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

As set out in the document titled: Mitigation of Global Warming the only dependable and economic source of non-fossil energy with the capacity to sustainably mitigate CO2 driven climate change is urban sited fuel breeding Fast Neutron Reactors (FNRs). These reactors should feature autonomous operation, passive temperature control, low coolant pressure and must not need a surrounding public safety exclusion zone.

The US nuclear electricity industry has become trapped in water moderated thermal reactor dogma. In a water moderated reactor loss of coolant causes the reactivity to decrease which stops the nuclear fission reaction. Existing US nuclear power reactor safety regulations rely on this phenomena.

In a liquid metal cooled FNR loss of coolant increases the reactivity which increases the fissile fuel temperature and accelerates the fission reaction. Stopping the nuclear reaction in a FNR requires increasing the coolant temperature or changing the fuel geometry. At the coolant boiling point is an FNR specific issue known as coolant void instability.

Liquid metal cooled FNRs with metallic fuel have a coolant void instability issue that in adverse circumstances could cause major equipment damage. There is typically only a small temperature difference between the design peak FNR fuel operating temperature and the fuel centerline melting point. During adjustment of the FNR temperature setpoint it is important to avoid fuel centerline melting and it is essential to avoid coolant boiling.

Due to the high neutron velociy in a FNR a change in FNR fuel geometry causes an almost instantaneous change in the corresponding fissile fuel temperature. However, from the perspective of an electronic temperature measurement and control the measured temperature will take several seconds to respond to a change in FNR setpoint. This temperature sensor transportation time delay can potentially lead to temperature oscillation. When there is a small change in fuel geometry the actual fuel temperature may oscillate, periodically overshooting the reactor setpoint, and causing fissile fuel melting.

Fissile fuel center melting concentrates zirconium along the core fuel rod center line and concentrates uranium around the outside of the central melt region.

To prevent fuel center line melting it is desirable to slow down the rate of response of the fissile fuel temperature to an externally imposed change in FNR nuclear fuel geometry.

A sudden change in the FNR nuclear fuel geometry, possibly caused by a military missile attack or a control system failure, can cause the FNR temperature setpoint to rise far above the melting point of the fissile fuel and/or above the boiling point of the liquid sodium coolant. Coolant boiling forms bubbles (cooling fluid voids) which will increase the reactivity further increasing the FNR temperature setpoint and hence further increasing the fuel temperature and the rate of coolant boiling. This is a positive feedback loop that can cause thermal runaway.

Once a FNR's temperature setpoint reaches the coolant boiling point the FNR temperature setpoint will spontaneously rise out of control due to the change in FNR reactivity caused by coolant void formation.

In a pool type FNR coolant boiling must be sustained for a long time before the increased coolant vapor pressure will structurally threaten the reactor enclosure. The main concern is fuel melting and fuel tube geometry distortion.

To avoid the coolant void instability it is necessary to sense circumstances that might lead to reactor set point temperature overshoot and to execute an immediate reactor shutdown to minimize fuel melting and to prevent coolant boiling.

Unless suitale protective measures are taken rapid fissile fuel melting can cause equipment damage before the mechanical reactor shutdown system has time to act.

This web page focuses on a new technology that allows liquid sodium cooled FNRs with metallic fuel to avoid damage due to fuel melting.

The thermal output power of a fission type nuclear reactor is controlled by controlling the reactor parameter known as reactivity. Reactivity is a mathematical parameter which indicates how fast the reactor's free neutron population will grow or decay and hence how fast the reactor's thermal output power will grow or decay.

The FNR reactivity at a particular fissile fuel temperature can be changed by changing the shape, composition or atomic concentration of the reactor's fuel assembly. In a FNR the change in reactivity with temperature is due to thermal expansion of the fuel assembly components, including the fuel, fuel tubes, liquid coolant and the fuel bundle structural steel. Thermal expansion changes the atomic concentrations.

The operating temperature of a FNR that has a pancake shaped core zone is a strong function of the FNR's core zone thickness. Changing this thickness by even a few mm results in a large change in FNR temperature setpoint. The FNR core zone thickness is carefully mechanically adjusted so that the FNR reactivity is zero at the desired FNR operating temperature.

The following graph shows a family of Reactivity versus Temperature curves.

The lowest curve is for a reactor that is turned off by full withdrawal of the movable fuel bundles. Note that it does not allow a positive reactivity at any temperature between - 100 C and + 900 degrees C.
The middle curve corresponds to a setpoint of 500 degrees C achieved by partial insertion of the movable fuel bundles. At the setpoint the reactivity is zero.
The top curve corresponds to thermal runaway caused by over insertion of the movable fuel bundles. For this curve there is no temperature at which the reactivity goes negative.

Consider an operating reactor:
If the FNR temperature falls below its setpoint the reactivity becomes positive causing the FNR thermal power output to exponentially increase with time until the thermal power is sufficient to restore the FNR temperature to its setpoint.

If the FNR temperature rises above its setpoint the reactivity becomes negative causing the FNR thermal power output to exponentially decrease with time until the drop in thermal power is sufficient to restore the FNR temperature to its setpoint.

In normal FNR operation the reactivity is close to zero and the thermal power output is nearly constant.

Provided that thermal expansion gives a FNR the necessary negative slope reactivity versus temperature characteristic the FNR temperature will maintain at its setpoint independent of the thermal load.

However, at the coolant boiling point coolant void formation causes the FNR reactivity to suddenly swing positive, causing thermal runaway. Similarly, there is a potential reactivity discontinuity at the fuel melting point.

In normal operation a sodium cooled FNR operates at about 500 degrees C at which temperature its reactivity is close to zero. At higher temperatures thermal expansion of the fuel assembly causes its atoms to move further apart resulting in negative reactivity. At lower temperatures thermal contraction of the fuel assembly causes its atoms to move closer together resulting in positive reactivity. Thus a FNR will normally maintain its setpoint temperature.

An important issue in sodium cooled FNRs is the ratio of the fuel assembly components. Some components such as Pu-239 and U-235 contribute positive reactivity. Other components such as Fe, Cr, U-238, Zr, and Na contribute negative reactivity. Remember that the net reactivity must be zero at the operating temperature and the slope of the net reactivity versus temperature curve must always be negative. At the coolant boiling point this slope may swing positive, causing an instantaneous rise in temperature setpoint and potentially violent coolant boiling. Likewise, there is serious concern about fuel melting potentially changing the fuel geometry. In this respect, during fissile fuel rod casting, Zr tends to concentrate along the core fuel rod centerline. That effect will lower the Zr concentration at the core fuel rod surface.

If a FNR has a pancake shaped core zone any axial change in the core zone thickness will affect the FNR temperature setpoint. If the temperature setpoint is suddenly increased due to a sudden increase in core zone thickness the fissile fuel will almost instantly adopt the new setpoint temperature and the surrounding sodium coolant will adopt that temperature as soon as the required heat can thermally transport from the fissile fuel to the sodium. If the new FNR temperature setpoint is above the boiling point of the sodium coolant, then the FNR will attempt to evaporate the sodium coolant. The contemplated FNR design has an atmospheric pressure liquid sodium coolant pool which provides abundant coolant thermal mass and has sufficient space for sodium vapor above the sodium pool so the main concern is prevention of damage due to fuel melting. However, if the reactor is not promptly shut down, the sodium pool temperature will rise and the sodium vapor pressure may eventually become high enough to blow the enclosing building apart.

It is essential to have reactor safety shutdown systems that recognize fuel overheating and quickly reduce FNR reactivity to force a FNR shutdown.

To minimize the consequence of an external overhead impact FNR's are mechanically designed such that any overhead impact causes the reactivity to decrease.

Urban residents need certainty that urban sited FNRs have no public safety issues.

Due to high neutron speeds FNR's rely on the negative slope of the fuel asssembly's reactivity versus temperature characteristic for passive fuel temperature control. Under normal circumstances this methodology maintains the desired temperature set point, which is usually chosen to be about 500 degrees C.

However, FNRs rely on a liquid coolant for heat removal. At some temperature above the normal FNR set point any low pressure liquid coolant will boil. Assume that a sudden unplanned event such as a missile attack deforms the reactor fuel assembly causing the reactor temperature set point to jump from 500 degrees 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 will boil forming bubbles (voids) in the circulating liquid coolant and the sodium vapor will exert pressure on its confinement. 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 fuel melting will cause a change in core fuel geometry.

Coolant void formation reduces the rate of heat flow away from solid fissile fuel which will contribute to fuel melting. Sustained boiling of the sodium coolant will raise the sodium pool temperature and 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.

In the FNR literature the term sodium void instability is a euphemism for this potential fuel melting/sodium boiling problem.

To address this problem there should be two independent fast acting reactor shutdown systems and there should be a means for minimizing fuel melting while the shutdown is occurring.

The technology required to avoid equipment damage due to the FNR coolant void instability 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.

Inherent Safety Aspects of Metal Fuelled FBR
Carbide as a Candidate Fuel for SFR
Measurement of Thermal Neutron Capture Cross Section of Cs
Na-23 Neutron Cross Sections

This web page last updated March 11, 2023.

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