Nuclear Hydrogen and Heat Application Research9-3

Design Criteria of HTGRs for Withstanding Extreme Events=Investigation of Technical Feasibility for Naturally Safe HTGRs=Decay heat

Conduction
Conduction

Oxidation
heatCore power density(MW/m3)16

Naturalconvection

Decay heat
1284CO
production

COoxidation
RPV

Air ingress

Surrounding
structures

Coolant channel diameter(mm)

0Radiation

Decay heat + oxidation heat

1008060

Design envelope
2.0Core radius (m)1.0

Design envelope forremoving
oxidation heat

Designenvelope

40
2000.01

Design envelope
for avoiding
CO combustion

0.1
1
Coolant channel flow area (m2)

10Fig.9-5 Physical phenomena in HTGR during extreme
eventsDuring extreme events that cause loss of coolant and massive
air ingress in HTGRs, the decay and residual heat in the core
are removed by physical phenomena such as thermal
conduction, radiation, and natural convection because of theintrinsic temperature differences. Carbon monoxide, generated
by the oxidation of graphite in air, is converted into carbon
dioxide by reacting with oxygen in the air.

Fig.9-6 Design envelope for self-stabilization during
extreme events
Design envelope which can remove core heat and suppress
carbon monoxide concentrations below the lower flammability
limit for extreme events that cause loss of coolant and massive
air ingress in HTGRs is clarified.

The Naturally Safe HTGR concept aims to intrinsicallymaintain safety; that is, under accident conditions, physicalphenomena ensure that radionuclides are retained withincoated fuel particles (Fig.9-5). We examined the conditions
that can suppress physical phenomena that would otherwise
degrade the integrity of fuel-coated layers. In particular, we
focused on core heat up due to radioactive decay, theoxidation of graphite, and the production of carbon monoxideby the oxidation of graphite with ambient air.
The temperature and carbon monoxide concentration in thereactor core were investigated as factors that can influencecore design parameters, such as core diameter, size of coolant
channels. The goal is to set HTGR core design parameters sothat, during a loss of coolant and massive air ingress, thetemperature and carbon monoxide concentration in thereactor core remain within safe limits.
A set of simulations clarify the range of average powerdensities and diameters of reactor cores that can suppress
core peak temperatures within the allowable limit of 1600 ℃.
Such control of core temperatures is to be purely performed
by inherent means, such as thermal conduction, radiation fromthe reactor pressure vessel (RPV) to surrounding structures,and natural convection in the reactor cavity (Fig.9-6). In
addition, the results show that indeed there are ranges ofdesign parameters that support the removal of oxidation heatand suppress the concentration of carbon monoxide in thecore below the lower flammability limit of 12.5% (Fig.9-6).
The simulation results indicate that, by fully utilizing
inherently safe characteristics, HTGRs have the potential toeliminate massive, uncontrolled radioactive releases fromnuclear plants to the environment.
We will explore the technical feasibility of Naturally Safe
HTGRs through research and development with the goal offostering public understanding.

Reference
Sato, H. et al., Analysis of Core Heat Removal Capability under DLOFC Accidents for HTGRs, Proceedings of 6th International Topical Meeting on High
Temperature Reactor Technology (HTR2012), Tokyo, Japan, 2012, paper HTR2012-8-003, 9p., in CD-ROM.
JAEA R&D Review 201297