XYLENE POWER LTD.

FNR AXIAL DISASSEMBLY

By Charles Rhodes, P.Eng., Ph.D.

SAFETY OF URBAN SITED FNRs
Urban residents need certainty that urban sited Fast Neutron Reactors (FNRs) present no threat to public safety.

This web page focuses on a technology known as Axial Disassembly which has two main objecctives:
1) In normal operation Axial Disassembly ensures that the FNR reactivity always decreases as the FNR fuel temperature increases. This feature enables a stable FNR operating temperature and ensures passsive high temperature fission shutdown.
2) Axial Disassembly also protects liquid sodium cooled FNRs with metallic fuel from fuel tube damage due to unanticipated step changes in fuel geometry that might otherwise cause fuel tube melting.
 

AXIAL DISASSEMBLY
FNR Fuel Axial Disassembly is a technology that gradually reduces the reactor reactivity as the fuel temperature rises and explosively further reduces the reactor reactivity at initial onset of internal sodium boiling.

An important constraint on FNR Fuel Axial Disassembly is the thermal behaviour of sodium, which is liquid at normal FNR operating temperatures. Liquid sodium has a large Thermal Coefficient of Expansion (TCE) which viewed in isolation tends to make the coolant sodium inject positive reactivity on an increase in temperature.

However, when a fixed mass of liquid sodium is confined within a metal tube which has a smaller TCE than that of liquid sodium the effect is to increase the apparent axial TCE of the contained liquid sodium. This axial effect is important in Axial Disassembly calculations. Also important is the radial temperature profile in the FNR fuel.

Due to thermal expansion of the fuel tubes and fuel bundles the cross sectional area of liquid sodium coolant in the core zone changes at as the coolant sodium temperature increases. This effect causes axial upward movement of coolant sodium with increasing temperature.

There are further issues. As the coolant sodium temperature increases the fuel tube and fuel bundle materials thermally expand. As the fuel ages the fuel tube material in the core zone swells.

Thermal expansion of the fissile fuel within the fuel tube with increasing temperature injects negative reactivity. However, with increasing temperature thermal expansion of sodium inside the fuel tube injects components of both positive and negative reactivity.

Much of this web page focuses on ensuring that under all circumstances, on increasing temperature the net change in FNR reactivity is negative. Further, at onset of internal sodium boiling core fuel must be instantly accelerated toward the fuel tube plenum to further reduce FNR reactivity.
 

FNR OPERATION:
Due to high neutron speeds FNR's rely on the negative slope of the FNR's reactivity versus temperature characteristic for passive temperature control. In normal operation this temperature control methodology maintains the desired average fuel temperature set point, which is usually chosen to be about 460 degrees C.
 

FNR PROTECTION:
Axial Disassembly protects liquid sodium cooled FNRs with metallic fuel from damage caused by unplanned excursions into prompt neutron criticality. On occurrence of a prompt neutron critical condition the fuel temperature rapidly rises and the fuel disassembles due to vaporization of contained and adjacent sodium. The sodium vapor initiates a liquid sodium pressure wave which blows the fixed fuel bundle metallic core fuel towards the fuel tube plenum, causing a change in fuel geometry which injects negative reactivity and hence stops the nuclear reaction. This protection technology avoids reactor damage due to sodium void instability and unplanned step changes in fuel geometry.

The Axial Disassembly apparatus also improves FNR temperature control by increasing the negative slope of the FNR's reactivity versus temperature characteristic at the FNR's normal operating point.

This Axial Disassembly technology relies on the relatively large temperature coefficient of expansion (TCE) of sodium, as compared to the TCEs of HT-9 steel, uranium and zirconium, to reduce reactor reactivity as the fuel temperature rises.

Axial disassembly operates by effectively increasing the vertical Thermal Coefficient of Expansion (TCE) of the core fuel rods. Core fuel rod thermal expansion increases the core zone thickness while keeping the number of core zone fissile fuel atoms constant. The core fuel rod TCE injects sufficient negative reactivity into the FNR to stop the exponential thermal power gain related to excursions into prompt neutron criticality associated with:
a) Sodium void instability,
b) Step changes if fuel geometry,
c) Unanticipated large drops in sodium coolant temperature.

Note that this protection may be only temporary. While this protection is in effect the corresponding movable fuel bundles should be partly withdrawn to ensure a sustained reduction of the FNR reactivity.
 

AXIAL DISASSEMBLY IMPLEMENTATION:
When Axial Disassembly is properly implemented a FNR should be protected against sodium void instability and against step changes in fuel geometry.
 

The core zone of the contemplated power FNR is about 11.3 m in diameter and about 0.4 m thick. This core zone contains ~___ fixed fuel bundles, each with ____active fuel tubes and ~400 ____ movable fuel bundles, each with ____ active fuel tubes. Increasing the core zone thickness while holding the number of fissile atoms constant reduces the assembly reactivity.
 

NEW FUEL

When the FNR core fuel is new the fuel is in the form of a solid rod with an outside diameter of about 0.85 X the inside diameter of the fuel tube. The annular space between the fuel rod and the fuel tube is filled with liquid sodium.

When the core fuel is new and has not yet absorbed any sodium, the linear TCE of the fuel rod will be small. In this case the fuel assembly material ratios will be critical in order to ensure that the initial reactivity versus temperature curve will have a negative slope at the operating point. Under these circumstances it may be impossible to protect against internal sodium void instability Which may occur at a sodium coolant temperature of about 700 ____degrees C. Remember that the hottest point on the fuel centerline must remain under 880 degrees C.BR>  

SWELLED FUEL:
After the FNR has been operating for a few weeks the fuel rod diameter will have swelled to equal the inside diameter of the fuel tube. This swelling is caused by formation of fission product gas molecules inside the fuel rod alloy lattice that occupy more volume than the TRU atoms that they replace.

When the fuel rod diameter expansion due to fission product induced swelling reaches about 15% the fuel swelling will stop due to formation of fission product gas leakage paths between the inside and the outside of the fuel rod. At this point the average fuel rod density will have decreased to about 70% of its original value. However, the fuel rod is still surrounded by liquid sodium under pressure. The sodium atoms are smaller than the gas molecules and hence will diffuse into the fuel along the gas leakage paths. The voids in this spongy mass will gradually fill with Na.

Note that Axial Disassembly protection relies on Na becoming embedded in the core fuel.

The result is a soft pseudo alloy fuel rod with a diameter equal to the fuel tube ID. This smeared fuel rod is composed of TRU-U-238-Zr-Na. This soft fuel rod is confined by the ID of the fuel tube which has its own TCE.

The result is that due to the large TCE of sodium on a sudden fuel temperature rise the axial fuel expansion is greater than it would be for a simple core fuel rod.The volumetric TCE of this soft pseudo alloy is a blend of the volumetric TCEs of the original core rod TRU-U-238-Zr and the sodium.
 

FUEL AXIAL DISASSEMBLY:
Core fuel linear thermal expansion and delayed neutrons are the primary safeguards against uncontrolled rapid power rise in a FNR. Hence it is crucial that the design of fast neutron reactors ensures that transients or accidents can not cause strong prompt neutron criticality.

A FNR with swelled fuel can safely manage small prompt critical excursions. In a FNR with fuel tube plenums a rapid power rise due to a small prompt critical excursion causes thermal expansion of the sodium that is embedded in the smeared core fuel. The embedded sodium has a large Thermal Coefficient of Expansion (TCE) which has the effect of increasing the axial TCE of the core fuel.

The thermal expansion acts equally on the fixed and movable fuel rods in terms of moving the core fuel rods toward or away from the fuel tube plenums, thus causing the reactor to become sub-critical before the thermal power dissipated by prompt neutron criticality is sufficient to cause physical damage.

Note that this protection relies on Na becoming embedded in the core fuel due to the core fuel swelling and venting gaseous fission products during the first few weeks of fuel operation. During that period gaseous fission products form in the core fuel and make the core fuel swell and become spongy.

The voids in this spongy mass then gradually fill with Na. The result is that on a sudden fuel temperature rise the axial fuel expansion is greater than it would be for a simple core fuel rod.

Thus, as long as there are no phase changes we can calculate the new effective linear TCE of the fuel rod, which due to the relatively large TCE of sodium will be larger than the TCE of the original solid fuel rod.

If the fuel temperature inside the fuel tube exceeds the boiling point of its contrained sodium there will be a sudden linear expansion of the fuel rod, which will reduce the FNR reactivity. Note that we cannot rely on this vaporization of sodium inside the fuel tube, because late in the fuel life the plenum pressure inside the fuel tube will increase the BP of sodium inside the fuel tube above the BP of sodium outside the fuel tube. Thus suppression of sodium void instability for sodium outside the fuel tube relies on simple thermal expansion of sodium inside the fuel tube.

Further, when the core fuel is new and has not yet absorbed sodium, the TCE of the fuel rod will be small. In this case the fuel assembly material ratios will be important in order to be certain to achieve a reactivity versus temperature curve with a negative slope at the operating point. Under these circumstances it may be impossible to protect against sodium void instabiity above about 600 degrees C.

At a temperature several hundred degrees C above the normal FNR set point liquid sodium inside and/or outside the fuel tube will boil.

Assume that a sudden unplanned event such as a missile attack changes the reactor fuel geometry enough to increase the reactor temperature set point from 460 C to 900 degrees C. The reactor fuel will almost instantly heat up to about 900 degrees C. However, at 900 degrees C the liquid sodium coolant inside the fuel tube will boil. The resulting sodium vapor will exert pressure on its confinement which will increase the length of the core fuel rods causing a drop in reactor rectivity.. The resulting sodium coolant voids increase the reactivity of the fuel assembly which further increases the reactor temperature set point and hence the fuel temperature. Due to the increasing fuel temperature coolant boils more quickly, amplifying the coolant void problem. Unless this problem is instantly addressed by axial core fuel disassembly fuel melting might cause a change in core fuel geometry.

Sodium void formation inside the fuel tube reduces the rate of heat flow away from solid fissile fuel which will contribute to fuel melting. Sodium void formation outside the fuel tube reduces the rate of heat flow away from the fuel tube which will further contribute to fuel melting.

Sustained boiling of the sodium coolant will raise the sodium pool temperature which will eventually cause sodium vapor pressure accumulation over the primary sodium pool.

As the fuel temperature inside the fuel tube rises the sodium vapor pressure inside the fuel tube will rise until ultimately the fuel tube will rupture.

Define:
R = reactivity
T = Temperature
L = axial thickness

dR / dT = [dR / dL] [dL / dT]
or
[(dR / dT)|R = 0] = [(dR / dL)|R= 0] [(dL / dT)|R = 0]

For reactor thermal; stability:
[(dR / dL)|R = 0] < 0
and
[(dL / dT)|R = 0] > 0

To avoid problems with sodium void instability we need:
[(dR / dT)|R = 0] < < 0
so that if sodium void instability adds a positive component to [(dR / dT)|R = 0],
in net:
[(dR / dT)|R = 0] < 0

To achieve this end we need to make [(dL / dT)|R = 0] as large as possible. That is an important function of the sodium inside the fuel tube.
 

TRANSIENT HEATING:
In prompt neutron criticality the rate of power rise is proportional to the degree of super-criticality and is inversely proportional to the neutron transit time T between successive fissions. This transit time T is given by:
T = 1 / [Vn Sigmafp Nfp]
where:
Vn = neutron velocity
Sigmafp = fast fission cross section of Pu-239 atoms
Nfp = average concentration of Pu-239 atoms in the reactor core

En = neutron kinetic energy
= 1.67 X 10^-27 kg X Vn^2 / 2
= 2 X 10^6 eV X 1.6 X 10^-19 J / eV
giving:
Vn = [(2 En) / (1.67 X 10^-27 kg)]^0.5
= [(6.4 X 10^-13 J) / (1.67 X 10^-27 kg)]^0.5
= 1.96 X 10^7 m / s

Sigmafp = 1.7 b
= 1.7 X 10^-28 m^2

From the web page titled: FNR CORE
Nfp = 1.616 X 10^27 Pu atoms / m^3
giving:
T = 1 / [Vn Sigmafp Nfp]
= 1 / [(1.96 X 10^7 m / s) (1.7 X 10^-28 m^2) (1.616 X 10^27 / m^3)]
= 1 s / [5.3845 X 10^6]
= 0.1857 us
= 185.7 ns.
 

At a prompt neutron growth rate of 1.001 / neutron cycle the number of cycles N required for the neutron flux to double is given by:
(1.001)^N = 2
or
N Ln(1.001) = Ln(2)
or
N = Ln(2) / Ln(1.001)
= 0.69314 / 9.995 X 10^-4
= 693.5

Thus at a neutron growth of 1.001 / cycle the fission power will double in:
693.5 X 185.7 ns
= 128,790 ns
= 128.8 us
= 0.129 ms
which is comparable to the time required for the gun powder to burn in a hand gun.

Thus as long as the degree of prompt neutron supercriticality in a FNR is small the dynamics of the core fuel are comparable to the dynamics of a pistol bullet in a gun. At temperatures below the boiling point of sodium the sodium expansion is simple thermal expansion. In the event of prompt neutron criticality the Na inside the core fuel vaporizes which causes both the fixed and movable fuel rods to rapidly axially expand toward the fuel tube plenum. This fuel disassembly introduces more steel and sodium coolant into the average neutron path which reduces the reactor reactivity, thus suppressing the minor prompt neutron critical condition. On cooling sodium vapor condensation, thermal contraction and gravity restore the original core fuel geometry.

Note that it is essential that the upper blanket rods slide freely inside the fuel tubes so that they will not prevent rapid core fuel rod axial disassembly. It is equally important to ensure that the internal pressure rating of the fuel tube walls is sufficient to withstand the pressure required for rapid acceleration of the stack of blanket fuel rods that is above each core fuel rod.
 

COMPUTATION OF ENHANCEMENT:
Fuel tube ID = R =
Fuel Rod initial OD = 0.85 R =
Fuel rod linear TCE =
Sodium linear TCE =
Fuel tube linear TCE =
Core Rod initial length = L =

IMPLEMENTATION:
The technology required to implement Fuel Axial Disassembly is the subject of a patent now being sought by FNR Power Ltd. This technology will be described on this web page at a later date.
 

This web page last updated June 16, 2023