For the JULES HOROWITZ REACTOR (RJH) which construction is planned at Cadarache, CEA and Technicatome designed a fuel element which has some similarities with the design of BR2 fuel, but nevertheless needs to be qualified full-size. BR2 appears to be the only reactor able to provide adequate irradiation conditions:
- 8 plates fuel element, generating almost twice the power of a BR2 fuel element
- 96 mm O.D. fuel element
- U3 Si2 fuel enriched at 27%
- Up to 485 W/cm² at the hot spot (500 W/cm² with uncertainties)
- Up to 50% burnup reached in 4 BR2 cycles
- Up to 18 m/s coolant velocity
The latter condition cannot be met with the circulation of BR2 alone and an additional pump is needed. The 96 mm outer diameter requires the use of the heavily modified central H1 channel of BR2. Standard channels, with their 80 mm usable diameter, are unsuitable. The modification consists in the replacement of the normal Beryllium plug by a composite plug, made of 70% aluminium and 30% water. This composition exhibits the optimum absorption/moderation/transmission properties for a fresh – reactive – RJH fuel.
When the burnup increases in the RJH fuel – which does not contain burnable poisons – its reactivity decreases and the aluminium plug must be replaced by a beryllium plug, in order to be able to continue the irradiation without too much detrimental effect on the other users of BR2.
In EVITA, water is taken above the core of BR2 in two unused channels at the periphery and injected in the central irradiation channel H1 by a booster pump. Hence, the ΔP provided by the primary circulation of BR2 is available for EVITA. However, the available ΔP is too low to allow the flow rate and velocity required by the experiment. Therefore, an additional pump is required to raise the ΔP in the chosen irradiation channel.
The water is taken in the upper plenum of BR2 via two unused peripheral channels. Instead of allowing the flow downwards through the reflector, the suction channels forces it upwards to a pipe connected to the suction side of a booster pump, where its available ΔP is increased. The booster pump is installed submerged on BR2 working floor, at the level of BR2 top cover, close to the access ladder. The pumped water is injected in a modified central channel H1 of BR2 containing the fuel to be tested. The water re-mixes with BR2 water in the lower plenum.
In the chosen configuration, there are always two pumps in series (BR2 primary pumps and EVITA booster pump) and no mobile flow restriction in the circuit. The main circuit of EVITA is completely enclosed in BR2 pool, thereby providing radiation protection for the personnel during the operation, insensitivity to leaks and protection of BR2 in case of piping rupture. EVITA takes profit of all safety systems and protections of BR2.
The benefits of this configuration outweigh the cost increase for the submerged pump:
- No penetration of piping through the pool walls is used
- No room occupation around the pool (space availability is limited there)
- No need to install radiation shielding
- No additional risk for BR2 in case of rupture of the piping
- Improved safety, when compared to non-submerged solution:
- The ΔP provided by BR2 primary circulation remains available in case of failure of the booster pump. So reduced cooling remains available to remove the decay heat of the tested fuel.
- EVITA being submerged in BR2 pool, a breach in EVITA piping will not lead to loss of coolant (but can lead to loss of flow).
- EVITA uses mostly BR2's own protection systems.
In this irradiation, the challenge is to operate BR2 with a fuel element in its centre, operating at a required power level, while still being able to operate the reactor at an adequate power level for the other users at the periphery. This is further complicated by the fuel design, which does not include the burnable poisons which would help to mitigate the reactivity decrease with burnup.
The reactivity compensation is achieved by the choice of materials surrounding the central fuel, a mixture aluminium-water which presents optimum absorption – moderation – transmission characteristics. Then, by removing absorbing targets laid in the six satellite channels surrounding the EVITA channel, it is possible to compensate the decrease of reactivity of the central fuel caused by burn-up accumulation and still operate BR2 at constant power. Finally, when the reactivity has sufficiently dropped, the aluminium-water plug is replaced by a beryllium plug.
The accuracy of the BR2 load determination before each cycle will be a critical element for this irradiation.
Fuel to be tested
The fuel to be tested is a full-size fuel element for the Jules Horowitz Reactor. The RJH fuel element is made of 24 curved plates assembled 3 by 3 in 120° sectors, forming 8 concentric circles. This is quite comparable to the BR2 design, but they are shorter (fuel length is 600 mm compared to 760), larger (outer diameter is 96 mm) and have a larger central hole (40 mm in place of 1"). BR2 has channels of 84,2 mm (3 1/3") and 203,2 mm (8") diameter. The irradiation of a complete fuel element requires the use of a modified 203,2 mm channel. The use of a channel of 84,2 mm would only allow the irradiation of an element reduced to 5 or 4 plates, which is not acceptable for qualification purposes.
The fissile material is U3 Si2 , enriched at 27 %, which does not qualify as LEU (> 20%) but rather "MEU". It does not include burnable poisons but there is a plate of Boral at its lower end to absorb the flux increase at the core outlet.