Existing Condition Analysis of Dry Spent Fuel Storage Technology

LI+Ning　XU+Lan　HAO+Jian-sheng

【Abstract】As in some domestic nuclear power plants， spent fuel pools near capacity， away-from-reactor type storage should be arranged to transfer spent fuel before the pool capacity is full and the plants can operate in safety. This study compares the features of wet and dry storage technology， analyzes the actualities of foreign dry storage facilities and then introduces structural characteristics of some foreign dry storage cask. Finally， a glance will be cast on the failure of away-from-reactor storage facilities of Pressurized Water Reactor（PWR） to meet the need of spent-fuel storage. Therefore， this study believes dry storage will be a feasible solution to the problem.

【Key words】Spent fuel； Dry storage； Spent fuel dry storage metal cask； Spent fuel dry storage concrete cask

0 Introduction

With the rapid development of nuclear power and the plants coming into service， how to reprocess high-level radioactive spent fuel discharged from reactor core becomes a prominent issue. Normally， after storage for a period of time in spent fuel pool， spent fuel will be sent to reprocessing plant directly or store in away-from-reactor spent fuel storage facilities for a while. The storage quantity of spent fuel in nuclear power plants has been near or above the capacity of storage pool， thus emerging the immediate need of transferring and reprocessing spent fuel. Otherwise， the plants could be shut down. Due to the insufficient reprocessing capacity of spent fuel， it is necessary to set up away-from-reactor spent fuel interim storage facilities.

1 Spent fuel storage technology

The international community presently adopts two methods as away-from-reactor storage of PWR spent fuel： wet storage（Fig.1） and dry storage. When adopting wet storage， spent fuel is kept in the spent fuel pool. The quality of the pool water is maintained through ion exchange technology to prevent fuel cladding from corrosion. This generates higher operation cost and large amounts of radioactive waste. The Fukushima incident in March 11th， 2011 forced people to reexamine the safety issue of wet storage.

Compared to the wet storage technology， dry storage technology is characterized as modularity， low operation cost， little radioactive waste and high resistance to accidents. As a result， the international community has been paying close attention to the technology and stepping up research on away-from-reactor spent fuel dry storage technology. The facilities in some countries have been successfully put into operation. The technology currently includes drywell storage（Fig.2）， vault storage（Fig.3）， cask storage， etc. Drywell is made of concrete with carbon steel vessel inside and a concrete plug on the top. Heat removal is normally accomplished by outside soil， but failed to meet the high heat load requirement of PWR spent fuel storage. Vault storage consists of modules， transfer cask reception building， charging hall and control area. Metal casks（Fig.4） and concrete casks are variations of the container storage systems. The technique comparison of wet storage and dry storage is illustrated in Table 1.

2 Global status in spent fuel dry storage

Having been applied for more than three decades， spent fuel dry storage technology has been making huge progress. According to the statistics from the International Atomic Energy Agency， dry storage facilities with about 40，000 tons of storage capacity had been set up by the end of 2012. There are 104 nuclear power plants in operation in the United States. Their total volume of spent fuel registers 70，000 tons with an annual growing rate of 2，000 tons. Dry storage is adopted for the away-from-reactor spent fuel storage and over 70% of the plants are equipped with dry spent fuel storage facilities in which about 20，000 tons of spent fuel is stored or soon to be transferred. In Japan， there are 50 nuclear power plants in operation， discharging about 900 tons of spent fuel annually. Wet storage is adopted and the pool capacity nears its full capacity. Fukushima Nuclear Power Plant set up its dry storage facilities in 1997 and has 20 metal storage casks. France enjoys 58 plants with one unit of dry storage facility. It stores spent fuel generated by Heavy Water Reactor with 180 tons in capacity. In Britain， there are 16 plants with High Temperature Reactor as the major reactor type. Wylfa has set up dry storage facility and Sizewell B Nuclear Power Station， owned by UK Power Networks， will take two years to construct a dry storage facility. In general， the facilities used in the countries are metal casks and concrete casks as the following introduction goes.

2.1 Metal casks

Metal casks can be used as dual purpose storage and transportation cask， and their variations have been developed. They consist of primary and secondary lid， shell and envelope， basket， neutron shielding and heat conductors. They could be made of forged steel， ductile cast iron or lead shield stainless steel and installed with baskets to support assemblies and ensure criticality safety. Metal casks are usually put on the concrete base and stored in open air. In German or some other European countries， casks are stored in buildings. Metal casks can be put at reactor to store fuel or used for transportation to avoid extra handling. They barely need maintenance due to their excellent heat conduction and sound security. However， the high cost of individual cask increases the total input of storage when it involves a large amount of spent fuel storage or a constant increment.

In Japan， the research on dry storage metal cask includes temperature analysis， creep test of fuel cladding， seal test， drop test and building collapse test. The typical structure of metal cask consists of upper and lower impact limiter， primary lid and secondary lid， forged steel shell and external steel envelope， basket and trunnion. There are metallic gaskets set both under the primary and secondary lid. Between the inner and outer shell lies the neutron shielding material. Heat conductors cover the outer wall of the cask. The upper and lower impact limiter are installed at both ends of the cask with bolts to absorb impact energy generated by drop accidents during transportation.

France also carries research on metal dry spent fuel storage cask. It consists of two shells， impact limiters installed at both ends， primary lid， secondary lid and basket. The primary lid is equipped with drain and vent port， multiple seals and neutron shielding material. Between the shells set fins and neutron shielding while outside the shells， trunnions and lateral impact limiters. TN24 metal dry spent fuel storage cask currently has ten variations， mostly used in the United States.

The U.S advocates an integrated approach to the management of spent fuel. That is to design and produce generic thick walled metal casks that can be used for both storage and transportation. The HI-STAR 180 transportation and storage cask designed by the Holtec International of the U.S. can store 32 and 37 PWR fuel assemblies. The HI-STAR 190 cask designed for Ukraines central storage facility can store 31 PWR or 85 RBMK fuel assemblies.

2.2 Concrete Dry Storage Cask

Since metal cask storage requires relatively thick shells to shield radiation from spent fuel， companies such as Holtec and AREVA TN thus developed concrete storage casks.

HI-STORM（Holtec International Storage Module） is Holtec Internationals ventilated and retrievable concrete system for the dry storage of spent fuels. The system is designed to remove heat by natural convection. Air comes into the cask from the air inlets at the bottom of cask， flows up through the annular space between the concrete vessel and the MPC， and then comes out through the air outlets near the top of HI-STORM overpack， taking away the heat. HI-STORAM consists of top lid， the cask body and MPC. During the loading operations， HI-TRAC（Holtec International Transfer Cask） is utilized. HI-TRAC consists of outer and inner shell， bottom lid， top flange， intermediate shell， and lead shell that provides gamma shielding.

The spent fuel loading operation of HI-STORM： First， placing MPC inside HI-TRAC， using lift yoke to lift and lower HI-TRAC into the spent fuel pool， loading spent fuel assemblies into MPC basket， and seal-welding MPC lid. Second， removing HI-TRAC from the water pool and placing it in the designated preparation area， using adapter mating device to open up the bottom lid of the HI-TRAC， inserting MPC into HI-STROM， and installing the HI-STROM top lid.

NUHOMS is AREVA TNs retrievable and horizontal concrete dry storage module. The storage module is made of reinforced-concrete and steel structures， providing neutron and gamma shielding. The dry storage canister of the module mainly consists of canister body and basket. The body is fabricated from welded stainless steel， providing leak tightness and shielding of radioactive substance and inner inert gases （after helium backfilling）. Grapple ring is designed at the bottom of the dry storage canister to send the canister into the concrete storage module or retrieve it from the storage module. At the top of the canister are outer cover plate， inner cover plate and top shield plug that have drain holes and air vents to facilitate draining and inflating operations. Inside the storage canister is basket for loading spent fuel assemblies. The geometrical structure of the basket can ensure the criticality control over wet loading， drying， leak test， transporting， and storing operations.

The decay heat is removed from the storage module by a combination of radiation， heat conduction and natural convection. To protect the concrete surface and facilitate heat conduction， aluminum plate is designed in the storage module. Air comes into the storage module from air inlet at the bottom， circulates through the storage canister and the aluminum plate， and then comes out through air outlet at the top. The storage module is installed on a cast-in-place reinforced concrete slab， without having direct contact with it.

3 Existing condition of spent fuel storage in China

At present， there are 26 operating nuclear power units in China， producing 300 plus tons of spent fuel each year. It is estimated that by 2020 the total spent fuel accumulated will reach 10， 000 tons， with annual additional spent fuel of 1，400 tons. By 2025， the figures will be 18，000 tons and 2，000 tons； by 2030 when no considering new nuclear power plant is established， 27， 000 tons and 2，000 tons. The spent fuel produced in the Unit One and Unit Two of the Tianwan Nuclear Power Plant has been stored in spent fuel pool. According to the unloading plan， by 2016 when the two units have undergone the ninth general overhaul， the storage capacity of the spent fuel pool will reach the upper limit of its capacity. The spent fuel pool of the Qinshan Phase One Nuclear Power Plant will be full by 2025， but only satisfy the storage needs within the designed life cycle. To guarantee the effective operation of the power plant during the 20 years of extension， the spent fuel must be removed from the spent fuel pool to intermediary storage facilities. Otherwise， the power plant may be shutdown.

To address problems concerning spent fuel since the Unit One and Unit Two were put into operation in 2003， the Qinshan No. 3 Nuclear Power Plant is constructing interim dry storage facilities， including 18 storage modules of steel and concrete structure. In each module， there are 40 steel storage canisters. Every canister can contain 10 fuel baskets that each can hold 60 fuel rods. Therefore， each storage module can store 24，000 fuel rods. Two modules are planned to be built in the Qinshan No. 3 Plant every five years， besides the two existing concrete storage modules built in 2009. Other units or plants in China have no dry storage facilities.

China has developed policies for closed nuclear fuel（下轉第229頁） （上接第224頁）cycle. According to the primary design of reprocessing projects development， by 2020 reprocessing demonstration project with capacity of 200 tons per year will be established. Around 2030， the first reprocessing factory with capacity of 800 tons per year will be set up. However currently， on-construction and to-be-constructed reprocessing factories are able to meet all the needs of spent fuel storage and processing.

4 Conclusion

Spent fuel dry storage， which has been internationally acknowledge as a safer， more enduring and flexible storage method in comparison to the wet storage， meets the needs of away-from-reactor storage of China. China starts late in studying spent fuel dry storage techniques and its studies mainly focus on the spent fuel of research reactors and heavy water reactors， not on the high burn-up spent fuel of pressurized water reactors. To promptly fill the gap between the rapid development of nuclear power and the increasing demand of spent fuel storage， and to ensure the safe operation of nuclear plants， it is necessary for China to take active part in international study on the actuality and future trend of spent fuel dry storage techniques， actively study the techniques， related regulations and standards， and develop and test key facilities. It is particularly necessary to accelerate the research and development of dry storage casks that will create great social and economic benefits. At present， our company is seizing the day to develop dry storage casks with our own intellectual property. We look forward to develop the solution for the increasingly greater mismatch between the spent fuel generation and limited storage capacity.

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[責任編輯：王楠]