Home Energy Physics Nuclear Power Electricity Climate Change Lighting Control Contacts Links


XYLENE POWER LTD.

FNR WALK AWAY SAFETY

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

THE WALK AWAY CONCEPT:
The concept of walk away safety is that if the appropriate operating and/or maintenance employees are not present or suddenly leave the FNR must always default to a safe condition.

The FNR facility consists of a common central heat source and 8 independent heat to electricity conversion systems. The heat outputs are connected in pairs to supply heat to four independent district heating loops. Each district heating loop has one local cooling tower and three remote cooling towers. In order to provide maximum electricity output in the summer all of the cooling towers must be fully functional. When not in the emergency cooling mode at all times at least one generator and its associated cooling tower and heat transport circuits must be fully functional to remove FNR fission product decay heat.

The FNR facility has multiple independent control systems:
1) Primary sodium Pool:
The primary sodium pool control system operates almost independent of the 8 heat to electricity conversion systems. The primary sodium pool features: a) Normal temperature control;
b) Shutdown system #1;
c) Shutdown system #2;
d) Emergency primary pool cooling.

2) There are 8 independent heat to electricity conversion systems, each with six heat transfer circuits and one turbogenerator. There are four on-site cooling towers, each which is connected to two turbogenerators. Each cooling tower is connected to one isolated quadrant of the district heating system.

NE Heat transport and power systems
Circuit (a)
Circuit (b)
Circuit (c)
Circuit (d)
Circuit (e)
Circuit (f)
Generator (a)
Circuit (g)
Circuit (h)
Circuit (i)
Circuit (j)
Circuit (k)
Circuit (l)
Generator (b)

3) NW Heat transport and power systems:
Circuit (a)
Circuit (b)
Circuit (c)
Circuit (d)
Circuit (e)
Circuit (f)
Generator (a)
Circuit (g)
Circuit (h)
Circuit (i)
Circuit (j)
Circuit (k)
Circuit (l)
Generator (b)

4) SE heat transport and power systems:
Circuit (a)
Circuit (b)
Circuit (c)
Circuit (d)
Circuit (e)
Circuit (f)
Generator (a)
Circuit (g)
Circuit (h)
Circuit (i)
Circuit (j)
Circuit (k)
Circuit (l)
Generator (b)

5) SW heat transport and power systems:
Circuit (a)
Circuit (b)
Circuit (c)
Circuit (d)
Circuit (e)
Circuit (f)
Generator (a)
Circuit (g)
Circuit (h)
Circuit (i)
Circuit (j)
Circuit (k)
Circuit (l)
Generator (b)

In each heat transport circuit if nitrate salt temperature is too low the salt drains to its dump tank.

Event triggering minumum power operation:
Loss of AC grid power

Events triggering a emergency primary sodium pool cooling include:
a) A primary sodium pool temperature high above its setpoint.

b) An imminent fire threat to the primary sodium pool.

Each shutdown state is a default state which is reached automatically without human intervention. In a warm shutdown the primary sodium pool maintains its temperature and the generators keep operating at minimum power. A cool shutdown occurs when a potentially dangerous condition affecting the primary sodium pool is anticipated or detected.

The safety concept is that there must always be enough cooling water stored on the reactor site to safely remove fission product decay heat by evaporation with minimal reliance on electic power. For example steam from one cooling circuit can be used to pressurize the cooling water tank of another circuit to allow that tank to provide cooling water for the other cooling circuits. Hence in principle only one cooling circuit of each power generator requires pressurized water injection.
 

NORMAL AUTONOMOUS OPERATION:
In the normal autonomous opertaion mode the entire FNR facility operates automatically. Absent an alarm there is nothing for anyone to do. The output power level is set by remote dispatch. The cooling towers act to regulate the district heating water temperature.
 

LOSS OF THE EXTERNAL AC GRID:
On loss of the external AC grid the generators all disconnect from the grid and revert to local frequency control. That local frequncy is phase locked to either the grid or a time base. Everything continues to operate as normal but the only generator load is the house load, so the secondary sodium pumps operate at minimum pumping rate. The nitrate salt pumps and local cooling tower wazter pumps continue operating as before.

Loss of Grid AC power means that the remote cooling tower and remote building water cooling pumps will no longer operate. It is necessary to power the local cooling towers off house power so that these local cooling towers continue to function when there is no grid AC power.

Typically each cooling tower has two of everything so that half of the cooling tower equipment is powered by one house power circuit and the other half is powered by the other house power circuit.

Thus on loss of AC grid power the FNR defaults to normal operation using the eight local house power circuits.
 

LOSS OF DISTRICT HEATING WATER
If there is loss of water from the district heating system the condensers will not work which implies that the affected generators will not work which leads to loss of two of the eight house power systems.
 

LOSS OF CITY WATER:
The main reactor does not rely on a continuous supply of city water. However, city water pressure may be required for support services such as flushing toilets, refilling emergency water tanks, etc. so loss of city water pressure is a condition that requires ongoing manual supervision until the condition is fixed.
 

LOSS OF HOUSE POWER:
There are eight house power circuits. In normal opertion these house power circuits continue operation after loss of AC grid power.

If there is loss of house power the related cooling tower water pump, secondary sodium pumps and nitrate salt pumps will immediately stop and the nitrate salt drains to its dump tank. Hence that system can no longer remove fission product decay heat.

Each of the eight house power systems requires heavy duty standby power to restart by melting the nitrate salt in each dump tank and circulating the salt prior to local house power generation. Generally this start power must come from either the AC grid, a large local diesel generator or one of the other seven house power systems. Thus, if possible we do not want an AC grid failure to precipitate house power failures. Loss of house power causes salt drain down in all the affected heat transfer circuits. On loss of power on all eight house power busses the FNR facility must default to a forced cold shutdown.
 

FORCED COLD SHUTDOWN:
On a forced cold shutdown the FNR no longer maintains temperature. The movable fuel bundles all fully withdraw. The nitrate salt circuits all drain down to thier dump tanks. If the primary sodium temperature rises above its trip point fission product decay heat is removed from the reactor by admitting water into the nitrate salt circuits. Natural circulation of the secondary sodium transfers heat from the primary sodium pool to water in the nitrate salt circuits which heat is vented as steam.
 

LOSS OF PRIMARY SODIUM POOL POWER:
In normal ongoing operation the primary sodium pool control system consumes very little power and is easily battery backed for a long period of time. The primary sodium pool control system does not lose power until long after all eight house power systems have failed. On loss of sodium pool control power the solenoid valves holding pressurized sodium behind the hydraulic pistons opens, causing gravity to withdraw all the movable core fuel bundles.

The primary sodium pool can have eight filter pumps, each of which is powered from a different house power circuit. These filter pumps can be off for a long period of time with little negative effect.

However, if the batteries for the primary sodium pool electronics become depleted the reactor must fail to a cold shutdown. These batteries should be charged by grid AC or any operating house power system.

If the primary sodium pool temperture becomes too high it is indicative of net primary sodium heating by fission product decay, which indicates a requirement for a forced cold shutdown.

In a forced cold shutdown the nitrate salt is already drained to its dump tank. Subject to sufficient secondary sodium presure water is admitted to the nitrate salt circuit to remove fission product decay heat from the secondary sodium. In the absence of city water pressure there must be an independent reliable source of air pressure sufficient to transfer water from an in=ground tank to the top of the sodium/salt heat exchangers.

Note that if the FNR has been operating for a significant lenth of time producing just house power the potential thermal power of the fission products will be low. However, care needs to be taken that emergency cooling water is not wasted.

Reconnection of a house power circuit to the AC grid requires resynchronization. Most such reconnections are manually supervised.

Recovery from a forced cold shutdown requires manual intervention.
 

POWER SYSTEM MAINTENANCE:
If only one heat transport circuit is involved:
Drain down the nitrate salt;
Drain down the secondary sodium sodium

If only one generator is involved:
Take generator to minimum power;
Disconnect generator from AC grid;
Turn off makeup water to steam generators;
Drain down the six connected nitrate salt circuits;
Drain down the six associated secondary sodium circuits;

If one cooling tower is involved:
Turn off one or both associated generators, as necessary for safe work.

This web page last updated November 11, 2021

Home Energy Physics Nuclear Power Electricity Climate Change Lighting Control Contacts Links