MAJOR TECHNICAL PARAMETERS
Parameter Value
Technology developer, country of origin
JAEA in cooperation with MHI, Toshiba, IHI, Hitachi, Fuji Electric, NFI, Toyo Tanso, Japan
Reactor type Prismatic HTGR
Coolant/moderator Helium / graphite Thermal power, MW(t) 30
Primary circulation Forced by gas circulators Primary coolant pressure,
MPa
4 Core Inlet/Outlet Coolant
Temperature, oC
395 / 850 (950 max.) Fuel type/block array UO2 TRISO ceramic coated
particle Number of fuel block in core 150
Fuel enrichment, wt% 3 – 10 (6 avg.) Average fuel discharged
burnup, GWd/tHM
22 (33 max.) Refuelling Cycle, days 660 EFPD
Reactivity control mechanism Control rod insertion Approach to safety systems Active
Design lifetime, years ~20 (Operation time)
Plant area, m2 ~200m × 300m
RPV height/diameter, m 13.2 / 5.5
Seismic Design (SSE) > 0.7m/s2 automatic shutdown Distinguishing features Safety demonstration test
Status Operational
1. Introduction
The High Temperature Engineering Test Reactor (HTTR) is Japan’s first High Temperature Gas-cooled Reactor (HTGR) established in the Oarai Research and Development Institute of Japan Atomic Energy Agency (JAEA). The HTTR has superior safety features by using coated fuel-particle, graphite moderator, and helium gas coolant. With the potential of supplying high temperature heat above 900°C, HTGR can be used not only for power generation but also for process heat in several industrial fields. JAEA conducted long-term high temperature operation (950°C/50days operation) to demonstrate the capability of high temperature heat supply.
It then conducted a loss of forced cooling (LOFC) test (at 30% power) to demonstrate the inherent safety feature of HTGR in 2010. The LOFC test simulates the severe accident in which the reactor coolant flow is reduced to zero and the reactor scram is blocked. The test result shows that the reactor could be shut down and kept in a stable condition without any operation management. JAEA has accumulated useful data for the development of future commercial HTGR system though the design, construction, and operation of the HTTR.
2. Target Applications
The objectives of HTTR are to: (i) stablish and upgrade the technological basis for the advanced HTGR; (ii) Perform innovative basic research in the field of high temperature engineering; and (iii) Demonstrate high temperature heat applications and utilization achieved from nuclear heat.
3. Main Design Features
Design PhilosophyIllustrated in the figure below, the reactor building is designed with five levels of three underground floors and two upper ground floors. The reactor building is 18.5 m in diameter, 30 m in height. The cylindrically shaped containment steel vessel contains the reactor pressure vessel, the intermediate heat exchanger, the pressurized water cooler and other heat exchangers in the cooling system.
HTTR 30MW
Reactor pressure vessel
Stand pipes Control rod
assemblyFuel
Carbon blocks Auxiliary
cooling pipe Main
cooling pipe Helium
Fuel kernel
Low density PyC SiC High density PyC
Fuel compact Fuel assembly
Coatedfuel particle
The reactor core is designed to keep all specific safety features within the graphite blocks. The intermediate heat exchanger is equipped to supply high temperature clean helium gas for process heat application systems.
The instrumentation and control system are designed to allow operations which simulate accidents and anticipated operational occurrences. As the HTTR is the first HTGR in Japan and a test reactor with various purposes, it incorporates specific aspects regarding safety design. JAEA established the safety design principles for HTTR in reference to the ‘Guidelines for Safety Design of LWR Power Plants’, but taking into account the significant safety characteristics of HTGR and corresponding design requirements as a test reactor.
Reactor Core
The HTTR reactor consists of reactor internals and core components. The reactor internals comprise the graphite and metallic core support structures and shielding blocks. They support and arrange the core components, such as fuel blocks and replaceable reflector blocks within the reactor pressure vessel (RPV). The core components are made up of the same prismatic blocks of 360 mm width across the flats and 580 mm in height, including replaceable reflector blocks, irradiation blocks, control rod guide blocks, and fuel assembly blocks. The 2.9m in height, 2.3m in diameter core is surrounded by the permanent reflector made of graphite.
The active core region consists of 30 fuel columns and 7 control rod guide columns while the reflector region contains 9 additional control rod guide columns, 12 replaceable reflector columns, and 3 irradiation columns.
Fuel
The HTTR employs the TRISO (Tri-structural isotropic)-coated fuel particles (CFPs) with UO2 fuel kernel.
There are four layers surrounding the fuel kernel, including a low-density porous pyrolytic carbon (PyC) buffer layer, followed by a high-density PyC layer, a SiC layer, and an outer high-density PyC layer. Approximately 13-thousand CFPs are fabricated in a graphite matrix of fuel compact. There are 14 fuel compacts in a fuel rod.
Each fuel assemblies contains 31 or 33 fuel rods.
The fabrication of the first-loading fuel for the HTTR started in June 1995. A total of more than 60-thousand fuel compacts, corresponding to about five-thousand fuel rods, were successfully produced through the fuel kernel, coated fuel particle, and fuel compact processes. The fuel rods were transferred to the reactor building of HTTR, where they were inserted into the graphite blocks to form the fuel blocks. In December 1997, 150 fuel assemblies were completely formed and stored in new fuel storage cells.
Reactivity control system
The HTTR contains two reactivity control systems, including a control rod system and a reserve shutdown system (RSS). The control rod system comprises of 16 pair of control rods made of B4C. Each pair of control rods can move individually by control rod drive mechanisms located in standpipes at the top head closure of the RPV. In the event of a scram, the control rods can freely fall into the core by gravity. There are 7 pairs of control rod in the active core and 9 pairs in the reflector region.
The RSS is located in the standpipes along with the control rod and can be inserted into the third hole of control rod guide block. The RSS consists of driving mechanism, hopper, guide tube, etc. The hopper contains B4C/C pellet. When the RSS is activated, the hopper is opened and the B4C/C pellets drop into the reactor by gravity.
The RSS was designed to be able to make the reactor subcritical from any operation condition at a temperature range from 27°C to 950°C.
Cutaway view of the HTTR
Air cooler Crane
Refueling machine
Service areas
Reactor containment vessel Pressurized
water cooler Intermediate
heat exchanger Service
areas Reactor pressure vessel
Double pipe
Cooling systems
The cooling systems of HTTR are composed of a main cooling system (MCS), an auxiliary cooling system (ACS), and a vessel cooling system (VCS). Under a normal condition, the heat of 30 MW from the reactor core could be removed by the MCS with two loading modes. One is a single loaded operation mode where 30MW thermal from the reactor is cooled by only the
primary pressurized water cooler. Another is a parallel loaded operation mode, in which 10 MW and 20 MW thermal are separately removed by the helium-helium intermediate heat exchanger and the primary pressurized water cooler, respectively. The helium-helium intermediate heat exchanger of HTTR is operated at the highest temperature in the world.
The ACS consists of the auxiliary heat exchanger, auxiliary gas circulators, and air cooler. The heat transfer capacity of the ACS is about 3.5 MW. The ACS automatically starts up when the reactor is scrammed and the MCS is stopped abnormally. The residual heat of the core can also be removed by the VCS without the activation of ACS.
4. Safety Features
The reactor delivers fully inherent safety due to three enabling design features:
- Helium coolant is chemically stable. It does not react chemically with fuel and core structures so that hydrogen gas is not produced by chemical reaction of fuel element in accident like LWR;
- The CFPs of HTTR have excellent heat-resistant property which can bear very high temperature condition over 2200°C without any fission product release. The HTTR is designed that the fuel temperature does not exceed 1600°C in any accident to prevent fuel damage;
- Graphite-moderated reactor core provides a negative reactivity coefficient, low-power density, and high thermal conductivity. Graphite core structure also can withstand up to 2500°C without any thermal damage.
The HTTR can remove the residual heat of the core inherently because of optimized low reactor power density and graphite core structure. If the forced cooling performance was lost in an accident, decay heat of fuel transfers to reactor vessel through the core graphite structure slowly by thermal conduction and radiation. The fuel temperature is kept below the design limit of 1600°C by this safety features. The HTTR does not need to consider the immediate accident management and to provide excess emergency safety system.
5. Plant Safety and Operational Performance
Various operational tests have been conducted to confirm the plant safety and operational:
a) Pre-operational test
Pre-operational test operation of the reactor cooling system was performed from May 1996 to March 1998. At the stage of the pre-operational test without nuclear heating the helium gas was heated by the gas circulators up to about 200°C at 2 MPa. Plant control systems were also fully checked. During the pre-operational tests, several improvements in the system were made in terms of securing its safety margin and easy operation. Their performance was finally confirmed in July 1999 after completing the actual fuel loading.
b) Start-up physics test
Fuel loading to the reactor started in July 1998, and the first criticality was attained on November 10th, 1998.
The fuel blocks were column-wise loaded from the outer fuel columns to the inner. The first criticality was achieved successfully with 19 fuel columns loading. After that, the other inner fuel columns were loaded and the full core criticality was achieved by December 1998. In the course of fuel loading, low power physics tests were also carried out for the 21, 24, and 27 fuel columns loaded core. These tests provide useful data for designing future annular cores of advanced HTGR.
c) Rise to power test
Rise to power tests were started in September 1999 when the reactor power was increased step-by-step to 10 MW(t), 20MW(t), and then finally to 30 MW(t). The 30 MW(t) full power and 850°C high reactor outlet coolant temperature were achieved in December 2001. Certificate of pre-operation test, that is, operation permit of the HTTR was issued in March 2002. The HTTR accomplished the maximum reactor outlet coolant temperature of 950°C in April 2004 in high temperature test operation. Operation permit for the high temperature test operation was issued in June 2004.
d) Safety demonstration test
The safety demonstration tests have been implemented from 2002 in order to confirm the excellent inherent safety of HTTR. In the first phase of the safety demonstration test, the control-rod withdrawal tests, the gas circulator tripping tests, etc. have been carried out demonstrating the safety of HTTR. From 2010, the second phase of safety demonstration tests, namely loss of forced cooling test (LOFC), was carried out. The LOFC test was initiated by tripping all three helium gas circulators of the HTTR while deactivating all reactor reactivity control systems to disallow reactor scram due to abnormal reduction of the primary coolant flow rate. The test results showed that the reactor power immediately decreased to almost zero and became stable
HTTR cooling systems