THE TN 24 DUAL PURPOSE CASK FAMILY FOR SPENT FUEL: FACTUAL EXPERIENCE AND TRENDS FOR FUTURE DEVELOPMENT

B. Kirchner
Transnucléaire
9, rue Christophe Colomb
75008 PARIS

ABSTRACT

As the policy for the spent nuclear fuel is not yet clearly defined in several important countries, a number of nuclear electricity generating Utilities have adopted a 'wait and see' attitude and need means to interim store increasing quantities of spent fuel for long time periods.

Among the various systems offered for interim storage of spent fuel, dual purpose metallic casks present a high safety level associated with a maximum flexibility, while being always competitive for Utilities which want to follow safety rules based on a wide international consensus and which are operating a small number of nuclear power reactors.

The paper will encompass three main stages covering the design, manufacture and operation of various TN 24 (Fig. 1.) forged steel casks that have been tailored to different types of LWR spent fuel.


Fig. 1. TN24 Cask, General View

Design: How to accommodate the different batches of spent fuel assemblies present in a given reactor pool taking into account the various shielding needs and interface criteria will be addressed.

This approach will provide means for an optimization of the spent fuel management strategy.

Manufacture: Combination of industrial partners around a forgemaster and a boilermaker in order to produce casks within a tight time schedule, with strict application of Quality Assurance rules, under the best price conditions and with the maximum benefits for local industry will be described by means of several examples.

Operation: Examples of cask operation will be reported.

As a conclusion, some statements will be proposed about how to balance safety concerns with economical considerations while selecting among the various interim storage systems offered.

INTRODUCTION

In many countries around the world, where electricity is generated from nuclear energy, the back end of the fuel cycle has not yet been clearly defined by the Authorities. As a consequence, the Utilities have generally adopted a 'wait and see' policy consisting in providing means to interim store all or a part of their spent fuel for long time periods.

In most West-European countries, the nuclear storage facilities must be justified against high level safety criteria. As a consequence, when storage duration is expected to be long, the selected system for spent fuel storage is generally based on dry metallic casks that must be designed to meet, first and foremost, the Type B(U) requirements of the IAEA Transport Regulations (1) which constitute the well known basis of an international consensus in matter of safety for highly radioactive materials. Moreover such transportable storage casks, also called 'dual purpose casks', provide a great flexibility in allowing to easily move the interim stored spent fuel whenever needed.

In many reactors of Western Europe, the burnup of the nuclear spent fuel has now reached very high levels, in the range of 45, 50 or 55 GW.d/MtHM, and in addition more and more MOX fuel assemblies are used. As a consequence, the neutron source, which is exponentially increasing versus burnup, now becomes very strong and requires highly efficient neutron shield, more especially as this strong neutron source is decaying very slowly with time.

For more than two decades, Transnucléaire has developed a cask technology based on a forged steel body (as main gamma shield) surrounded by a hydrogenous resin layer (as main neutron shield). As long as fuel burnup was low enough, several other cask technologies, that provide a weak adaptability to various spent fuel characteristics, and in particular those showing limited performances in neutron shielding, could nevertheless be competitive. But today, and tomorrow still more, the 'forged steel / hydrogenous resin' technology appears to give the most efficient solution, as regards both safety and cost, to achieve high capacity casks. This is mainly the result of the forged steel technology additionally providing a great flexibility to cope with any specific characteristics of spent fuel assemblies because the gamma and neutron shielding functions are basically independent. This allows the conceptual design to be easily adjusted to maximize the cask capacity within given constraints of dimension and weight.

This ability to maximize the cask capacity provides important benefits for both types of cask use, transport or storage, in reducing the number of cask loading/unloading operations and in-site/off-site shipments for a given amount of spent fuel concerned: this results in lower exposure for the operators, together with money savings.

The numerous cask designs that have been developed and manufactured based on this technology to serve the various needs related to the spent fuel of different Utilities constitute the TN 24 cask family. We will now describe the main features common to this cask family regarding design, manufacture and operation.

THE MAIN FEATURES OF THE TN 24 DESIGN

Defining the Contents the Cask is Designed for

Due to their initial use for transport needs, spent fuel casks are often designed to be versatile so as to cover wide ranges of irradiation characteristics of the spent fuel assemblies (SFA) contained.

When the main purpose is storage, this view is not adequate, because the requirement to cover an eventual small number of SFAs with high burnup and short cooling time would impose greater shielding thicknesses that might drastically reduce the cask capacity, while being quite useless for a possibly large majority of SFAs with lower burnup and/or longer cooling time. On the contrary the casks designed for storage should be tailored as closely as possible to definite batches of SFAs with rather homogeneous irradiation characteristics in order to maximize the cask capacity in each case. Indeed, as soon as one cask design can be amortized on five or six units, the money saving due to the increase of cask capacity will be generally much higher than the additional cost for development and licensing .

This advantage may be further enhanced when the expected loading plan of each cask unit can be considered from the early stage of cask design, based on the knowledge of irradiation characteristics of the SFAs that will be present in the reactor pool at the time of loading. As the SFAs are generally put in dry storage after more than five years of cooling, the corresponding characteristics are well known more than five years before cask loading, which provides enough time to prepare optimized cask loading plans, to develop optimized cask designs accordingly, to have them licensed, and to manufacture the necessary cask units in due time. Using this spent fuel management method, it is even possible to limit the number of cask designs that will be necessary to serve a given reactor. According to our experience, when the matter is well managed, two or three cask designs are sufficient to efficiently cover the interim storage need for all SFAs produced by one reactor during 20 years of operation. Even then, these two or three cask designs are quite similar, only differing by the cavity diameter and by a few centimeters in the neutron and gamma shielding thicknesses so that development and licensing will be much simplified.

In addition to the substantial savings already mentioned, this method is also beneficial in terms of safety and environmental quality. In fact the constant effort toward the largest possible cask capacity reduces, for a given amount of spent fuel, the number of cask loading/unloading operations (and therefore the total dose uptake to the operators), the number of loaded cask shipments (which is certainly advantageous to the public), and the number of casks standing on the storage pad (which is worthwhile to both the Utility and environment).

But it should be pointed out that all these significant benefits are only possible for a cask technology that is really flexible regarding the cavity diameter and shielding material thicknesses. Such a flexibility is easily achieved by a forged steel body (whose soundness and cost will not be affected by a change in diameter and thickness) surrounded by a layer of resin poured between the forged body and an outer steel shell (whose manufacturing process is quite independent of its thickness). Of course a similar flexibility is not at all possible when the technology involves a structural and/or shielding material that must be poured, because the soundness of such a cast metal will be closely related to its cooling conditions after pouring, which therefore can only be qualified for a precisely defined geometry, so that any change in geometry will require a re-qualification of the manufacturing procedure.

Increasing the Thermal Performances Without Detriment to Main Cask Features

When designing a new cask concept, it often happens that the aimed capacity must be reduced not due to lack of shielding or to excessive weight, but for thermal reasons. Although there is room enough for the aimed number of SFAs with the required thicknesses of shielding material around the cavity, the residual heat power appears to be 20% or 50% too high compared to what could be dissipated in the ambient atmosphere in satisfying the temperature limitations imposed to the cask materials and/or contents.

In such a case the cask designer generally decides to fix additional cooling fins to the outer cylindrical surface of the cask body. But sometimes this kind of solution is not possible because it would increase too much the cask overall diameter which would then exceed the maximum dimension available for cask handling at reactor site. Moreover, such protruding fins could create contamination problems during wet loading at reactor pool.

In order to significantly increase the heat dissipation capability with a low impact on the overall outer dimensions, Transnucléaire has proposed a method for which a patent has been applied for.

Designing a Square Shaped Cask

Sometimes the space available to move a heavy cask within the reactor pool building is so limited, for instance when crossing a gate, or due to the size constraints associated with the lifting beam or transport vehicle, that a good solution may consist in taking advantage of the square array of the basket lodgements intended for western type LWR fuel to design a cask with a squared general shape in its cross section.

For this purpose it is possible to re-arrange the internal cavity into a quasi-square cross section by putting additional shielding plates machined on a lathe so as to match the cylindrical internal cavity surface, while the outer surface of the forged shell is also squared by machining the steel shielding in excess.

This method, which has been patented, can be easily implemented by simple machining operations, using an appropriate jig, without detriment to the structural resistance of the product, whereas a square configuration obtained with a material processed by pouring would jeopardize the axi-symmetry of the casting and therefore the material soundness justification.

This interesting design solution is used in several TN 24 concepts.

A New Fixation Mode for the Bottom

Another innovation that has been patented and introduced into several TN 24 concepts is related to the bottom fixation mode. The idea is to design the bottom in a similar way as the lid, i.e. with a flange able to withstand hard impacts, the bolts being replaced by a weld with an equivalent strength which also provides for leak tightness.

As a consequence a full penetration weld limited to a small fraction of the bottom thickness is amply sufficient because, even in the case of hardest impacts, regardless of the orientation, the solicitation to the weld is weak or nil. Moreover, for licensing purposes, a justification similar to that of the lid will apply.

An Efficient Technology for the Basket

In addition to this efficient technology of cask body, Transnucléaire has also developed for 15 years a particularly efficient technology for the internal arrangement which uses boronated aluminum. Several basket concepts based on different principles and using this kind of material with various chemical compositions have been patented.

Boronated aluminum presents several significant advantages to build a spent fuel cask basket. First of all, it is particularly efficient to control criticality. In addition, it has a high thermal conductivity allowing to dissipate a large heat power within a small volume. It also features good mechanical characteristics which might be of interest in some cases. Lastly it is a low density material so that all preceding functions are ensured by a light basket. Thanks to these interesting characteristics, boronated aluminum has been used to manufacture more than one hundred baskets equipping 100 t class casks fully qualified against IAEA Transport Regulations (Safety Series n 6) (1) and currently used for 15 years by most nuclear countries which have to implement important programs of spent fuel shipments on either a national or international basis. Let us recall that, contrary to the former Transnucléaire basket concept based on a pure aluminum casting incorporating neutron poison sintered plates, all boronated aluminum based baskets produced today can be thoroughly dried in far less than one day even when the residual heat power is very low, and generally within 3 to 6 hours.

It is quite regrettable that, in the USA, boronated aluminum has not yet been accepted as structural material to be used in any basket for spent fuel transport casks, even when based on an appropriate qualification of the material (4) associated with a full justification of the design. Indeed this potentially represents a serious loss in terms of safety. As a consequence, the few transport casks available for spent fuel in this country have a very limited capacity so that, for a given amount of spent fuel to be moved, the shipments will be significantly more numerous than what would be required in any other country, therefore increasing in the same proportion the risk of accident, the dose uptake to the operators and the exposure of the public and environment. This is all the more surprising as in many cases the structural role of the basket does not involve any safety function, including criticality, so that even if the basket were to collapse for any reason, nuclear safety would nevertheless be ensured.. Since almost no spent fuel has to be moved for the time being in this country, the drawback is still limited. Hopefully a more enlightened approach will be taken before the need for many spent fuel shipments arises.

Among the different basic principles available to design a fuel basket, one is particularly interesting: it is directly derived from the under moderated configuration of the reactor core which obviously has generally been designed with a view to be safely controlled against criticality. Accordingly a cask basket concept (or a pool rack concept) that would use a similar configuration, i.e. a similar geometry with a similar moderation ratio and a higher poisoning ratio than that existing one in the core under shut down conditions, would be even safer from criticality point of view than the core itself after shut down. As a result, a basket (or a rack) made from a simple assembly of contiguous boronated aluminum tubes (intended to provide additional neutron poison within the fuel medium, and to collect the residual heat power from the fuel assemblies and dissipate it toward the periphery when under dry conditions) associated with poison rods inserted inside the guide-tubes of the fuel assemblies when there exist, will constitute the safest means to guarantee under every circumstances the subcriticality of fuel assemblies when they are outside the reactor core. Indeed this is the only conceptual design that can be intrinsically safe even when, due to any external impact of any intensity, the basket (or rack) were to deform, or collapse, or rupture, because with such a tight configuration, even when ruptured, the poisoned material will remain in place within the fissile medium where the moderation ratio (and consequently reactivity) will have further decreased. Moreover it has been checked through a test using a full scale model that, under precisely defined dry conditions, such a basket constituted by an assembly of contiguous tubes has excellent thermal performances.

It is easy to see that, compared to the above concept, some existing basket designs are not intrinsically safe, including those where the boronated material needed for criticality control is not used as structural material. This is still much more true when considering those designs where credit is given to burnup in fuel assemblies or, what is worse, to boron in pool water. Obviously it is quite easy to design a basket (or a rack) using this kind of trick and to correctly demonstrate in a good safety analysis report that it allows to control criticality under every circumstances provided it is loaded with the specified fuel assemblies. But the danger lies precisely here. A low burnup fuel assembly is not visibly different from a high burnup fuel assembly. Even when loading procedures are implemented under Quality Assurance and require the use of a burnup-meter, a human error cannot be ruled out. If we consider that many reactor pools are filled with numerous fuel assemblies of different initial enrichments and various irradiation characteristics to be loaded into several types of casks, some with and some without burnup/boron credit, then the most elementary cautiousness recommends to take into account the fact that the probability of a human error during cask loading might be high. Therefore wouldn't it be quite irresponsible and also inconsequent to use a highly hazardous concept based on burnup/boron credit with a view to realize some poor savings whereas an intrinsically safe way exists to do the job at a reasonable cost?

It may be worth recalling a recent incident occurred during a canister loading operation where the boronated pool water reacted with the canister zinc coating and produced hydrogen that exploded. Fortunately, the amount of boron that had escaped from the fuel medium by precipitation as zinc borate was not yet enough to additionally produce the criticality flash.

MANUFACTURE OF TN 24 CASKS

All casks of the TN 24 family take advantage of both basic technologies, 'forged steel / hydrogenous resin' for the cask body and 'boronated aluminum' for the basket, under various conceptual designs. Up to now four different types of TN 24 casks have been manufactured and/or put in operation:

TN 24 P for 24 intact PWR 900 fuel assemblies with a 35 GW.d/MtU burnup and a 5 year cooling time, or 48 consolidated PWR 900 fuel assemblies with a 30 GW.d/MtU burnup and a 7 year cooling time ; it has been licensed for transport in France and for storage in the USA ; one unit has been manufactured in Japan and delivered to Idaho National Engineering Laboratory (INEL) where its thermal and shielding performances have been tested as part of a DOE demonstration program (2) (3)

TN 24 D for 28 PWR 900 fuel assemblies with a 36 GW.d/MtU burnup and an 8 year cooling time ; it has been licensed in France for transport and in Belgium for transport and storage ; 8 units were manufactured in Belgium using Italian forgings and have been in operation at Doel nuclear power plant (Belgium) to serve reactor N 3 ; another two units are currently under manufacture

TN 24 XL for 24 PWR 1300 fuel assemblies with a 40 GW.d/MtU burnup and an 8 year cooling time ; in a near future it will be licensed in France for transport and in Belgium for transport and storage ; 6 units are under manufacture in Belgium using Italian forgings to serve Doel N 4 reactor

TN 24 G for 37 PWR 900 fuel assemblies with a burnup around 40 GW.d/MtU and a cooling time exceeding 9 years ; licensing in France for transport and in Switzerland for transport and storage is in progress ; 4 units are under manufacture, 2 units for delivery in September 1998, one unit in September 1999 and one unit in September 2000.

Several other TN 24 concepts have been prepared for many other needs and will be developed in the coming years.

To build forged steel casks of the TN 24 family, the manufacturing processes are well known in many countries around the world. They mainly include the know-how of forgemaster on one hand, and that of boilermaker on the other hand, in the same way as for the hundred casks of the TN 12 family that have been produced over a period of 15 years by a dozen manufacturers belonging to six different countries, strictly controlled according to Quality Assurance rules.

This means there is a long experience of fabrication available for this type of casks, and also a fair competition possible in many countries. A combination of a forgemaster in a country with a boilermaker in another country has also been implemented successfully.

Therefore it has been proved that manufacture of TN 24 casks can be achieved in most countries, with strict application of Quality Assurance criteria, under market controlled conditions of price and with the maximum benefits for the local industry.

OPERATION OF TN 24 CASKS

For the time being, the most important practices of TN 24 cask operation have taken place in the USA where many different tests (dry loadings and unloadings, inserting poison rods inside guide-tubes, leak tightness tests, dose rate and temperature measurements, cask handling and transfer to storage pad, etc.) have been implemented on only one cask (TN 24 P), and in Belgium where a more limited number of tests (wet loading, leak testing, dose rate and temperature measurements, cask handling to reactor pool and back, cask transfer to storage building) have been implemented on a larger number of units.

But it should be noted that both the design of cask interfaces (orifices, handling attachments, etc.) and operation procedures take advantage of the long experience gained with the hundred casks of the TN 12 family operated for 15 years at about 40 nuclear power sites located in Western Europe and in Japan as well as at La Hague reprocessing plant.

Therefore it has been proved that TN 24 casks can be operated safely taking advantage not only of the various auxiliary equipment design features extensively experienced in the past, but also of a few incidents that occurred here and there and corresponding corrective actions that lead to improve both the design features and operational procedures.

CONCLUSION

Although the countries where reprocessing has been chosen for the back end policy have henceforth less need of interim storage system, these countries are those where most experience has been gained about spent fuel transports and casks, which provides the best know-how to develop efficient and safe systems for long term interim storage.

Since Chernobyl any major mistake involving criticality hazard or activity release in the environment is not at all acceptable to the public, wherever it might happen. In a certain sense, all nuclear countries are co-responsible in that matter toward the world public opinion.

For the time being, all knowledge required to safely manage the large amount of spent fuel that has been produced for 20 or 30 years by some 400 nuclear reactors generating electricity in the world are available, so that, provided the decision makers honestly select solutions based on factual considerations without blindness or 'a priori', then the back end strategies appropriate to each particular case can be implemented quietly, step by step, without endangering the public or environment and without worrying the media.

Hopefully all Chief Executives of nuclear Utilities everywhere in the world, with the assistance and under the control of their Competent Authorities, will keep conscious of the huge stakes involved and, within their choices for the selection of a spent fuel storage system, will favor a reasonable safety level instead of poor money savings.

REFERENCES

  1. Regulations for the Safe Transport of Radioactive Material, 1985 Edition (As Amended 1990), IAEA, Vienna 1990
  2. The TN-24P PWR Spent-Fuel Storage Cask: Testing and Analyzes, EPRI NP-5128 April 1987
  3. Testing and Analyzes of the TN-24P PWR Spent-Fuel Dry Storage Cask Loaded with Consolidated Fuel, EPRI NP-6191, February 1989
  4. Use of Boron in Structural Materials for Transport Packagings, by V. Roland and H. Issard, Proceedings of PATRAM '95, Las Vegas, December 1995