Table I. Main gamma sources
Table II Main neutron sources
Table III Maximum mass
Table IV Thermal dissipation capacity
Bernard Lenail and Vincent Roland
Transnucleaire
Paris, France
ABSTRACT
The TN 81 is the new dual purpose cask for high level vitrified waste from reprocessing. This new generation cask is another step in the recycling optimization. It accepts the higher neutron sources from reprocessing of higher burnup LWR spent fuel and shorter cooled vitrified high level wastes. It uses containment and shielding options, that are fully validated through the experience which Transnucléaire has gathered on the TN 28, TS 28 and TN 24 casks during the last ten years. It also complies with ICRP 60 neutron quality factor. The TN 81 accepts 25% more heat power within the same dimensional and weight limits as the current casks and almost the double of the dose rate contributing isotopes. This competitively priced cask will be available by 1999. Initially aimed at the German and the Swiss market, its features and versatility make it a worthy candidate for other needs in terms of transport and interim storage of high level wastes.
INTRODUCTION
Reprocessing contracts always provide for the return to the fuel owner of the wastes arising from reprocessing and specially High Activity Wastes (HAW). These HAW are vitrified then enclosed in stainless steel canisters that are intended to be interim stored during several decades either in a vault system or in a dual purpose cask. Each cylindrical canister holds 150 liters (400 kg) of glass, in which the activity corresponding to more than 1.3 t of spent fuel discharged from a reactor has been concentrated.
Other papers (Ref. 1 to 3) have been published about the tools developed by Transnucléaire for transport and for interim storage of HAW. These are the TN 28 VT for routine transport of HAW and the TS 28 V dual purpose cask, that is being used to transport and store HAW.
While these casks have demonstrated their good performance and behavior, more experience has been gathered on cask design, and the following new challenges must be met:
REVIEW OF EXISTING HAW CASKS AND OPERATIONAL EXPERIENCE
Two HAW casks designed by Transnucléaire are currently in operation:
Both types of casks are based on the Transnucléaire technology that combines a heavy forged body for mechanical strength and main gamma shielding and proprietary resin for neutron shielding. Their maximum decay heat dissipation is in the region of 45 kW depending on the storage conditions. They have been described in several papers and we shall now talk of their first operational experience. For leaktightness the TN 28 VT uses elastomer O rings and the TS 28 V uses metallic gaskets. The two models are designed for a load of 28 canisters. They can also be loaded with 20 hotter canisters only, when equipped with another basket.
Loading of the Casks
Cask loading is performed with an automatic remote controlled handling system at the NPH facility in La Hague.
The cask is deposited vertically onto a transfer lorry that moves along rails on the floor of a shielded cell. The cell is then closed, the lid remotely lifted off, and the cask is moved to its loading position where it is tightly connected to a lock installed in the ceiling. The canisters are automatically loaded into the cask basket partitions through the lock.
When four canisters are stacked in each of the seven partitions, the cask is disconnected from the lock. The lorry moves the cask back below its lid, which is lowered into place. Openings of limited size, made in the closure slab of the cell ceiling, give access to the lid from above while limiting operator exposure. The bolts are tightened and the leaktightness is checked.
The cask is then lifted up from the cell, a first check of radiation level is made and the transport shock absorbers are bolted on so that the cask is made ready for shipping; Extensive checking are run according to transport regulations and to the criteria of the receiving facility.
Shipping of the Casks
TN 28 VT. The TN 28 VT is placed on a shipping frame that is transported by road and rail to the Cherbourg harbor, then loaded in a dedicated ship scheduled for Japan. It is unloaded there, the canister condition is checked before they are transferred into the storage positions of a vault type intermediate storage facility.
The maiden voyage of the TN 28 VT focused some media attention during its first travel from La Hague to Japan in early 1995: the excellence of the design, its good shielding and thermal behavior together with its outstanding easy handling were demonstrated fully then.
TS 28 V. The TS 28 V is tied down onto a transport frame placed onto a heavy haul trailer. It travels by road to Valognes terminal and is transferred on a dedicated railwagon that transports it through France into Germany, headed to the Gorleben intermediate storage facility.
Once there it is prepared for long term intermediate storage: a secondary lid is bolted on, in order to provide an additional leaktightness barrier. The space between the lids is pressurized and connected to pressure sensors for permanent monitoring. The fact that the interlid pressure is set above inside pressure and atmospheric pressure provides a zero release system, since nothing can flow from the inside to the outside.
The maiden transport of the TS 28 V to Gorleben made the headlines in Europe during May 96 and the excellent ability of the cask under all operating conditions was demonstrated.
The operational interfaces were thus fully validated.
The techniques for developing, testing, licensing and operating the Transnucléaire HAW casks, a work of ten years of dedicated engineering, are now a tool ready for new developments.
MAIN CHARACTERISTICS OF THE NEW TN 81 DUAL PURPOSE CASK
Specification
The TN 81 is a transport/storage cask and, as such, complies with the Safety Series n°6 IAEA Radioactive Materials Transport Regulations for B(U) F packages.
The TN 81 accepts the maximum sources from Cogema and BNFL reprocessing plants in term of decay heat and of shielding needs.
Because the presence of HAW from Magnox fuel yields a rather different spectrum from that of waste from reprocessing of UO2 LWR fuel only, the optimization of the cask requires special attention.
The TN 81 complies with the Gorleben requirements for surface dose rates limitations at storage when measured with ICRP 60 calibrated meters, i.e. with a near doubling of the neutron equivalent dose rate with respect to current standards. The IAEA SS n°6 limitations for equivalent dose rates during transport are also complied with.
The TN 81 can dissipate 56 kW decay heat while keeping the vitrified HAW below the maximum allowable temperatures.
One of the challenging characteristics from HAW canisters is that, unlike for spent fuel casks where the burnup profile displays a marked maximum at cask mid height, the cask must be designed for a constant source on the full height of the basket: thermal dissipation and dose rates have to integrate this specifics. The lid bolting area is to be given special care for operator protection.
Tables I to IV illustrate the progress that has been made since the TS 28. For the purpose of Tables I and II, the isotopes are taken one by one. The dose rate limits are identical.
Table I. Main gamma sources
Table II Main neutron sources
Table III Maximum mass
Table IV Thermal dissipation capacity
Because the handling and transport interfaces are similar, only minor modifications in this field are allowable. One of the challenges is to reach the shielding performances while keeping a steady mass.
Because the outer dimensions are almost identical due to interface reasons, the ability to dissipate such an increased decay heat while keeping the glass canisters at the allowable temperature obviously requires additional heat exchange surface: this is obtained by extra outer fins. Furthermore the design work concentrated on keeping the temperature gradients along the heat transfer paths as low as possible.
The evolution of the specifications is quite significant. The main features of the TN 81 are described hereafter (Fig. 1)
Fig. 1. The TN 81
Cask.
CONFIGURATIONS
The TN 81 cask is designed to be transported as a Type B (U) F package with either the primary lid or the secondary lid. It can be handled horizontally by 4 trunnions and vertically by 2 trunnions. The lower trunnions serve as axis for tilting the cask on conventional transport conveyances.
The TN 81 is normally stored in vertical position without bottom shock absorbing cover. In the normal use, the cask is equipped on the storage pad with the secondary lid and a monitoring system, the volume between primary and secondary lid being pressurized with helium to a pressure exceeding that of the cavity.
Body
The approx. 20 cm thick body is a forged carbon steel shell assembled to a thick forged bottom by heat shrinkage and leaktightness weld. The mechanical strength of the cask stems principally from this forging. The outer face of the body is fitted with bolted aluminum boxes acting as radial heat conductors and containing the neutron shielding resin. These boxes are extruded aluminum profiles that also feature integral outer cooling fins. Thus heat transfer gaps are kept to a minimum. The bottom side is extended by an integral drum made of thick plates containing neutron shielding. The cask sits on the drum for storage.
The primary lid is a semi-recessed forged steel disk, flange bolted, topped by a neutron shielding compartment and a handling pintle. The secondary lid is a welded assembly, pressure resistant component composed of a flat head, a shell and a bolted flange. All gasket seats are protected by local stainless steel weld overlays. All external surfaces exposed to atmosphere are protected by painting for easy maintenance.
Basket
The TN 81 basket is made from 6 W-shaped copper plates, bolted together so that they create 6 peripheral and 1 central loading positions. Copper has been chosen for its outstanding heat conduction.
The ends of the plates are bent on several centimeters to allow fitting and bolting onto the curved inside of cask body along the height of the cavity generatrix. The purpose of this bolting is to control the gaps for heat transfer from basket to cavity.
For easier installation inside the cavity, the basket is divided into 3 sections of equal height that are stacked.
Containment
A closure system consisting of the cask body itself and of several lids is installed in the following order:
Despite their actual leaktightness, the contribution to containment by the canisters themselves is not taken into account for gaseous products and the hypothesis is made that at last one is defective under normal operation conditions. Long term leaktightness is assured by the use of metallic gaskets that bear on stainless steel seats.
Leaktightness of the containment in any normal operation configuration is better than 10-8 Pa.m3/s. In other words, the leaktightness level is such that it would take more than 100 years of operation to have a possibility of release.
A service lid might be added in case further interim storage should be preferred to final disposal of canisters or refurbishing of the primary lid.
Shielding
Neutron shielding is achieved through the use of a proprietary polyester resin, the efficiency of which has been demonstrated by several operational casks. One of the advantages of the resin is that its thickness can be adjusted with respect to the exponentially increasing neutron sources stemming from reprocessing of higher burnup spent fuel. Because the implementation of the neutron shielding is independent from that of the main gamma shielding, the ICRP 60 recommendation, that features a neutron quality factor of 20 instead of 10 can be complied with by working on its thickness.
Furthermore, with a density of approximately 2, the resin makes a sizeable contribution to the overall gamma shielding.
The main gamma shielding is made up by the thick steel forging that is the main component of the body and by lead inserted in the outer aluminum extruded profiles.
Benchmarks made with the existing casks allow for optimization of the cask shielding: it is the careful balance between steel, lead and resin that make possible the high performance of the cask within the set mass and dimensional limits.
Heat Dissipation
The decay heat is transferred from the canisters to the copper basket by radiation and by conduction by the helium gas filler. Then by conduction through the basket, the forged shell, the aluminum boxes (Fig. 1). Ultimately from the finned outer surface to the atmosphere through natural convection and heat radiation.
The minimization of gaps and material transitions keep the gradients as low as possible.
The heat dissipation evaluation is also based upon actual tests results from the existing HAW casks.
Transnucléaire has tested on a scale one partial model the behavior of the aluminum fins and that of the lead beneath the resin under fire condition, in order to make sure that no major shielding loss occurs.
CONCLUSION
Transport/storage cask technique progresses with every new cask, with every benchmark that helps improving theory on the basis of operation. The TN 81 is the HAW cask of the nineties as the TS 28 V and the TN 28 VT were the casks of the eighties. Its outstanding neutron and overall shielding performance will carry it well into the 21st Century.
The quest for an improved design is also results in improved pricing: the TN 81 features manufacturing improvements and costs significantly less to manufacture than current casks.
Even with the current sources, the TN 81 casks carries the advantage of lower dose rates to the public, the operators and the environment. In addition, it accepts in batches of 28 canisters sources that were to be transported not by batches of 28 canisters but by batches of 20 canisters with the older types of casks: the TN 81 improves safety by reducing the number of transports by a factor of 1.4.
REFERENCES