Helium Cooled Pebble Bed Concept Research Articles

A new HCPB breeding blanket for the EU DEMO: Evolution, rationale and preliminary performances

The Helium Cooled Pebble Bed (HCPB) Breeding Blanket (BB) is one of the 4 BB concepts being investigated in the EU for their possible implementation in DEMO. During 2014 the former “beer-box” BB concept based on the ITER’s HCPB Test Blanket Module suffered several design changes so as to meet the different counteracting nuclear, thermohydraulic and thermomechanical requirements. These studies evidenced that the concept is too rigid to meet the tight TBR requirements imposed for the EU DEMO (i.e. TBR≥1.10). Additionally, the complex manifold system with unbalanced helium mass flow in each of the 2 parallel cooling loops made the concept thermohydraulically complex. However, parametric studies during 2015 revealed that the HCPB concept have potential for a better nuclear performance, as well as margin for a significant simplification of the cooling internals by redefining the cooling plates and the architecture of the blanket, building a symmetric flow scheme.This paper describes the new HCPB concept based on an integrated FW with the breeding zone thermohydraulics and helium manifold systems. The former complex manifold backplates have been compacted and integrated in the cooling plates, releasing ≈300mm of radial space that can be used now to increase breeder zone, the neutron shielding, to reinforce the Back Supporting Structure (BSS) or basically to reduce the reactor size. Detailed neutronic analyses have yielded a TBR of ∼1.20 for the baseline design. Initial analyses show a correct thermohydraulic behavior. Preliminary thermomechanical analyses also indicate that the design can potentially withstand an in-box LOCA at 9 MPa at a level C according to the RCC-MRx code. Future consolidation activities are described, which shall lead to a concept meeting the BB requirements.

RAMI analysis for DEMO HCPB blanket concept cooling system

A preliminary RAMI (reliability, availability, maintainability and inspectability) assessment for the HCPB (helium cooled pebble bed) blanket cooling system based on currently available design for DEMO fusion power plant is presented. The following sub-systems were considered in the analysis: blanket modules, primary cooling loop including pipework and steam generators lines, pressure control system (PCS), coolant purification system (CPS) and secondary cooling system. For PCS and CPS systems an extrapolation from ITER Test Blanket Module corresponding systems was used as reference design in the analysis. Helium cooled pebble bed (HCPB) system reliability block diagrams (RBD) models were implemented taking into account: system reliability-wise configuration, operating schedule currently foreseen for DEMO, maintenance schedule and plant evolution schedule as well as failure and corrective maintenance models. A simulation of plant activity was then performed on implemented RBDs to estimate plant availability performance on a mission time of 30 calendar years. The resulting availability performance was finally compared to availability goals previously proposed for DEMO plant by a panel of experts. The study suggests that inherent availability goals proposed for DEMO PHTS system and Tokamak auxiliaries are potentially achievable for the primary loop of the HCPB concept cooling system, but not for the secondary loop. A sensitivity analysis is also presented to explore results dependency on key estimated parameters and analysis assumptions.

Requirements for helium cooled pebble bed blanket and R&D activities

This work aims to give an outline of the design requirements of the helium cooled pebble bed (HCPB) blanket and its associated R&D activities. In DEMO fusion reactor the plasma facing components have to fulfill several requirements dictated by safety and process sustainability criteria. In particular the blanket of a fusion reactor shall transfer the heat load coming from the plasma to the cooling system and also provide tritium breeding for the fuel cycle of the machine. KIT has been investigating and developed a helium-cooled blanket for more than three decades: the concept is based on the adoption of separated small lithium orthosilicate (tritium breeder) and beryllium (neutron multiplier) pebble beds, i.e. the HCPB blanket. One of the test blanket modules of ITER will be a HCPB type, aiming to demonstrate the soundness of the concept for the exploitation in future fusion power plants. A discussion is reported also on the development of the design criteria for the blanket to meet the requirements, such as tritium environmental release, also with reference to the TBM.The selection of materials and components to be used in a unique environment as the Tokamak of a fusion reactor requires dedicated several R&D activities. For instance, the performance of the coolant and the tritium self-sufficiency are key elements for the realization of the HCPB concept. Experimental campaigns have been conducted to select the materials to be used inside the solid breeder blanket and R&D activities have been carried out to support the design. The paper discusses also the program of future developments for the realization of the HCPB concept, also focusing to the specific campaigns necessary to qualify the TBM for its implementation in the ITER machine.

Neutronics experiment on a helium cooled pebble bed (HCPB) breeder blanket mock-up

A neutronics experiment has been performed in the frame of European Fusion Technology Program on a mock-up of the EU Test Blanket Module (TBM), HCPB concept, with the objective to validate the capability of nuclear data to predict nuclear responses, such as the tritium production rate (TPR), with qualified uncertainties. In the experiment, the TPR has been measured using Li 2CO 3 pellets located at various depths at two symmetrical positions at each depth, one in the upper and one in the lower breeder cassette. Three independent measurements were performed by ENEA, TUD/VKTA and JAEA. The neutron flux in the beryllium layer was measured as well using activation foils. The measured tritium production in the TBM ( E) was compared with the same quantity ( C) calculated by the MCNP-4C code using a very detailed model of the experimental set up. A sensitivity and uncertainty analysis has also been performed to evaluate the calculation uncertainty due to the uncertainty on neutron cross sections. The results of such analysis, carried out using both Monte Carlo and deterministic codes, shows that the uncertainty due to cross sections amounts to 4–5% (2 σ). C/ E ratios were obtained with a total uncertainty on the C/ E comparison of 9–10% (2 σ). The C/ E comparison shows that the calculation of the tritium production is underestimated by about 10% in average and is within the combined 2 σ uncertainty margin of the measurements and the calculations. The obtained results indicate that design calculations of the tritium breeding ratio provide conservative results for the fusion reactor employing a HCPB type breeder blanket.

Neutronic design issues of the WCLL and HCPB power plant models

This work addresses neutronic design issues of the two near term reactor models of the EU Power Plant Conceptual Study (PPCS) employing the water cooled lithium lead (WCLL) and the helium cooled pebble bed (HCPB) blankets for the tritium and the power production. Detailed three-dimensional torus sector models have been developed for the two PPCS reactor variants to enable proper design calculations with the MCNP Monte Carlo code. It is shown that both blanket concepts can provide tritium self-sufficiency although the tritium breeding potential of the HCPB concept is higher. The HCPB blanket also shows a higher energy multiplication which results, together with a higher thermal efficiency in the power conversion system, in a smaller fusion power required for the same net electrical power. The shielding efficiency of the two reactor models is shown to be similar. A total thickness of 1 m in for the inboard blanket/shield system is required to allow the operation of the power reactor over the anticipated lifetime of 40 full power years.

An improved European helium cooled pebble bed blanket

The Helium Cooled Pebble Bed (HCPB) blanket is one of the two European breeding blanket concepts for DEMO. In the framework of selecting reference blankets for the European Power Plant Conceptual Study (PPCS) performance limits of the HCPB concept have been explored. This paper shows the improved, moderate-technological-extrapolation HCPB blanket design including recent changes to the breeding zone that counteract a recuperator effect in the cooling plates. It highlights the attractive features of the design, like single-size Beryllium beds, large admissible neutron wall loads (NWL) and much improved electrical efficiency. In addition to two reference blanket cases proposed in the study, a third blanket based on the small NWL in the PPCS reactor is put forward. Finally, steam cycle efficiencies calculated with an up-to-date power plant code correct previous values for the improved HCPB blanket upwards by about 2%.