IAEA Education and Training
Seminar/Workshop on Fast Reactor
Science and Technology
October 1 – 5, 2012
Centro Atómico Bariloche, Argentina
Gases as Fast Reactor Coolants
Dr Richard Stainsby
AMEC
Booths Park, Chelford Road, Knutsford,
Cheshire, UK, WA16 8QZ
Phone: +44 (0)1565 684903, Fax +44 (0)1565 684876
e-mail: [email protected]
Contents
 A brief history of gas cooling of reactor cores
 Thermal hydraulic, neutronic and material consideration of using


gases as reactor coolants
 Thermal hydraulics
 Neutronic considerations
 Material/chemical considerations
Power cycle options
Helium demands of Generation IV reactors
2
A Brief (UK) History of Gas Cooling of
Reactor Cores
 1947, Windscale Piles, UK




- Military
plutonium production
Atmospheric air cooled, graphite
moderated reactors
Low temperature
Open cycle
Large pumping power requirement
3
Carbon dioxide – 1953, Magnox reactors
 Closed cycle
 Pressurised to reduce pumping


power
Higher temperatures with lower
oxidation rate than for air
Temperature high enough for
commercial electricity generation
4
Calder Hall Nuclear Power Station
(Magnox) (2 out of 4 reactors shown)
5
Carbon dioxide – Advanced gas cooled
reactors (AGRs) (late 1960s onwards)
 Generation II technology
 Enriched fuel
 Stainless steel clad oxide fuel,
 Coolant outlet temperature 650oC
 Good quality superheated (and reheated) steam (comparable to

quality from a highly optimised coal plant)
High thermal efficiency – 42%
6
Architecture of a carbon dioxide cooled
AGR
7
Problem with carbon dioxide that
became apparent in AGRs
 At AGR temperatures (~600oC), CO2 dissociates into carbon
monoxide and oxygen under the combined action of heat and
radiation (radiolytic dissociation).
2CO2 ↔ 2CO +O2
 The free oxygen oxidises the graphite and metallic structures
 The rate of oxidation becomes unacceptable at higher temperatures
 Dissociation can be minimised by dosing with CO and mitigated using
methane which, itself, dissociates;
CH4 ↔ C + 2H2
Giving the complete reaction as
CO2 + CH4 ↔ C + 2CO +2H2O
The carbon gets deposited to “repair” the graphite, the CO builds up to a
equilibrium concentration to limit further dissociation and water vapour is
extracted by the coolant treatment system.
8
High temperature reactors – Helium
cooling
 Helium was adopted for HTRs to avoid the dissociation problems

associated with carbon dioxide.
These reactors are Dragon (UK, OECD), AVR and THTR-300
(Germany), Peach Bottom and Fort St Vrain (US), HTR-10 (China),
HTTR (Japan) – outlet tempertures of 750oC to 950oC.
 Both carbon dioxide and helium have been proposed as gas-cooled
fast reactor coolants
9
Thermal Hydraulic, Neutronic and Material
Considerations of Using Gases as Reactor
Coolants – Thermal hydraulics
Core inlet temperature of T1 ,core outlet temperature of T2, coolant mass flow rate of m , the amount
of heat that can be transported is q:
q  m cP T2  T1  ,
cP is the specific heat capacity - a physical property of the particular coolant.
The mass flow rate of coolant (in kg/s) is the volumetric flow rate, Q (in m3/s), multiplied by the
coolant density  (in kg/m3), so
  c T  T  .
qQ
P
2
1
For 2 cores, both have same power, both with the same inlet and core outlet temperatures - one cooled
by CO2 and the other cooled by helium, we get;
Q He  He cP ,He
1 
,
QCO2 CO2 cP ,CO2
the ratio of the volumetric flow rates is
Q He CO2 cP ,CO2

.
Q CO2
 He cP ,He
10
Comparison of pumping power
The pressure drop over the core is;
p  k Q 2 ,
where k is a pressure loss coefficient.
The power consumed (in Watts) by pumping the coolant through the core, W, is
simply given by the pressure drop multiplied by the volumetric flow;
W  Qp  k Q 3 .
Assuming that the geometry of the two cores is identical, this results in both cores
having the same value for k, so the ratio of pumping power for the two cores is;
2
3
3
CO
c
WHe
 He Q He
2 p ,CO2


3
2
3
WCO2 CO2 Q CO

c
He P , He
2
11
Comparison of pumping power
Demonstration: core inlet temperature of 300oC, core outlet temperature of 650oC
(an assumed core average temperature of 475 oC) , pressure of 40 bar (absolute).
cP ,CO2 = 1.15 kJ/kg/K and cP ,He = 5.195 kJ/kg/K.
Assuming perfect gases, density ratio is simply the ratio of the molecular weights
(M~ 4 kg/kmol for He and 44 kg/kmol for CO2)
Ratio of pumping power for the two gases is therefore;
2
3
2
3
CO
c
WHe
44
1
.
15




p
,
CO
 22 3 2     
  121 0.01085  1.3126
WCO2
 He cP ,He  4   5.195 
12
Comparison of heat transfer
performance
 For any working pressure a CO2 cooled core will require less
pumping power than a He cooled core
 In practice the important parameter is peak clad temperature and not
core outlet temperature,
 High conductivity of He should (naively) increase heat transfer coefficient
by factor of 6
But
 Smaller mass flow reduces the Reynolds number so overall improvement is
actually about a factor of 1.5
 Conclusions
 He is a superior gas thermal hydraulically (highest h)
 but, costs more to pump at a given pressure, or costs more to contain if
used at a higher pressure
13
Improvement of “poor” surface to
coolant heat transfer
 Can’t change conductivity of the gas (other than to use a different gas)
But
 We can change the Nusselt number, Nu, and we can change the heat
transfer area.
 Nusselt number is changed by changing the geometry and/or by changing
the amount of turbulance,
 The area is changed by changing the geometry or by extending the
surfaces (fins)
14
Enhanced clad surfaces (AGR pins)
•Rib roughened pins have been proposed for metallic-clad GFR cores in the past.
•At the moment they are not considered for ceramic clad
15
Rib-roughend pin for General Atomics
GCFR concept
16
Heat transfer enhancement for ribroughened surfaces
Nusselt numbers for turbulent flow over CO2 cooled pins
100000
Nusselt number
10000
1000
Transverse ribbed
100
Smooth
Longitudinally finned
10
1000
10000
100000
1000000
10000000
Reynolds number
17
Neutronic considerations
 Neutrons interact with the atoms of the coolant.
 The strength of the overall effect is governed by the probability of a




particular interaction (absorption or scattering) and the number density
of the coolant atoms.
Absorption removes neutrons from the system.
Scattering causes the neutrons to “bleed” energy thus slowing them
down (moderation).
Both of these mechanisms add negative reactivity
If the coolant is removed (lost or “voided”), the loss of negative
reactivity is equivalent to an insertion of positive reactivity.
18
The probabilities of absorption and
scattering: microscopic cross sections for
fast reactor coolant atoms (in barns)
Absorption
Nuclide
Scattering
Fast
Thermal
Fast
Thermal
4He
1.0E-20
9.9E-21
3.7
0.85
12C
1.2E-3
3.4E-3
2.4
5.0
16O
1.2E-2
1.9E-4
2.8
4.0
23Na
2.3E-3
0.53
3.1
3.1
19
The number densities of atoms and the
overall effect: macroscopic cross sections

Nuclide
(g/cm3)
M
(g/mol)
N
Absorption (fast neutrons) Scattering (fast neutrons)
(1/cm3) Micro (barns)
Macro
Micro
Macro
(cm-1)
(barns)
(cm-1)
4He
0.00257
4
3.87x1020
1.0E-20
3.87x10-24
3.7
1.43x10-3
12C
0.007718
12
3.87x1020
1.2E-3
4.65x10-7
2.4
9.30x10-4
0.010291
16
7.75x1020
1.2E-2
9.30x10-6
2.8
2.17x10-3
0.838
23
2.19x1021
2.3E-3
5.05x10-5
3.1
6.80x10-2
(as CO2)
16O
(as CO2)
23Na
Macroscopic cross sections to fast neutrons for helium, carbon and oxygen (as CO2)
compared with liquid sodium for P=40 bar and T=475oC
20
Material/chemical considerations




Of the two gases, helium is chemically inert whereas CO2 can dissociate.
Generally, the damaging mechanism with dissociated CO2 is high
temperature oxidation as opposed to traditional “wet” corrosion associated
with water cooled systems.
 The oxidation rates in CO2 cooled reactors are generally lower than in water
reactors,
 Much experience in CO2 cooled thermal reactors on management and limitation of
oxidation,
Whilst helium is an inert gas, there is still the possibility of chemical attack of
the structural materials.
 With most common structural metals, they are protected by a thin, self-repairing,
oxide layer that forms naturally in an oxygen containing atmosphere.
 In an inert atmosphere, if the oxide layer is damaged, there is no oxygen available to
repair the layer.
An associated problem is tribology in a helium environment.
 Surfaces which slide against each other, e.g., bearings, valves and valve seats, and
screw threads can effectively weld themselves together (diffusion bonding).
 This occurs through of the exchange of metal atoms, by diffusion, through the oxidefree surfaces under the action of contact pressure, heat and time.
– A particular problem for safety systems, such as decay heat removal system
valves.
21
The Safety Performance of Gas Cooled
Fast Reactors


The main safety advantages with gas coolants are
 their single phase behaviour
 low reactivity insertion due to voiding of the coolant.
 Optical transparency and electrically non-conducting
The main disadvantages are
 the low density creating the requirement for pressurisation- inreases the likelihood and severity
of a LOCA
 the inability to form a pool, a problem when trying to ensure that the reactor core remains bathed
in coolant within a breached primary circuit,
– following a severe accident it is easier to manage the core debris if immersed in a pool of
liquid coolant, both in terms of cooling and restricting the release of fission products into the
containment building.
 the non-condensable nature of coolant that is lost from the reactor circuit - a problem for pressure
loading the containment building.
 low thermal inertia means that the reactor core will heat up rapidly if forced cooling is lost

The low thermal inertia of the coolant is of particular significance in a fast reactor core.
 The core itself possesses little thermal inertia, so the fuel temperatures rise rapidly following a
loss of forced circulation of the coolant.
 The compact core makes the “conduction cool down” heat path insufficient to remove the decay
heat within the fuel temperature limit.
 Convective cooling is required, either by restoration of forced cooling (through a back-up cooling
system or dedicated pumped decay heat removal system), or by natural convection.
22
Power cycles for gas cooled fast
reactors
 Traditionally gas cooled thermal reactors (even HTRs) have been



coupled to steam (Rankine) cycles.
Early concepts for GFRs also featured Rankine cycles.
More recent HTR projects (PBMR, GT-MHR, GTHTR-300) propose
coupling with gas turbine cycles.
Gas turbines make the best (most efficient use) of the high
temperature heat source.
 Two different modes of coupling
 Direct cycles – the reactor primary coolant drives the turbine directly,
 Indirect cycles – the primary coolant transfers its heat to a secondary
coolant via a heat exchanger, and the secondary coolant drives the trubine.
 (PBMR), GT-MHR and GTHTR-300 are direct cycle concepts.
23
GT-MHR direct
cycle concept
24
Direct “Brayton” gas turbine cycle
pre-cooler
intercooler
turbine
generator
shaft
reactor
LP compressor
HP compressor
recuperator
25
Helium demands of Gen IV reactors
 Current designs for HTR’s
 PBMR, 400 MWth, direct cycle gas turbine – helium inventory
4 Tonnes
 HTR-PM, 250 MWth, indirect steam cycle – helium inventory 2.44 tonnes
 Approximately 1 tonne per 100 Megawatts of thermal power, or 25 tonnes
per Gigawatt of electricity
 The rate of roll-out of these systems is not certain, but …
 The UK produces 20% of its electricity by nuclear power (about 10

GW) - if this was replaced by HTRs we would need 250 tonnes of
helium.
If all of the UK’s electricity came from HTRs, we would need 1250
tonnes of helium to fill them initially.
26
Estimates of leakage rates and impact
on demand
 The target leakage rate for Dragon was 0.1 % of inventory per day.
 Although not measured it was assumed from individual component tests
that the actual rate was much lower 3x10-7 % / day – although anecdotal
evidence suggests it was much higher !
 Commercial plants will have more leak paths than an experimental reactor
– e.g., more shaft seals, and more cable and pipe work penetrations, plus
more fuel charging and discharging operations.
 If we take the Dragon limit of 0.1% / day – this means the inventory is



replaced every 1000 days (~ 3 years) – for a 60 year reactor life, its
inventory would be changed 20 times.
Hypothetically, satisfying the current UK electricity demand over 60
years could require up to 20000 tonnes of He (say between 2020 and
2080)
Demand in China ? HTR-PM development is proceeding well and
deployment is expected to be widespread.
The higher power density of large Gas Cooled Fast Reactors should
require only about 25%-50% of the inventory of HTRs for the same
power output
27