for a steel screen,BRC can be increased to 1.02 if the shape of the core is changed, keeping the same volume, so

that the height and diameter would each be 250 cm; then Keff = 1.03 is achieved with the plutonium content decreasing from

9.2 to 8.5%;

• the median neutron energy is 0.22 MeV with water with supercritical parameters and 0.19 MeV with lead coolant;

• the effect of water loss from the working state is positive and equals 1.48% of the reactivity; using a 30 cm high

water interlayer at the end, followed by a 50-cm absorber zone (B4C with density 1.8 g/cm3, placed in the fuel elements

instead of the fuel) decreases the water-loss effect to 0.75% of the reactivity with uranium or to 1.1% with a steel screen;

increasing the absorber zone by 15 cm in the direction of the core decreases the water-loss effect to 0.9%;

• the temperature coefficient of the fuel is negative and equals –2.2·10–5 1/K;

• flooding the reactor with 300 K cold water increases the reactivity by 17%; this can be compensated by the con-

trol rods and by adding boric acid to the emergency water tank or cadmium or gadolinium absorber into the fuel.

In summary, the neutron-physical indicators of the fast reactor BR-VSP cooled with water with supercritical param-

eters are satisfactory. In the future it would be desirable to take measures to decrease the positive effect of water loss from

0.7–0.9% to zero or a negative value, as done in fast reactors.

Reactor for Burning Weapons or Power Production Plutonium. The construction of this reactor is close to that

of an RBMK uranium–graphite channel reactor but with a substantial change in the components of the core. The graphite is

replaced with gas or air, and the zirconium in the channel tubes and fuel-element claddings is replaced with steel. The latter

makes it possible to increase the maximum burnup up to 10% h.a.,which will improve the cost-effectiveness of the RBMK

fuel cycle and also increase the efficiency of the system cooled with water with supercritical parameters.

The reactor will be able to operate in a continuous-reloading regime without a large excess reactivity. This is impor-

tant for burning plutonium. Steel in the channel tubes and fuel-element claddings can be replaced by a composite material

“double carbon”– graphite filaments in a graphite matrix [5]. Such a replacement can be done after radiation resistance is

confirmed. The composite material has advantages over other structural materials, including lower neutron absorption,which

is important for a fast reactor.

In the present version,the 18-fuel-element assembly of RBMK fuel elements was replaced with a 36-fuel-element

assembly with fuel elements with shorter diameters. This solution makes it possible to decrease the height of the core and the

diameter while decreasing the power to 0.8 GW. The outer diameter of the fuel elements is 10.5 mm,and the fuel diameter

657

is 9.5 mm. The fuel consists of plutonium dioxide with the lower density 8 g/cm3 to ensure deep burnup,mixed with deplet-

ed uranium dioxide with the same density or with magnesium oxide with density 3.5 g/cm3.

Two cooling variants were examined:with RBMK coolant parameters and with supercritical parameters. In the first

case, an 88× 4 mm channel tube is made of stainless steel and is inserted into a second steel tube with dimensions 100× 4 mm.

The second tube provides protection from the possible destruction of neighboring channels if the primary tube cracks and is

penetrated. We note that the possibility that neighboring channels will be destroyed if the channel tube ruptures with no pro-

tective tube in place is not obvious and requires study. The tube need not be hermetically sealed; it is inserted in a medium

with pressure 0.1 MPa and cooled on the exterior side by circulating gas filling the volume of the reactor space between the

tubes. If the structure is hermetically sealed, the outer tube will become the second channel tube. Inserting a heat-conducting

indicator between the tubes will make it possible to remove heat from the tube to the channel and to determine any leaks in

the primary tube according to the increase in the content and activity of the indicator in the water of a specific circuit in the

circulation loop. Heat is removed from the steel endface reflectors using the RBMK scheme with the split graphite bushings

replace with steel bushings.

In the second case, water under pressure 25 MPa cools the fuel assembly placed in a 110 × 15 mm steel channel tube

with wall thickness 15 mm. The second (protective) tube with dimensions 144 × 15 mm is made of a composite material and

does not greatly influence the physical characteristics of the core.

The cell corresponding to the fuel assembly was calculated by the Monte Carlo method assuming a medium with an

infinite radius [6]. Lateral leakage was taken into account with a margin in Keff, ∆Keff,1 = 0.15. The core – fuel elements,chan-

nel tubes,and end reflectors – was studied along the height. Average burnup of 5% h.a. was taken into account with a margin

in Keff equal to ∆Keff,2 = 0.05. Thus,the total margin in Keff for lateral leakage and burnup was taken to be Keff,crit,1 = 1.2

andwas taken into account in both cooling variants – with RBMK coolant or water with supercritical parameters. In the lat-

eral case, the steel volume in a single 110 × 15 mm channel tube was twice the steel volume in the two 88 × 4 and 110 × 4 mm

steel tubes. The excess steel can be taken into account in Keff with margin ∆Keff,3 = 0.15. Then,when the cooling system with

RBMK coolant with steel tubes is replaced with cooling with water with supercritical parameters, criticality is reached with

Keff,crit,2 = 1.35. For a similar transition from an RBMK system with tubes consisting of a composite material to cooling

with water with supercritical parameters, another increment ∆Keff,3 = 0.15 will be required and then the critical level is

Keff,crit,3 = 1.5. These margins will make it possible to study both cooling variants using the same data,presented in Figs. 3–5.

658

Keff

0 0.3 0.7

1

2

3

4

γH2O, g/cm3

1.7

1.5

1.3

Fig. 3. Keff versus water density without cobalt inserted into the core,

with channel tubes consisting of a composite material with gMgO = 50,

60, and 80% (1, 2, 3) and steel with gMgO = 80 (4) and magnesium

oxide as a fuel diluent.

The calculations showed that a reactor with or without a steel reflector can operate with 25 cm or larger spacing of

the technological channels with both cooling schemes. A 30 cm thick steel reflector is adequate. Figures 3–5 show Keff versus

the water density in the range 0–0.9 g/cm3 with different degrees of dilution of plutonium dioxide with magnesium oxide or

depleted uranium dioxide and with or without cobalt added into the core. In all cases,for a high content of weapons plutoni-

um and relatively small amount of diluent added, the water-loss effect is positive and flooding with cold water decreases the

reactivity. For a high mass fraction of the diluent,the water-loss effect becomes negative, and the effect of flooding with cold

water is positive. In the intermediate situation between these two cases,a negative water-loss effect and a negative effect of

flooding with cold water can be obtained (see Fig. 3,gMgO = 60%) or the reactivity can be almost constant in the entire water

density range (see Fig. 5,g238UO2= 0.7). A negative water-loss effect is preferable. Estimates are presented in Tables 1 and2.

659

Fig. 4. Keff versus water density with cobalt inserted into the core, with steel

fuel-element claddings and channel tubes,magnesium oxide fraction 50,60,

70,and 80% (1, 2, 3, 5) and 70% with cadmium between the tubes (4).

Fig. 5. Keff versus water density with depleted uranium with mass fraction

0, 30, 60, 70, 80% (1, 2, 3, 4, 5) as the fuel diluent in a system with steel

channel tubes.

Keff

0 0.3 0.7

1

2

3

5

4

γH2O, g/cm3

1.4

1.2

1

0.8

Keff

0 0.3 0.7

1

2

3

5

4

γH2O, g/cm3

2

1.6

1.2

For a reactor with weapons plutonium,two steel channel tubes,and RBMK coolant,the temperature coefficient was

estimated with the fuel temperature increased or decreased by 200 K. The temperature coefficient during reactor operation is

negative and equals αT = –1·10–5 ∆Keff/K. The negative temperature coefficient of the fuel and hence the power coefficient

are important in a reactor which burns plutonium,just as operation with continuous reloading.

Thus,this paper reflects the initial development of two new types of fast reactors. The reactors can be cooled with

boiling water or water with supercritical parameters and power-production or weapons plutonium can be used as fuel.

The following can be noted for a BR-VSP fast reactor cooled by water with supercritical parameters:

• according to the main neutron-physical parameters, the reactor operates similarly to a fast power reactor; the neu-

tron spectrum is shown in Fig. 2, the median energy is 0.22 MeV and 0.19 MeV in BR-SKV and BREST, and the internal

breeding ratio BRC > 1;

• with respect to thermohydraulics with water with supercritical parameters,core applanation can be decreased from

the generally used large value 3–5 to 1,which will decrease the plutonium load and increase BRC;

• the positive effect of reactivity with loss of coolant (water loss) still cannot be decreased below +0.7%; this effect

can be compensated by passive introduction of absorbers into the core, which are hydraulically confined above the core by the

water flow; when unsealing occurs with loss of pressure, a membrane is passively ruptured in a tank for emergency flooding

of the loop with borated water and reactivity from water loss and flooding of the core with cold water is suppressed; the pos-

sibility of decreasing the effect due to loss of coolant to approximately zero, as in modern fast reactors,has not been ruled out;

• similarly, with respect to neutron physics,the two variants of an RBMK-type fast reactor without graphite and with

plutonium (RBMK-Pu) were found to be operable – with fuel consisting of a mixture of weapons plutonium dioxide and mag-

nesium oxide, cooling with water with supercritical parameters,and with mixed fuel based on weapons plutonium and deplet-

ed uranium dioxides with cooling with boiling water, just as in RBMK reactors;

• both variants have negative water-loss effects and positive effects of reactivity with flooding with cold water; the

latter can be suppressed by compensating organs or by the presence of boric acid in the emergency tank;

• in both variants,the reactor was found to be serviceable with the 25 cm spacing of the technological RBMK chan-

nels maintained over a square lattice;

• the higher neutron absorption in a variant with depleted uranium compared with magnesium resulted in a much

higher plutonium load – 19.3 instead of 9.6 metric tons with cooling with boiling or supercritical water (see Table 2); corre-

spondingly, the rate of utlization of weapons plutonium is 4.4 and 1.8 metric tons/yr owing to the higher rate of denaturation

in the first variant;

660

TABLE 1. Indicators for Reactors with the RBMK Cooling Scheme (boiling water, 7 MPa) or Cooled with Water with

Supercritical Parameters (VSP, 25 MPa)

Indicator VSP-1 (Fig. 3) RBMK-1 (Fig. 4) VSP-2 (Fig. 4) RBMK-2 (Fig. 5) VSP-3 (Fig. 5)

Keff,crit 1.5 1.2 1.35 1.2 1.35

Mass fraction,%:

MgO 75 60 50 – –

PuO2 25 40 50 25.6 36.5

UO2 – – – 74.4 63.5

∆Keff, %:

water loss –2.7 +5.2 +3.6 –5.1 +3.4

flooding with water +5.2 +0.7 –7.0 +3.3 –1.0

Water density, g/cm3 0.1 0.52 0.1 0.52 0.18

PuO2 volume fraction,% 12.7 22.6 30.4 25.6 36.5

• the quality of denaturation, i.e., the isotopic composition of the plutonium approaches in the fast spectrum the equi-

librium value 239/240/241 = 60.8/30.4/4.14 [7]; the approach can be characterized by the residence time in the neutron field

or the mass of fission products,which grows with time and is referred to the plutonium mass,since the total number of neu-

trons produced is fixed by the thermal power and the time; in the variants indicated this quality indicator was 1.9 times high-

er in the variant with magnesium,primarily because of the lower mass of the loaded plutonium,since the accumulation of

fission products over the run is almost the same (4 and 3.8 metric tons); in the variant with magnesium oxide, the plutonium

load and Keff can be increased to ensure burnup while maintaining a negative water-loss effect (see Fig. 3),while in the case

with depleted uranium the chosen variant is close to the cases with a positive water-loss effect (see Fig. 5);

661

TABLE 2. Indicators for RBMK-Pu Cooling Variants with the Reactor Operating in a Regime Utilizing Weapons

Plutonium,Boiling Water, or Water with Supercritical Parameters

Parameter Boiling water Water with supercritical parameters

Fuel type UPuO2 PuO2MgO

Fuel density, g/cm3 8 4.07

Mass fraction,g, %:

PuO2 25.6 25

UO2 74.4 –

MgO – 75

Volume fraction,%:

PuO2 25.6 12.7

MgO – 87.3

∆Keff, %:

water loss –5.1 –2.8

flooding with water +3.3 +5.2

Fuel volume, m3 10.7 10.7

Mass,metric tons:

UPuO2 85.6 –

PuO2 + MgO – 43.6

heavy atoms 75.3 9.6

plutonium 19.3 9.6

fissioned atoms,metric tons/yr 1 0.7

Efficiency, % 31.2 45

Power, GW:

electric 0.8 0.8

thermal 2.56 1.78

Fraction of consumable h.a.,% 0.05 0.39

Mass of fission products,g/cm3 0.35 0.35

Fuel run, yr 4 5.4

Mass,metric tons/yr:

generated 0.4 0

burned plutonium 1 0.7

denatured plutonium 3.41 1.08

utilized plutonium 4.41 1.78

• replacing zirconium with steel made it possible to increase the average burnup substantially from 5 to 39% h.a.

with 0.35 g fission products/cm3 in both variants; eliminating zirconium increased reactor safety in serious accidents,as hap-

pens in other fast reactors.

The implementation of each of the three reactors studied entails the following:

• the BR-VSP power reactor is a serious goal if it is decided that water with supercritical parameters is to be intro-

duced into the reactor arena; it can operate as a power reactor with a uranium screen with BR = 1.2 or without a uranium

screen with BRC = 1.02; operation could be admissable with a small positive void effect of reactivity or this effect must be

reduced to zero;

• the fast RBMK-Pu reactor for utilization of weapons plutonium with cooling with boiling water using the RBMK

scheme is closest to implementation in a variant with depleted uranium in mixed fuel; one such reactor utilizes 88 metric tons

of weapons plutonium in 20 yr, while in 11 operating RBMK reactors, as calculated in [8],50–60 metric tons of weapons

plutonium are utilized in 20 yr; the reactor operates in an open cycle and then switches to a closed cycle and can burn dena-

turized weapons or power-production plutonium from irradiated VVÉR and RBMK fuel; in the distant future, when nuclear

power will be shut down, such a reactor will be useful for burning plutonium from the final reactor loads;

• RBMK-Pu with fuel from weapons or power-production plutonium dioxide and the inert additive magnesium oxide

or a different reactor without breeding, just as in BR-VSP, involves the prospect of using water with supercritical parameters

as a reactor coolant and development of PuO2–MgO fuel for high burnup; Fig. 4 shows that the placement of 60 metric tons

of cobalt in the RBMK-Pu core decreases Keff substantially and the problem of commercial production of isotopes must be

considered separately from burning of plutonium (weapons or power-production).

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3. MCNP-4B, Manual, LA-13625M (1997).

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